Long range directional microphones-myth and reality

There are a lot of myths about directed long range microphones. You can here like they can reach distances of 100, 200 and more meters, other say that this is a myth and these numbers are commercial purposes. Lets try mathematically find a proof and see real situation.

Introduction to long range microphones

When talking about directed microphones usually we have in mind that sound sources are in open air, and there is no reverberation effects. So the only factor is distance of sound source object from microphone. Along the distance sound power drops significantly and ir longer ranges it interfere with other sounds like wind and other noises in atmosphere.

So when distance is about 100m sound pressure drops more than 40dB(comparing to distance equal to 1m). So if sound level is 60dB then from 100m you will hear 20dB. Sound level 20dB is less than other environmental noise and many common microphones are not sensitive enough for such sound level.

So we can say, that directed microphones must have:

  • High sensitivity and selectivity from environment noises even if they have highe level than real sound;

  • High directivity in order to able exclude noise signals that are higher than useful sound signal. Directivity means ability to attenuate noise signals, that come from other directions than sound source object.

Practically speaking to comply these requirements with one microphone is quite difficult task. There were other solutions like creation of low directive microphones with high sensitivity or highly directive microphones with low sensitivity. This is why there are several constructions of directional microphones.

Types directional microphones

There are four general types of direction microphones:

  • Parabolic;

  • Flat with Phase grid;

  • Microphones with running wave;

  • Gradient.

Parabolic microphones

Parabolic microphones have parabolic shape that reflects incoming sound waves to one focal point where microphone is located. This is one of most known and commonly used directional microphones

parab.gif

Diameter of parabolic mirror may reach from 200 to 500mm. Working principal is simple – sound waves incoming along direction axis are reflected towards microphone in parabola focal point. At this point sound level “amplifies” because of sounds added with same phase. So the bigger diameter of mirror is the bigger amplification may be reached. Other sounds coming from different angles aren’t amplified much by this effect because of different phases of each reflections in focal point. Parabolic microphone has high sensitivity, but directivity isn’t very high.

Flat directed microphones

Flat directed microphones are based on idea of receiving sound from multiple points located in one plain surface which is perpendicular to incoming sound.

flat_direc_mic_2.png

The picture above isn’t exact drawing of such microphone, but it gives an idea how does it look like. Where wave guide holes are there can also be microphones located where signals can be summed electrically or waveguides where sound-waves are summed at one point and then converted to electrical signal. Mechanical or electrical signals have to be same phase. If all sound signals entering waveguides are unidirectional then they will have same phase and summing will give maximal result. If sound direction isn’t perpendicular then it will have different phases in each hole and summing will weaken them. The bigger angle the lower noise level. One good advantage of flat microphones comparing to parabolic is that they are easier to hide, because the flat surface can be a suitcase or even a wall.

Microphones with running wave

Microphones with running wave or so called pipe microphones are different because it receives sound not perpendiculary but along wave direction.

Runnig_wave.png

Main part of this microphone is a waveguide pipe with diameter 10 – 30mm with special cells located over all length of pipe. It is obvious that that sound incoming into each hole will be added with same phase. If sound is incoming with different angle then the phase in each hole will differ because of different sound speed inside pipe would result in loosing its power.

Pipe length usually is from 15 – 230mm to 1m. The longer waveguide is the bigger sensitivity of microphone.

Gradient Microphones

Gradient microphones are different from phased receivers, where same phase signals are added to get more sensitivity. Gradient microphones are based on calculation by direction. But this method is limited by sensitivity of discrete microphones. Calculation of signals also weakens signal but summing noises. But main advantage is that this method allows constructing small sized directional microphones. Simplest gradient microphone is so called first order microphone:

gradient_microphone.png

This construction consist of two high sensitivity microphones near each other. Output signals of both microphones are subtracted from each other. And finally the diagram cos(Q) is calculated where Q is angle of incoming wave. By this diagram sounds can be filtered for one direction. Usually there are 2nd and 3rd and higher order gradient microphones with better characteristics.

Comparing directional microphones

Directional microphones can be compared by working distance by common conditions. For open area with independent noise direction working distance R is related with:

SNR=Ss-Sn-20lgR+G-Sp

where G – direction coefficient of microphone(dB), Sp – microphone sensitivity threshold(dB).

Coefficient G can be calculated by formula:

G.png

Where Q- wave angle, φ angle in polar coordinates.

F.png

L – length of waveguide, l – sound wave length. When L=l then for Running wave microphone:

G=4L/l;

For Flat microphone:

G=4p(S/2l);

where S – aperture area, l- sound wave length.

For gradient microphone:

G=n(n+1);

n- order of microphone.

