TL testing down to 10Hz

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Re: TL testing down to 10Hz

Postby seb » Sat Sep 25, 2010 2:07 pm

One of the problems you have is that the sound power output from the speakers in the pit will change when the pit gets a lid. Hence your "noise reduction" or "insertion loss" test won't be accurate unless you know how the sound power output has changed.

Keeping all the amp settings etc the same won't be enough. The physics of the situation are such that the sound power output of the speaker(s) will be different when radiating into an unenclosed space (pit with no lid) compared to when the lid is put on.

However, don't let me dissuade you from learning something. Your proposed test is not a transmission loss test, it is an A, B, C, D etc comparative test. I am pretty sure the 10Hz results will be useless for anything meaningful but that doesn't mean I wouldn't do it if I had the opportunity. By testing 1x13mm drywall, 2x13 drywall, 3x13 drywall, you could compare your A, B, C deltas to actual published TL data and may be able to come up with some useful comparisons (but not at 10Hz).

In the real world, 10Hz TL data depends on panel sizes, panel resonances, panel structural damping, none of which (in your proposal) will be anything like real-world scenarios. 10Hz TL is not mass-law dependant and so will vary installation to installation due to the factors I have mentioned. Your testing might also be a handy Green Glue comparison as well - GG has not published any single panel tests - the only tests published are for single-stud frame walls clad both sides. The one published test that has drywall decoupled from the studs (Resilient Furring Channel) shows only improvements at coincidence and not at low frequency.
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Re: TL testing down to 10Hz

Postby bert stoltenborg » Sat Sep 25, 2010 3:22 pm

seb wrote:One of the problems you have is that the sound power output from the speakers in the pit will change when the pit gets a lid. Hence your "noise reduction" or "insertion loss" test won't be accurate unless you know how the sound power output has changed.

Keeping all the amp settings etc the same won't be enough. The physics of the situation are such that the sound power output of the speaker(s) will be different when radiating into an unenclosed space (pit with no lid) compared to when the lid is put on.



Could you explain this?
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Re: TL testing down to 10Hz

Postby Scott R. Foster » Sun Sep 26, 2010 10:15 am

cause of horn loading?
SRF
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Re: TL testing down to 10Hz

Postby bert stoltenborg » Sun Sep 26, 2010 12:33 pm

You change the acoustical impedance when you put something in front of the speaker, but to me that seems to be the essence of measuring...
:roll:
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Re: TL testing down to 10Hz

Postby seb » Tue Sep 28, 2010 3:51 am

At low frequency, a small microphone will not have much effect but putting a lid on a pit will def. change the acoustic impedance that the speaker inside the pit sees. I was looking for some references but a very busy week :cry: Certainly in the testing that NRCC did for the acoustic transfer of noise from air-conditioners mounted in the ceiling of an office, it was found that the effective sound power output of the Fan Coil Unit mounted in the enclosed ceiling space was different from that tested for the same Unit in a large reverberant room.
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Re: TL testing down to 10Hz

Postby The Byre » Wed Sep 29, 2010 3:51 pm

Thanks for this. This is EXACTLY the type of comment I was looking for!

I was worried from the start, well worried is perhaps not the right word, but I was aware that putting a lid on the pit will alter the acoustics of the whole test, including the loading of the test speaker driver.

The only way around this, would be to place test mics inside the chamber and test for changes within the pit and develop work-arounds, though I am not yet aware of what those work-arounds will be. Perhaps using smaller spears, for whom a 2 cubic meter space will not act as a noticeable load, or at least, as a load that can be taken into account. I shall know soon enough, once I have placed my first board on the pit and tested the insides, both with and without test piece.

As for testing real world scenarios, one of the (many!) objectives, is to come up with non-real-world measurements - i.e. the TL for materials without their structures. Drywall (plasterboard) in a 100 x 200 cm surface (set into sand held down by foundation blocks) and compared to the same material, set onto a wooden frame, say 2" by 4".

The surrounds to the pit are boulder clay and the pit and the floor are 18" concrete. As I have stated above, I envisage using sand and foundation blocks to hold the test pieces. The pit is already there, it's the rest of the building that is being held up by other work (studio customers) and the weather. I see 1m by 2m as being the minimum size, as anything smaller will be totally unduly effected by the inertia of methods of fixing. It is also the largest area I can get with a 10cm overlay, when using a single sheet of 120cm by 240cm board, which is the industrial standard.

Anyway, it will be a while, before I am ready!
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