TABLE OF CONTENTS When airborne sound insulation is measured using broad-band noise, the sound pressure level to be measured in the receiving room is not always sufficiently high above the background noise. In these situations it is necessary to increase the power output from the loudspeaker; this can be done by limiting the broad-band noise to an individual frequency band. However, the increase in level may still not be sufficient. This problem can be overcome by using a signal that is commonly referred to as MLS (Schroeder, 1979). This allows the measurement of signals (e.g. sound pressure, vibration) at low levels, as well as reverberation times in the presence of high background noise. MLS measurements are well-suited to reverberation time measurements as they determine the impulse response of an acoustic system; this facilitates use of the integrated impulse response method as well as reverse-filter analysis (Section 3.8.3.2.2).
The signal processing for MLS measurements is more complex than with broad-band noise measurements. Modern analysers usually automate the MLS measurement process so that it is not essential (although it is beneficial) to have an in-depth knowledge of the signal processing.
To reap the benefits of using MLS with sound insulation measurements, it is necessary to accept longer measurement times, restrict changes in the environmental conditions during the measurement, and prohibit the use of moving microphones and loudspeakers. This section starts with an overview of MLS in its application to sound insulation measurements and then reviews the limitations of the...
