Signal processing

Historically, sinusoidal signals were used for measurement of the most common building acoustic properties like airborne sound insulation and reverberation time. However, as the obtained result may change considerably by even a small change in the frequency, it was soon realised that band limited noise, with a bandwidth of one- or one-third octave, was more convenient to obtain the mean value for a certain frequency range. The wanted property for the noise is the spectral distribution, the unwanted is the stochastically distribution of the result due to the randomness of the excitation. The system to be measured can, however, in most cases be regarded as deterministic, linear and time invariant. This allows general signal theory to be applied for the measurement.

Stochastic signal analysis methods for the measurement of sound transmission phenomena started to be developed around 1960, but lack of available computing power excluded the use of these methods outside the most equipped research laboratories. The recent development of digitising circuitry, powerful personal computers and the use of digital signal processing components in sound measuring equipment for field use, have made the application of measuring equipment based on extended digital signal analysis readily available even in handheld instruments.

Norsonic applied the new measurement technique in Nor840 in the form of the MLS-option (Maximum Length Sequence). The “swept-sine method” is implemented in Nor140 (Nor118/Nor121) as an option. These methods are less sensitive to extraneous noise than the classical methods and can be used for extending the measurement of low levels by 10–30dB downwards. This may be important for the measurement of the level in the receiving room as well as measurement of the reverberation time in large rooms or in rooms heavily polluted by extraneous noise.