Does the attenuation measurement for the cut off frequency of 80, 100, 125 Hz & 250 Hz will vary much or not for Air Intake System? (Speaker noise is 60 to 120 dB)
I am trying to figure out the inner size of Anechoic chamber. I would like to measure Attenuation (with FFT & Microphone) for Air Intake system. Now, as the cut off frequency will increase the inner size of anechoic chamber will reduce.
(1) So for different size of anechoic chamber (different cut off frequency), the measured attenuation will vary? If yes, how much?
(2) does panel model(vibration measurement on a surface (eventully,noise)) will affect by the different size of anechoic chamber
True that the size of Anechoic Chamber is inversely propotional to cut-off frequency. But, there are innovative designs which help you gain lower cut-off frequency with similar room size {various wedge size & design}.
If you wanted to measure the 'transmission loss' of the Air Intake System, you can always use the 4-microphone method instead of anechoic termination. Its quite accurate & less prone to environmental boundary conditions.
Since we don't measure sound pressure level or intensity, I don't think panel mode measurement will affect the anechoic chamber size. The accelerometer response to the hammer input have no connection with the room wall surface, neither are there any reflection concerns.
The measured attenuation below cut-off frequency can be innaccurate. However, you can still rely on it if the ratio of direct noise signal to reflected noise signal is high. & I have no idea how this can be measured
For 4-microphone method, refer this SAE paper {2003-01-1653}:-
'A review of current techniques for measureing muffler transmission loss' - Z. Tao and A.F.Seybert
Your last concern would be the frequency range of interest. Air Intake Systems normally deal with 50-1000Hz, sometimes upto 1800Hz {turbo}. The 4 microphone method works well in those frequency ranges. Its limit is the microphone spacing.
Rather like with engines, there is no substitute for cubic inches. The larger the semi-anechoic chamber is the better it will be.
You can have the best designed absorbers in the business, but for most practical purposes you cannot mount them where they'll do most good (in the middle of the room) and you tend to mount them where they'll do the least good (at the nodes of the mode shapes).
I do agree, an audio survey of the chamber is a good idea - if I were ever involved in specifying one again I'd decide what my most critical test was, and duplicate that with a sound source (B&K make one), and then define the allowable errors from the free field attenuation at my measurement locations, vs frequency.
Hmm, I wonder if this is one area where active systems might work.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
I'm about to do the very same thing myself: I'm testing some of our air intake systems, using a loudspeaker as a driver.
Not having an anechoic chamber available (Yet!), I'm going to do a "free-field" test: I'm going out in the field out back, away from any buildings. It's free!
LOL!
I can remember years ago measuring an almost silent generator in similar "free-field" conditions - our drive-by site is in the middle of a field. Unfortunately the nearby traffic (air and surface) neccesitated testing in the small hours of the morning. Imagine a couple of NVH engineers wandering around a field in the dark, trying to find a silent generator to measure!
