Natural Science Forum / Physics / Acoustics / April 2005

Here we have the situation where a simple variable is measured, the
decay of sound at a point in space.

The question becomes "what does this mean".

For instance, when I make an internet connection in dial-in, one of the
handshaking protocols is where a pulsed tone "ping" is sent to my
computer, and the rate at which it decays in subsequent round trips is
an indication of the transmission loss of said connection, and therefore
indirectly implies the bandwidth that this specific connection can purvey.

In a closed and very diffuse room, the decay rate from a "ping" implies
the amount of sound absorption that is contained in that closed room,
and Sabine's equation may be applied when that absorption is relatively
minor, etc.

The fact that sound persists in the room from an emitted spoken
syllable, also of a "ping" character, for a finite time while it decays
has a second meaning; that dynamic (real) speech depends on a rapid
succession of emitted syllables to convey information. When the
reverberation time is so long as to not change reverberant level much
between syllables, "speech interference" is said to occur. Thus, english
proceeding at 50 milliseconds per syllable means, for instance that in
this one 20th of a second, sound in a room with an RT60 of one second
will diminish 60/20= 3 dB between syllables. One might then infer that
the signal-to-noise ratio for speech in that room is only 3 dB (not very
good).

In an open space, the decay of echoes whose level diminishes with time
can be measured OK. But it's hard or impossible to define the volume to
be assigned to that space, so that this real observed decay cannot be
used in a Sabine formula to determine absorption area.

But that same real observed decay can be used to determine speech
interference.

Angelo campanella