Sujet Placement enceintes monitoring pres de murs: revetement absorbeur, diffracteur ?

Citation : 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 omnidirectionally.

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 2).

Figure 2.
Figure 2. The frequency response dips caused by a wall reflection.

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 controls.

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 Genelec speakers.

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.

Summary
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.

Citation : Et bien voilà le possesseur des cartons à oeufs que tu as montrés en exemple, Samsong.

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