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Cheap, innocent looking piezo elements do okay as greetings cards sounders. They seem to cause grief when used as contact mics. They seem to promise a lot, but easily sound rough as commonly applied. The problem with piezo guitar pickups and contact mics is that they are not well matched to typical audio inputs. By their nature they can generate a lot of signal, but they cannot drive a 50 kilohm typical line input. The pickup needs to work into a much higher impedance, typically 1 megohm or so.

Two solutions are proposed - the cheap and cheerful FET buffer which probably meets all the requirements of 90% of users who are using the devices on outside structures, and the high performance version for those who want higher performance and lower noise. First of all though, why do these things sound rotten the way they are most commonly used?

The sound to voltage conversion isn't noted for its high quality - most piezo contact mics are tuned speaker elements used in reverse. The brass disc on which the element is glued is designed to resonate at the design frequency of 2-4kHz so that a large audio output is achieved with a small power input. This will tend to lead to a peakiness at mid-frequencies, which is why I glue these to a magnet to try and spread this peak and make the coupling to the source better.

However, the main reason these have gotten a bad rap is that many people couple them into a standard audio load, which loses low frequencies.

The reason why these devices often sound tinny is because the piezo sensor presents its signal through a series capacitance which is small, typically 15nF or less. When wired to a normal 50 kilohm line input this forms a 200Hz high-pass filter, which eliminates the bass.
If wired to a consumer plug-in-power microphone input of about 7 kilohms impedance, the result is a 1kHz high-pass filter. Hence the reputation for poor bass performance....."
A piezo element can be thought of as a sound-dependent voltage source in series with a large capacitance of about 15nF. Here the trouble starts. You need to put this into a load which is higher than the impedance of the series capacitor at the lowest frequency of interest. If that is 20Hz, since the impedance of the capacitor is 1/2 pi freq*C then you want that to be > 530k. I've terminated mine in 3.9M because that's what I had to hand.

So what to people do? They go and stick this into the line input of their recorder, typical impedance 50k, or the plug-in-power mic input of their recorder, typical impedance about 7k, and they start to grouch that this damn thing sounds tinny. Which is does - from the frequency response you can see that the mic input starts losing low frequencies below 1kHz (red line) and the line starts to go below 200Hz (green line).

We don't really like that very much. This tends to be compounded with the problem that these things aren't really designed as mics and resonate at 2-4kHz anyway unless you stick them down so you get a harsh ugly result. What you need is a high impedance input, which gets you the blue line in the plot above.

You could try and equalise in post production. The trouble with that approach is you need the specs of the mic and amplifier input to know where to start your 6dB/octave LF boost, so you will probably have to tune that by ear if you don't know these. Plus you will boost all the noise and hum at low frequencies. With the mic input you are looking at putting in a 25dB boost by the time you get to 50Hz which is just asking for hum problems, and even the 8dB boost on the line input version is more than doubling any hum you may have.

These things are very much like the crystal mics people used to use with valve open reel recorders in the 1950s/60s. Valves do high impedance really well, and inputs were often in the area of 1 megohm input impedance. The move to transistor circuitry meant consumer audio inputs tended to end up around 50k which is reasonably easy to do. Crystal mics also used the piezoelectric principle, and were valued for their low cost and high output voltage which overcame the preamplifier noise better. Transistorising audio gear killed off the crystal mic as they sounded tinny and harsh with the lower input impedances for the same reason.
cheap FET buffer high performance version

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Cheap, innocent looking piezo elements do okay as greetings cards sounders. They seem to cause grief when used as contact mics. They seem to promise a lot, but easily sound rough as commonly applied. The problem with piezo guitar pickups and contact mics is that they are not well matched to typical audio inputs.

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Homemade Instruments - 03 How To make a piezo microphone

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DIY piezo pickup

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Mechanosensitive Piezo Channels in the Gastrointestinal Tract - PubMed (nih.gov)

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Record. Everything.

The NT-SF1 records the entire spherical sound field. Every sound, every direction, perfectly. This means that later, in post-production, you can manipulate the sound. Alter the mic directivity, position and rotation. Create a full 7.1.4 surround mix. Create a fully head-tracked 360-degree soundscape for immersive video. Whether recording opera, a swamp or a feature film, the NT-SF1 offers unparalleled quality and flexibility.

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The NT-SF1 records the entire spherical sound field. Every sound, every direction, perfectly. This means that later, in post-production, you can manipulate the sound. Alter the mic directivity, position and rotation. Create a full 7.1.4 surround mix. Create a fully head-tracked 360-degree soundscape for immersive video.

