Metal Foams: A Design Guide
Metal Foams: A Design Guide
Metal Foams: A Design Guide
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172 <strong>Metal</strong> <strong>Foams</strong>: A <strong>Design</strong> <strong>Guide</strong><br />
and aluminum the sound velocity is about 5000 m/s. The wave velocity, v, is<br />
related to wavelength s and frequency f by v D s f. To give a perspective:<br />
the (youthful) human ear responds to frequencies from about 20 to about<br />
20 000 Hz, corresponding to wavelengths of 17 m to 17 mm. The bottom note<br />
on a piano is 28 Hz; the top note 4186 Hz. The most important range, from an<br />
acoustic point of view, is roughly 500–4000 Hz.<br />
Sound pressure is measured in Pascals (Pa), but because audible sound<br />
pressure has a range of about 106 , it is more convenient to use a logarithmic<br />
scale with units of decibels (dB). The decibel scale compares two sounds<br />
and therefore is not absolute. Confusingly, there are two decibel scales in use<br />
(Beranek, 1960). The decibel scale for sound pressure level (SPL) is defined as<br />
� �2 � �<br />
prms<br />
prms<br />
SPL D 10 log10 D 20 log10 ⊲12.1a⊳<br />
p0<br />
where prms is the (mean square) sound pressure and p0 is a reference pressure,<br />
taken as the threshold of hearing (a sound pressure of 20 ð 10 6 Pa). The<br />
decibel scale for sound power level (PWL) is defined by<br />
� �<br />
W<br />
PWL D 10 log10 W0<br />
p0<br />
⊲12.1b⊳<br />
where W is the power level and W0 is a reference power (W0 D 10 12 watt if<br />
the metric system is used, 10 13 watt if the English system is used). The two<br />
decibel scales are closely related, since sound power is proportional to p 2 rms .<br />
In practice it is common to use the SPL scale. Table 12.1 shows sound levels,<br />
measured in dB using this scale.<br />
Table 12.1 Sound levels in decibels<br />
Threshold of hearing 0<br />
Background noise in quiet office 50<br />
Road traffic 80<br />
Discotheque 100<br />
Pneumatic drill at 1 m 110<br />
Jet take-off at 100 m 120<br />
The sound-absorption coefficient measures the fraction of the energy of a<br />
plane sound wave which is absorbed when it is incident on a material. A<br />
material with a coefficient of 0.9 absorbs 90% of the sound energy, and<br />
this corresponds to a change of sound level of 10 dB. Table 12.2 shows<br />
sound-absorption coefficients for a number of building materials (Cowan and<br />
Smith, 1988)
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