In the previous articles, we discussed sound transmission, but from this article onwards, we will be covering "sound absorption."
There are various reasons for sound absorption, but the following three points can be considered as the main reasons.
- This involves applying sound-absorbing treatment to the interior materials that make up a space to control the reverberation within that space.
- This involves applying a sound barrier to the sound propagation path, such as on the sound source side of a soundproof wall, to reduce the sound pressure level on the receiving end.
- Inserting sound-absorbing material into the air layer of a hollow double wall or similar structure to improve the sound insulation of the wall.
Thus, sound-absorbing materials are often used primarily to maintain quietness in a sound field, and their sound-absorbing performance is generally evaluated by the random incidence sound absorption coefficient.
This time, we will explain the measurement method and calculation formula.
Random incidence sound absorption coefficient is the sound absorption coefficient under the condition that sound is incident uniformly on the sound-absorbing material from all directions, and a reverberation chamber is used as the measurement environment. Other methods include perpendicular incidence sound absorption coefficient using an acoustic tube, and oblique incidence sound absorption coefficient under the condition of incidence at a certain angle. These may be used when there are constraints such as only being able to prepare small samples of material, or depending on the sound field conditions.
Now, to measure the random incidence sound absorption coefficient in a reverberation chamber, as shown in Figure 1 on the next page, a sample of sound-absorbing material (in the case of a flat material) with an area of 10 m² to 12 m² (or 2/3 times the area of (V/250) if the reverberation chamber is larger than 250 m³) is placed on the floor of the reverberation chamber under the conditions of a reverberation chamber specified in the corresponding standard, JIS A 1409 "Method for measuring sound absorption coefficient in a reverberation chamber: 1998" (ISO 354:1985).
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Figure 1. Measurement of random incident sound absorption coefficient in a reverberation chamber.
The sound absorption coefficient is calculated from the reverberation time T, which is determined by actual measurement, using the following procedure.
First, we calculate the equivalent sound absorption area A using the following equation (1).
.................................（1）
A: Equivalent sound absorption area (m²)
V: Volume of the reverberation chamber without a sample (m³)
c: Speed of sound in air (m/s)
T1: Reverberation time (s) of the reverberation chamber without a sample.
T2: Reverberation time (s) of the reverberation chamber with the sample inside.
Next, we determine the sound absorption coefficient αs from the equivalent sound absorption area and the area of the sample.
.................................（2）
αs: sound absorption coefficient
S: Area of the sample (m²)
Furthermore, when dealing with furniture such as chairs or sound-absorbing materials (such as rolls of glass wool) rather than flat materials, it is common practice to calculate the equivalent sound-absorbing area A using equation (1) and evaluate the performance based on A per unit.
JIS A 1409 specifies details beyond just the shape and volume of the reverberation chamber, including the number of reverberation time measurements and the reading of the reverberation waveform. Please refer to it for further information.
The derivation of equation (1) will be explained next time.
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The Japanese Industrial Standards Committee (JISC) website allows you to view PDF files containing detailed information corresponding to JIS standard numbers. From the page accessed via the following link, please click "Database Search" - "JIS Search" (the year is omitted when entering information in the search field).
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(Excerpt from the email newsletter issued on April 21, 2011)
