Practical course: room acoustical measurement

Introduction

The discussion of the acoustical quality of a room is based on the impulse response. For given source and receiver position the impulse response contains all possible information about the system - the impulse response represents a finger print of the room for that particular source and receiver position. From the impulse response, several objective criteria can be evaluated such as


The objective room acoustical criteria are frequency dependent. The evaluation of the quantities is usually done in the octave bands from 125 Hz to 4 kHz.


Impulse response measurements in the Tonhalle Zuerich


The impulse response measurements were performed in 2005. As a source an omnidirectional loudspeaker was installed on the stage, the three receiver positions E1, E2 and E3 are indicated on the plan below.

The impulse response for point E2 can be heard here (be aware of the large dynamics of the signal).

The measured impulse responses were filtered in octave bands and squared. For the evaluation these squared impulse responses are avaliable in the 500 Hz and 2 kHz band:


Task

For the three receiver positions E1, E2 and E3, the room acoustical criteria RT, EDT and C80 shall be evaluated in the two octave bands 500 Hz and 2 kHz.

RT and EDT are calculated by evaluating the Schroeder backward integration. The RT measures the time for a decay of 60 dB. As such a dynamic is seldom avaliable usually a decay of 30 dB is used (the corresponding time is then multiplied by 2). However if the dynamics is not sufficient for 30 dB, the decay range has to be lowered correspondingly. EDT evaluates the decay of the top 10 dB (time is multiplied by 6).

C80 corresponds to the ratio of the integrals of the squared impulse response, evaluated from 0 till 80 ms and from 80 ms till the end. It has to be noticed that time 0 equals the moment of arrival of the direct sound

As will be seen, the dynamics of the impulse response is rather low. Therefore strategies have to be developed to handle the disturbing noise. Of help is the fact, that the impulse responses were determined with a correlation technique with maximum length sequences. From that follows that any disturbing noise is equally distributed over the complete impulse response.


2011-06-07, Kurt Heutschi.