This time, we will consider the factors that determine the sound pressure level of sound transmitted to the receiving room in a typical room-to-room situation, based on the formula for calculating sound transmission loss measured using [reverberation chamber - reverberation chamber].
It is easy to imagine that when the sound pressure level in a receiving room is high, improving the sound insulation of the partition wall between rooms will reduce the sound pressure level of transmitted sound. However, other factors also affect the sound pressure level in the receiving room. Understanding the relationship between each factor and the resulting sound pressure level inside the receiving room allows for considering several options during the design phase or when countermeasures are needed.
In the measurement column "Fundamentals of Sound Measurement - Part 12: Noise Evaluation, Part 8 - Relationship between Acoustic Power Level and Room Sound Pressure Level," we explained the relationship between acoustic power level and room sound pressure level. This time, we will review and explain it again from there.
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Figure 1: Sound transmission from the sound source room to the sound receiving room.
Assuming that the power level of the sound source in the sound source room shown in Figure 1 is Lw and the equivalent sound absorption area of the room is A1, the sound pressure level inside the room can be calculated using the following equation (1) (see emm102 for the derivation of equation (1)). However, equation (1) is only valid at points where the room has high reverberation and is a certain distance away from the sound source.
.................................（1）
Substituting equation (1) into equation (2), which we derived as the sound transmission loss in the previous measurement column;
.................................（2）
The sound pressure level L2 in the receiving room is given by equation (3).
.................................（3）
Lw: Sound source power level (dB) in the sound source room
L1: Room sound pressure level (dB) in the sound source room.
L2: Indoor sound pressure level (dB) in the receiving room.
A1: Equivalent sound absorption area of the sound source room = 0.16 V1 / T1 (m²)
A2: Equivalent sound absorption area of the sound receiving room = 0.16 V2 / T2 (m2)
F: Parting wall area (m 2)
R: Sound transmission loss (dB)
V1: Sound source chamber volume (m 3)
T1: Reverberation time of the sound source room (s)
V2: Volume of sound receiving room (m 3)
T2: Reverberation time in the receiving room (s)
Equation (3) shows that the sound pressure level L2 in the receiving room is influenced by the power level Lw of the sound source in the sound source room, the acoustic transmission loss R of the partition wall, the equivalent sound absorption areas A1 and A2 in the sound source room and the receiving room respectively, and the area F of the partition wall. Equation (3) is valid under the condition that sound propagates from the sound source room to the receiving room only through the partition wall.
While the [reverberation chamber-to-reverberation chamber] used to measure sound transmission loss is assumed to satisfy this condition, in real spaces such as the partition walls between apartments in multi-unit buildings or the walls between hotel rooms, sound insulation performance is significantly affected not only by the partition walls but also by sound transmitted through side walls and ceilings as structure-borne sound. This transmission through paths other than the partition walls is called side-path propagation, and reducing structure-borne sound transmitted through side-path propagation is a crucial factor in achieving sound insulation performance as designed.
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(Excerpt from the email newsletter issued on March 24, 2011)
