Room Modes - Bang & Olufsen BeoLab 90 Technical Sound Manual

measurement with a microphone is not
necessarily representative of what
you'll hear. This is because a
microphone does not have two ears.
Also, the direction the reflection comes
from will change how you perceive it. A
sidewall reflection sounds di erent
from a floor reflection. This is because
you have two ears – one on each side
of your head. Your brain uses the
sidewall reflections (or, more precisely,
how they relate to the direct sound) to
determine, in part, how far away a
sound source is. Also, since, in the
case of sidewall reflections, your two
ears get two di erent delay times on
the reflection (usually), you get two
di erent comb-filter patterns, where
the peaks in one ear can be used to fill
in the notches in the other ear and vice
versa. When the reflection comes from
the floor or ceiling, your two ears get
the same artefacts (since your two
ears are the same distance to the floor,
probably). Consequently, it's easily
noticeable (and it's been proven using
science!) that a floor or ceiling
reflection has a bigger timbral e ect
on a loudspeaker than a lateral (or
sideways) reflection.

16.2 Room Modes

Room modes are a completely di erent
beast – although they exist because of
reflections. If you pluck a guitar string,
you make a deflection in the string that
moves outwards until it hits the ends of
the string. It then bounces back down
the string, bounces again, etc. etc. As
the wave bounces back and forth, it
settles in to a total result where it looks
like the string is just bouncing up and
down like a skipping rope. The longer
the string, the lower the note, because
it takes longer for the wave to bounce
back and forth on the string. You can
also lower the note by lowering the
tension of the string, since this will slow
down the speed of the wave moving
back and forth on it. The last way to
lower the note is to make the string
heavier (e.g. by making it thicker) –
since a heavier string is harder to
1
Whether the pipe is closed (capped) or open only determines the characteristic of the reflection – there will be a reflection either way.
2
Do a search for "tritone" or "diabolus in musica".
move, the wave moves slower on it.
The air in a pipe behaves exactly the
same way. If you "pluck" the air in the
middle of a pipe (say, by clapping our
hands, or coughing, or making any
noise at all) then the sound wave
travels along the pipe until it hits the
end. Whether the end of the pipe is
capped or not, the wave will bounce
back and travel back through the pipe
in the opposite direction from whence
1
it came. As the wave bounces back
and forth o he two ends of the pipe, it
also settles down (just like the guitar
string) into something called a
"standing wave". This is the pipe's
equivalent of the skipping rope
behaviour in the string. The result is
that the pipe will "resonate" or ring at
a note. The longer the pipe, the lower
the note because the speed of the
sound wave moving in air in the pipe
stays the same, but the longer the
pipe, the longer it takes for the wave to
bounce back and forth. This is basically
how all woodwind instruments work.
What's interesting is that, in terms of
resonance, a room is basically a big
pipe. If you "pluck" the air in the room
(say, by making sound with a
loudspeaker) the sound wave will move
down the room, bounce o the wall, go
back through the room, bounce of the
opposite wall, etc. etc. (Of course,
other things are happening, but we'll
ignore those.) This e ect is most
obvious on a graph by putting some
sound in a room and stopping
suddenly. Instead of actually stopping,
you can see the room "ringing"
(exactly in the same way that a bell
rings when it's been hit) at a frequency
that gradually decays as time goes by.
However, it's important to remember
that this ringing is always happening –
even while the sound is playing. So, for
example, a kick drum "thump" comes
out of the speaker which "plucks" the
room mode and it rings, while the
music continues on.
54
Time (sec)
Figure 16.8: The concept of the e ect
of a room mode and Active Room Com-
pensation. See the associated text for
an explanation.
Figure 16.8 shows the concept of the
e ect of a room mode and how it's
dealt with by Active Room
Compensation. The sound coming out
of the loudspeaker is shown on the top
plot, in black. The response of the
loudspeaker and a single room mode is
shown below, in red. You can see there
that the room mode keeps "ringing" at
one frequency after the sound from the
loudspeaker stops.
There are two audible e ects of this.
The first is that, if your music contains
the frequency that the room wants to
resonate at, then that note will sound
louder. When you hear people talk of
"uneven bass" or a "one-note-bass"
e ect, one of the first suspects to
blame is a prominent room mode.
The second is that, since the mode is
ringing along with the music, the
overall e ect will be muddiness. This is
particularly true when one bass note
causes the room mode to start ringing,
and this continues when the next bass
note is playing. For example, if your
room room rings on a C#, and the bass
plays a C# followed by a D – then the
room will continue to at C#, conflicting
with the D and resulting in "mud". This
is also true if the kick drum triggers the
room mode, so you have a kick drum
"plucking" the room ringing on a C# all
through the track. If the tune is in the
key of F, then this will not be pretty.
In order for the loudspeaker to
compensate for the e ect of the room
mode, it has to not only produce the
2