As a rule of thumb rooms that have ratios of dimensions to the following numbers will tend
to have a more linear spread of room modes therefore be more neutral sounding.
1 : 1.14 : 1.39
1 : 1.28 : 1.54
1 : 1.6 : 2.33
Reverberation Decay of listening rooms (Rt60)
In simple terms, reflectivity is the apparent "liveliness" of a room. Professionals prefer the
term
reverb time or Rt-60.
Rt-60 defined, is the amount of time (in seconds) it takes for a
pulsed tone to decay to a level 60dB below the original intensity. A live room has a great
deal of reflectivity, and hence a long Rt-60. A dead room has little reflectivity and a short
Rt-60. An ideal room needs some reflections. The upper frequencies should have an Rt60 of
about 0.5 seconds whereas the low mid and bass frequencies can be over 1 second. High
frequency reflections are easily controlled by absorptive materials and furnishings such as
sofas, carpets and heavy curtains. It is much harder to control low frequencies due to the
much longer wavelengths. Some degree of control can be achieved by using "bass traps
"
in
room corners. World class recording studios spend thousands of dollars in one studio
control room to keep bass frequencies tamed. So in a domestic environment it is almost
impossible to get perfect acoustics in a listening room.
There are ways of measuring Rt60 of a room with computer analyzers but the simplest and
most intuitive way is to use the "hand clap" approach. If you clap you hands once you will be
able to hear the sound of the clap reflection back, this is reverberation of the room. If the
room has a lot of hard surfaces such as wood, stone or glass, then the sonic reflections of the
hand clap will be more intense. The more intense the reflections are, the more they will
interfere with the sound emanating from the speakers. This will deteriorate the sound
quality. A typical environment with bad acoustics such as a railway station can reduce sound
quality so much as to make the dialogue of an announcer almost unintelligible.
Sound bounces or reflects off all solid surfaces, so that when the sound source such as a
loudspeaker stops producing sound, the reflections continue for a period of time until the
energy is absorbed. Or to put it another way, the room functions as an energy store, returning
the acoustic energy to the air at some point after the initial event.
Rigidity and Mass
Rigidity and mass both play significant roles in determining how a given space will react to
sound within. They have a strong relation to the low frequencies - both qualitatively and
quantitatively. Low frequencies can be tremendously powerful, capable of flexing walls,
ceilings and occasionally, floors. Flexure of this type is termed diaphragmatic action.
1-5