Measurement of Sound Pressure Levels at Low Frequencies in Rooms (NT TR 385)

Abstract

Comparison of available methods and standards with respect to microphone positions. Proposal for new procedures.

Within a Nordtest project, a comparison has been made of all methods found in a literature survey (24 methods) for the determination of the sound pressure level in a room. New principles for the location of the measurement microphone were tested. The existing general purpose methods intended mainly for the measurement of A-weighted sound pressure levels show an unacceptable scatter between measurement results when the sound is dominated by low frequency sound or if the sound is measured in octave or third octave bands. The reproducibility (i.e. the statistical uncertainty in the determination of the sound pressure level), is in the order of 15 dB at low frequencies. The methods generally underestimate the C-weighted, octave and third octave band values below 200 Hz. Arithmetic averaging increases the uncertainty at low frequencies. At frequencies higher than the 250 Hz octave band, most of the methods work satisfactorily except for some specialized corner methods. Some new methods based on moving microphones with large radii, or many fixed distributed positions or the use of a specified corner location all perform satisfactorily, both with respect to deviation from the true room average (bias error) and to scatter (reproducibility). The project group outlined a new method, based on the concept of scanning the corners of the room for the highest C-weighted level (as implemented in the present Swedish standard SS025263), and including this measurement in the spatial average. It seems that this concept reduces the measurement effort and the scatter, it also improves the correlation to the room average. This new method was tested in an Inter-Nordic Round Robin comparison with 5 laboratories, where very good reproducibility was achieved. The bias error (defined as the deviation of the measurement result from the average of 6 room positions measured in the central part of the room) was very low. The new principles for the location of the measurement microphone were proposed to CEN TC 126/WG1 which is working on new EN standards that will replace the current national standards/methods.