Building Acoustics

Sound absorbers 175

2

ln sin.

2

b

d

πε

π

⎡ ⎛⎞⎤

Δ=− ⋅⎢ ⎜⎟⎥

⎣ ⎝⎠⎦

(5.40)

The quantity b is the width of the slot, and ε is the rate of perforation b/C, where C is the
c-c distance between the slots. Similarly to the use of conically shaped holes instead of
the normal cylindrical ones to enhance the absorption, one will obtain the same effect
using a slatted panel where the slots are wedge-shaped instead of using the normal
rectangular slats. However, this necessitates another prediction model (see Vigran
(2004)).

63 125 250 500 1000

Frequency (Hz)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Abso

rption

facto

r

d = 200 mm

100 mm

50 mm

d c-c =10

t=1 b=.15

Figure 5.15 Absorption factor of resonance absorber. Aluminium plate with micro-slits. Measured and
predicted results for normal incidence. The cavity depth d is indicated on the curves. Solid curves – measured.
Dashed curves – predicted.

A recent development is using microperforations in the form of narrow slits of
width of some tenth of a millimetre, again utilizing the “natural” viscous losses for
obtaining the necessary resistance component (see section 5.4.1.3). We shall illustrate
this by showing measured and predicted results where the measurements were performed
in a standing wave tube. The tube had a square cross section with side length 200 mm,
which limits the measurement range upwards to approximately 850 Hz. (Why is that ?)
A cross section of the plate used in shown in the insert to Figure 5.15. As indicated,
the thickness of the aluminium plate used was 1.0 mm, and the slits made by laser were
only 0.15 mm wide and 10 mm apart. Figure 5.15 shows measured and predicted results
using a cavity depth of 50, 100 and 200 mm, respectively. As seen from the results, using
a model based on Equations (5.36), (5.21) and (5.40), taking the perforation rate into
account, predicts the general shape very well. However, this model presupposes that the
plate itself does not move and cannot predict the excursions showing up in the frequency
range 100–150 Hz, particularly pronounced at cavity depth 200 mm. These are due to
mechanical resonances in the “bars” making up the plate.