Transmission loss question

[SomptingGuy]

Not a "mechanical" acoustics question I know, but acoustics anyway...

Measuring TL across a single-input, single-output component is well defined. But say I have multiple outputs. It strikes me that all I need to do is put anechoic terminations on all the outputs and then measure from input to each output in turn. Then add them up. For example, if I measure one at 40dB and one at 50dB, the combined TL is:

-10log10(10^(-50/10)+10^(-40/10)) = 39.6dB

(i.e. less TL than the lower one)

My concern is that maybe I shouldn't have an anechoic termination on the output I'm not currently measuring. Has anyone done this sort of thing before or got any good pointers?

TIA

 

[GregLocock]

I think you are on the right track, but have a strong suspicion that what you are doing is theoretically incorrect.

For a start, if the output from the two outlets is coherent then all bets are off.

Cheers

Greg Locock

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

 

[SomptingGuy]

Thanks for the reality slap Greg. If I had a pound for every time I've reminded people that combining coherent signals is very different from combining incoherent signals, I'd have a small pile under my desk. I've been thinking more though.

TL is defined in terms of acoustic power flow through a component. It doesn't matter what happens once it's left the component. If you do have coherent signals that cancel or reinforce in the free field, who cares? It doesn't alter the internal acoustic power flow. However, I think it's a pretty meaningless thing to try to measure.

 

[SomptingGuy]

I'm going to combine the pipes smoothly into one with twice the area, even though there are two in reality. That way I get some idea of the TL of the "whole" muffler. Hopefully it won't affect the acoustic properties too much and will give a useful, rather than meaningless answer.

 

[GregLocock]

To be honest I never got very far measuring transmission loss of intakes and exhausts, although I did see one of my clueier co-workers tuning an intake on a TL rig. Since I usually had access to a car and a chassis dyno it was easier to work directly on the car than a rig.



Cheers

Greg Locock

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

 

[SomptingGuy]

We never use TL either. But many of our customers do, so we have to get involved. Personally I can't see the point of measuring it on a cold exhaust muffler with zero flow.

 

[Rob45]

Sompting Guy wrote:
"Personally I can't see the point of measuring it on a cold exhaust muffler with zero flow."

I see the point: it gives the clueless something to put on a print!

 

[GregLocock]

Well, I suppose as a production validation test it might work.

Cheers

Greg Locock

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