Microphones at longer wavelengths than their size?

[The Bill]

Why are microphones pretty good at picking up sound that is much longer in wavelength than the size of the microphone? 1khz sound has a wavelength of around a third of a meter, varying a bit depending on atmospheric conditions. Yet a 1cm diameter electret microphone can pick it up reasonably well.

Is there something different about sound from radio waves that let's small microphones work well where small antennas wouldn't work as well for picking up long wavelength signals?

Or do I appear to be fundamentally misunderstanding something?

 

[Vanadium 50]

Do you have the same problem with ears? That will influence the likely helpful answers.

 

[Delta2]

I am not sure at all about my answer but here it goes

I believe the main reason is that the microphone is placed near the source of sound waves, while a receiving antenna for radio waves is usually placed miles away from the source of radio waves that is the transmitting antenna. So the EM wave that hits the receiving antenna is usually weak, while the sound wave that hits the microphone is quite strong. If we use big microphones we would able to catch sound signals that are from sources hundreds of meters or even kilometres away. (I remember now seeing a documentary about elephants and how they are able to catch weak sound signals with their big ears).

 

[NTL2009]

A barometer is small and it can respond to pressure changes that occur over days. Sound waves are compression waves.

 

[The Bill]

Okay, that's some intuition. But what is the physics?

12cm waves from a microwave oven don't pass through 1mm holes in the window shielding. Well, I know that it isn't quite that absolute and it's really an exponential decay of intensity outside the shielding. But anyway... Yet 1.2m wavelength sound waves can pass through a 1cm hole without seeming like exponential decay is happening.

Is there some fundamental difference between sound and electromagnetic wave theory, or am I missing some difference between the situations? Or something else?

I'd like answers that reference the actual physics. I'm not saying not to answer with intuition at all, but I won't be satisfied with this thread unless I get some solid physics answers that can be verified or falsified.

 

[NTL2009]

As I understand it, and mentioned above, the sound wave is a compression wave. So the microphone diaphragm responds to the high and low pressure of the wave as it passes. I do not think that the size of the diaphragm has to have a relationship to the wavelength, it is only measuring the pressure at a specific point.

If the diaphragm was at an angle to the compression wave, size might come into play - I could envision where the diaphragm was exposed to the entire wave, it would average out the pressures, and exhibit no/little overall motion.

Example: A 1.414 cm diameter microphone diaphragm at a 45 degree angle to a 1 cm wavelength sound wave would 'see' the high and low pressure points average out to zero across that diaphragm.

https://www.physicsclassroom.com/class/sound/Lesson-1/Sound-is-a-Pressure-Wave

Someone else will need to compare this to an antenna, I would probably mess that up, but I think this diagram from wiki helps:

https://en.wikipedia.org/wiki/Antenna_(radio)#Resonant_antennas

 

[A.T.]

Is there some fundamental difference between sound and electromagnetic wave theory

https://en.wikipedia.org/wiki/Longitudinal_wave
https://en.wikipedia.org/wiki/Transverse_wave

 

[berkeman]

Okay, that's some intuition. But what is the physics?

As discussed in this thread: https://www.physicsforums.com/threads/does-the-size-of-an-antenna-matter.960025/#post-6088212

A receive antenna (or microphone) can be physically small compared to a wavelength, but can be made to resonate reasonably efficiently at the lower frequencies.

 

[NTL2009]

As discussed in this thread: https://www.physicsforums.com/threads/does-the-size-of-an-antenna-matter.960025/#post-6088212

A receive antenna (or microphone) can be physically small compared to a wavelength, but can be made to resonate reasonably efficiently at the lower frequencies.

But I think the comparison kind of breaks down here. With a common microphone, we don't want resonance, we want a fairly flat response across the audio band of interest. If it resonates, it would accentuate certain frequencies (yes, that might be done to a degree to tailor the sound, but generally we want flat).

The prime motivation for a microphone is not to couple efficiently - to do that, you would want resonance with the frequency of interest. As in my barometer example - if I design a microphone with a sealed back chamber, it could pick up audio, and also pick up changes in barometric pressure over days , weeks, months. But it would be an infinitesimally small fraction of a wavelength of those signals. I'm pretty sure that microphones of that design actually do provide for a bit of leakage on the back side, so as to not be affected by environmental pressure changes.

