Archive for January, 2018

21
Jan
18

Piezos Pt 3 – Amplifying and creating instruments

After preparing the discs and the buffer/amplifier in Part 2 of this series, I looked around for different instruments that could effectively be amplified and recorded with the use of piezo elements.

I also tried a few inexpensive commercial piezo contact mics, like these:

The top one (Cost: approx £1.50) has quite a large piezo disc inside a plastic cover and a sticky pad to fix it to the surface which is to be amplified; the bottom one (Cost: £1.25) is built into a (not-too-strong) plastic clip, with a foam pad to protect the disc.

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The more solidly the piezo is connected to the sound-producing surface, the better the sound obtained.  In other words, it’s best if the disc can be glued to the surface.  I did this with some of the non-valuable items:

However, I wasn’t keen on making this permanent addition to my instruments, and  looked for different ways of making temporary connections.  Each of these could be useful in different circumstances:

This double-sided tape is described as ‘removable’, and is less likely to damage either the piezo element or the instrument it’s stuck to, so is a good choice – although, at about £7.00, was rather expensive.   I hope to make use of it elsewhere in the house!  It’s also more suitable for a one-off performance or recording.  I’ve also read that Blu-Tac works in a similar way, although my experience is that it can leave marks; elsewhere I’ve read that insulating tape can be used – that might also be less sticky than conventional sellotape, but unlike the double sided tape, would have to go over the top of the piezo disc.

I also bought some clamps of different sorts:

The spring clips were very strong, so I would guess I’d have to be careful using them so as not to damage the piezos.  I have read of people using soft pads – made of felt or foam rubber, for  example – to put over the piezos when using them with strong clips.

In this way I had a variety of different methods of attaching the piezos to items I wanted to amplify or record.  The items themselves could be either acoustic instruments that just needed appropriately amplifying; or items that were not musical instruments, but which could be amplified – perhaps changing their sound, or revealing a hidden sound in the process – by attaching a piezo.

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As they are used to pick up sounds from vibrations in solid surfaces, the best acoustic instruments to work on would be those with sounding boards, such as guitars, zithers or other stringed instruments, several of which I had in my collection; as for non-musical instruments, this would be a matter of experimentation!  The advantage of using a piezo contact mic in these examples would be that, unlike a conventional microphone which picks up airborne sounds, the contact mic wouldn’t easily be affected by the nearby sounds of the player, other instruments, or external noises in the recording environment – e.g. traffic or the people next door.

First of all, the more conventional instruments.  These are just a few of the various things I tried the piezo mics with.  At the top are bells and a rainstick; at the bottom are a rattle (not perhaps, strictly speaking, an instrument!) and a zither.

The following sound file illustrates how these sounded:

In these experiments only the zither was recorded in stereo by using two piezos, but a stereo effect would certainly bring something to some of the others – for example the rainstick.

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Next, some uses of the piezos that created new instruments in themselves.  Both of these made sounds that, when picked up by the piezos were very different from the way they sounded in the room.

The first one uses a small snare – normally used for a snare drum.  This was purchased very cheaply (about £1.30) and attached to a wide, flat tin.  A 50mm piezo disc was superglued to the middle of the tin.

The second one is just a 50mm piezo disc with 4 lengths of piano wire soldered to it.  I’m not 100% certain of the diameter of the wire: it said ‘G0’  (i.e. ‘G zero’) on it, and was bought from a (classical) music shop about 30 years ago.  If it means Music Wire gauge 0, this would make it about 9mm, something like a thin top E guitar string, which is about what it seems to be.  The 4 lengths are approximately 6″, 9″, 12″ and 18″.

This sound file illustrates first the ‘snare’ instrument, then the ‘string’ instrument:

It’s surprising how different the sound through the piezo is, compared to the natural sound, especially the one with the soldered strings, which makes hardly any noise at all.  The small preamp also plays a part in preserving the lower frequency sounds.

The picture below shows, on the left, the three – I don’t know what to call them – strikers or activators, which I used to make the sounds from the snare instrument: the one on the left is a home made beater or mallet, made from a length of dowel and a wooden bead; the middle one is a wooden coffee stirrer, much more delicate; and the one on the right is a small cleaning brush.

Activators IMG_1281

It’s a good idea to make a collection of these if you’re going to make piezo instruments, as the way you interact with the instrument can make a big difference to how it sounds.  The picture on the right shows some more things I use, as well as pipe cleaners, fire-lighting spills and small emery boards.  This writer has a very impressive collection: https://skoglosa.wordpress.com/2015/05/06/contact-mic-guidelines/!

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The instruments required quite a bit of physical construction, since a framework was needed to support the sounding parts.  I bought some small square trays to serve as the bases, and cut lengths of 2x2cm wood for the uprights.

After glueing and screwing these together, I was able to attach the sounding parts:

The two on the right are the ones described earlier; the one on the left is simply a sandpaper disc with a 35mm piezo glued to the back.  The narrow space underneath the base can be seen in this picture.  The batteries and electronics would have to fit in this space.

