Where can I find sound transmission coefficient (STC) values for
various grades of commercial felt? For example, I'm considering using
F3 felt to dampen the echoes behind stereo speakers placed in alcoves,
and it would be nice to know what the STCs are for various thickness
of this felt.

Dear inbalance:

After conducting an online search, I was unable to find the Sound
Transmission Coefficients for various grades of any felt, commercial
or otherwise. This is most likely because the STC is far more rarely
used as a measurement of sound absorption than the Sound Aborption
Coefficient, so no one has actually conducted measurements of this
value.

I have, however, found the values for the Noise Reduction Coefficients
for the various grades of commercial felt. As you probably already
know, judging from your question, the value of the Sound Aborption
Coefficient is frequency based. The NRC is the "arithmetic mean of
sound absorption coefficients at the 1/3-octave band center
frequencies 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05
metric sabin per square meter."

Source: National Instruments

[ http://zone.ni.com/devzone/nidzgloss.nsf/webmain/D6BE16166C47FF83862568C6005CBA03?OpenDocument&node=DZ59414_US
]

The NRC coefficients given for SAE feltare as follows:

Properties                F-1  F-2  F-3  F-5  F-7 
Noise Reduction Coef (1") 0.50 0.50 0.52 0.58 0.58 

Source: Boston Felt

[ http://www.bostonfelt.com/sae_tech.htm ]


I hope these numbers meet your needs. If you need any clarifications,
please don't hesitate to ask. I would be more than happy to assist you
further.

Best Regards,
blader-ga

---

Clarification of Answer by blader-ga on 05 Jun 2002 16:48 PDT

Dear inbalance:

I have found an additional link which may be of interest to you. The
graph on the top of page three of the PDF file linked below gives the
graphs of the absorption coefficients for various thicknesses of felt
based on frequency. The specific type of felt, however, is not given.
Just thought it might help as well. =)

[ http://www.me.psu.edu/lamancusa/me458/8_rooms.pdf ]

If I find any more relevant links, I'll be sure to let you know. 

Best Regards,
blader-ga

Very prompt answer, and helpful references!
Thanks, blader