Building a Piezo hydrophone

[Firestein]

Hey guys this is my first pose here .
So i need to build a hydrophone for a project, to pick up sound in the audible range inside an aquarium ( preferably 1khz to 20khz or maybe 1khz to 10khz) ,
I just need to receive very simple waves to measure their attenuation ( maybe sine waves or pulses ) .
So i ve seen some patents for DIY hydrophones that use a piezo transducer inside a plexiglas enclosure. The problem is i don't really know which piezo to use . The most common ones have 4khz resonance frequency . Does that mean i can't use it for the entire range that i need ?
Do you have any other ideas otherwise ?
Thanks a lot

 

[audioguru]

You do not want a piezo "buzzer" that has a transistor oscillator inside a 4kHz resonant enclosure, instead you need a piezo transducer (speaker) without an enclosure that can be used as a poor sounding microphone.

 

[alec_t]

What water depth? That will determine the housing/waterproofing requirements and hence perhaps the most appropriate transducer.

 

[Firestein]

It's inside a tank , so maybe 10cm below the surface.

 

[Firestein]

You do not want a piezo "buzzer" that has a transistor oscillator inside a 4kHz resonant enclosure, instead you need a piezo transducer (speaker) without an enclosure that can be used as a poor sounding microphone.

What is the difference between a buzzer and a piezo transducer ? I am talking about piezo disks ( or piezo diaphragms ? ) ( that i was intending to glue to a plexiglas surface and connect to a preamplifier circuit )
I am kinda new to this so bear with me

 

[rjenkinsgb]

If you use a bare piezo transducer, that has a brass plate as the "diaphragm", you could just glue the brass side solidly to the wall of the tank.
It will give an output voltage according to the flexing of the tank wall from the sound vibration.

Glued on, any resonance of the disc itself will be swamped by that of the surface it is fixed to.

Just be sure to use a very high impedance buffer on it. They do give a good output voltage - tens of volts, if tapped directly!

 

[Firestein]

If you use a bare piezo transducer, that has a brass plate as the "diaphragm", you could just glue the brass side solidly to the wall of the tank.
It will give an output voltage according to the flexing of the tank wall from the sound vibration.

Glued on, any resonance of the disc itself will be swamped by that of the surface it is fixed to.

Just be sure to use a very high impedance buffer on it. They do give a good output voltage - tens of volts, if tapped directly!

Ye i thought of that , the only reason i am doing it with a plexiglas box is because the quality of the glass of the tank is kinda poor so there was no transmissions .
Also what do you really mean by swamped ? will get uniform response over the entire range or ?
Many thanks

 

[rjenkinsgb]

Also what do you really mean by swamped ? will get uniform response over the entire range or ?

The limiting factor will be the response of whatever material sheet or panel the piezo is attached to; the disc itself cannot resonate separately, so its resonant frequency is then irrelevant.


Edit - how about using a small moulded hard plastic "tub" style container (the type of thing as wide as it is tall) with the piezo disc bonded to the inside of the base? That could work as a reasonable waterproofing body and you would only need to partly submerge it.

 

[Firestein]

As far as waterproofing goes , i kinda followed the patent already :/ . I thought glass would be more suitable than plastic but i don't really know . I guess i shoulda tried that now that i think of it .
I used Gaskets and PVC tube , pressed against two plexi surfaces. I wanted to be a little big so i can connect all the wires on the inside .