42 Bedrock SMxx operating manual © Bedrock Audio BV, 2019
8. Room acoustics
8.1. Reverberation Time meter (RT60)
The reverberation time (or RT60) is among the most commonly specified acoustic meas-
ures. Unfortunately, reverberation time measurements are also among the most complex
to carry out accurately. RT60 measurements are prone to errors, which cannot always be
detected by the measuring device.
If you are new to measuring reverberation times, we recommend that you read
this section of the manual carefully, and preferably also consult external litera-
ture and training materials on reverberation time measurements.
Standardized procedures for measuring the reverberation time of a room are specified in
ISO-3382-2. Generally speaking, there are two different ways to measure RT60: the im-
pulse response method and the interrupted noise method. Both methods are standardized
through ISO-3382-2. The impulse response method uses a carefully controlled, known
test stimulus of short duration (sweeps, chirps, pseudo-noise sequences or gunshot-like
impulsive sounds). By normalizing the recorded sound in a room relative to the source
signal, the impulse response of the room is obtained. From this impulse response, the RT60
is calculated.
Starting with firmware version 2.4.0, the impulse response method (which previously not
supported) has also been implemented on the SMxx. The test signal to be used is a loud,
short impulsive sound (such as the sound from a starter gun). Do not use sweeps or chirps
as your test stimulus.
If you are using impulsive noise as your test stimulus, even the slightest move-
ment of the microphone during the test may render the measurement inaccu-
rate. The use of a tripod is absolutely necessary.
The interrupted noise method, which is also supported by the Bedrock SMxx, measures
the decay of the sound field if a sound source is suddenly switched off. This method is
more robust against slight inaccuracies or inconsistencies in the placement of sound source
and microphone. In small rooms, no other tools than the Bedrock BTB65 talkbox and the
SMxx may be needed. In larger rooms, a larger (spherical) loudspeaker and amplifier will
be needed. For the interrupted noise method, the following steps are taken in preparation:
• Place a source of continuous sound (preferably pink noise) in the room for
which the RT60 needs to be measured.
• Place the Bedrock SMxx at some distance from the source (stay clear from
walls and other flat surfaces)
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