Fundamentals of Sound Measurement - 22nd Lecture: "Sound Transmission and Absorption" Part 1 - Measurement and Theory of Acoustic Transmission Loss Using a Reverberation Chamber (1) -

Over the past six installments, we have discussed the theory and measurement of reverberation time. As already mentioned, reverberation time is the most fundamental quantity that describes the characteristics of the sound field in a room. It is an important physical quantity that is measured not only when directly indicating the reverberation of a room, but also when calculating the acoustic performance of materials, such as the sound absorption coefficient (reverberation room method sound absorption coefficient) and sound transmission loss (reverberation room - reverberation room method). In order to achieve appropriate reverberation and quietness according to the intended use of the room, the design of reverberation time and the blocking of external noise are important. Sound insulation and sound absorption technologies are used to block noise.

Therefore, starting from this time, we will be dealing with sound transmission and absorption measured based on reverberation time. First, we would like to theoretically understand the meaning of the formula for calculating sound transmission loss measured using a reverberation chamber. Understanding this theoretically will make it easier to intuitively grasp what factors affect the sound in a room and to what extent they affect it when you experience it in the field. There will be some mathematical formulas, but we hope you will understand at least the general flow. The explanation of the theory will be in subsequent sessions.

There are two methods for measuring sound transmission loss in a test chamber: one uses a test facility combining two reverberation chambers, and the other uses a test facility combining two reverberation chambers. In the former case (Figure 1), sound (random noise) emitted from the sound source chamber diffuses within the chamber, and sound enters a sample placed on the partition wall between the two chambers from all directions. The sound then penetrates from the sample to the receiving chamber, where it diffuses. At this time, the sound transmission loss is determined from the average sound pressure level of the sound source chamber and the receiving chamber, and the reverberation time of the receiving chamber.

The measurement method for [reverberation chamber to reverberation chamber] is specified in "JIS A 1416:2000 "Method for measuring the airborne sound insulation performance of building materials in a laboratory" (ISO 140-1:1997)". Please refer to the standard for details on measurement conditions, etc. The sound transmission loss is calculated using the following formula 1. The measured values are L1, L2, and T, as explained above.

(dB)

Here;
L1: Average indoor sound pressure level (dB) in the sound source room.
L2: Average sound pressure level (dB) in the receiving room.
S: Sample area (^m²) equal to the open sample area.
A: Equivalent sound absorption area of the sound receiving room (^m²)

Furthermore, the equivalent sound absorption area A is calculated using the following formula.

（m²）

Here;
V: Volume of the sound receiving room (^m³)
T: Reverberation time of the receiving room (s)

Next time, I will explain the process of deriving Equation 1.

Let's also touch upon the measurement using the other combination, [reverberation chamber - anechoic chamber] (Figure 2). The process is the same as with the two reverberation chambers up to the point where sound is transmitted to the receiving chamber. Acoustic intensity measurements are performed on the transmitted sound to the receiving chamber, and the radiated power from the sample surface is measured. The acoustic power level at the sample area is calculated, and the acoustic transmission loss is determined from this and the average sound pressure level at the sound source. Furthermore, the measurement method for [reverberation chamber - anechoic chamber] is specified in "JIS A 1441-1:2007 "Method for measuring the airborne sound insulation performance of buildings and building components by acoustic intensity method - Part 1: Measurement in the laboratory".

The sound transmission loss R is calculated using the following formula:

　　　　　R=L_p-L_w+10 logS - 6　　(dB)

Here;
​_Lp: Average sound pressure level (dB) within the room where the sound source is located.
L _w: Acoustic power level (dB) transmitted through the sample aperture.
S: Sample area (^m²)

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).

JIS A1416:2000 "Method for measuring the airborne sound insulation performance of building materials in a laboratory" (ISO 140-1:1997)
JIS A1441-1:2007 "Method for measuring the airborne sound insulation performance of buildings and building components by acoustic intensity method - Part 1: Laboratory measurements"

Ono Sokki "Measurement of Acoustic Transmission Loss Using the Reverberation Chamber Method"

(Excerpt from the email newsletter issued on January 27, 2011)