Natural Science Forum / Physics / Acoustics / August 2008

Matt

There is work on absorption over railway ballast, but I can't locate it.
However, search under the name Keith Attenborough

A very old paper is

The sound-absorbing properties of some common out-door materials
G W C Kaye and E J Evans 1940 Proc. Phys. Soc. 52 371-379   doi:
10.1088/0959-5309/52/3/307

Which you can find on

www.iop.org/EJ/article/0959-5309/52/3/307/prv52i3p371.pdf

but it will cost you $30

Good Luck

Geoff

I Googled

Gravel+Sound+Absorption

to obtain:
/////////////////quote\\\\\\\\\\\\\\\\\\\\\\
"The sound-absorbing properties of some common out-door materials

G W C Kaye et al 1940 Proc. Phys. Soc. 52 371-379   doi:
10.1088/0959-5309/52/3/307  Help
           
      PDF (586 KB) | References | Articles citing this article
       
G W C Kaye and E J Evans
Physics Department, The National Physical Laboratory, Teddington, Middlesex

    Abstract. The sound absorption coefficients of pure sounds of
frequencies from 125 to 4000 cycles per second have been measured at
random incidence for specimens of some commonly occurring out-door
materials, among them, gravel, turf, sand, ashes, railway-track ballast
and snow. They mostly share the common characteristic of increased
absorption with rising pitch. Some of the materials (e.g. loose gravel
soil and ashes) are highly absorbent, snow is remarkably so, while
others (e.g. compressed gravel and wet sand) are indifferent absorbents.
Some practical aspects are discussed, and also the influence of nearly
grazing incidence, such as often obtains with out-door sounds, in
raising the degree of absorption.

Print publication: Issue 3 (1 May 1940)
Received 25 January 1940

\\\\\\\\\\\\\endquote///////////

    Seems to be the same reference cited by others.

    The comment about ashes is apt, as the porosity must be realtively fine
to affect any significant low frequency absorption. A "green" use of
Clinkers?

For yor model, I'd use 0.5 as an input.....

maybe o.25 up to 250 Hz, then 0.75 above.

    Angelo Campanella

If you're looking at sound propagation, is the concept of an
absorption value meaningful?  That 1940 paper may be very good for
what it is, but it predates modern understanding of outdoor
propagation.  It's not as simple as just an absorption coefficient.

In his 1996 tutorial on "Sound Propagation Outdoors"  (J. Acoust. Soc.
Am. Vol 100, No. 1, July 1996) Tony Embleton gives a flow resistivity
value of 1500 to 4000 MKS Rayls for limestone chips .01 to .025 m.
Which I think comes out to the same size as your gravel.  There may be
more data in other JASA articles by Tony and his NRC colleagues.

Ken Plotkin

M. David Egan, "Architectural Acoustics" (McGraw-Hill 1988 edition -
the b&w hardbound one with the crappy binding), data from Table on
page 52-53:

Gravel, loose and moist, 4 in thick
f: alpha
125 Hz: 0.25
250 Hz: 0.60
500 Hz: 0.65
1 kHz: 0.70
2 kHz: 0.75
4 kHz: 0.80
NRC: 0.70

Three references are provided in the table, two of which might contain
the above, but none of which I have in my library. See text for more
details.

Savant