Single Acoustic Barrier
A single layer acoustic barrier.
A single partition between two rooms.
The transmission loss of an infinite single barrier can be predicted by the mass law:
Where
t = thickness of the barrier [m]
ρ = density of the barrier [kgm-3]
f = frequency [Hz]
ρ0 = density of air [kgm-3]
c0 = speed of sound in air [ms-1]
The mass law shows that doubling the thickness of a barrier increases the transmission loss by 6dB.
The mass law under predicts the transmission loss at very low frequencies. A better expression derived by Sewell [Sewell, E. C., "Transmission of Reverberant Sound Through a Single-Leaf Partition Surrounded by an Infinite Rigid Baffle", Journal of Sound and Vibration, Vol. 12, pp. 21-32, 1970] was based on theoretical considerations:
Where
k = wavenumber [m-1]
A = area of barrier [m2]
U(Ω) = shape factor correction for non-square barriers
m = ρt = mass per unit area of barrier [kgm-2]
fc = coincidence frequency [Hz]
Where
Y = Young's modulus of barrier [Nm-2]
The mass or Sewell law is only valid below the coincidence frequency. Cremer [Cremer, L., "Theorie der Schalldammung dunner Wande bei schragen Einfall", Akustica Zeitschrift, Vol. 7, pp. 81-103, 1942.] devised an expression for the transmission loss at frequencies above the critical frequency over fifty years ago.
Where
η = critical damping
The mass or Sewell law is only accurate below half the coincidence frequency. However, good agreement with experiment has been obtained using a prediction method proposed by Sharp [Sharp, B. H., "Prediction Methods for the Sound Transmission of Building Elements", Noise Control Engineering, Vol. 11, pp. 53-63, 1978] suggested using a linear interpolation between the transmission loss found with the mass or Sewell law at one-half the coincidence frequency and the transmission loss found with Cremer's expression at the coincidence frequency.
| dB Frequency (Hz) |
See also: Acoustic Barriers, Composite Acoustic Barriers, Double Acoustic Barrier, Holes in Acoustic Barriers.
Subjects: Noise & Vibration