Passive amplification of a specific audio frequency range(microphone)

  1. Hi, I want to passively amplify a specific audio frequency range of a microphone.
    When I say passively I mean by non-electronic means - no op-amps or transistors.
    Like a speaker can be designed for specific frequency range - a woofer/tweeter/etc...
    I presume this works in the other direction - If a make a large membrane it will respond more to a lower frequency range relative to a small membrane.
    If I place a microphone in the other side of the membrane(opposite to audio source) I will accomplish this goal.

    I am looking for something not to just filter, as in a band-pass filter, a specific frequency range, but to amplify it so I get maximum power conversion from audio power to electronic signal - maybe some structure that will resonate in that frequency range.

    If you know a good book/website on the subject or an advance method you've encountered, as in adding ridges to that membrane or using a loudspeaker as a means of focusing sound onto a microphone instead of radiating sound or other ideas that are clever, I will be glad to hear.

    By the way I want to use a piezo transducer as a microphone and it's resonance freq. is 180Hz...

    Oh and one more thing - reduced size is an advantage!
    Last edited: May 13, 2013
  2. jcsd
  3. berkeman

    Staff: Mentor

    One solution is a parabolic microphone:

  4. sophiecentaur

    sophiecentaur 13,293
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  5. berkeman

    Staff: Mentor

    There's a really cool exhibit at the Exploratorium in San Francisco (well, I'm assuming that they moved this exhibit when they changed buildings recently). There are two chairs facing each other about 30 meters apart, and behind/around each chair, there is a parabolic reflector about 3 meters in diameter. The two reflectors face each other across the room.

    When you sit in the chair, your head is at the focus of the reflector. When your partner in the other chair 30 meters away talks in a normal voice, you can hear them like they were right in front of you! Very impressive demonstration of the power of a parabolic reflector! :smile:
  6. The problem here is that parabolic microphones have bad amplification in low frequencies due to their shape... you need a really big paraboloid to focus large wavelength sounds(low frequency). Any other solutions? all I could find about these parabolic reflectors is basic no 'patent' to reduce their size and keep the same effect.
  7. davenn

    davenn 3,392
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    yeah that's cool,
    The Parkes Radio Telescope Observatory in NSW state of Australia has a similar setup as part of their interactive visitor display.

  8. sophiecentaur

    sophiecentaur 13,293
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    For some years I had two 1.4m parabolic dishes mounted on opposite facing walls in my lab at School. A good demo and it cost nothing to acquire the dishes. Very disturbing if you walked through the focus and some kid on the other side of the room was telling her mate something dodgy!
  9. Bobbywhy

    Bobbywhy 1,908
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    Since you have a specific frequency of sound you want to amplify passively (180 Hz), why not just make a resonant cavity and place your microphone inside it? For this low frequency, you do not get "reduced size" because the laws of Physics dictate the physical size of the cavity. Consider this an "Engineering Challenge" and discover some new, innovative way to solve this!
  10. sophiecentaur

    sophiecentaur 13,293
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    Up to a point but a Helmoltz Resonator can be made much smaller than one wavelength in size. What you really want is not so much 'Amplification' (which I regard as necessarily involving an active device) but effective matching the impedance of the waves to detector.
  11. I found something about Helmholtz resonance but to manufacture a cavity like that is difficult without specialized equipment... I am still googling for a solution...
  12. sophiecentaur

    sophiecentaur 13,293
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    "Specialist equipment"? A box with a hole in it constitutes a Helmoltz Resonator, in principle. Take the sub-woofer in your Home Cinema setup; that is acting like a Helmholtz resonator (ported cavity). It resonates at, say 20Hz (wavelength about 15m) but has a low Q factor, I think.
    I don't think you are likely to come up with a very simple solution; what you are after could be a a bit demanding if taken to the extreme.
    Perhaps if you gave us a bit more detail / context? How big does this thing have to be, for instance? Are you trying to detect a continuous tone at low amplitude? You could use a mechanical resonator (reed) attached to the diaphragm to get a magnification of, perhaps 100 times as long as the source is on for a sustained time.
  13. That means tearing apart my sub-woofer...
    and some detail about what I plan to do:
    Well the piezo speaker is going to "monitor" the presence of a 180Hz audio wave if I place it in a certain location. For example if I place it in near my living room's window for 1 day I can see how much 180Hz sounds came from the street to my living room throughout the day - like a street noise monitor but for 180Hz and not just 'noise'.

    When I said "Specialist equipment" I meant that according to the equation in the website I mentioned above,
    I need a certain diameter, certain length and volume and all that stuff to make a resonator for my specific frequency - I can't just find a box and hooray I am done - 20Hz with low Q is still not best for 180Hz and sub-woofer are big.
    I want this to be as small as possible - something you can put in your pocket and as accurate as possible for someone without the "Specialist equipment" - things 'in principle' will perform worse or not at all...

