Piezos Pt 2 – Contact microphones

Continuing my experiments with piezos, which I began in Part 1 of this series, the first plan I had was to use piezos as contact microphones.  This would enable me to amplify small string and percussion instruments, and, with additional circuitry, create new ambient electronic instruments.

The first step would be to wire the elements to cables and jacks.  It’s best I had read, to use shielded wire to do this, especially where the cables are long, so I thought it would be easier to buy and reuse cables with jacks already attached.  These are generally only about £1.50 – £2.00, so I bought some 2m leads with mono 3.5mm and 6.35mm jacks, and cut them in half.

I soldered the stranded shield to the outside of the disc and the core to the centre, using lengths of different diameter shrink tubing to strengthen the connections and make sure the cut wires were kept apart.  The already-attached leads were colour-coded black and red to indicate their function.

At this point I tested the discs to make sure they were picking up sounds before moving on to the next stage.

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The next thing I did – again, a procedure suggested by Nic Collins – was to seal and further strengthen the piezo discs with rubberized paint.

There don’t seem to be many of these on the market these days, and although – compared to the other components in these projects – it was rather expensive (having had to be imported from the US), I opted for the one Nic Collins used: Plasti-Dip.

There is a cheaper Ronseal product, which is advertised as a liquid rubber for sealing flat roofs, but I wasn’t sure if it would be the right consistency.  Ronseal has a very annoying website which doesn’t allow you to search for products by name, and I couldn’t find it there, but it’s called Isoflex , and you can get more details by searching suppliers’ websites.

The Plasti-Dip looked rather thick, but the piezo discs are very thin and it wasn’t at all difficult to dip the ends in and cover them to a level just above the shrink tubing – the tin, as can be seen in the picture, is tall and thin, presumably for this reason.

The Plasti-Dip did the job perfectly, and I’ll be looking for other uses for it in projects and around the house, as hardly any of it was used up!  I bought the 400ml tin, as, at around £18 it seemed better value than the small tin at about £14 for 250ml, but even 250 ml would be a lifetime’s supply at the present rate.

By the way, although there are many colours available, I chose black because of its lack of visibility, in case the microphones would be used in situations where they needed to be discreet – e.g. in the presence of wildlife.  This would potentially be important in one particular future project.

Nic Collins recommends putting a small piece of insulating tape over the solder connections on the backs of the discs to give them extra protection against breaking off – which would be a great shame after all the soldering and dipping – but I forgot to do this, so I hope the dipping is enough to keep them together.

After dipping them I hung them up to dry overnight and dipped them again the following day to improve the seal

After two dips they looked fine.  I was worried that too much paint would dull the sound too much: although there would be a number of applications where I would use the piezos without dipping – or with a single dip – Nic Collins certainly mentions two dips and says that three would cause dampening of the sound pickup.  I was thinking that these ‘stand alone’ piezos might be used in a context where they needed to be waterproof, and one dip didn’t seem to seal them enough, so I left it there.

The following picture shows two sizes of piezo mic, 18mm diameter on the right, 27mm in the centre.  The one on the left has two elements connected in parallel.  I thought this might be useful to record oddly-shaped objects or objects with a large surface area.

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I had read in more than one place that, although piezos can be used as contact mics just by plugging them into an amplifier or recorder, they work much better if the signal is run though a buffer circuit first.  The buffer needn’t necessarily amplify, but the low-frequency response would in any case be significantly improved.

A very useful series of articles starting here or here gives much greater detail on this.  I adapted the low noise preamp from that site, here or here:

Features of the circuit: the two diodes protect the opamp inputs from damage by restricting the maximum voltage that they can receive from the piezos.  The TL072, which I used – or the lower noise pin-for-pin replacement, the NE 5532 – is a dual opamp, so two of these circuits can be built using the same chip.   (The TL074 is a quad version, from which four  circuits can be made).  The pin numbers in brackets are those used by the second circuit built around the TL072: the second circuit is identical, and the only place where the two circuits meet is at the point marked A, the half supply voltage point created by the voltage divider – these two 100k resistors don’t need to be repeated for the second circuit.

