absorption of a long thin duct

[Bob]

Imagine a duct,
6"x6"x10' (15cm x 15cm x 3 meters). 6"x6" of airflow.

Build the duct with a perforated metal sheet absorber (sheet steel with thousands of 1/8" holes (3mm), with 2" (5cm) to 4" (10cm) of rigid rockwool behind it, then a couple layers of drywall, another 2" of rockwool and then plywood, but having 6"x6" of air path).

If there are no bends or curves,

and you speak into one end,
can a friend at the other end hear you?

I'm guessing yes. But it's been asserted that they can't.

It's been asserted that the drywall increases attenuation through the assembly.

[bert stoltenborg]

Such a duct has a sound attenuation. If the diameter gets smaller the high frequencies are attenuated better as the direct sound is influenced.
A 3 meter duct with 140 mm wide spacing has 50 dB attenuation in the mid frequencies.
I can post the exact data tonight.

[Bob]

A 3 meter duct with 140 mm wide spacing has 50 dB attenuation in the mid frequencies.

Yes. I found that independently Aug 13 with an excel spreadsheet estimator (not real data).
And I found some real data for a duct silencer of similar diameter that had 2dBA drop per foot of length, which is about 20dB attenuation over 3 meters.

And that wide ducts (18") have significantly less attenuation than narrow (6") ducts.
And your "mid frequencies" is right too, peak near 1k hz, and drop off in attenuation to both higher and lower frequencies.

[bert stoltenborg]

This is a table with measurements, Bob.
It's from Merford and the silencers are of the TROX brand.
Tussenschakeldemping means you put such a thing between source and receiver.
Spleetbreedte = gap width

[Bob]

Thank you Bert.

[kimve]

This is a table with measurements, Bob.
It's from Merford and the silencers are of the TROX brand.
Tussenschakeldemping means you put such a thing between source and receiver.
Spleetbreedte = gap width

Isn't this for splitter sound attenuators, Bert?

In attachment some data I have on rectangular ducts with and without sound absorbing layer.

[bert stoltenborg]

Thanks!