Spaciousness
Spaciousness is a unique condition of reflected sound. It occurs when there are very time delayed reflections of the direct signal entering the ears from either side of the room. Spaciousness in a listening room is a highly valued sonic accent because it removes the feeling of listening in a small room. The room feels opened up and yet at the same time, not fully open, as if there were no walls at all.
It’s that the feeling of a large flat wall nearby is eliminated, replaced with an indistinct presence of something out there. It’s the kind of reflections we get when we are in a forest, where we are surrounded by tall round sound reflectors. The space around us doesn’t sound like open space and it also doesn’t sound like a hard wall bounded space. This space sounds like it is open but partially, intermittently occupied.
Spaciousness has an acoustic formula. It is created by time delayed, low level lateral reflections from multiple reflecting sources at different lateral angles to the listener. Early lateral reflections are delayed between 25 and 60ms while late lateral reflections occupy the range of 60ms and beyond. Early later reflections allow the sound stage to wrap around the listener while later lateral reflections create the spaciousness effect. Earlier reflections are 10 dB and later reflections are 20 dB below the direct signal.
If a speaker is 10 from the listener and a side reflection flat wall surface is 17’ from the listener, the flat wall reflection is only 20 Log 10/34 = -10 dB lower than the direct signal which is too loud. Flat wall segments cannot be used to simulate spaciousness in the small dimensions of listening rooms.
A cylinder shaped reflection spreads sound out laterally. The lateral reflection of sound off a cylinder of radius r is Ld = Lr – 10Log d/r, where d is the distance from the center of the cylinder to the listener. The lateral expansion of sound off a cylinder is -3 dB per doubling of distance. That’s why line source speaker arrays are popular for projecting sound into the distance, compared to point source speakers where the sound level drop off is -6 dB per doubling of distance.
A cylinder shaped surface, instead of a flat wall, is located 17’ to the side of a speaker and 17’ to the side of the listener. The speaker as before is 10’ from the listener. The sound level at the face of the cylinder is 20 Log 10/17 = -4.6 dB below the direct signal. This is reflected by a 1’ radius cylinder. The sound drop off from the cylinder to the listener is Lr – 10 Log1/17 = -15 dB.
The overall drop off of sound between the speaker, to the cylinder then to the listener is Lr = -4.6 dB -15 dB = -20 dB below the direct signal. A flat wall reflection over the same path is -10 dB down and too loud.
But the room width to develop the timing for lateral reflections is 42’ which is too wide for typical listening rooms, which are about 16’ wide. |
