Propagation of Sound Indoors - the Room Constant
For a continuing sound source in a room the sound level is the sum of direct and reverberant sound. The sound pressure level for a receiver can be expressed as:
L p = L N + 10 log (D / (4 π r 2) + 4 / R) (1)
where
L p = received sound pressure level (dB)
L N = sound power level from source (dB)
D = directivity coefficient (typical 1 for receivers in the middle of the room)
R = room constant (m 2 Sabin)
π = 3.14 ....
r = distance from source (m)
Room Constant
The room constant express the acoustic property of a room.
R = A / (1- α m)
= Σ S i α i / (1- α m) (2)
where
R = room constant (m 2 Sabine)
A = total room sound absorption (m 2 Sabine)
α m = mean absorption coefficient of the room
S i = individual surface area in the room (m 2)
α i = absorption coefficient for individual surface in the room
Example - Received Sound
The sound power generated from a machine is 90 dB. The machine is located in a room with total sound aborption 12.2 m 2 Sabine and mean absorption coefficient 0.2.
The room constant can be calculated as
R = (12.2 m 2) / (1 - 0.2)
= 15.3 m 2 Sabine
For a receiver located in the middle of the room with a directivity coefficient 1 and distance 2 m from the source - the received sound pressure level can calculated as
L p = (90 dB) + 10 log (1 / (4 π (2 m) 2) + 4 / (15.3 m 2 Sabine ))
= 84.8 dB
Related Topics
• Acoustics
Room acoustics and acoustic properties, decibel A, B and C, Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation.
• Noise and Attenuation
Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.
Related Documents
Maximum Sound Pressure Levels in Rooms
Maximum recommended sound pressure levels in rooms like kindergartens, auditoriums, libraries, cinemas and more.
Noise from Machines - Enclosures and Sound Pressure Level Reductions
Barriers and enclosures to reduce noise sound pressure levels from machines.
Sound - Attenuation and the Directivity Coefficient
The attenuation in a room depends on the location of the sound source and the receiver - and the room constant.
Sound - Reverberation Time
The sound reverberation time indicates how long it takes until the sound pressure level in a room is decreased with 60 dB.
Sound - Room Absorption Characteristics
Mean absorption coefficients and reverberation times in typical rooms.
Sound - Room Absorption Coefficients
Room sound absorption and absorption coefficients for common materials like plaster walls, brickwork, plywood panels and more.
Sound - Room Attenuation in Direct Sound Fields
Room size, absorption characteristics - and attenuation in direct sound fields.
Sound Intensity
Acoustic power of sound per unit area.
Sound Pressure
Sound Pressure is the force of sound on a surface perpendicular to the propagation of sound.