It will be interesting to gain experience from persons applying a hydrophone for the measurement of sound in air. Initial experiments seems to indicate that a hydrophone performs different in air compared to water, probably due to grater impedance mismatch between the impedance of the transmission medium and the sensor.
Yes, impedance mismatch creates a huge difference in sensitivity when taking a hydrophone designed for underwater into air.
Is it possible to design a sensitive acoustic receiver that performs almost equally well in air as in water? I think it is possible, but it would be very challenging. You would have to, in effect, render acoustic vibrations optically, by somehow viewing suspended particle motion directly.
Note incidentally that when humans hear sound underwater (as noted above), it is not so much from sound entering the inner ear under direction from the outer, as it is sound entering the inner ear through the entire head, because acoustic vibrations underwater couple very well with the head, indeed, with the entire body (low impedance mismatch). This is why plugging your ears underwater does virtually nothing to hearing sensitivity underwater, but wearing a thick close-fitting neoprene diver's hood does reduce hearing sensitivity somewhat. Nothing like this happens when bringing a hydrophone into air, however, because the mechanical structure of the hydrophone generally has no points of low impedance mismatch with air to offer unimpeded entry to its acoustic vibrations into the responsive components of the transducer.
Ronald
Interesting discussion.
I agree with the concept of impedance mismatch and that with Ronald for the reduced impedance mismatch of human ear underwater due to coupling of the acoustic vibrations to the inner ear through "LARGER" areas of head and the body.
Well why can't we think of the same conditions here too - Why can't we provide the "HEAD" and the "BODY" to our hydrophone here in the air and evaluate how it sounds?
What I mean to say is that can we attach some kind of mechanical structure well-resonating to a specific or a range of frequencies of interest to the hydrophone so as to simulate the human head-body situation and see how it performs in the air?
My measurement on a hydrophone formed as a sphere seems to indicate that structural vibrations lead to an unwanted response. The response seems to be similar to placing an ideal microphone in a reverberant room. The overall sensitivity is as expected, but the sensitivity varies with the frequency as it does in a reverberant room.
The response may be linked to the design of the probe where the sphere consists of two half-spheres and vibrations may be reflected from the joint between them. I assume this is a larger problem when the hydrophone is operated in air than in water since the water probably will damp the unwanted vibrations. I therefore assume the hydrophone will have a different directional response in air than in water and not only based on the wavelength scaling.
Ole-Herman,
You still haven't told us why you want to use a hydrophone in air.
I agree with everyone on the impedance mismatch. Hydrophones just weren't designed to work in air.
If you are looking for a weatherproof or immersible microphone, there are a number available. Just search the web on "waterproof microphone". Some are fully immersible, although they probably don't work very well as hydrophones, just as hydrophones don't work very well in air.
A weatherproof mic in air will give you a much better signal-to-noise ratio and a much more predictable amplitude-frequency response and directional characteristic than a hydrophone in air.
If you want a transducer that will work in both media, I suggest you use both a hydrophone and a waterproof mic and mix them after the preamps. If you set the gains correctly, the impedance difference between the two media should effectively switch between the two transducers automatically.
A noteworthy special case is when you wish to measure the pressure, rather than displacement, of an acoustic wave. Standing waves created in closed tubes can be used for hydrophone calibration, for example.
Because air, particularly in the open, is highly compliant, it only takes a slight pressure to cause a very large particle velocity. So a very loud sound in open air has a very small pressure component.
That's another way of saying the impedance doesn't match well.
Secondly, sound propagation speed in a hydrophone element is much higher than in air, so a hydrophone structure that might be relatively isotropic in water (with a similar propagation velocity) will demonstrate highly frequency-dependent directional characteristics at all but very low frequencies.
That's another way of saying the received sound will suffer from reverb-like effects.
But, in closed systems, where pressure is the quantity you wish to monitor, and at relatively low frequencies, a hydrophone can work quite well -- better than a microphone if you don't want the volume of a small test chamber to change significantly as the pressure is measured.
Ole-Herman, there is an opinion that piezoelectric hydrophones like https://www.bksv.com/en/products/transducers/acoustic/microphones/hydrophones/8105
show the similar responsivity in air and water. But I believe that it might be true only in the certain frequency range - at low frequencies.
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