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Hydrophone (underwater microphone) sound/acoustic signal recording

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Member level 2
Feb 20, 2011
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Hi everyone.

I'm no expert in digital signal processing. I would like to record underwater sound using hydrophone (electret base). The hydrophone working frequency range is 10 Hz to 100 kHz signal but my interest is to record frequency within the range of 10 kHz to 30 kHz. The problem is I have to build everything from scratch including amplifying, filtering, AD converting, and last but not least recording so that I can fit it to the main PCB board.
1. Could anyone suggest the best way to do this?
2. Which one first amplifying or filtering? What type filtering is suitable?
3. What is maximum sampling rate needed for ADC? 2 x 30 kHz or 2x100 kHz?

Thank in advance.

Since you want to record up to 30kHz, your sampling rate should be twice that fast.

A feasible sequence is:

(1) bandpass filter (10k to 30k) consisting of RC highpass and RC lowpass
(2) transistor amplifier
(3) ADC
(4) writing to storage device.

There is a theoretical a reason you would not amplify before filtering. It is because undesired frequencies will be amplified, and perhaps force you to cut down gain.

However there is also a chance the hydrophone signal is unusable unless it is first amplified. Then the sequence will become: preamplify/ filter/ amplify/ ADC.
Which one first amplifying or filtering? What type filtering is suitable?

You had not informed the application intended, but depending on the distance of the source of the sound to be recorded, to the microphone cell, you certainly will face to the need of some kind of frequency compensation performed either in hardware (pre emphasis) or at further digital processing. The water media attenuate the sound not only according to the distance travelled, but also acts as a low pass filter.
After long I have time to continue my project. Actually my intention is to localize underwater sound source (which is a pinger) using hydrophone array i.e. at least four for 3D source position (x,y,z). It is assume that the source distance is around 50m from the array. How to implement frequency compensation?

- - - Updated - - -

Thank for your explanation but sorry for very long time delay. Is there any differences between amplify and preamplify? How to know when we need preamp?

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