Archive for the 'Field recording' Category

17
Aug
19

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.

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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.

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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].

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

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17
Aug
19

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.

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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.

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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.

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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.

27
Jul
18

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.

 

21
Jul
18

Piezos 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:

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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.

Contact mic preamp circuit

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.

In my next post in this series, I’ll try out some improved methods of recording closer to where I want to in the water.

09
Jun
18

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.

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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.

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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.

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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.

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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]




andymurkin

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