Placing free-standing control room monitors
remember the baffle effect
wall reflections change the frequency response
exact symmetry produces accurate stereo imaging
angle the monitors towards the listener
height does matter
integration to images in TV/film monitoring
The baffle effect
An enclosed dynamic loudspeaker drive unit has theoretically ideal working
conditions only if its frontal radiation space is either a full or a half
space (i.e. spherical or hemi-spherical radiation).
In practice the radiation angle decreases as the frequency increases
first because of the finite size of the front baffle of the speaker cabinet
and secondly because of the size of the drive unit itself. For a medium
sized speaker cabinet having front baffle dimensions of 300 x 500 mm the
loading of the drive unit will change from full space characteristics to
half space characteristics between frequencies 200...600 Hz. The level
of the sound will increase by 6 dB above this threshold frequency, and
the loudspeaker then predominantly radiates towards instead of radiating
If the speaker is placed near a wall, the wall presents an extension
of the baffle of the loudspeaker at low frequencies, and this step change
in the radiated level decreases.
Usually near field monitors are designed to radiate correctly in free
field. The step change described above is compensated for in the design
of the speaker. If this speaker, then, is placed near a boundary, its frequency
response will be modified, and we experience a bass boost. The Genelec
monitors have versatile room
response controls to take care of these changes.
Wall reflections interfere with the direct sound
A free standing speaker is usually surrounded by boundaries that generate
reflections(walls, the ceiling and the floor). These boundaries act as
acoustical mirrors to the speaker's radiation, enhancing or cancelling
the direct sound, depending on the phase difference between the reflection
and the direct sound at the listening position.
The problems of boundary refelections diminish as the frequency increases.
This is because the directivity of a conventional loudspeaker increases
with frequency. This is why the boundary reflections mostly cause problems
at low frequencies.
The most common problem at low frequencies is the interference between
the speaker's direct radiation and the reflection from the wall behind
the speaker. At low frequencies this reflection is in phase with the direct
sound. The phase of the reflected sound lags more as the frequency increases.
Finally, at some frequency the reflection will be delayed so much as
to be in opposite phase relative to the direct sound. Depending on the
relative amplitudes of the direct and reflected sounds, a cancellation
dip (typically 6...20 dB deep) will occur in the frequency response (figure
To overcome this problem we can position the speakers far enough from
the wall to move the first order interference dip below the lower cut-off
frequency of the speaker. To move the dip down to 30 Hz, the distance needed
is 2.8 meters.
Note that when you have a stereo pair, they must have exactly similar
frequency responses to produce exact and accurate stereo imaging. The boundary
reflections change the frequency responses of the speakers differently
if they are at a different distance from the boundaries. You should take
great care to place the loudspeaker pair to an exact symmetry in the listening
room to maintain similar frequency responses for both speakers. This also
implies that the listening room itself should be symmetrial along the axis
between the speakers to fulfill this requirement. This is usually true
for modern control room designs.
The second method is to push the speaker as close to the wall as possible
to decrease the time delay of this reflection relative to the direct sound.
This moves the interference problem to a higher frequency, where the speaker's
own directivity decreases the rearward radiation and in this way the amplitude
of the reflected sound attenuates effectively relative to the direct sound.
Then, you should compensate the resulting boost of the bass frequencies
by using the room response
Aim the monitors towards the listener
The loudspeaker naturally becomes more directive as the frequency increases,
and the speaker has a certain acoustical axis where the response has been
optimized. The response of the Genelec speakers has been specially optimized
for a wide listening area. The monitors should be directed towards the
listener. The frequency response at the listening position should be measured,
and the room response controls should be adjusted to obtain correct balance
of the frequency response, if needed. You can find this adjustment in the
Sometimes it is not possible to place the monitors to the same height
with the listeners ears, although this is desirable. If you have to place
the speakers high, you should aim them towards the listener. This removes
the effects of the increaseing directivity as the frequency increases.
Integration to images in TV and film monitoring
The height of the speakers is particularly important when you are mixing
a film or a TV programme. Then, you should also pay attention to the width
of your stereo base. For film mixing, it is customary to set the width
of the stereo base to equal the width of the picture. For TV work, however,
you set the loudspeakers to approximately the standard stereo base although
the TV picture is much smaller. These aspects seem to be a topic of continuous
debate, and they should be, because the decisions on the width of the stereo
base significantly affect the sound mix.
Battling the room modes
An enclosed dynamic speaker behaves like a pressure source, and placed
near the wall will excite standing waves in room. The only effective way
to overcome this problem is to heavily damp the rear wall of the control
room at low frequencies with absorption material.
High quality nearfield monitoring requires an exactly symmetrical loudspeaker
placement. The monitors should be carefully aimed towards the listener,
and the stereo base width and the height of the monitors should be set
according to the purpose of the mixing task.