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Mic Anything!! Can You Make a Guitar Pickup From an Old Watch ...

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A piezoelectric mic or a crystal microphone uses a thin strip of piezoelectric material attached to a diaphragm. The two sides of the crystal acquire opposite charges when the crystal is deflected by the diaphragm. The charges are proportional to the amount of deformation and disappear when the stress on the crystal disappears.

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How to use Piezo mics

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Cheap, innocent looking piezo elements do okay as greetings cards sounders. They seem to cause grief when used as contact mics. They seem to promise a lot, but easily sound rough as commonly applied. The problem with piezo guitar pickups and contact mics is that they are not well matched to typical audio inputs. By their nature they can generate a lot of signal, but they cannot drive a 50 kilohm typical line input. The pickup needs to work into a much higher impedance, typically 1 megohm or so.

Two solutions are proposed - the cheap and cheerful FET buffer which probably meets all the requirements of 90% of users who are using the devices on outside structures, and the high performance version for those who want higher performance and lower noise. First of all though, why do these things sound rotten the way they are most commonly used?

The sound to voltage conversion isn't noted for its high quality - most piezo contact mics are tuned speaker elements used in reverse. The brass disc on which the element is glued is designed to resonate at the design frequency of 2-4kHz so that a large audio output is achieved with a small power input. This will tend to lead to a peakiness at mid-frequencies, which is why I glue these to a magnet to try and spread this peak and make the coupling to the source better.

However, the main reason these have gotten a bad rap is that many people couple them into a standard audio load, which loses low frequencies.

The reason why these devices often sound tinny is because the piezo sensor presents its signal through a series capacitance which is small, typically 15nF or less. When wired to a normal 50 kilohm line input this forms a 200Hz high-pass filter, which eliminates the bass.
If wired to a consumer plug-in-power microphone input of about 7 kilohms impedance, the result is a 1kHz high-pass filter. Hence the reputation for poor bass performance....."
A piezo element can be thought of as a sound-dependent voltage source in series with a large capacitance of about 15nF. Here the trouble starts. You need to put this into a load which is higher than the impedance of the series capacitor at the lowest frequency of interest. If that is 20Hz, since the impedance of the capacitor is 1/2 pi freq*C then you want that to be > 530k. I've terminated mine in 3.9M because that's what I had to hand.

So what to people do? They go and stick this into the line input of their recorder, typical impedance 50k, or the plug-in-power mic input of their recorder, typical impedance about 7k, and they start to grouch that this damn thing sounds tinny. Which is does - from the frequency response you can see that the mic input starts losing low frequencies below 1kHz (red line) and the line starts to go below 200Hz (green line).

We don't really like that very much. This tends to be compounded with the problem that these things aren't really designed as mics and resonate at 2-4kHz anyway unless you stick them down so you get a harsh ugly result. What you need is a high impedance input, which gets you the blue line in the plot above.

You could try and equalise in post production. The trouble with that approach is you need the specs of the mic and amplifier input to know where to start your 6dB/octave LF boost, so you will probably have to tune that by ear if you don't know these. Plus you will boost all the noise and hum at low frequencies. With the mic input you are looking at putting in a 25dB boost by the time you get to 50Hz which is just asking for hum problems, and even the 8dB boost on the line input version is more than doubling any hum you may have.

These things are very much like the crystal mics people used to use with valve open reel recorders in the 1950s/60s. Valves do high impedance really well, and inputs were often in the area of 1 megohm input impedance. The move to transistor circuitry meant consumer audio inputs tended to end up around 50k which is reasonably easy to do. Crystal mics also used the piezoelectric principle, and were valued for their low cost and high output voltage which overcame the preamplifier noise better. Transistorising audio gear killed off the crystal mic as they sounded tinny and harsh with the lower input impedances for the same reason.
cheap FET buffer high performance version

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Cheap, innocent looking piezo elements do okay as greetings cards sounders. They seem to cause grief when used as contact mics. They seem to promise a lot, but easily sound rough as commonly applied. The problem with piezo guitar pickups and contact mics is that they are not well matched to typical audio inputs.