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After the physical construction, it was time to add the electronics.  I’ll describe this in the next part of the series.

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20
Jan
18

Electret microphones Pt 1- General

After starting my recent piezo project, I decided to look into electret microphones – there are many situations in which a piezo-type contact mic is less suitable than a mic which detects sounds in the air.

Electret capsules are very cheap, and need just a few additional components to get them to work.  I bought a number of these for about 10p – 11p each:

It’s possible to get them without any wires – long or short – attached, but I preferred these.  The smaller ones, illustrated at the top, were very small – 4mm in diameter; the larger ones were 10mm.  I also bought a few that were in between, at 6.5mm.

Most people working in electronic music will be aware of the importance of microphones, and  I have some quite expensive ones for different amplifying and recording purposes; but there are various situations in which a very low-cost method of picking up and amplifying sometimes quite small sounds can be all that’s needed.

One renowned electronic music composer for whom the microphone became extremely important for a time was Karlheinz Stockhausen.

In summer 1964, Stockhausen said, ‘I searched for ways to compose – flexibly – also the process of microphone recording. The microphone, used until now as a rigid, passive recording device to reproduce sounds as faithfully as possible, would have to become a musical instrument and, on the other hand, through its manipulation, influence ALL the characteristics of the sounds . . .’

At the same time, he had been experimenting with a large tam-tam (a percussion instrument very similar to a gong), ‘using a great variety of implements – of glass, cardboard, metal, wood, rubber, plastic – which I had collected from around the house.’

‘One day’, he continues, ‘I took some equipment from the WDR Studio for Electronic Music home with me. My collaborator Jaap Spek helped me. I played on the tam-tam with every possible utensil and during this, moved the microphone above the surface of the tam-tam. The microphone was connected to an electrical filter whose output was connected to a volume control (potentiometer), and this in turn, was connected to amplifier and loudspeaker. During this, Jaap Spek changed the filter settings and dynamic levels, improvising. At the same time, we recorded the result on tape.

‘The tape recording of this first microphony experiment constitutes for me a discovery of utmost importance . . . Actually, this moment was the genesis of a live electronic music with unconventional music instruments. On the basis of this experiment I then wrote the score of Mikrophonie I. Two players excite the tam-tam using a great variety of implements, two further players scan the tam-tam with microphones . . . Two further players – seated in the auditorium at the left and right of the middle – each operate an electrical filter and two potentiometers. They, in turn, reshape the timbre and pitch . . . dynamic level and spatial effect . . . and the rhythm of the structures . . .’

The score includes instructions for the placing and movement of the microphones, just as it includes instructions for the tam-tam players and the filter operators, so the microphones can be regarded as essential instruments in the performance of the piece.

One of the interesting features of the use of microphones in the piece is, as Stockhausen wrote: ‘normally inaudible vibrations (of a tam-tam) are made audible by an active process of listening into [them with a microphone].’ The reviewer Albrecht Moritz (http://home.earthlink.net/~almoritz/mikrophonie1.htm) states: ‘There are several passages in Mikrophonie I where this process is exclusively employed, foregoing stronger excitement of the tam-tam which would produce the commonly heard sounds. A result is that, if you would play back these passages to persons whom you would leave in the dark about the source of the sounds, probably most or even all of those listeners – including musicians – would not be able to guess it.’

Elsewhere Moritz says that ‘the audibility of most sounds that are created on the tam-tam in Mikrophonie I appears to strictly depend on the microphonic amplification. Among these are scratching noises, produced by treating the surface with not only metallic, but also other kinds of objects. Strangely “rolling” sounds can be generated on the surface, sounds evocative of rustling of silver paper, and many other astounding sounds . . . Quite frequently there are dark, roaring, sometimes growling, yet in volume often rather soft, undercurrents of sound that appear to stem from only local resonances of the tam-tam plate, generated by gentle use of a beater or as a result of other treatment, a sound phenomenon most likely audible only because of microphonic amplification as well.’

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These electret capsules won’t work, however, just by connecting them to a mixer or amplifier – they have a small built-in preamp inside them which needs power to operate it.  This means the positive lead to the capsule must have a few volts of power running to it – 3v to 9v, typically – for the capsule to work.   An interesting article on this topic can be found at http://www.epanorama.net/circuits/microphone_powering.html.

The minimum circuitry required to get a sound from the capsule is this:

Using a prototype of this circuit with the Taurus amplifier, the level of the signal was perfectly good enough without any additional circuitry.

Sometimes, however, more output is needed, and I found a suitable circuit which was simple but effective at http://www.instructables.com/id/Pre-amp-to-electret-mic/.  It was based on a single transistor, a 2N3904, obtainable at less than 5p each by buying a bag of 50, plus 3 resistors and 2 capacitors.   The circuit for one channel looked like this:

I made up a batch of them all at the same time on a spare piece of veroboard.  These 12 circuits cost no more than about £2 – £2.50 altogether.

