Handbook for Sound Engineers

Test and Measurement 1611

• “The monitor system produced 105 dBA-Slow at the
  performer’s head position for broadband program.”


• “The ambient noise with room empty was NC-35
  with HVAC running.”

In short, if you read the number and have to request
clarification then sufficient information has not been
given. As you can see, one-number SPL ratings are
rarely useful.
All sound technicians should own a sound level
meter, and many can justify investment in more elabo-
rate systems that provide statistics on the measured
sound levels. From a practical perspective, it is a worth-
while endeavor to train one’s self to recognize various
sound levels without a meter, if for no other reason than
to find an exit in a venue where excessive levels exist.

46.3.2 Detailed Sound Measurements

The response of a loudspeaker or room must be mea-
sured with appropriate frequency resolution to be char-
acterized. It is also important for the measurer to
understand what the appropriate response should be. If
the same criteria were applied to a loudspeaker as to an
electronic component such as a mixer, the optimum
response would be a flat (minimal variation) magnitude
and phase response at all frequencies within the
required pass band of the system. In reality, we are usu-
ally testing loudspeakers to make sure that they are
operating at their fullest potential. While flat magnitude
and phase response are a noble objective, the physical
reality is that we must often settle for far less in terms of
accuracy. Notwithstanding, even with their inherent
inaccuracies, many loudspeakers do an outstanding job
of delivering speech or music to the audience. Part of
the role of the measurer is to determine if the response
of the loudspeaker or room is inhibiting the required
system performance.

46.3.2.1 Sound Persistence in Enclosed Spaces

Sound system performance is greatly affected by the
sound energy persistence in the listening space. One
metric that is useful for describing this characteristic is
the reverberation time, T 30. The T 30 is the time required
for an interrupted steady-state sound source to decay to
inaudibility. This will be about 60 dB of decay in most
auditoriums with controlled ambient noise floors. The
T 30 designation comes from the practice of measuring
30 dB of decay and then doubling the time interval to
get the time required for 60 dB of decay. A number of

methods exist for determining the T 30 , ranging from

simple listening tests to sophisticated analytical meth-

ods. Fig. 46-5 shows a simple gated-noise test that can

provide sufficient accuracy for designing systems. The

bursts for this test can be generated with a WAV editor.

Bursts of up to 5 seconds for each of eight octave bands

should be generated. Octave band-limited noise is

played into the space through a low directivity loud-

speaker. The noise is gated on for one second and off for

1 second. The room decay is evaluated during the off

span. If it decays completely before the next burst, the

T 30 is less than one second. If not, the next burst should

be on for 2 seconds and off for 2 seconds. The measurer

simply keeps advancing to the next track until the room

completely decays in the off span, Figs. 46-6, 46-7, and

46-8. The advantages of this method include:

1. No sophisticated instrumentation is required.


2. The measurer is free to wander the space.


3. The nature of the decaying field can be judged.


4. A group can perform the measurement.

A test of this type is useful as a prelude to more sophis-

ticated techniques.

46.3.2.2 Amplitude versus Time

Fig. 46-9 shows an audio waveform displayed as ampli-

tude versus time. This representation is especially mean-

ingful to humans since it can represent the motion of the

eardrum about its resting position. The waveform shown

is of a male talker recorded in an anechoic (echo-free)

Figure 46-5. Level versus time plot of a one-octave band

gated burst (2-second duration).

Figure 46-6. A room with RT 60 <2 seconds.