Picking up ultrasonic signals on a hydrophone

Question

I bought two Aquarian H2A (3.5mm version, not XLR) hydrophones for receiving faint ultrasonic signals (29 KHz) being generated by an underwater sensor node.

I am running into some issues.

• I tried to use the hydrophone port on two Windows computers (one Intel NUC, the other a generic desktop), recording through Audacity. While they are able to pick up normal voice at 96 KHz mode, they are unable to pick up any signal being generated by the sensor node (even in the 5k-15kHz frequency spectrum.) Boosting the signal through the control panel makes the signal buried in noise, such that even filtering in MATLAB cannot recover the signal. I suspect the sound cards in generic computers are too noisy to pick up the signals from the hydrophone, or cannot record anything beyond 20KHz, even though the audio driver supports 96 KHz sampling.

• I connected the hydrophone to a USB Audio DAC. The problem is the DAC doesn't support 96 KHz sampling and is limited to 44-48 KHz, hence the maximum frequency I can set on the sensor node to 22-24 KHz. The DAC performs amazingly well. When combined with amplification and filtering in MATLAB, it is able to detect the faint signals from the sensor node. However, I can't set the frequency of the sensor node to 29 KHz (which is the resonant frequency of the transducer.)

• I tried using the Zoom U22 audio interface suggested on the Aquarian website. The interface takes only XLR/0.25 inch connectors so I used a 1/8 inch to 1/4 inch converter to connect the hydrophone to the audio interface. While the interface can sample at 96 KHz, it doesn't even detect normal voice. I suspect it's an impedance mismatch issue, but it is weird that cheap audio cards inside computers can pick up human voice (although not the faint signal from the sensor node) and the interface can't even do that.

What is the best solution to try? The audio DAC seems to work best so far but has limited sampling rate.

Answer 1

The data for both products is lacking!

These products are meant to be used underwater. They will perform very poorly in air due to the acoustic impedance mismatch (air to transducer).

The H2a hydrophone requires power which I assume uses phantom power that some audio products provide. Do your recording devices supply phantom power? I was unable to find meaningful information on the electrical interface to the H2a.
The output level is fairly low (sensitivity of -180 dBV/uPa which is good for a hydrophone) which means you need to use the microphone input or use a preamplifier (20 to 40dB) if using a line level input. If you place the hydrophone in a bucket of water, splashing your hand in the water should provide a strong signal. Jingling keys near the hydrophone in air will also produce a strong signal, especially around 30 kHz.

The ceramic ring transducer is meant to be potted to provide environmental protection from water. Ring transducers are usually air backed (center portion is a hollow cavity). While the resonant frequency is 29 kHz, it will work at lower frequencies. As a projector, you can expect a sensitivity slope at frequencies below resonance of +6 dB per octave. Again, this transducer is designed to be used underwater.

The image below shows the general frequency response of piezoelectric hydrophones and projectors.

Graph is adapted from "Underwater Electroacoustic Measurements", Bobber.

Answer 2

What is the best solution to try?

You should get the H2a-XLR hydrophone and run it with the Zoom U-22 interface. The Aquarian website explicitly states this is what you should be doing: on the H2a web page (link you provided) it states "Due to limitations in the quality of the preamps that are built into [smartphones, tablets, and computers], we recommend using the H2a-XLR hydrophone and the Zoom U-22 for connecting to these devices [...]" In other words: even the web page for the H2a recommends you should instead use the H2a-XLR when interfacing with the Zoom U-22.

The Zoom U-22 webpage says the same thing: "Use directly with our H2a-XLR or H3x hydrophones."

(Note: the H2a and H2a-XLR have different specifications for their output impedance; that is probably the key difference.)

I suspect the sound card's in generic computers are too noisy to pick up the signals from the hydrophone, or cannot record anything beyond 20KHz, even though the audio driver supports 96 KHz sampling.

It's a safe bet that both these things are true:

1. The analog-to-digital converter has an analog anti-alias filter in front of it. Since the sound card of a computer is designed for audio, and human hearing is limited to about 20kHz, that's probably the cutoff frequency of the anti-alias filter. [Side note: why sample as high as 96kHz if the analog cutoff is at 20kHz? Because the higher sample rate relaxes the performance requirements for the analog filter. It is a very common practice to run the A/D converter at 4x or even 10x the maximum analog input frequency.]
2. The sound card is designed for "normal" audio (human speech and music), not low level stuff. So its noise performance is probably pretty poor. One rule of receivers for audio and radio frequencies is applying more gain does not improve the signal-to-noise ratio... in fact it typically makes the SNR worse (because the amplifier itself adds some noise too). Looks like that's what you're encountering here. And the Aquarian website says the same thing in their comments about "limitations in the quality of the preamps", as noted above.

Bottom line: get the H2a-XLR hydrophone. It's always to begin with the vendor recommended setup.