    If I had a 3D printer I could print one :)

    And by the way does the shape of the resonator matter(round/square/etc)? the equation requires only volume but I guess shape has some meaning and can improve the performance of the resonator.
    Last edited: May 14, 2013
  14. sophiecentaur

    sophiecentaur 13,293
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    If you want to achieve this without even as much as making yourself a wooden box then you are bound to be disappointed, I'm afraid. Things like this are not on supermarket shelves and would need some significant practical input from you.
    One thing that is important for a low loss resonator is rigid walls and lots of mass. People used to go wild about mounting loudspeaker drive units inside concrete pipes at one time for this reason. I can't imagine your notional 3D Printer would do you any good at all because you would be stuck with the available fabrication material, which would be low density - you are clearly of the recent generation, imagining that a high tech solution would be necessarily less trouble and would work better. It certainly would not be cheaper at this stage in history.
    Google around some more and you should get some idea of just how big a resonator would need to be for a good resonance at 180Hz - it gets worse as you go smaller, if I remember right. How 'big' can you go for?
    I have one big question for you about all this. If you are planning to monitor the street sounds during the day then why does this need to be a passive device? Will it not be associated with (i.e. connected to) some electronics, which will be very much 'active'? So it could be a powered device.
  15. It needs to be a low power device - portable and possibly battery operated so less active amplifying means less power. The voltage the piezo will output goes into a circuit that converts the voltage to a DC voltage relative to the amplitude of the sound wave and then A/D converter sends this voltage to a microprocessor and then possibly with USB to a PC...
    by small I mean something portable that you can hold in the palm of your hand - actually I was looking for something that could fit in a 16mm diameter tube that has a maximum length of 15cm - so it's portable like a pen and easy to handle.

    You talked about pipes is it possible to make a pipe - lets say made out of common hardware store material(maybe PVC - rigid like you said) that will act as a resonator without adding the cavity itself?
    Last edited: May 14, 2013
  16. sophiecentaur

    sophiecentaur 13,293
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    I have two hearing aids which last for about two weeks between battery changes. Power consumption for a well designed device is hardly relevant. How are you proposing to record the signals that this detector will be gathering?
    This is a fairly demanding engineering job (if you really are concerned about power consumption) so you must approach it as such. There is seldom any point in 'bending the project to fit a pet solution'
    Starting from now, I can guarantee I could come up with a very low power amplifier before you had even been down to the hardware store for the parts you'd need to do the job passively.
    btw, there is no way that a suitable 180Hz resonator could be made with a cavity the size of a pen.
    You appear to be confused about the 'resonator'. The pipe would be the resonator - but it would have to be a reasonable size - unsuitable for your brief
  17. Well I plan on using a tiny solar cell(maybe a bunch in parallel to increase current) to power it if this answers your question about the power consumption.
    I have experience with A/D converter and made 3 working projects one with A/D, D/A, a microcontroller and FPGA working together in the same project so I can manage this section of the problem. In this project the samples from the A/D will be stores in some memory with a suitable capacity for the application.

    I know electronics more than acoustics and I want passive amplification so I will learn something new and I also like low power devices.

    For example if I want this to be used as a wireless sensor network to monitor 180Hz in various places for a long period of time then power consumption is an issue(using the solar cell removes the need to replace a battery)...

    plus I like things I make to be scalable so simple is better - If I make this simple with passive amplification - then improving it and keeping power consumption and complexity low is an advantage - especially if I plan to make multiple units.

    I love innovative ideas and always like to hear ones and like to think of some - for example I looked into mimicking bat ears as bio-inspiration but It seems they are suitable for high frequency(above 10KHz) - they actually look like these reflectors you and other users mentioned...

    You said I need some practical input, that's not a problem I have improvised some things before that people told me "that wouldn't work well or at all, it's not a professional solution..." - and in the end they worked and I am still using them...

    You are really inquiring on all aspects of the project - this shows your someone who knows what he is doing :) thank you for that.
    Last edited: May 14, 2013
  18. sophiecentaur

    sophiecentaur 13,293
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    The "bat ears" stuff will have shown you the difficulty when you scale things down to 180Hz. More like Elephant's Ears lol.

    If you are planning a wireless network then why are you concerned with the extra few hundred microAmps drain involved in doing this electronically? This link gives a circuit (of which you would only need the first two transistors, followed by a simple band-pass LC filter) which would do just what is needed. Current drain would be laughably low. There are loads more examples out there.
  19. Do you also remember my small size requirement?
    I have an A/D, micro-controller and a serial-to-usb module take space along with the piezo(by the way it is possible to lower it's res freq - by adding mass to it according to its datasheet) itself that needs to be inside too. also the solar cell that I want to use is connected to a circuit which charges a super-cacapitor(it's a small one - 10mm diameter) and has a nice step-up converter with it and they all take space.

    I know I blah blah a lot about all these requirements but I have a bigger plans for this so this has to been done small.

    And most important - using all these transistors to amplify is not as interesting...
    And yes I have looked into elephants too as they communicate with low pitch tones but think of this! humans hear down to 20Hz and whales to 10Hz their ear structure is massively different in size but still a humans ear hears those low frequency sounds - here something about whale ears:

    I say no to saying "NO" - we most find the answer!

    A thought I had is to attach the piezo directly to a structure wall as buildings vibrate from low frequency vibrations like earthquakes - maybe to my living room's wall or window frame but I don't know their resonance frequency... this article backs this up: - "By placing a microphone on a reflecting surface it will become twice as sensitive as it would be if it was placed well away from the wall, because the sound pressure level on that boundary is twice as high. The result is more output and/or less self-noise, which can be very useful."

    one more - a male humans voice has a fundamental frequency of about 180Hz maybe looking more on how it is produced can lead to something.
    Last edited: May 14, 2013
  20. sophiecentaur

    sophiecentaur 13,293
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    What actual specification do you have for this device? It would be an idea if you sorted out exactly the sound pressure levels you plan to deal with because that would give you an idea of how necessary it would be to invent a special, frequency-selective transducer. You talk in terms of using existing structures for gathering the sound energy. That could be fruitful, if you were lucky, but it makes the point that a suitable device for use 'anywhere' would need to be a similar size.
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