Audio Engineering

Room Acoustics 857

At certain specifi c frequencies, the refl ections are perfectly in phase with each other and
cause a resonance peak in the room frequency response. The resonant frequencies are
directly related to the dimensions of the room, the fi rst resonance occurring at a frequency
corresponding to the half wavelength of sound equal to the spacing between the walls
or fl oor and so on (see Figure 29.5 ). Resonances also occur at the harmonics of the fi rst
(fundamental) frequencies. At frequencies related to the quarter wavelength dimension,
the incident and refl ected waves are out of phase with each other and, by destructive wave
interference, attempt to cancel each other out, resulting in a series of nulls or notches in
the room frequency response. Room modes therefore give rise to a harmonic series of
peaks and dips within the room frequency response.

At different positions throughout the room, completely different frequency responses
can occur depending on whether a peak or null occurs at a given location for a given

(a)

(b)

(c)

(d)

Distance apart L λ/2

Room wall Room wall

Distance between reflecting

surfaces

Figure 29.5 : Formation of room modes.