Voltage from mic - Page 1
Forum section: electronics / beginners

author: DDRE
author_rank: Newbie
timestamp: February 25, 2015, 01:01:10 pm
content: Hi there,I am a beginner and recently got interested with microphones and it's working. I know that when we speak into a electret microphone, a minute voltage is produced. This voltage generated is too less for my voltmeter(Old one that malfunctions most of the time) to give readings.I just wanted to request a few members of this forum to give readings of voltages produced by an electret mic when an audio source is placed 1.5m away from the mic and the loudness of the audio source is 60dB.This would be of great help as I am currently trying to collect some results for conducting further tests.Thanks for all your help!DDRE

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 25, 2015, 01:12:21 pm
content: An electret microphone capsule does not actually output ANY voltage. It acts as a variable resistance and can only "modulate" an externally-applied voltage.It has a transistor inside which conducts more or less depending on the waveform of the audio hitting the diaphragm. The transistor is required because the diaphragm capacitor is extraordinarily high impedance and the transistor forms an "impedance converter" to bring it down to a couple thousand ohms for better survival out in the Real World.Now, there are some electret mic capsules which are "raw" and have no impedance converter transistor inside. They actually DO output a voltage (after a fashion) but it is so infinitesimal and so astronomically high impedance, you will never be able to measure it with any conventional test equipment.

author: DDRE
author_rank: Newbie
timestamp: February 25, 2015, 01:29:19 pm
content: Hello Richard,As the electret mic doesn't produce any voltage with a sound source, is there any electronic equipment which produces voltage when there is an audio source near it? If so, how high will the voltage be?Thanking You,DDRE

author: G7PSK
author_rank: Super Contributor
timestamp: February 25, 2015, 01:37:50 pm
content: You need to look at moving coil microphones or possibly ribbon mic's. and piezo microphones to get an actual voltage output the piezo mic's will give the greatest voltage output.

author: BradC
author_rank: Super Contributor
timestamp: February 25, 2015, 01:47:58 pm
content: Quote from: DDRE on February 25, 2015, 01:29:19 pmAs G7PSK stated, moving coil is the go. You can also try crystal mics, but you won't get anywhere near the sensitivity as you will with a moving coil. As for the voltage, it depends on the impedance of your circuit. As mics are AC devices, it is very common to use transformers to deliver the required matching to a circuit.Having said that, 60dB at 1.5M is not a lot of sound. You'll want a very sensitive mic with a BIG diaphragm to get more than a couple of mV into any form of load. My Shure SM58 will generate from ~10-100mv depending on how loud I'm screaming into it at the time.Is there any reason you can't use some form of amplifier?

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 25, 2015, 01:51:23 pm
content: A dynamic microphone directly produces voltage. The diaphragm moves a coil back and forth over a magnet.That is how generators work, etc. By moving coils of wire in relation to magnetic fields.And a ribbon microphone is a half-brother to the common diaphragm dynamic microphone.It uses a single ribbon of very thin aluminum to wave back and forth through a strong magnetic field.But, because the ribbon microphone is only "one turn", the voltage it produces is exceedingly low.So virtually all ribbon mics use a step-up transformer from the extraordinarily low impedance of that aluminum ribbon, up to something usable in the Real World.As noted by others, a crystal microphone will also generate a voltage by the piezoelectric effect. But those are pretty rare.Note that even the highest-output dynamic microphones put out only very low millivolts of signal even in the strongest acoustic field.That is why we use microphone preamps virtually everywhere you find any kind of microphoneThe mic preamp typically amplifies the signal by 40dB (100x) and boosts the "mic-level" signal up to "line-level" (something approaching 1V)

author: DDRE
author_rank: Newbie
timestamp: February 26, 2015, 05:33:42 pm
content: Hello Everyone,Thanks for the inputs and feedback.I just wanted to get these values/ideas to see whether it is possible to generate usable electricity from the ambient sounds in close proximity(1.5m is close proximity, 60dB is normal human voice loudness). If this was possible by a small device(i.e. mic), then we could use this to power something momentary or power a small device when sound reaches the mic.I thought that even if If we get values just >20-25mV consistently, then this voltage can be stepped by by a transformer and can be amplified using an IC to power a device.Are there any other types of devices/contraptions that can probably generate this much voltage within these parameters?Are there any suggestions for probably getting this much voltage from any type of transducer/mic/speaker?Or should I start by making a thin generator based of the design of a speaker? If so, how big should the the diaphragm be to just get usable voltage?Thanks for all your help!DDRE

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 26, 2015, 05:57:41 pm
content: I fear that you will discover what others before you have discovered.1) There is extraordinarily little actual ENERGY in you average everyday sound waves.2) The cost (in terms of mechanical and electrical efficiency) is prohibitively high to make any practical generator.You can use a speaker as a "microphone", and there are specialized applications where that is rather common.For example, picking up the specific sound of a kick-drum.But, in that example, there is a large amount of energy as long as you are very close (an inch or two) away from the drumhead.You might even be able to "see" average ambient sounds on an oscilloscope, but only with the massive amplification in the scope.The only "energy harvesting" scheme that has claimed success is people who are using the proliferation of RF fields to gather microscopic amounts.But, frankly, there is widespread skepticism that even that premise actually produces anything practical.

author: AG6QR
author_rank: Frequent Contributor
timestamp: February 26, 2015, 06:49:23 pm
content: Quote from: DDRE on February 26, 2015, 05:33:42 pmVoltage is only part of the power equation. Power is voltage times current. Energy is power integrated over time. Transformers can boost voltage, but they can only do so at the expense of current. Transformers always deliver slightly less power output than input, as they trade voltage for current, or current for voltage.And the phrase "amplified using an IC" suggests a similar misunderstanding. There is no IC that can amplify total power. There are certainly ICs which amplify signals, but they always require more power input (usually from a DC source) than the power contained in the output signal they produce. ICs don't contain energy sources, so the law of conservation of energy tells us that the total power output of an IC cannot be greater than the power input.In this way, ICs are like transformers. They can be used to manipulate certain characteristics of a signal in very useful ways, but they don't produce more power output than they consume as input.As Richard Crowley states, there is very little energy in ambient acoustics, at least under normal circumstances. If you want to harvest free ambient energy, I suspect light is a few orders of magnitude easier to efficiently harvest than sound.