Electret microphones and a parabolic reflector

One final – well, maybe not final, we’ll see how it goes! – type of microphone I wanted to try while out field recording was a parabolic dish or reflector.  I planned to use electret microphones in the way described in the series of articles beginning here.

Strictly speaking, the three-dimensional shape of the parabolic reflector is called a paraboloid, and the adjective is paraboloidal. A parabola is the two-dimensional shape and the distinction between this and a parabaloid is like that between a sphere and a circle, according to the Wikipedia.  However, in informal language, the word parabola and its associated adjective parabolic are usually used in place of paraboloid and paraboloidal.

So, this is the shape of the dish.  Note that there is a point marked ‘focus’.

Diagram by Melikamp – Own work, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=28181019

So, now we know exactly what we’re talking about!

*

As with some of my other recent experiments, it’s not so much the microphone itself as the way it’s mounted that’s significant; and the significance of the particular shape of the parabolic dish is that all the sound captured within it is reflected back and focused on a single point a few centimetres from the centre of the dish. The effect of this is to naturally amplify the sound captured – and amplify it by quite a lot.

This diagram illustrated how the sounds coming into the dish are all focused on the same spot – the spot where the microphone is placed, facing back into the dish.

Own work assumed (based on copyright claims)., Public Domain,
https://commons.wikimedia.org/w/index.php?curid=3222214

In addition to this, the captured sound is from a restricted area, directly in front of the dish, so what it allows you to do is pick out an individual sound source – a person, bird or animal, machine or natural feature – some distance away and record it without having to get too close, which may cause disturbance, or resort to extreme amplification, which may cause noise or instability.

This is basically the audio equivalent of using a telescope – and, indeed, astronomical telescopes – not just optical, but also radio – use parabolic reflectors to focus light or electromagnetic waves, as do satellite TV dishes.

This photograph from the Wikipedia, showing the receiver from the MERLIN array at the Mullard Radio Astronomy Observatory, Cambridgeshire, is essentially a giant version of the parabolic reflector microphone, and illustrates the reflector’s features: the shape of the dish and the focus point – usually in the centre (although typically on the edge of a TV satellite dish).

Photograph y Cmglee – Own work, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=33433585

The idea of using a parabolic reflector to gather sound from a distance has been going for a long time – since classical antiquity, in fact, as the Wikipedia points out, when the mathematician Diocles described them in his book On Burning Mirrors, and it has been claimed (although probably wrongly) that Archimedes used parabolic reflectors to set the Roman fleet alight during the Siege of Syracuse in 213–212 BCE.

In the UK, as far back as the First World War, giant concrete ‘sound mirrors’ were erected on the south and east coasts. Before the invention of radar, using these structures to listen for the sound of their engines was the most effective way of detecting the approach of enemy aircraft.

The caption to the above photograph – also from the Wikipedia – says: ‘On the pipe in front of the acoustic mirror was a trumpet-shaped ‘collector head’, a microphone which could pick up the reflected engine sound of Zeppelins approaching from the sea. Wires passed down the pipe to a listener seated in a trench nearby with a stethoscope headset, who would try to determine the distance and bearing of any enemy airships.’

[Photograph by Paul Glazzard, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=1667862 – ‘WW1 Acoustic Mirror, Kilnsea, East Riding of Yorkshire, England. Rare 4.5 metre high concrete structure near Kilnsea Grange, northwest of Godwin Battery, a relic of the First World War.’]

This photograph from the same source shows 3 ‘Listening Ears’ together, near Greatstone-on-Sea, Kent.

[RAF Denge photograph by Paul Russon, CC BY-SA 2.0,
https://commons.wikimedia.org/w/index.php?curid=1511357]

A great collection of photographs of a whole range of these sound mirrors from Selsey to Sunderland by Joe Pettet-Smith is featured on this page from the BBC website.

*

Normally, commercial parabolic microphones are extremely expensive, although excellent ones are available from companies such as Telinga and Wildtronics.

As usual, I tried to do things on a budget, but finding a suitable parabolic dish proved difficult – bearing in mind that the parabolic shape itself is the important thing, as explained above, and a plastic bowl of some other type wouldn’t work as well.

Other factors included size and weight. The reason for the large size of the coastal ‘sound mirrors’ was not just the aim of collecting sound over a large distance; the size of the dish also determines how easy it is to detect low-frequency sounds. In the case of the sound mirrors, the low frequencies of aircraft and airship engines were a priority. This also has to be borne in mind with the portable reflector, which will inevitably be more suited to higher frequency sounds.  This partly explains its popularity amongst those who go out to record birdsong.

As far as weight is concerned, you have to take into account that the dish might have to be carried for quite a while in the field. Wildtronics, in particular, make a point of stating the weights of their dishes, to the extent of naming their thinnest variety Feather Light, and emphasising that it can be folded or even rolled for transportation. There’s heaps of information online about satellite TV dishes – and you’d think a second hand one of these would be a good bet, cost-wise – but nothing about how much they weigh.  However, they look heavy to me, and their particular design style, with the focus point well outside the rim of the dish, makes it seems as if they’d be difficult to wind-proof.

At the other end of the scale, I almost went for this hand-held item below.  However, although it’s much bigger than it looks – some 25cm (10″) diameter – and despite more positive than negative reviews on Amazon, it really did seem a little too expensive (around £25) and a little too small to me, and would almost certainly not be that effective – I’m looking out for a cheaper second-hand one on eBay to give it a try, though!