The majority of piezo buffer circuits I found seem to use FET transistors, but these are quite expensive in comparison to the TL072, TL074 or NE5532.  I also read in one of the articles referred to above that ‘the manufacturers of FETs don’t control their parameters well . . . The gate-source voltage needed to bias the transistor into the linear region can vary between 0.25V and 8V, which leaves a good 7.75V down to a hopeless 0.4V for the transistor and load if used with a typical NiCad 8.4V PP3.  You’ll have to get more FETs than you need and throw out the dogs . . . Design manuals get all sniffy about that sort of thing because selecting FETs obviously adds to the cost if you are mass producing something. That’s not the case here, and there’s just no way to cope with a manufacturing tolerance which can throw more than 90% of the battery voltage away in variations in manufacture without screening the bad ‘uns.’

So I decided to stick with the opamps – the most recent batch of TL072’s I bought were between 4p and 5p each; the NE5532’s were more expensive at about 20p.

So, as soon as the microphone assemblies were ready, I made up one of these circuits with a TL072 and tested it out.

I plugged it into the line input of my MacBook, and it seemed to produce little background noise.  I attached two of the piezos which I had prepared as above, and clipped them to a plate, recording the sound in the Audacity app as I tapped the plate with a pen. As a comparison, I turned off the buffer and plugged the piezos directly into the computer.

The following short sound file illustrates the difference the buffer makes.  The output was a little louder with the buffer than without so I adjusted the recordings to be more or less the same volume.  First heard are the taps with the piezos plugged directly into the computer; then the taps using the buffer.  The difference is quite striking.

The buffer does amplify the sound, but I’ve tried to minimise this in the recording: nevertheless, there is a noticeable increase in the lower frequency response when the buffer is used.  The sound is much fuller, so, although the small expense and time involved in making buffers does add to the cost and the effort of piezo projects, I think it’s probably worthwhile for the improvement in the quality of the results.

[Edit: Listening back to this recording just now, I realise there’s a bit of a buzz in the background.  I don’t know what was causing that on this recording, as the preamp is normally pretty silent and doesn’t create noise

In the end I used a simplified version of the buffer, looking like this:

This seemed equally effective, but with a lower component count].

In Part 3 of this series I’ll describe some particular projects in which I used the piezo elements as microphones, mostly with the buffers.

Piezos Pt 1 – General

Piezos, Piezo Sensors, Piezo Transducers, or Piezo Elements are small, cheap components that can be useful in several different ways to the electronic musician.  I had some ideas for ways I’d like to use them, and this series will describe some projects in which piezo elements were employed.

A transducer is a device which changes energy from one form to another – for example, tape heads and record pickup cartridges are transducers as they change magnetic signals on the tape or movement in the grooves of a record into electrical signals; microphones and speakers are transducers because in the first case they change movement in the air into electrical signals , or in the other they change electrical signals into movement in the air.

Piezo elements are transducers because they can transform physical movement into electrical signals like a pickup cartridge, or electrical signals into movement in the air like a speaker.  They do this not by sensing magnetic fields, like a tape head or guitar pickup, but by the movement of crystals, and this is what a piezo element has inside it.

Piezos work especially well when attached to something which will vibrate and produce electrical signals which can be amplified, or can amplify the vibration of signals fed into it.

In everyday life, they’re usually found in mobile phones, in buzzers or in place of speakers in smaller children’s toys.  This gives a clue as to the different ways in which they can be employed in electronic music circuits: as microphones, as speakers, or as triggers for switches.

Much of the information below was gleaned from Nic Collins’ book Handmade Electronic Music, and his series of videos on YouTube called Hack of the Month Club.

First of all, this is what piezo elements look like if you buy them from a components supplier, or take one out of a phone or musical toy:

or sometimes like this, if they come in the form of a sounder or buzzer – in this case, the element can be carefully removed from the plastic surround.

They vary in diameter from 10mm to 50mm.  I have used some of the smaller ones, but most of the ones I have are 18 or  20mm, and 27 or 35mm.

