acoustic measurements in a noisy environment 4

[mostclueless]

I'm faily new to acoustics and was wondering whether I can take legible reading/gain a accurate signal in a fairly noisy environment. If I can identify the frequency that I'm looking for can filters be used to reduce eliminate unwanted noise sufficiently?

 

[Strong]

Clueless,

If the frequency of interest is a pure tone (sine wave), then you do FFT frequency analysis with a narrow filter (lots of line of resolution) and use spectrum averaging to make the tone standout with random background sound being averaged to a low level. Other tricks include:
1) Get closer to the source
2) Wait for change (or turn off) background sound
3) Use a directional microphone
4) Use a pressure gradient microphone or intensity probe

Walt

 

[GregLocock]

Another vey powerful technique is to use gated analysis. If your problem signal is periodic, then attach a trigger to the object, and use that to trigger your data acquisition. Average the data in the TIME domain and all noise that is not correlated to the trigger will slowly average out.

This is not a very common technique, admittedly.

If you describe more exactly what you are trying to do we can be more helpful, possibly.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Rob45]

Yet another useful tool is the particle velocity transducer from Microflown; it measures particle velocity directly instead of calculating it from the intensity, and is especially well suited for use in a noisy environment (they have an example of this exact thing on their website).

In order for it to work, you need a good idea of the position of the noise source.

And another technique useful for measurements in noisy environments is order tracking, when you have a tachometer or speed signal related to the noise source. You filter out non-synchronous noise.

- R

 

[SomptingGuy]

... noisy (i.e. uncorellated) or reverberant? If it's just plain noisy, the above suggestions will help. If it's reverberant, you'll need to think about acoustic intensity.

 

[Rob45]

Acoustic intensity is what you get if you add particle velocity to sound pressure.
That's why the particle velocity transducer (i.e., flowmeter) is so cool - you get to skip a step. or you get to measure intensity "directly."