[Edit: I recently managed to get hold of one for only just over £20, and made a few modifications to it to make it a practical device to use.  I’ve written it up here]

So, in the end, I went with a UK firm I found, who make a decent reflector at a very reasonable price – OK, more expensive than most of my other projects, but reasonable indeed in the world of commercial parabolics. This was Innercore; or Parabolic Microphone, who make a 50cm ABS plastic reflector for about £65 with an integral stem for microphone mounting, a rubberized hand grip and a standard tripod mounting thread. I also bought their spandex wind shield for an extra £10, as I know from experience that wind can be a real destroyer of decent recordings in the field. As the dish is white, a black cover would, in any case, be a good thing from the point of view of concealing the dish – avoiding disturbing wildlife, and so on.

*

When the dish arrived, it was exactly as described, and, as well as the windshield, even included a microphone, User Guide and cable ties to attach the microphone to the central stem.

The central stem had the focal point clearly marked.  As the picture shows, a handle was attached on the back.  The dish, made of ABS plastic, as I said, was surprisingly light and could comfortably be carried for some time; but in the base of the handle is a hole with a standard 1/4″ thread in it, which would fit a photographic tripod.  I have a couple of handheld devices with 1/4″ threads on the top, capable of folding into a small tripod, which could prove useful.

I also acquired a light, but full-sized tripod, which could be used in the same way.

*

The main task, however, was to attach the microphone inside the dish – or, in this case, microphones, as I wanted something of a stereo effect.

This was unlikely to be pronounced, as the only sound entering the microphones would be that captured by the dish.  I have seen 3-microphone systems where two of them are forward-facing, recording ambient sound in Left/Right stereo and one is facing into the dish, recording the sound on which the disc is focused; but I decided to go with my standard 2-microphone set-up and not worry too much about creating mixers for extra microphones or how different the left and right recordings were.

So, to this end, I used a standard twin-phono socket lead, cut the plugs off one end, drilled a suitable diameter hole in the dish, threaded it through and soldered two small electret capsules to the end.

I wanted some small ones, and the only ones I had left after the two binaural projects, – the dummy head and the dummy ears – were a type called WM-61A.  The Panasonic WM-61A was a very popular and often-used quality electret, now no longer manufactured and consequently becoming more expensive; these were not advertised as ‘Panasonic’, and were not expensive, so their quality was not guaranteed . . .

To fix them to the central stem I used an old Allen key and a jubilee clip – the Allen key only because it had a right-angle shape with some straight sides, and would therefore fix fairly solidly in place. You can also see in this picture the blue band which marks the focal point of the parabolic dish.

I attached the lead and capsules to the stem, close to the focal point marker, with cable ties.

The small felt pad on the end of the stem was to protect the wind shield, which was quite thin, and, I thought, could be damaged by the pointed stem.

The final thing to be done was a little wind-proofing.  Firstly, I took a spare microphone windshield, cut a small hole in the end, and pulled it over the Allen key mount, covering the two capsules.

Finally, I pulled the spandex cover over the front of the dish, covering the whiteness of the plastic as well as helping to keep wind out of the dish.

The parabolic reflector was now ready for testing.

I went to a local nature reserve and made recordings in different areas: woodlands, a river path and a small lake with wildfowl.  The following extracts are typical of the results.  The first recording illustrates the difference in what is picked up when the dish is turned in a different direction.

By and large, though, I had to turn the recording levels up too high, and there was too much noise.  The first and third recordings are just as they came out; the second and fourth are the same recordings with noise reduction applied.

The noise reduction makes them just about acceptable to use, but this is contrary to the purpose of the reflector disc, which is supposed to amplify the sound naturally, without the need for noise-making electronics.

So I’m going to have to do more research and find out the cause of this: are the electret capsules at fault?  Are they badly placed within the dish?  Are the sounds I’m trying to capture too faint?  I’ll report back on any improvements I manage to make.

Edit: I recently tested the dish out again in my back garden, and was much more pleased with the results.  I think I was too ambitious the first time out, and trying to capture sounds that were just too far away, turning up the preramp gain beyond what was reasonable.

In the following recording you can hear just a little directionality as I move through 360 degrees:

Binaural Recording, Pt 3

After my second experiment in binaural recording, described in Part 2 of this series, the third area of my research focused on the importance of the ears, rather than the head, to the quality of the recorded sound.  The pdf article I had orginally read on the subject had stressed the importance of the shape of ears, rather than the presence of the head, and at least one range of well-known commercial binaural microphones, the 3dio, consists of ears without a head.

So, I set about sourcing a pair of realistic ears.  Some of these can be prohibitively expensive, but I found a type that were more reasonably-priced (a little under £6 for the pair, from China), moulded in silicone, and intended for acupuncture, it said, or medical study.

When they arrived I was surprised, as they were heavier, softer and more flexible than I was expecting – nothing like the hard, fixed ears of the dummy head.  Much more realistic, and softer even than real human ears.

Unfortunately, as you can see here, the right one – on the left in the picture – had been rather crushed in the post, with the ear lobe bent right over:

The insides of the plastic bags seemed quite moist, so I was hoping that if I pulled the ear lobe back and stored the ear upside down, with a bit of weight on it, it would still be malleable enough to return to something like its original shape.  Perhaps when taken out of the bags, the ears would then harden up in the right shape.

In the  end I had to use a dab of hot glue to pin the ear back to its proper place.  The ears became less moist, but not really less floppy after being out of their bags for a few days.

*

The next part of the plan was a pair of ear muffs, readily available from Chinese sources on eBay at about £1.  They come in all sorts of colours and finishes, but the ones I picked on had a fur interior and a faux-leather finish on the outside:

The reason I chose this type was that the fur would provide a certain amount of sound deadening, and the faux-leather would take hot glue.

More hot glue?  The glue, of course, was needed to attach the silicone ears to the ear muffs!