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A microphone designed to be in physical contact with the object producing sound. A contact mic receives and derives its audio signal from mechanical vibrations instead of airborne sound waves. These are sometimes called “piezo” or “transducer” mics, but “contact mic” is the most accepted name.
What is "Contact Microphone"? (sweetwater.com)

pat

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https://www.instructables.com/Make-a-Contact-Microphone/

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Sound Field website

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SoundField | Microphones and Processors with unique surround sound capabilities

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FREEDMAN ELECTRONICS GROUP BREAKS GROUND INTO AMBISONIC AUDIO - PURCHASES 360-DEGREE SURROUND MICROPHONE MARKET LEADER, SOUNDFIELD, FROM TSL PRODUCTS

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Here is a link to a piezoelectric microphone working under water.

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Vesper’s piezoelectric MEMS microphone works under water

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This is a website that talks about what the basics of carbon microphones are, what they are used for and the advantages and disadvantages are.

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Microphone Tutorial Includes:
Microphone basics Microphone types Microphone specs Microphone directionality Dynamic microphone Condenser microphone Electret microphone Ribbon microphone Crystal / ceramic microphone Boundary / PZM microphone Carbon microphone How to buy best microphone Microphones for video Microphones for vocals/ singing

The carbon microphone is not widely used these days, but it has been included here, more for the sake of interest and completeness.

The carbon microphone was developed in the 1870s by Englishman David Edward Hughes. It was the first reliable form of microphone and it was widely used for many years before being supplanted by other types that gave much higher levels of performance.

Carbon microphone basics

The basic concept behind the carbon microphone is the fact that when carbon granules are compressed their resistance decreases. This occurs because the granules come into better contact with each other when they are pushed together by the higher pressure.

The carbon microphone comprises carbon granules that are contained within a small contained that is covered with a thin metal diaphragm. A battery is also required to cause a current to flow through the microphone.

When sound waves strike the carbon microphone diaphragm it vibrates, exerting a varying pressure onto the carbon. These varying pressure levels are translated into varying levels of resistance, which in turn vary the current passing through the microphone.

Construction of a carbon microphone

The varying current can be passed through a transformer or a capacitor to enable it to be used within a telephone, or by some form of amplifier.

The frequency response of the carbon microphone, however, is limited to a narrow range, and the device produces significant electrical noise. Often the microphone would produce a form of crackling noise which could be eliminated by shaking it or giving it a small sharp knock. This would shake the carbon granules and enable them to produce a more steady current.

Carbon microphone applications

Carbon microphones were an ideal choice of microphone in the early days of the telephone. They were widely used in telephone applications because they gave a high output which meant no amplification was used.

Carbon microphones were used in telephones like this vintage British GPO 300 series telephone

As radio started to be used, the carbon microphone was initially used there as well - for broadcasting as well as communications purposes. However their use in broadcast applications soon came to end because of the drawbacks of noise and poor frequency response. Other types of microphone started to become available and their use was preferred because of the better fidelity that was available. The use of the carbon microphone persisted for many years for communications purposes as they gave a high output and they were robust. The poor frequency response was not an issue.

The carbon microphone was used for telephones up until the 1970s and 1980s, but even there it became possible to use other types of microphone more conveniently. Also the crackle and noise of the carbon microphone had always been an issue and when other types of microphone became available at a low cost they started t be used, despite the requirement for additional electronics needed.

Carbon microphones are now only used in a very few applications - typically only specialist applications. They are able to withstand high voltage spikes and this property lends itself to use in a small number of applications.

Carbon microphone advantages & disadvantages

As with any form of microphone there are advantages and disadvantages.

Carbon microphone advantages

High output

Simple principle & construction

Cheap and simple to manufacture

Carbon microphone disadvantages

Very noisy - high background noise and on occasions it would crackle

Poor frequency response

Requires battery or other supply for operation

The carbon microphone has a number of advantages, but today the disadvantages normally outweigh the positives and as a result they are rearely used..

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Microphone Tutorial Includes: Microphone basics Microphone types Microphone specs Microphone directionality Dynamic microphone Condenser microphone Electret microphone Ribbon microphone Crystal / ceramic microphone Boundary / PZM microphone Carbon microphone How to buy best microphone Microphones for video Microphones for vocals/ singing The carbon microphone is not widely used these days, but it has been included here, more for the sake of interest and completeness.

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he carbon microphone depends for its operation on the varying resistance of a carbon element when subjected to varying pressure. The usual arrangement of this type unit, for best fidelity, consists of two carbon buttons one on either side of the diaphragm. This metal diaphragm - in a properly built carbon microphone - is stretched and air damped so that the effects of self-resonance vibrations are negligible, giving a reasonably uniform output at all ordinary audio frequencies.

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Carbon mics are those funky ones in the big circle that have coil around them used in the 20's and 30's

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Carbon mics also known as button mics