100 Chapter 5
5.2.1.2 Impedance Tube Testing Methods
The laboratory methods generally involve the use of an
impedance tube to measure absorption of normally inci-
dent sound—i.e., sound arriving perpendicular to the
sample. There are two standard methods to measure
absorption in an impedance tube: the single-microphone,
standing wave method; and a two-microphone, transfer
function method.^2 In general, impedance tube measure-
ments are relatively inexpensive, relatively simple to
perform, and can be very useful in research and develop-
ment of absorber performance. In the standing wave
method, for example, the normal absorption coefficient
(Dn) can be calculated from
(5-1)where,
Ii is the incident sound intensity,
Ir is the reflected sound intensity.
While the cost and time saving benefits of the imped-
ance tube method are obvious, care should be taken
since the normal absorption coefficients are not equiva-
lent to the Sabine absorption coefficients discussed in
the previous section. In fact, unlike DSAB, Dn can never
be greater than 1.0. In one set of experiments, DSAB was
as little as 1.2 times and as much as almost 5.0 times
greater than Dn.^10 Regardless, there is no established
empirical relationship between DSAB and Dn. Normal
absorption coefficients should not be used to calculate
the properties of a space using standard reverberation
time equations.
One main advantage offered by normal absorption
coefficients is that they offer an easy way to compare the
performance of two absorbers. Reverberation chambers
have inherent reproducibility issues (explained in more
detail below). The impedance tube can overcome this to
some extent. One limitation of the impedance tube is
frequency range; large tubes are needed to test low
frequencies. Another is that tests of resonant absorbers
tend not to produce accurate results, because of the small
sample size.
5.2.1.3 Other Absorption Testing Methods
Many methods can be employed for the measurement of
sound absorption outside the confines of a laboratory test
chamber or impedance tube.^2 Of course, both the rever-
beration chamber method and the impedance tube
methods can be adopted for use in the field. In fact,
Appendix X2 of ASTM C423 provides guidelines for
carrying out the reverberation method in the field.^5
When the sound impinging on an absorber is not
totally random—as is the case, more often than
not—there may be better methods for describing its
performance. One of these methods, described by Brad
Nelson,^11 involves the analysis of a single reflection by
means of signal processing techniques. Although
Nelson’s method describes the measurement of absorp-
tion at normal incidence, his method can be extended to
determine the in situ absorption coefficients of a material
at various angles of incidence, which can be particularly
useful for the analysis of absorbers that are being used
for reflective control in small rooms. Nelson’s method
was employed by the author to determine the in situ
angular absorption coefficient (DT) of two different
porous absorbers, the results of which are shown graphi-
cally in Fig. 5-2 for reflections in the 2000 Hz band. The
results at least partly confirm what has often been
observed in recording studios: sculpted acoustical foam
tends to be more consistent in its control of reflections at
oblique angles of incidence relative to flat,
fabric-covered, glass fiber panels of higher density. Or,
to put it another way, the glass fiber panel offers more
off-axis reflections than the acoustical foam panel. Of
course, the relative merits of one acoustical treatment
over the other are subjective. The important point is that
the differences are quantifiable.DnIi–Ir
Ii-------------=Figure 5-2. Angular absorption coefficients (DT) of two
absorbers for the 2000 Hz^1 / 3 -octave band.0 o 15 o 30 o 45 o 60 o
Angle (Incidence = Reflection)1.000.800.600.400.200.00Angular absorption coefficientABA. Flat, fabric-wrapped,
glass fiber panel
96 kg/m^3 (6.0 lb/ft^3 ).
B. Sculpted foam panel
32 kg/m^3 (2.0 lb/ft^3 ).