The purpose of this was to be able to use the ears in different contexts, either attached to a dummy head, or used on their own, relying on the ear-shape for the binaural effect, rather than the whole head.

Accordingly, I first obtained a cheap polystyrene dummy head and inserted some long nails to act as a support for the ear muffs:

It took a few tries to get the exact locations for the nails, but I fairly soon had them in the right place to hold the ears at the correct angle:

I then cut a small block of polystyrene to a suitable size to fit the ears on as a free-standing unit.  I estimated that 14-15cm would be a suitable separation distance for the two ears and trimmed the polystyrene accordingly:

The final and most important stage was to add the electret capsules inside the ears and attach a twin phono lead for the output to the preamplifier.

I used smaller capsules than the ones I had used in the dummy head, as it was difficult to make holes in the silicon ears.  In the end I used my polystyrene cutter, which has a small heated blade, inserted the cable between the silicon block and the ear muff backing, attached the capsules and then pushed them down into the ear canal towards where the ear drum would be.

These photographs illustrate this process:

You can also see a certain amount of melted silicone debris, which I had to remove.

After tidying this up and testing the sound was OK, I superglued the cables along the back of the ear muffs to keep the microphones from moving, and the project was finished.  As soon as there is some better weather, I’ll be able to go out and test the three different binaural systems together.

Ultrasonic Field Recording

I had been looking for ways of recording sound from scenes which was not immediately obvious.  I had developed an inductor-based recording device which could pick up electrical noise, and the next area I wanted to explore was ultrasonic sound – that is, sounds which are too high for our ears to hear.

A recording of sounds which are too high for our ears to hear, of course, would not be too interesting – you wouldn’t be able to hear anything on the recording, either; but I got the idea of what to do with them from a type of device available commercially in finished or even kit form – a Bat Detector.

Bats make noises which are mostly ultrasonic – too high for our ears to hear – when they hunt and communicate.  What the classic bat detector does is pick up these ultrasonic sounds, amplify them and lower their pitch by an octave or two, so we can hear them.

But not only bats make noises in the ultrasonic region, many noises around us contain ultrasonic elements which we can’t hear as well as elements which we can hear.  Lowering the pitch of the ultrasonic elements would enable us to appreciate the fullness of sounds which at the moment we only hear part of.

There are several ways of changing the pitch of ultrasonic sounds, and there are many circuits available on the internet utilising these methods for producing a bat detector.  The three main ways are:

1. Frequency Division, in which the very high-pitched bat sounds are converted into square waves which can be digitally divided by – typically – 10, to produce a much lower sound, within the range of human hearing. For example, a bat making calls at 50kHz, when divided by 10, will sound at 5kHz – quite high, but well within our normal hearing range of 20Kz to 20kHz.

2. Time Expansion, where sounds are recorded digitally in the bat detector at a high sampling rate, then played back at a slower rate.  This is a common method of pitch-changing in the world of digital sampling (and the method I used in the computer-based Black Widow sample manipulator).

3. Heterodyne, which works on the same principle as the electronic musical instrument, the Theremin: the high-pitched sounds are mixed with equally high-pitched sounds produced by an oscillator inside the bat detector. The aim is to tune the bat detector to produce a slightly different pitch to the bat, as the output is designed to be at a frequency which is the difference between the two pitches. In this case, if the bat is making calls at 50kHz, for example, and the bat detector is tuned to 45Hz, sounds will again be heard at 5kHz.

This latter turned out to be the method I used – in fact, I bought a bat detector kit.  The best commercially available bat detectors are very expensive, providing a wide frequency range to work within, and often providing more than one way to hear the bats.  I should make it clear at this point that I wasn’t particularly focusing on bats – I wanted to listen to any ultrasonic sounds that might be in the vicinity; but I wanted as wide a variety of sounds as possible to be detected and brought into hearing range.

So I found a neat-looking and very reasonably-priced kit, the Franzis Bat Detector, which was available from a number of sources, all at around £20-£25.  It comes in an attractive and quite sturdy cardboard box, which can serve as the container for the circuit when made up, with art work for the two potentiometers required – for frequency and volume – and holes behind which the speaker is attached.

Inside the box is a very small PCB, no more than 3″ long, already populated with about 25 tiny SMD (Surface Mount Device) components:

Together with the board is a plastic bag containing a handful of non-SMD components, which you solder in place yourself.  These include a battery clip (not shown), a 5v voltage regulator, a few capacitors, an LM386 i.c. amplifier, the two potentiometers and an ultrasonic receiver – in appearance rather like a large electret microphone capsule.

Together with the components, there was a nicely-produced and informative booklet, containing general information about bat detection, an explanation of how the circuit works, a circuit diagram and comprehensive instructions for assembly and testing.

*

Beginning at the front end of the circuit, there are many different types of ultrasonic receiver available, and the majority are rather expensive.  The one that came with the kit is the most popular of the more reasonably-priced ones, but is designed to work best at 40kHz, with quite a narrow band of frequencies in which it works at its greatest efficiency.  This is because it is designed to work in precisely that way, usually being paired with an identical-looking 40kHz ultrasonic transmitter, and commonly used together in detection or distance measuring applications.  I was concerned that this frequency restriction might limit the sounds the detector was able to pick up, bats or otherwise, but there is a certain amount of pickup outside the intended operating range.

*

This particular circuit would probably not suit it, but a possible alternative to the ultrasonic receiver in some circumstances would – surprisingly – be an electret capsule.  Although these are sold for use in ordinary microphones, some have good ultrasonic capabilities.  It’s hard to know which ones, and to what extent they might be useful in this regard, as figures are not normally released for frequencies above 20kHz, the normal extent of human hearing.

