Audio Engineering

858 Chapter 29

frequency. Variations of 10–15 dB within the room response are quite common unless
these modal resonances are brought under control. Figure 29.6 presents a frequency
response trace of a monitor loudspeaker in a poorly treated control room.

There are three types of room mode: the axial, oblique, and tangential, but in practice
it is the axial modes that generally cause the most severe problems. Axial modes
are caused by in and out of phase refl ections occurring directly between the major
axes of the room, that is, fl oor to ceiling, side wall to side wall, and end wall to
end wall.

Figure 29.7 shows the variation in sound pressure level (loudness) that occurred within
a poorly treated control room at the fundamental and fi rst harmonic modal frequencies.
Subjectively, the room was criticized as suffering from a gross bass imbalance, having
no bass at the center but excess at the boundaries, which is exactly what Figure 29.7
shows, there being a 22-dB variation in the level of the musical pitch corresponding to
the fundamental modal frequency and 14 dB at the fi rst harmonic.

When designing a control room or studio, it is vitally important to ensure that the room
dimensions are chosen such as not to be similar or multiples of each other, as this will
cause the modal frequencies in each dimension to coincide, resulting in very strong
resonance patterns and an extremely uneven frequency response anywhere within
the room.

5

0

 5

 10

 15

 20

20 50 100 200 500 1 K 2 K 5 K 10 K 20 K 40 K

dB

Prominent room modes at 33, 55, 90, 120 and 240 Hz

Response peak @ 500–1-6 K due to poor absorption/wall reflections

Room

before

treatment

5 dB

Figure 29.6 : Frequency response trace of a monitor loudspeaker in a poorly

treated control room.