With an antenna, you are trying to couple efficiently at the target frequency. You can demodulate to get the audio, and that audio can be fairly flat. They just seem very different to me.

 

[Nugatory]

Is there something different about sound from radio waves that let's small microphones work well where small antennas wouldn't work as well for picking up long wavelength signals?

As I understand it, the difference is in the way that the wave interacts with the detector (microphone or antenna).

A sound microphone is detecting pressure changes at a single point in space. We do that by positioning a diaphragm at that point and measuring how the diaphragm deflects in response to pressure changes there. The wavelength is irrelevant because we're only looking at the pressure at one point and we don't care that the pressure will be very different a half-wavelength away. We do care about the frequency, but only to the extent that there are mechanical limits to how quickly the diaphragm can move.

A radio antenna works by measuring the voltage differential between two different points on the antenna. These points cannot be more widely separated than the length of the antenna, and if that distance is small compared with the wavelength, the difference in the electric field intensity and hence the voltage difference between these points will be small. The second diagram posted by @NTL2009 illistrates this.
(Be aware that my description ignores much complicated and interesting physics. It's OK as a handwaving exercise, but there's lot more than that in the working of an antenna).

 

[tech99]

Why are microphones pretty good at picking up sound that is much longer in wavelength than the size of the microphone? 1khz sound has a wavelength of around a third of a meter, varying a bit depending on atmospheric conditions. Yet a 1cm diameter electret microphone can pick it up reasonably well.

Is there something different about sound from radio waves that let's small microphones work well where small antennas wouldn't work as well for picking up long wavelength signals?

Or do I appear to be fundamentally misunderstanding something?

There are parallels between sound and EM waves.

First of all, an antenna can be used for receiving or transmitting. It has the same relative gain in either role and the same radiation pattern. With a loudspeaker, it can in principle function equally as well as a microphone, when its efficiency and pattern will be the same. Now recall what makes a good bass speaker - it is usually big. The reason for this is that the sound from the back of a cone is 180 degrees out of phase with that from the front, and so if in free air, sound comes round to the forward direction and causes cancellation. A large housing allows the rear radiation to be delayed by 180 degrees so it reinforces the front radiation.

We can make a loudspeaker housing small by sealing the back space and using friction material in the box to dissipate the rearward radiation., but the efficiency is very poor. It is akin to using a short antenna with a resistor at its end.

If we want to obtain good radiation with a small structure, it is possible for a single frequency. We now have to produce huge vibrations of the cone, or very large antenna currents. This requires us to resonate the structure to cancel out reactance - inertia in the case of the LS, and capacitive reactance for the antenna. But the frequency range is now very small.

For some receiving-only applications, it is possible to use a transducer to just measure the passing wave. For instance, an electric field sensor with an amplifier for EM waves, or a pressure transducer for sound. In these cases we do not extract power from the wave - we just measure its amplitude. The electret element is in this category, where it is sensitive to pressure, and any short fall in sensitivity can be made up by an integrated amplifier.

As a matter of interest, there are two categories of microphones, pressure activated and velocity activated. These are equivalent to active antennas sensing either the electric or magnetic field of a passing wave.
Regarding the ear, it is not very well coupled to air at ordinary frequencies due to its small size, but is is apparent that the sensitivity has evolved to be just enough to discern the noise of the quietest environment. Try using an ear trumpet however! Not only is there increased directivity at higher frequencies, the mechanical impedance of the ear drum is now matched to the medium, improving efficiency, and there is increased collecting area.

A large horn can also be used for EM waves in a similar way. The human ear trumpet, or pinna, starts to become efficient at frequencies above a kilohertz, when its dimensions together with the head (which forms part of the antenna structure), are greater than the wavelength, and one can only wonder at the beauty of the design of bat ears, a tapered reflector of immense sophistication, optimised for ultra sound, or the huge acoustic lens of the sperm whale.
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