[Edit: I have usually got the best sound out of the larger diameter ones, so these are the ones I use for preference, unless space dictates a smaller one has to be employed.  This means I usually choose the 35mm, which cost quite a bit less than the 50mm ones; but I’ve occasionally used the 50mm ones for special purposes].

When I buy them, I prefer the ones with the leads already soldered on.  This saves a job – and it’s quite tricky to do the soldering effectively – and they’re still very cheap.  The last batch I bought worked out at about 12p each for the 35mm diameter ones, and just 6p each for the 18mm ones.

Perhaps the first piece of music to use transducers in its realisation was Cartridge Music by John Cage, composed and first performed in 1960. As described on the webite of the John Cage Trust: ‘The word ‘Cartridge’ in the title refers to the cartridge of phonographic pick-ups, into the aperture of which is fitted a needle. In Cartridge Music, the performer is instructed to insert all manner of unspecified small objects into the cartridge; prior performances have involved such items as pipe cleaners, matches, feathers, wires, etc. Furniture may be used as well, amplified via contact microphones. All sounds are to be amplified and are controlled by the performer(s).’

Another composer who famously used transducers was David Tudor. Tudor – who was closely associated with John Cage – created a piece called Rainforest – originally in the mid 1960’s, but it went through a number of changes during the rest of the decade as Tudor’s techniques and equipment developed. The piece was based on the idea of making objects other than speakers vibrate, picking up the sounds they made with microphones and then filtering and mixing the resultant sounds.

‘My piece Rainforest IV‘, Tudor explained, ‘was developed from ideas I had as early as 1965. The basic notion, which is a technical one, was the idea that the loudspeaker should have a voice which was unique and not just an instrument of reproduction, but as an instrument unto itself . . .’

‘. . . I eventually acquired some devices called audio transducers. They were first developed for the US Navy because they needed a device which could sound above and under the water simultaneously . . . I had them in 1968 when MC [choreographer Merce Cunningham] asked me for a dance score and I decided that I would try to do the sounding sculpture on a very small scale. I took these transducers and attached them to very small objects and then programmed them with signals from sound generators. The sound they produced was then picked up by phono cartridges and then sent to a large speaker system.’

‘Several different versions of this piece were produced. In 1973 I made Rainforest IV where the objects that the sounds are sent through are very large so that they have their own presence in space. I mean, they actually sound locally in the space where they are hanging as well as being supplemented by a loudspeaker system. The idea is that if you send sound through materials, the resonant nodes of the materials are released and those can be picked up by contact microphones or phono cartridges and those have a different kind of sound than the object does when you listen to it very close where it’s hanging. It becomes like a reflection and it makes, I thought, quite a harmonious and beautiful atmosphere, because wherever you move in the room, you have reminiscences of something you have heard at some other point in the space.’

(from An Interview with David Tudor by Teddy Hultberg in Dusseldorf, May 17-18, 1988, http://davidtudor.org/Articles/hultberg.html).

A reviewer present at a performance of Rainforest described the appearance and sound of the piece as follows: ‘The entire piece sounds at first like an ethereal insect chorus, but the layers gradually disperse into patterns of jagged counterpoint, which in the performance seemed to harmonize perfectly with the movements of the dancers . . .

‘Most of the sounds are created by sine tones being reverberated through a forest of suspended metal containers, pieces of junk that function as “biased” loudspeakers imparting their own timbral colouration to the sounds which pass through them. These sounds are picked up by contact microphones, fed back into Tudor’s mixing and filtering controls, and then recycled back into the expanding forest of increasingly hybrid noises. The array of metal containers usually fills an entire gallery, and spectators are invited to walk around and put their heads inside the containers.’

(Roger Sutherland, Musicworks, Number 75, Fall 1999, http://moderecords.com/catalog/064tudor.html)

Some modern performances of Cartridge Music will use piezo elements instead of cartridges, although this might be considered cheating. Piezos, on the other hand, are ideal for achieving the kinds of effects employed by Tudor in Rainforest, and the different projects I planned with them will hopefully cover these uses and more.

Part 2 of this series of articles is here.