# I'm trying to compare 2 acoustic sensors

1. Jun 7, 2009

### WarpedWatch

Hi all,

Through my tears, I'm trying to compare two acoustic emission sensors. The first sensor has a specification that says its sensitivity is −150 dBre 1V/μPa. The other sensor says its sensitivity is -70 dB ref 1V/μbar. (Note the different units.)
Wikipedia tells me that 1 Pa = 10-5 bar. So I'm guessing 0.1Pa = 1μbar .

Okay, so it's been a long, long time since I used dB for anything. Frankly, using dB to compare things has never made any sense to me. And with that negative sign thrown in there, I'm really confused. Are they telling me that the sensor provides negative voltage output, or is it a negative sign that gets plugged into the "20 log rule". (When I plug that negative into the 20 log rule, I get insanely small numbers. ) :uhh:

For "20 log rule", I'm looking at this on Wikipedia: en.wikipedia.org/wiki/Decibel

If I ignore the negative and just plug and chug, I get something like this for the first sensor: Pressure needed to get 1 Volt = 10150/20 = 31.6 Pa

and this for the second sensor: Pressure needed to get 1 Volt = 1070/20 = 3162 μbar = 316 Pa

They seem to be off by an order of magnitude, but ... I noticed other sensors, which happen to have integral pre-amps, have a sensitivity rated at -24 dB ref 1V/μbar. So what to do??? Anybody understand how this sort of specification is used? I'm trying to figure out how much voltage I can expect the sensors to output vs. some input of pressure. I expect the output to be in the millivolt range. but...

many thanks,
Mark

2. Jun 7, 2009

### vk6kro

The log bit is OK.

If you take the log of 3 / 1 you get 0.477
If you take the log of 1 / 3 you get -0.477

So, all the negative sign is doing is telling you that the voltage involved is smaller than the reference assuming you are using 20 log (voltage / reference voltage)

However the data seems pretty odd.
A pascal is already a very small unit. One Pascal is like 100 ml of water spread out over a square meter. Not much pressure.
A micropascal is extremely small.
To have a pressure that is 150 dB less than a micropascal gives crazy results. Could be a misprint.