Building Acoustics

214 Building acoustics

The quantity ΔAj in the last equation is the A-weighting factor^1 for the frequency band
having centre frequency j. The frequency bandwidth may be octave or one-third-octave,
whichever is appropriate.

Figure 6.5 Sound transmission through a window. Sketch showing sound pressure levels outside and inside
before and after A-weighting.

The quantity of interest, the difference in the A-weighted sound pressure levels, is
given by
Δ= −LLppAAoutAin() ()Lp, (6.17)

a quantity that we will be able to calculate knowing the spectrum of the outside noise.
Here one uses a standard spectrum for road traffic noise. When expressing this spectrum
in frequency band values in such a way that a summation after A-weighting gives zero
dB, i.e.
() Aout
10 10 1 or ( Aout) 0,

L jj

p

j

L

⎡⎤−Δ

⎢⎥≡ ≡

⎢⎥

⎣⎦

∑^

we arrive at the sound reduction index for traffic noise

A out

()0 10

AA() ()10lg10 .Ain

jj

p

LR

L

pp

j

RL L

−

= ⎛⎞⎜⎟

=Δ ⎯⎯⎯⎯⎯→− =− ⋅

⎜⎟

⎝⎠

∑ (6.18)

The spectrum level values Lj is given by

LLj=−Δ( out)j A.j (6.19)

These values are tabled in ISO 717 Part 1 for one-third-octave bands as well as for
octave bands. We may then readily calculate RA if we have laboratory measurement data

(^1) The A-weighting curve is specified in the IEC standard for sound level metres; see references to Chapter 1.
L
L
in
out
A
S
L (dB)

R

Frequency

L

Lin

out

After A-weighting