Over the past five installments, we have discussed reverberation time from a theoretical standpoint, covering the definition of a diffuse sound field, the derivation of Sabine's reverberation time theory formula, its limits, and the derivation of Eyring's formula. In this and the next installment, we will explain the history and outline of ISO 3382, the standard for measuring reverberation time.
ISO 3382, "Measurement of reverberation time in auditoria," established in 1975, specified methods for measuring the reverberation time of auditoriums such as music halls, theaters, and auditoriums. Twenty years later, it was revised to ISO 3382:1997, "Measurement of reverberation time of rooms with reference to other acoustical parameters," which added content reflecting the development of acoustic measurement technology using digital signal processing. Furthermore, the scope of the standard was broadened to include not only auditoriums but also rooms used for speeches and music, or spaces targeted for noise reduction measures.
Since 1980, the design and evaluation of auditoriums, including concert halls, which have seen a surge in construction, have come to utilize not only reverberation time but also room acoustics calculated from impulse response. In the 1997 standard, it was proposed to include these room acoustics in the main body of the standard, but some aspects were still in the research stage, and ultimately, they were only included as reference in an annex. The "other acoustical parameters" in the standard name refer to these room acoustics.
Now, regarding measurement, the biggest difference between the current standard (1997) and the previous standard (1975) is the specification of a method for calculating reverberation time from the impulse response (impulse response integration method). This impulse response integration method was first shown in a paper published by Schroeder in 1965 [1], but it became widespread after the 1980s when sound pressure waveforms could be handled digitally in a general-purpose manner. I will leave the explanation of this method for next time, and this time I will explain the conventional orthodox noise discontinuation method that has been used for a long time.
The noise discontinuation method is a technique for determining the attenuation curve by using broadband noise or band noise as the sound source and directly recording the sound pressure level after it is stopped. Before digital recording became possible, attenuation curves were recorded using level recorders, but nowadays, in most cases, the data is either converted to digital, imported into a PC and processed with software, or processed similarly using dedicated equipment.
Next, let's explain how to determine the reverberation time from this decay curve.
As previously reported, reverberation time is defined as the time it takes for a continuous sound to decay by 60 dB after it has stopped. However, depending on the relationship between the ambient noise level of the measurement environment and the speaker output level, it may not be possible to ensure a 60 dB signal-to-noise ratio across all frequencies. This is common to the impulse integration method, but as shown in Figure 1 below, linear regression using the least squares method is applied to the decay from the initial level of the decayed waveform from -5 dB to -35 dB, and the time equivalent to 60 dB decay at that slope is determined. The reason for not including the initial decay is that initial reflections arrive discretely, so the decayed waveform does not decay uniformly, and this tendency is particularly strong in the low-frequency range. Also, when the sound decays and the level waveform matches the ambient noise level, just before that, the decayed waveform is affected by the ambient noise, and rises by a value equivalent to the dB sum of the decayed sound and the ambient noise, causing the decay curve to slope in a flat direction. Therefore, the range from -5 dB to -35 dB, which is unaffected by ambient noise and excludes the initial reflection component range, is considered.
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Figure 1: Method for determining reverberation time from decay curves
Furthermore, if the dynamic range is narrowed due to constraints from either the sound source equipment or the measurement environment, it is permissible to perform the same processing within a range of at least 20 dB, from -5 dB to -25 dB, to determine the reverberation time. If it is necessary to clarify this difference, T30 is indicated when the reverberation time is determined from attenuation between -5 dB and -35 dB, and T20 when it is determined from attenuation between -5 dB and -25 dB. For the former measurement, it is necessary to secure a sound source that can obtain a sound pressure of 45 dB or more (35 dB or more for the latter) above the ambient noise level.
Furthermore, this standard provides detailed specifications for measurement-related items, including requirements for receiving-side measuring instruments such as microphones, recording and analysis equipment, and level attenuation recording devices, as well as measurement points, the number of measurements, and descriptions of measurement results. Therefore, please refer to this standard when actually measuring reverberation time.
Recently, ISO 3382 has been split into two standards: one for auditoriums such as music halls and auditoriums, which is a standard for room acoustic evaluation indices, and another for reverberation time in other general rooms, as follows:
ISO 3382-1: 2009 Acoustics -- Measurement of room acoustic parameters Part 1: Performance spaces
ISO 3382-2: 2008 Acoustics -- Measurement of room acoustic parameters Part 2: Reverberation time in ordinary rooms
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○ [1] MR Schroeder, "A new method of measuring reverberation time" J. Acoust. Soc. Am., vol. 37, pp 409-412, 1965
Ono Sokki DS-0232 Sound Insulation/Sound Absorption Characteristics Measurement Software
(Excerpt from the email newsletter issued on November 18, 2010)