However, one capsule known to have a good response even in the environs of 100kHz is the Panasonic WM-61A, one which has been tested and used in this way.  Unfortunately, this particular capsule was discontinued more than a decade ago, and remaining ones are getting more and more expensive, even if they can be found.  Some are still advertised on, for example eBay, but I was put off by warnings of possible fakes, whose frequency response would not necessarily be the same.

A good currently available alternative is the Primo EM258 from FEL Communications.  At over £5 each, these were 20 times as expensive as the unbranded electret capsules I’d bought before for other more conventional microphone projects, but they are not much more expensive than genuine Panasonic WM-61As these days, and have been tested and shown to have a good ultrasonic response (better, in fact than the Panasonics, according to FEL).

JLI Electronics manufacture the JLI-61A, which is intended as a direct replacement for the WM-61A, but it wasn’t clear that this was available at a reasonable price in the UK.  In the US this would be a good  alternative, at half the price of the Primo.

FEL, incidentally, also advertise a potentially excellent alternative, a tiny SMD-style MEMS microphone.  Normally these things are practically invisible to the naked eye, but FEL have installed one on a breakout board like this:

The microphone itself is a Knowles SPU0410LR5H-QB, as the legend on the PCB suggests, with a sensitivity to ultrasonic frequencies up to 200kHz and beyond.  It was almost twice as expensive as the Primo electret, but would, no doubt work very well, and that price, just under £10, is not at all unreasonable compared to current good quality alternatives.

Alternatively, the cheapest way to obtain a second ultrasonic detector – other than ordering direct from China – might be to purchase a module like this:

Its purpose is distance measurement – the device on the left, marked ‘T’, is an ultrasonic transmitter, the one on the right, marked ‘R’ is a receiver.  It would be a few moments work to detach the receiver from the board, and attach it at the beginning of the circuit.

*

As for the construction, I planned to fit the circuit inside one of the small boxes I had previously used for microphone preamps; so I connected the small PCB to sockets in the box for power and audio out – I intended to use headphones instead of the speaker supplied.  I also added an extra socket for a line out from the wiper of the volume control.  Together with an appropriate preamp (for example, the one I use for my contact mics), this would enable me to record the ultrasonic sounds I was picking up.

 

I attached the ultrasonic detector to the front of the box with hot glue, and attached a pair of 2.5M bolts for the two aerials, which had threaded bases (in the end I only used one aerial).  I also added an extra socket for attaching an external aerial or detector; a plug here would disconnect the internal ones.

The remaining parts of the circuit were: a buffer/amplifier for the ultrasonic detector; a high-frequency oscillator, based on 555 integrated circuit; a mixing/heterodyne circuit, and an audio amplifier, based on an LM386 (the only part of the circuit which uses the full 9v available from the PP3 battery).

*

The difficulties with the first part of the circuit  – the buffer/amplifier – if you were to build it yourself, are to do with size.  The specified transistor for this amplifier, the BC849C, is a tiny, tiny 3-pin SMD device.  I didn’t have a 5p handy – physically the smallest coin currently in use – but I did have a 1p, which is only a little bigger, and a BC849C, and this is how they compared:

It would be quite a task trying to attach wires to this miniscule component – but at least they’ve separated the 3 pins onto separate sides.  I obtained this one just as an example – the actual one used in this circuit was already happily soldered to the PCB by the makers!

*

The mixer section is based on a CD2003 chip, ‘originally developed for radio receivers’, as the kit booklet says, ‘the core of an AM/FM radio with oscillators, mixer stages, intermediate frequency amplifiers and demodulators for the two ranges’.  In this design, only the AM preamp and AM mixer stage are used.  According to the booklet, the i.c. ‘offers a total amplification of 40 dB and a suppression of the input signal of -20 dB. The output of the mixer provides a low pass filter for an additional damping of the input signal.’ – a very handy chip for the task in hand.  It is possible to get hold of these, but they are not nowadays common or cheap – except from China, it appeared.

*

After connecting everything together, I plugged the headphones in and tested the detector out, using the preamp I had constructed for the piezo contact mics.

The device worked well: I detected ultrasonic sounds from jangling a bunch of keys and from rubbing my finger and thumb together – everyday sounds known to have a significant ultrasonic component – and outside in the evening I was pretty sure I detected some genuine bats.

This view of the front of the device shows the two potentiometers which need to be accessed when the device is in use: the tuning control on the left, which adjects the frequency of the internal oscillator and effectively ‘tunes in’ the high frequency noise picked up by the ultrasonic detector; and on the right the volume control.

This view of the back shows, on the left, the unit with the 9v battery attached (with velcro, in my usual way); and on the right, without the battery, but showing the caption for the switch, indicating that either the ultrasonic ‘mic’ or the attached aerial can be selected; a 3.5mm plug in the ‘EXT AERIAL’ socket on the front disconnects the switch, so a different size or kind of aerial can be used.

The following sound file gives examples of some quick recordings I made with the ultrasonic detector.  Unfortunately, it’s now late November – much later than when I first tested the circuit out – so no bats flying around.  Instead, I just went round the house for 10 minutes, picking out a few promising locations.

So, you can hear fingers rubbing together, the laptop, TV set, some unexplained radio-tuning type sounds, a low-energy light-bulb, jingling keys, and water streaming slowly into the sink from a tap. The laptop, TV set, radio-tuning and low-energy light-bulb were recorded with the aerial, the others with the ultrasonic microphone-type detector.

The keys are particularly interesting, and I look forward to trying this on some natural sounds outside – especially bats when they emerge from hibernation next spring.

Binaural Recording, Pt 2

After trying the commercial microphones in Part 1 of this series on Binaural Recording, I thought I should try something more home-made – although this also involved a large-ish initial purchase.