When G is known then it is enough for calculating SNR value. But in most cases this may result in wrong results. This is why it is better to calculate relative non absolute values of distance. Using this ideology microphones can be compared with human ear. Then we can write:

R=R0 · 10 · 0.05·(G-G0)-0.005 · ΔSp;

where R0 – distance of hearing sound with human ear, R- distance with directional microphone, G0 – ear directional coefficient. ΔSp – sensitivity difference between ear and microphone.

diagram.png

In the diagram we can see that when G=15dB(value for good microphone) then distance is about 3 times bigger than ear. Practically speaking if we compare human ear and directional microphone in city (noisy area) then values would be like human ear can hear human speech at distance about 2- 4m and directional microphone can from about 6 – 12m. Outside city where noise level is low, ear can hear at distance 10m while directional microphone from more than 30m.

Of course there are more advanced methods like digital multichannel filtering and using high sensitivity sensors where threshold may reach -15dB. Sensitivity can also be increased by increasing size of antenna.

As we mentioned at the beginning calculation shows that reaching 100 or 200 distance with directional microphones is quite difficult task. Normally there are directional microphones in market that can effectively register human speech (76dB) at distances about 50m.

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Source: http://vrtp.ru

ScienceProg

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18 Comments:

  1. Hey keep up the good work on this site, the content is just awesome… I have you set up on my google reader, so when ever there is new content its on my homepage, and I love it.

    Kinda odd your forums arnt just packed with people, but what ever!

    -Brent

  2. Thank you,
    What do you mean packed with people? Could you give me a hint that I could improve the situation. Thank you.
    SPman

  3. Hi, I was curious about your calculation of G for the flat microphones. What does the variable p represent? Is S total or individual aperture area?

  4. I wear dual hearing aids, and like most H-A users, I am at a complete loss in areas with high “background” noise. Filtering as the hearing aid mfgs. do does not really work because the background noise is almost always other people talking. Filter them out and I filter out the person that I am trying to hear. I would like to add a very directional mic to my H-A input in place of the normal mics. The input part is easy because I have inductive inputs and a neck loop to couple to the H-A’s. After reading your report, it seems as though a flat directed mic would work best in this case. What do you think? Has anyone else asked this question?

    Thank you, Dave Capone, Harwick PA

  5. Pingback: Audival – Podcasting Tips & Tricks → Archive » Minimizing background noise during recording

  6. hi any ideas on good quality directional microphone
    to plug into a small digital recorder for bird song

    • The 12/04 “Nuts and Volts” has a good design for a parabolic microphone using a dish from Edmund Scientifics (about 30 bucks). Scientifics has up to 2 foot dishes. I used the hardware (dish and handle) with a Primo Microphone (EM172). A Radio Shack electret mike worked well and I was satisfied until I tried the Primo low noise mikes and they are only two wire mikes, too. Ditched the Shack mike. I started out with an Olympus WS-400 digital recorder and got really got good results but I like the DM-420 and DM-620 better because I also use the recorder to learn bird calls and songs and they will randomize playback.. Here is the link to the Primo microphones. http://www.frogloggers.com/FORMgallery4.htm

  7. This is a reply to tim: it seems http://www.bestparabolicmicrophones.com sells what you are looking for

  8. Hi: Im looking for advice on a parabolic microphone. Am trying to devise something for recording animal calls that could be 1000 meters away or further. Also will need a digital recording device and software for cleaning up recordings. Ideas?

  9. Bob, I am afraid that for 1000 meters away you will need a totally customized microphone and that will cost you pretty good money.
    As long as I could verify, those guys at bestparabolicmicrophones.com are the only on the market making available more than 30Megs of downloadable archives for their microphones and also offering customization capabilities.
    I would suggest you to contact them…

  10. Hi i work as a public space cctv operator… I would like to know if it would be possible to fot a diretional microphone to a cctv camera housing and connect the feed to a control room usinf the installed fibre optic cable

  11. Mike, the closest product that I can suggest matching your description is this: http://www.spectradome.com/product_info.php/cPath/58_86/products_id/707/language/en

  12. I’m trying to put together a devise to amplify wildlife calls and record responses , as a scouting defice, the recording defice would not need to be very clear or crisp, just enough to indicate the presence of wildlife via there return vocalization. Please help, where do I start?

  13. hi I am looking for a device which can pick a sound from group of people who talks at a time like a trading pit is there any one who can give me a clue ?
    thanks

  14. thanks for this page. it helped a lot on my project on directionality of sound waves

  15. Thanks for the work. Finally I found an article that brings some maths too. Big THANKS!

  16. Alright, so the graphs and equations are cool, I guess, but what is the conclusion? “It can be a difficult task.”?? That just sounds redundant. Instead of the really exciting equations, how about a yes or no and the best equipment to actually use to get it done. Or even videos of field testing to show these different microphones and your theories put to the test. “It can be a difficult task” is not very conclusive.

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