Introduction to Acoustics

316 Part C Architectural Acoustics
Part C 9.5
• Real-time (1/3 and 1/1 octave) analyzers or sound
level meters with sufficient memory for storage of
a series of short time-averaged root-mean-square
(RMS)values
• Fast Fourier transform (FFT) analysers capable of
recording adequately long records
• Tape recorders, preferably a digital audio tape (DAT)
recorder, or even MP3 recorders given that the data
reduction does not deteriorate the signal.
Of these, real-time analyzers and sound level meters
will process the signal through an RMS detector before
9.5 Prediction of Room Acoustic Parameters
Beyond theoretical calculation of the reverberation time,
the prediction techniques presented in this section range
from computer simulations and scale model measurements
to simple linear models based on empirical data
collected from existing halls.
In all cases, the main objective is the prediction of the
acoustic conditions described in terms of the acoustic parameters
presented in Sect. 9.3. Just as we can record the
sound in a hall, both scale models and numerical computer
simulations can also provide auralizations,which
are synthesized audible signals that allow the client and
architect to listen to and participate in the acoustic evaluation
of proposed design alternatives. Such audible
simulations will often have much greater impact and be
more convincing than the acoustician’s verbal interpretation
of the numerical results. Therefore, the acoustician
must be very careful to judge the fidelity and the degree
of realism before presenting auralizations.
9.5.1 Prediction of Reverberation Time
by Means
of Classical Reverberation Theory
Reverberation time as defined in Sect. 9.3.1 is still the
most important objective parameter for the characterization
of the acoustics of a room. Consequently, the
prediction of T is a basic tool in room acoustic design.
Calculation of reverberation time according to the
Sabine equation was the first attempt in this direction and
today is still the most commonly used. However, during
the 100 years since its invention, several other reverberation
time formulae have been suggested (by Eyring,
Fitzroy, Millerton and Sette, Metha and Mulholland Kut-
storage, which means that not the full signal, but only
energy with a limited time resolution will be recorded.
Today, calculation of the objective parameters will
always be carried out by a PC or by a computer built into
a dedicated instrument. A large number of PC-based systems
and software packages particularly suited for room
acoustic measurements are available. Some systems
come with a specific analog-to-digital (A/D), digital-toanalog
(D/A) and signal-processing card, which must be
installed in or connected to the PC, while others can use
the sound card already available in most PCs aswellas
external sound cards for hobby music use.
truff and others), all with the purpose of correcting some
obvious shortcomings of the Sabine method.
The Sabine equation has been described in
Sect. 11.1.4. In larger rooms, the absorption in the air
plays a major role at high frequencies. If we include this
factor, the Sabine equation reads:
T = 0.161V
Sα∗ , (9.21)
+ 4mV
where V is the room volume, S is the total area of the
room surfaces, α∗ is the area-weighted average absorption
coefficient of the room surfaces
α ∗ = α = 1 �
Siαi
(9.22)
S
i
and m accounts for the air absorption, which is a function
of the relative humidity and increases rapidly with fre-
Fig. 9.11 Two types of omnidirectional sound sources for
room acoustic measurements: blank pistol (in the raised
hand of the seated person) and an omnidirectional loudspeaker
(icosahedron with 20 units)