What I bought was this handsome life-size mannequin head, intended for work in a hat shop:

The idea, of course, was to install microphones in the ears of the dummy head.  It was made of a fairly hard, but not too brittle, plastic (PVC, I believe it said in the eBay listing), which seemed to be a couple of millimetres thick.

There were several things I particularly liked about this style of head: first of all, the realistic appearance – the whole point of binaural recording is realism, so the closer the recording device resembled the human head, the better.  Unlike some mannequin heads, however, this one wasn’t painted to look like a real person – that would be too spooky! . . .

In particular, the ear was quite well-fashioned:

A big part of the way we hear things is because of the size and shape of our ears, so the accuracy of the ears of the dummy head would have an effect on the quality of the recordings.  For similar reasons, some dummy heads for recording include shoulders, as sound will bounce off these into the ears.

Finally, the underside of the base had a socket which would make it possible for the head to be mounted on a pole or stand, so as to be set at an appropriate sitting or standing height,  whichever was required for a particular recording situation.

*

My task in this case was essentially to drill suitable holes in the mannequin’s ears, and insert a pair of electret capsules.  I began by soldering a pair of capsules to the cut ends of the twin phono lead I had left after removing 10cm of one end for the previous project.

The capsules were like these:

The lead with the three connections to the side of the capsule is the Ground lead, the other is the signal.  I connected the two capsules this way, with some shrink tubing to make the joints stronger and stop them short-circuiting.

Turning to the mannequin head, the base was only attached by a dab of glue on one side, so came off easily with a little twisting and a cut with a craft knife.

It was a fairly quick procedure to drill 3 holes in the head: one in each ear, slightly smaller than the size of the electret capsules, and another, larger one at the back for the leads to exit from:

I pushed the lead in through the hole in the back, ran a big blob of hot glue round the front edge of the electret capsules and stuck them just behind the ear holes.  I chose hot glue as it’s easy to remove in case the capsules need replacing at some point in the future; it didn’t matter if a bit of the glue came over the front edge of the capsule as the actual hole on the front which the sound goes in through is very small, just a millimetre or so, right in the middle of the capsule.

Looking inside through the base, you can see how the electret capsules are stuck inside the ear, and the cables are held in place with more hot glue:

This took only a matter of minutes, and the final result looked like this:

As you can see, the microphones are held discreetly in the ear holes, and the twin phono lead, which connects to the preamp, exits from the large hole in the back of the neck, where it’s held in place by further hot glue.

*

The cost of this project was £10.50 – about half as much as the first one.  The mannequin head was £9.50; the electret microphone capsules about 25p; and the half phono lead was 75p.

The only other thing to consider is whether the head should be filled – and, if so, what with – to more accurately reflect the fact that human ears are separated by more than air.  The human brain is about three-quarters water and has the consistency of jelly or tofu; it’s quite heavy, but soft and squishy, and you can’t really pick it up until it’s been preserved in some way, which most brains we see pictured have been.

So what the best thing would be to fill the head is difficult to decide, given that jelly or tofu would soon go off.  In one article that I read the dummy head maker installed the microphones then filled the head with liquid silicone, which gradually set solid.  That seemed to be a good plan, although there’d be no way of getting to the microphones again if there were a problem with the capsules or the wiring.  My thinking is it would be sufficient to use something sound deadening, like wool or felt, to ensure that the microphones would only be picking up sound from outside.

[Edit: This is what I did, filling the empty head with a pyjama jacket, which I had been bought but had never worn].

*

In the third part of this series, I’ll complete the final project, do some recording and compare the results.

In the meantime, here’s a short extract from the first recording I made with the head:

Binaural Recording, Pt 1

I’d done some field recording with conventional microphones, and after recording with contact microphones and hydrophones, as described in this post, I decided it would be interesting to try binaural recording.

Binaural recording is an attempt to record in as likelike a way as possible.  Since both the size and shape of our ears and the fact that they are placed on opposite sides of our head are important factors in establishing the quality of the sound we naturally hear, binaural recording attempts to replicate this by, most usually, placing microphones within the ears of a dummy head.

‘Lifelike’ aspects which could be captured by binaural or dummy head recording include time differences in the arrival of sounds at one ear or the other, and types of frequency-dependent level differences and distortions which vary with the direction of the sound source.  These would allow a listener using headphones to gain extra information about the precise location and distance of sounds they were listening to; information which would not as readily be apparent if the recordings were made with conventional microphones or played back via loudspeakers.

This article [note: it’s a pdf] refers to three ways in which binaural recordings preserve ‘cues’ as to sounds’ direction and location.  These are, in order of importance: the shape of the ear; the time difference between sounds arriving at one ear, then the other; and, least significantly, it says, the presence of the bulk of the head between the two ears.

(This isn’t a universally-held view: a number of binaural recording devices feature two microphones, side by side, and separated by a sound-absorbing panel.  A notable example of this is the Jecklin disc, which has a diameter of 35cm, is covered with sound-absorbing foam or fleece, and placed between two omnidirectional microphones).

I experimented with three ways of creating binaural recording devices, comparing the results in terms of quality, practicality and cost.  I intended to use the pre-amp I had originally designed for use with electret capsules, and which I had built into a handy case when developing an inductor pickup, so I didn’t include this in the cost.  The pre-amp – which in any case was very cheap and simple – looked like this:

and the case like this:

The preamp is stereo, so inside there are two of the circuits above.  The inputs are phono sockets; the output, a 5-pin XLR, is compatible with the connecting lead of my recording device, a Marantz PMD-660.  The 3.5mm mono socket was for a 9v power source; the velcro on the top of the case was to mount the holder for a PP3 battery.

