Design of an Semi-Anechoic chamber

[hamerish]

This is for a anechoic chamber for testing the exhaust noise from a particular engine, it will just be an engine test cell with the dyno in an adjacent room.

As the dominant noise from an engine is the engine itself and the exhaust, how would one separate them? also is there any info on where one would measure the exhaust noise? I know some vehicle companies use 45 degree 250mm from the exhausts outlet, but have found no reason to as why.

 

[Danger]

Hi, Hamerish.
I have no expertise whatsoever in this field, (although I can do just about anything with a domestic car that's old enough to not have solid-state parts). Two things, however, sprang to mind immediately upon reading your post. I would pipe the exhaust out of the building with a flex hose wrapped in sound-deadening material such as cotton or fibreglass insulation, then measure it at the output end.
You could also look into the more expensive option of using an "electronic muffler" which outputs an inverse version of the noise and thus cancels it out. Knowing what frequencies and decibels are required to do so should give a reasonable idea of just what it is that you're canceling.

 

[Bobbywhy]

If you transfer the exhaust somewhere else in a tube your measurements will not be made according to regulations and so would not be useful. The tube itself would attenuate and change the sound. You do not need to create any kind of anechoic chamber. Nowhere can I find any regulatory standard requiring the “separation” of engine noise from exhaust noise. If that became necessary you’d need either a large physical divider to isolate the two sources or an acoustic signal processor with some complex digital filtering.

Instead of trying to “re-create the wheel” it is a simple matter to see how others already do this.

A step-by-step procedure of how to measure exhaust noise, including the equipment and specifications necessary:
http://www.ehow.com/how_6150114_measure-exhaust-noise.html

California’s regulations, including exactly necessary equipment, placement of microphone, and technical specifications:
http://www.bobsmuffler.com/noise.htm

Many more examples appear during a simple Google search.

Bobbywhy

EDIT: Cool idea from Danger: use an "active noise cancellation" system! Such exotic stuff is found in certain Bose headphones and in secret underwater acoustic submarine warfare!

 

[AlephZero]

I know some vehicle companies use 45 degree 250mm from the exhausts outlet, but have found no reason to as why.

One reason for the standards specifying an "off-axis" measurement is to avoid the microphone being contaminated with smoke and soot, or even flames from a defective exhaust system backfiring.

The choice of position is not so important as consistency between different measurements. If somebody wants to try to design a noisy exhaust with an acoustic baffle plate that "kills" sound radiation at 45 degrees to pass the test, good luck to them

 

[Bobbywhy]

Microphone placement during exhaust noise measurement must always conform to the same standard in order to obtain accurate comparisons to standards and absolute acoustic intensities. The two example references given above in post number three describe this placement, and appear to be identical.

From "ehow":
3. Place the microphone of a decibel meter even with the level of the center of the muffler exhaust opening. Locate the microphone 19 1/2 inches behind the tail pipe at a 45-degree angle to the opening. The microphone cannot be in the direct gas path from the exhaust.

From "Bob'sMuffler":
(b) Microphone Location. The microphone for the sound level meter shall be at the same height as the center of the exhaust outlet. The microphone shall be no closer to the pavement than 8 in. (203mm) when the exhaust outlet is lower than this height. The microphone shall be positioned with its longitudinal axis parallel to the ground, 20 inches plus or minus 1 inch (508mm plus or minus 24mm) from the nearest edge of the exhaust outlet, and 45 degrees plus or minus 10 degrees from the axis of the outlet. For exhaust outlets located under the vehicle body, the microphone shall be located at the specified angle and at least 8 in., (203mm) from the nearest part of the vehicle. For exhaust outlets that make an angle of 45 degree or less with the longitudinal axis of the vehicle, the microphone shall be outboard of the exhaust axis. For exhaust outlet angles of more than 45 degrees, the microphone shall be on the side of the exhaust axis to the rear of the vehicle.

 

[Danger]

It seems that I misunderstood the second question of OP's post; I took it to mean a physical separation of the sources rather than just an acoustic one, hence the suggestion of a hose. Sorry.