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Graph 2. A flat reverberation time (RT)
Graph 3. High bass RT (red) and high treble RT (blue)
5.2
Room Modes, Reflections and Wall Behind the Loudspeaker Cancellations
Room Modes
Every room (with the exception of a perfect anechoic chamber) has a set of resonant frequencies.
These frequencies and their relative strength in the sound field are defined by room geometry and
surface materials. Frequencies below 300 Hz are most critical due to their long wavelengths and
so these frequencies will excite room modes quite easily. The fewer the number of modes the more
audible they will be. Basically the number of existing modes depends on the room dimensions and
the frequency band in question. Large rooms have more modes than small rooms. So, in general,
this favours using large rooms instead of small, up to some limit. As the room size increases, so
does the reverberation time. At the same time the distance between walls increases and reflections
start to sound like echoes, especially if the walls are not acoustically treated.
Most Home Theaters have parallel walls which leads to strong rooms modes. In that case the
incident sound and its reflections off these walls may constantly reinforce each other, creating a
resonance. In such a case the pressure minimum and maximum are found on specific, permanent
locations in the room. The distance between the two walls determines the set of resonant frequen-
cies in that space. When a loudspeaker is driven in such a space, room modes are excited. To avoid
this, loudspeakers should not be placed in a pressure maximum. For example, if the room height is
H, positioning the loudspeaker precisely at height H/2 coincides with a pressure minima in the verti-
cal plane. Placing the loudspeaker at 2/5 ... 2/6 room height minimizes troubles from hard ceiling
and hard floor.
This could help problems with the lowest modes, which are most difficult to absorb. In general,
the number of room modes depends on the proportions of the room, and so the Home Theater
designer should avoid planning room proportions with precise integer ratios.
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