*

The first, and simplest, method involved purchasing the microphones!  I found that Roland made a very useful-looking pair of microphones resembling ear-buds, the idea being that you would wear them while recording – no need for a dummy head when your own head could do the job!   This set (CS10-EM) sounded as if it would be particularly effective, as the microphone earpieces also contained earphones, enabling recordings to be monitored while being made.  The downside, however, was the cost: over £70 on Amazon – a reasonable price, I suppose, for a potentially very useful pair of microphones, but not in the price-range for projects in this blog.

However, I noticed that Roland also made a similar pair of microphones for use with GoPro cameras, the WPM-10 WearPro:

Although this set didn’t have actual earphones for monitoring, the cost was considerably less: just under £20, including postage; so I invested in a set.  When it arrived, it contained a choice of different-sized earpieces, so it was possible to select the best fit for your ears; and, as can be seen from these drawings, it also included a pair of foam covers that might have an effect – albeit a small one – on pick up of wind noise.  Having fitted them, it didn’t look to me as if they would stay on for very long, though, so I didn’t plan on using them.

Because of their intended purpose, the plug on these microphones is not an audio plug, it’s a mini USB.

Except that it isn’t a simple mini USB plug at all.  A standard sized USB plug has 4 pins; a mini- or micro-USB has 5; this one is the size and shape of a mini-USB, but has 10.  I suppose it could be called a proprietary connector, as a number of manufacturers use them, but not always in the same way.  GoPro uses them like this:

The connections in the row along the bottom are the standard mini- or micro-USB set: +5v, Data-, Data+, ID and Ground.  ‘ID’ is the one omitted from the standard-sized USB connection: using standard-sized USB connections, the ‘in’ socket on a host device (e.g. a laptop) should be the narrow, rectangular one, Type A; the ‘out’ socket on a peripheral device (e.g. a printer) should be the square one, Type B.  With mini- and micro-sized sockets, there is no distinction in shape between these ‘in’ and ‘out’ sockets, so the job of distinguishing them is done by the ‘ID’ pin.  If the socket is performing the job as a peripheral – e.g. a camera, the ID pin is not connected; if as a host, e.g. a laptop, it is connected to Ground.

In the case of the GoPro, the device responds differently to different resistances between the ID pin and Ground.  As shown in the diagram, a resistance of 100k between ID and ground causes the device to function as a video and audio source, and it can be plugged into an external video/audio receiver; a resistance of 330k, and it will receive signals from a microphone.  In both cases, the presence of a resistance between ID and Ground allows the upper 5 pins to come into operation; their specific use is shown in the diagram above.

(I have seen it suggested that a resistance of 33k allows both these functions at the same time, but that was not confirmed by experiment in the article where I read it).

The reason for the resistor shown with a dotted line is that the conventional place to connect the 33k/100k/330k resistor would be the case of the plug, but one experimenter who posted a video on YouTube had difficulty with this, and used the Ground pin in the centre of the upper level instead and confirmed that this worked fine.

*

However, I don’t have a GoPro, and this is not the way I intended to use the microphones – in fact, it was the exact opposite.  What I needed to do was remove the mini-USB plug altogether and attach instead two phono plugs, so that the microphones could be used with the preamp shown above, for recording on my Marantz machine.

I was banking on the fact that these microphones would be a pair of electret capsules, and would receive sufficient power from the preamp to operate correctly.

So, as I had done before, instead of buying a pair of separate in-line phono plugs, I bought a 2m twin phono lead – it was only about £1.50 and would, when chopped in half, make two single-ended twin leads.

I took one of the leads and the WearPro microphones:

and cut off the mini-USB plug from the microphone lead.  As expected, this left two signal leads, Red for right, Yellow for left, and two unenclosed ground connections.

I didn’t need a whole metre of extra cable, and my next project would require as much of the 2m as could be spared; so I cut off one pair of phono leads with about 10cm of cable.  In this case the two signal leads were red and white, and the two grounds were black.

All I had to do was connect these 4 leads together, red to red, yellow to white and black to copper, and test the microphones with my recorder.

The test was fine, the microphone in my left ear was recording to the left channel of the recorder, the microphone in my right ear was recording to the right channel; so I sealed the wiring with shrink tubing and duct tape.

I used duct tape because the quality of the electrical insulating tape I’ve been coming axross recently has been terrible: neither flexible enough or sticky enough.  After I took the lower picture I added an overall layer of duct tape to bind the two wires securely together.

*

The cost of this project was £19.75 – the microphones were £19 and half the phono lead was 75p. Afterwards I felt slightly guilty at being lazy and buying the microphones.  It would have been possible to buy a pair of headphones or earphones and adapt them by gluing electret capsules on the outsides and transferring the wiring from the speakers to the microphones.  The electret capsules would have cost no more than about 20p each, and the price of a pair of not-very-good headphones or earphones would have been minimal, so it could have been done for a quarter or a third of the price.

However, this was a neat solution, took only a few minutes to finish, and the resulting set up looks good.  The quality comparison of my different binaural systems – and sound files – will come later, after I’ve finished all three projects, but this one is going to have the advantage when it comes to practicality, as it’s very simple and discreet, certainly the best solution for situations in which I don’t want to be advertising the fact that I’m recording.

This was my first recording with these microphones. I started in the hallway, walked out of the house into the shed, out of the shed, brushing past some bushes, picked up and started filling the watering can from the water butt, then turned and walked back towards the house. No noise reduction has been applied to the recording, which shows you can get a decent clear sound from the WPM-10s.

As soon as I had completed the project, I took the remaining part of the phono lead and moved on to the next one, which is described in Part 2 of this series, here.