I took some of these and made up some stereo circuits, adding an output socket, 9v battery clip and volume control pot to each one:

With this I was able to experiment with different microphone combinations. No other components were needed, except the electret capsules themselves.  In some cases I connected the two wires from the capsule directly to input and ground on the board, at other times I attached input sockets.

In all cases I was surprised at the quality of the sound I was able to get for such a low cost.  The circuit also worked with the piezo mics/pickups I had made earlier, although the output didn’t seem to have as much lower frequency content.

In the next article in the series I’ll describe some of the practical applications for which I used these electrets.

 

 

15
Jan
18

The Blue Cow

Having just finished a couple of projects designed for automatic control via the Bigfoot sequencer, I was looking for a similar toy that could be played manually.

The Blue Cow seemed to be exactly the kind of thing I was looking for. which could be worked on with a combination of modification and circuit-bending.

There are buttons which make the sounds of a cow, cowbell, cat, dog and pig; two switches – the ducks’ beaks – which play a short musical sequence and light up the 6 LEDs; and three mechanical controls.  One, a snail which moves slowly across the cow’s back, makes no electronic sound, but there are two others which do: a rotating ball which makes the cowbell sound as it turns, and a rotating control with finger-holes, which moves the cow’s tail and plays a tune with a bell-like tone.

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I opened the back of the device and took a close look at the PCB inside.

The first thing I noticed was an extra switch which hadn’t been used.  It had the word ‘sheep’ next to it, so I eagerly connected a pair of wires to it and, sure enough, a baa emerged.  So the first thing I did was add an extra switch on the front of the cow to produce this new sound.

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After that I searched for the resistor which would affect the pitch of the sounds.  It took a while, but I found it, removed it from the board, and attached two wires in its place, running to a potentiometer so that the pitch could be varied.

In this view of the circuit board 1 indicates the previously unused switch, and 2 indicates the position of the pitch/timing resistor.  The wires now connected to these two sections are on the underside of the board.

It took a little experimentation to find the correct value for the potentiometer, to ensure sufficient pitch variation, but not to increase or decrease the resistance so much that the device crashed.  This is the normal thing when replacing a timing resistor.  In this case, a 250k potentiometer was the best value, with a 100k trimmer to adjust the minimum resistance.

The potentiometer went on the front of the cow.

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I unscrewed  the sections behind the 4 main sound buttons, and checked the additional circuit boards which were connected to the main board by a series of red wires.  There were 14 of these these wires, which went directly to the sound-producing chip.

Some of these wires went to LEDs beneath the buttons, so  I tested the others by connecting them in pairs, and found various combinations which would produce almost all available the sounds, including the newly-discovered sheep.  If I included one of the connections of the switches under the ducks’ beaks, all the sounds were available, and I began thinking of extra ways to access them.

First of all I added four tilt switches, so different sounds would be produced as the device was moved around.  These were a simple and cheap type – about 10p each – but very effective.  Constructed in a small can with two legs, not unlike an electrolytic capacitor in appearance, an internal connection is made when the can is tipped from horizontal to vertical.

I glued the switches inside, left, right, top and bottom, with a different sound connected to each.  The arrows in the following picture indicate two of the switches in situ.

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One awkward thing about the circuit – especially with the tilt switches in place – was that there was no on/off switch: the power was on – and the device ready to make noises – as soon as batteries were inserted.

So, I decided to add one.  I found a nice one, described as an SPST illuminated rocker:

The odd thing about it, as an SPST, is that it has 3 contacts.  It took me some time to work out, but its normal function is to switch +v from one outside contact to the centre; the other outside contact – not connected to the switch – is for a 0v connection to the internal LED.  In this case, the power lead from the batteries connects to the ‘off’ side of the switch, the lead to the circuit board connects to the centre, and the 0v lead connects to the ‘on’ side of the switch.  In this way, when the switch is turned to the ‘on’ position, power is connected to the circuit board and the switch lights up – very attractive!

I liked these switches so much, I bought several of them at about 30p each.  They were labelled as suitable for 12v – their origin is for use in the automotive industry – but lit up fine from the Blue Cow’s 4.5v.

There’s quite  variety of illuminated switches available, all of which would beautify a project where a +V switch was required.  There were also square ones:

although I always find these more difficult to mount than round ones.  The one on the right looks interesting, as it’s a centre off with a latching switch one way and a momentary switch the other.

One UK outlet has a whole variety of these, including toggle switches, push switches and pull switches:

At this time I also added a small SPDT switch to change the output from the small internal speaker to 4mm external speaker sockets or to an audio out socket.

In the  following picture 1 = the internal/external speaker select switch; 2 = the speaker select switch inside; 3 = the external speaker sockets; 4 = the audio out socket.

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Here’s a short demo of the Blue Cow.  In this case it was connected to an external speaker and the sound recorded via microphones.




andymurkin

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