National standard TCVN 10615-2:2014 on acoustics – Measurement of room acoustic parameters – Part 2: Reverberation time in ordinary rooms

This standard specifies three accuracy grades for measurement: survey, engineering, and precision. The fundamental differences relate to the number of measurement positions and consequently to the time required to carry out the measurements. Annex A includes additional information on the measurement uncertainty of the reverberation time (RT60). The introduction of the survey measurement option is intended to encourage room reverberation time (RT60) measurements to be conducted more frequently, if convenient. This shows that a simple measurement is still better than no measurement.

There are several reasons for measuring the reverberation time (RT60). Firstly, the sound pressure level from noise sources, speech intelligibility, and personal perception in a room depend heavily on the reverberation time (RT60). Rooms may include domestic rooms, stairwells, conference rooms, industrial plants, classrooms, offices, restaurants, exhibition centers, sports halls, and railway and airport terminals. Secondly, the reverberation time (RT60) is measured to determine the correction term for room absorption inherent in many acoustic measurements, for example sound insulation measurements according to ISO 140 (all parts) and sound power measurements according to ISO 3740.

In some countries, building regulations specify the required reverberation times (RT60) in classrooms and other types of rooms. However, in the vast majority of rooms, this is left to the design team to specify and design a suitable reverberation time (RT60) for the intended use of the room. This standard also contributes to the general understanding and acceptance of the reverberation time (RT60) regarding the quality and usability of a room.

In this standard, two different evaluation ranges are assessed, namely 20 dB and 30 dB. However, the 20 dB range is preferred for several of the following reasons:

a) The subjective evaluation of the reverberation time (RT60) is related to the early part of the decay;

b) To estimate the steady-state sound level in a room from its reverberation time (RT60), this is consistent with using the early part of the decay;

c) The signal-to-noise ratio is always an issue in field measurements, and is always difficult or impossible to achieve with an evaluation range greater than 20 dB. This requires a signal-to-noise ratio of at least 35 dB.

The traditional measurement method is based on the technique of visual inspection of individual decay curves. With modern measuring equipment, standard decay curves are not displayed and this can lead to the risk that abnormal curves are used to determine the reverberation time (RT60). For this reason, Annex B introduces two new measures to determine qualitatively the degree of non-linearity and curvature of the decay curve. These measures can be used to issue warnings when the decay curve is non-linear, and therefore the results must be flagged as unreliable and not yielding a unique reverberation time (RT60).

The use of rotating microphones during the measurement of decay curves was considered by the working group, and this procedure was deemed to have no clear physical significance and is therefore only accepted for the interrupted noise method and only when the result is used for the correction term.

For other reverberation time (RT60) measurements, TCVN 10615-1 (ISO 3382-1) covers auditoria and performance spaces, and ISO 354 covers sound absorption coefficient measurements in reverberation rooms. Neither TCVN 10615-1 (ISO 3382-1) nor ISO 354 is suitable for measurements in rooms like those mentioned above. Therefore, this standard addresses the gap between measurement standards concerning the acoustic properties of buildings.

This standard will not repeat the technical issues of TCVN 10615-1 (ISO 3382-1), but will only address the measurement of reverberation time (RT60) in other types of rooms.