Inductor Pickups 2 – An outdoor pickup

In the first post in this series, I tested some pickups using various inductors, which picked up electrical noises from my laptop.

One of my long-term projects is field recording, which I began by using a Marantz PMD-660 solid-state recorder.  This track is based on the first series of recordings I made, at a nearby lock – although much of the track uses these recordings altered by Karlheinz Essl’s application fLOW, which I have discussed before:

I improved later recordings with some good quality microphones which I bought from a guy connected with the Wildlife Recording Society who makes them himself.

I decided after a while to expand the range of field recordings I could make, by creating stereo hydrophone and contact mics.  I’ve described these in use here, and written about constructing them earlier in the blog.

Finally, I decided to add a fourth type of recording, using inductors of the type I had experimented with earlier.

I used the same transistor-based preamp I had experimented with – the one I originally made for the electret elements.

As with the hydrophone and contact mics, I put the preamp in a small plastic case with appropriate in and out connectors, and a 3.5mm socket and velcro patch for the external 9v battery.

I wanted the inductor pickups to be a little more robust for outdoor use, so I used two of the ‘telephone pickup’ coils I described in the first post, without removing them from their plastic cases.

I attached them to a shielded stereo phono lead and for a holder, I chose a folding plastic ruler.  The idea of this was that the two coils could be moved closer together or further apart to maximise the stereo effect of any electrical sounds they picked up.

With the help of some epoxy adhesive and superglue I attached the telephone coils to the plastic ruler.  As they were still inside their plastic containers, they would be sufficiently robust and weatherproof for outdoor use.

I went out recording by the river the other day and came across a lamp post and a large electrical box, connected with some flood gates.  The inductor recorder picked up these sounds:

This device may have a limited application in comparison to standard microphones or the other recording devices I’ve made recently – the hydrophone and contact mics referred to above – but it has a place in my collection and I’m sure in time I’ll find plenty of interesting sources of electrical noise to record on my travels.

 

Piezos Pt 8 – Hydrophones and contact mics used outdoors

Finally this summer I was able to try out my hydrophones and contact mics outdoors, in making some field recordings in and beside a nearby river.

I began with the hydrophones, which I described in the last post in this series.  My experience with the 50mm piezo discs was that there was too much pickup from the cables and the wooden float structure in the bath, so I took the mics made from the 35mm discs.

These worked fine in absolutely still water, but whenever there was the slightest wind or wave – which was virtually all the time – there was too much noise from the wind or water hitting the cables and the wooden float, so the wooden structure had to be discarded.

I was rather restricted then in only being able to record where I could reach and dangle the mics in the water, but the recordings were much better, and even worked well with the discs lying on the river bed.  I believe the sound at the end of this recording is a water snail munching on a reed stem: I saw the snail, which was quite large, an inch or two in length, and dangled the microphone close to it; this is what I heard:

*

I then turned to recording with the contact mics.  The basic format of these, and the original preamp I used, are described in this post, Pt 2 of this series.

Experimenting beforehand with the sturdy clips and clamps I had bought for outdoor use – pictured in Pt 3 of this series, here – I found that pressure on the ‘crystal’ side of the disc, where the leads are attached, had a tendency to cause distortion, so I decided for the outdoor version of the contact mics I would use the same ‘sandwich’-style construction as for the hydrophones, with 2 discs mounted back-to-back (or front-to-front – that is, with the crystal side inside), but kept apart by a ring of silicone sealant around the edge:

The difference in this case, though, was that the leads were attached to only one of the discs – I removed the leads from the other one as I had not experienced the same noise problems as I had with the hydrophones, and it wouldn’t therefore be necessary to have the double leads from each mic and the 4 channel balanced preamp which the hydrophone required.

The preamp was not the same one I had used for the experiments in the posts referred to above – although it was similar.  This time I used a standard inverting op amp amplifier.  The resistor on the input was 1M and the resistor between the input and output was 10M, providing amplification of up to 10 times, but the output volume was controlled by a dual-gang 10k log potentiometer.   I was intending to use my usual TL072 dual op amp, which is pretty low-noise and would have worked fine, but in the end used an NE5532 as I had just bought some of these and they are known for being especially low-noise.  They are also a pin-for-pin replacement for the TL072.

I enclosed the circuit in a small plastic box, very similar in appearance to the hydrophone preamp.  This design was easy to hold in the hand and manipulate the volume control while monitoring with headphones.

As previously mentioned, it was quite a windy day when I went out, so I attached the contact mics to tree branches to test them out – one mic would be near the end of the branch, the other one closer to the trunk.  One disc would be held tight to the branch (the side to which the leads were attached), and the clamps pressed firmly onto the disc without leads.  This side, of course, needn’t have been a piezo disc at all, but I used them because they were exactly the same size as the discs with the leads, and these particular discs were very inexpensive, so I didn’t feel it was too much of a waste of resources.  If you didn’t want to use up piezo discs like this, you could use any fairly rigid substance – plastic or glass, for example – as long as it was roughly the size of the disc and wouldn’t deform when clipped or clamped.

As I hoped, there was no distortion caused by pressure on the crystal layer, and the recordings came out well, capturing the movement of the trees in the wind.

 

Piezos Pt 7 – Hydrophones

While making the piezo contact microphones described earlier in this series, I decided to adapt a couple of them for the specific purpose of recording sound underwater in rivers and ponds.

In this case, I wanted them to be compatible with my Marantz PD660 recorder, which records in stereo via two XLR external microphone sockets.

For my first attempt, I began by using 3m twin shielded cables, which had two 6.35mm mono jacks on one end – what was on the other end didn’t matter, as these connectors were cut off and the ends attached to piezo elements.  As with the others, the shields were attached to the brass outsides, the cores to the centres, and they were given two coats of Plasti-Dip.

In this way I was able to record in stereo, but my attempts were bedevilled by extraneous noise.  I also realised that underwater recording required much greater amplification than I was getting from the simple preamp I had used  for the simple percussion instruments I had been making.

Looking around the internet to see what others had done, I came across the idea on this site of using two piezos back-to-back for each channel.  Actually, this is probably more accurately described as front-to-front, as it requires the two piezos to be sandwiched together with flexible silicone sealant.

Having read somewhere else that the air gap behind a piezo affects what it picks up, I decided it would better if there was some air between the two piezos, rather than completely filling the space between them with the sealant.  Hence, I just put a ring of sealant round the edges before sticking the discs together.

The main thing is to keep the centres of the two discs from touching.

To connect the discs to the recorder, I used a 3m twin XLR cable.  Each channel used two discs: the black leads from the discs were connected together, and the two red leads were connected to the left and right connectors of the XLR lead.

In fact, I made two sets, to see if there would be a difference between 35mm discs and 50mm discs; the 35mm discs are shown above.  The 50mm discs came with no wires attached, so I had to solder the wires on myself – a bit of a tricky job, to make sure they attached firmly, but without damaging the delicate central area of the disc with the crystals in it.  When I had done the soldering and checked the strength of the joint, I superglued the wires to the outer edge of the discs to make sure there would be no stress on the soldered joints which might cause the wires to become detached while in use.

In the case of the 50mm discs, where there was more space, I stuck a small lead weight on the edge of one disc in each pair before putting them together.  This, I hoped, would make them more stable in the water.  Once again, I used the same principle of putting the sealant round the edges of the discs only, and then putting them together like a sandwich.

I connected the wires to a twin XLR cable as before, and gave the discs two coats of Plasti-Dip.

*

In yet another place I had read that underwater signals might require up to 100 times amplification, so I put together a TL072-based buffer amp somewhat similar to the preamp I had used for the piezo-based percussion instruments, but with two major differences:

Firstly, it was designed to amplify the input signal by a factor of 100, with a volume control to reduce the output if necessary; secondly, it was in the form of a ‘differential’ amplifier, so that the signals on the left and right leads of each channel were amplified and added together, producing a simple two-channel stereo output from the four inputs.

Each channel looked like this:

After checking the prototype (pictured above) was working satisfactorily, I improved the connections with 2 core shielded cable and put the circuit into a small box.  I changed the output from the 3.5mm socket shown to a 5 pin XLR socket because this matched the professionally made apparatus I use for field recording with conventional microphones.  There was no room for a battery in the box, so I used the 3.5mm socket for a power socket.

For absolute minimal noise, the closer the preamp is to the piezos the better, and I have seen designs where the circuit was contained in a waterproof housing in the water.  However, mine is designed so the preamp box is well away from the water, and the output volume control can be used while it’s in operation, as well as the input volume control on the recorder.

*

The first problem when it came to testing the hydrophones was how to ensure that they floated in the correct place; and the second was how to maintain a consistent stereo image.

Clearly, the hydrophones would require some sort of framework to which they could be attached, and from which their depth underwater could be adjusted.  The framework would need to be buoyant and, as I discovered, potentially very wide.  The speed of sound in air is about 750mph or 350 metres per second, and a distance between the microphones of about 9-12 inches or 25-30cm is usually enough to get a satisfactory stereo image.  However, underwater, the speed of sound is more like 1,500 metres a second, getting on for four and a half times as fast!

This meant that the hydrophones would need to be spaced up to four and a half times the distance apart to get the same stereo effect as two conventional microphones recording in the open air.

I thought about different methods of creating a frame 3 – 4 feet or 1 – 1.5 meters long, and easily foldable for transportation, and came up with the idea of using a folding wooden ruler.  I found one which was a metre long, about the minimum necessary length, which folded on brass hinges into 5 sections of 20cm each.  This would easily fit into a bag and could be carried to the recording site before being opened out and placed in the water.  It cost about £2 (although postage was £3!).

The first thing I did with the ruler was cut a number of slots in it:

The purpose of these slots was so that the hydrophone leads could be threaded through them, stay in position, and allow the piezo elements to stay at the right depth in the water beneath the framework.

I tested that the slots were suitable for this purpose:

and then – since I had the tin in the workroom – gave the framework a coating of Plasti-Dip.  I’m sure it would have been fine without it, but the Plasti-Dip will hopefully give the wood a little extra protection, make it easier to dry and increase its life.

Later, because of using a wider construction with two piezos sandwiched together, I cut narrower channels to connect the slots to the edge of the framework, so the wires could fit in and the piezos wouldn’t need to pass through the slots.

*

The yellow balls in the above picture need an explanation!  Although the wood would be quite buoyant, the framework would need to be supported in the water.  I looked to see what anglers might use, and came across these small ‘bubble floats’:

These are hollow plastic and have a small bung in them so they can be partly filled with water.  This gives them a little weight so they can be cast into an ideal position in the water, but will still float.  I thought these would ensure the wooden framework would remain on the surface and the hydrophones could be positioned at an appropriate depth beneath.

In the end, I used them without any water in them; empty, they were able to support the framework slightly under the surface.  As can be seen in the picture, in a restricted space such as a bath or small pond, the end sections of the framework can be folded in.

*

Having fitted the floats, it was time to test the device, for which purpose I filled my bath, floated the framework with the piezos attached, and ran the tap.  The sound was pretty good, and the noise minimal:

I’ll post again when I’ve had a chance to try it out in the field!

[Edit: the wooden float wasn’t very successful in the field.  See next post in the series]