Placing Loudspeakers correctly in Control Rooms

  • consider the loudspeaker radiation space
  • wall reflections change the frequency response
  • follow the free-standing placement recommendations
  • exact symmetry produces accurate sound stage imaging
  • angle the loudspeakers towards the listener
  • height and width matters in TV/film monitoring
  • battle room modes

 

The loudspeaker Radiation Space

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 loudspeaker cabinet and secondly because of the size of the drive unit itself. When a loudspeaker is then placed near a wall, the wall presents an extension of the baffle of the loudspeaker at low frequencies, its frequency response will be modified, and we experience a bass boost.

> To learn more, read here further about Radiation Space.


Wall reflections interfere with the direct sound

A free standing loudspeaker is usually surrounded by boundaries that generate reflections (walls, ceiling and floor). These boundaries act as acoustical mirrors to the loudspeaker's radiation, enhancing or canceling the direct sound, depending on the phase difference between the reflection and the direct sound at the listening position.

The problems of boundary reflections diminish as the frequency increases. This is because the directivity of a conventional loudspeaker increases with frequency. This is why boundary reflections mostly cause problems at low frequencies.

The most common problem at low frequencies is the interference between the loudspeaker's direct radiation and the reflection from the wall behind the speaker. At low frequencies this 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.

> Read here to learn how to overcome the Wall behind the Loudspeaker Cancellation.


Free-standing placement recommendations

Although flush mounting loudspeakers offers many benefits it is usually expensive and in most cases, especially in small installations, loudspeakers are placed free standing. As stated before, reflections from various boundaries will characterize the performance of free standing loudspeakers.

To learn more, read here how to correctly place in a control room:
> Free-standing loudspeakers
> Free-standing loudspeakers combined with subwoofer


Symmetry for accurate imaging

Considering the simple case of 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 loudspeakers differently if they are at a different distance from the boundaries. You should take great care to place the loudspeaker pair in an exact symmetry in the listening room to maintain similar frequency responses for both loudspeakers.

As the positioning of tables, screens, racks, etc, is critical for stereo installations, it is even more truth in multichannel control rooms. Early reflections, with high amplitude in relation to the direct sound, can smear the coherence of the sound image and compromise the localisation of the sources in space. To avoid this, all reflecting surfaces between the loudspeakers and the listening position should be minimised. Of course equipments are inevitable, however symmetrical positioning of the equipment is essential.

Also, note that the listening room itself should be symmetrical along the central axis between the loudspeakers to maintain accurate sound stage imaging. This is usually true for modern control room designs.


Aim the loudspeakers towards the listener

The loudspeaker naturally becomes more directive as the frequency increases, and the loudspeaker has a certain acoustical axis where the response has been optimized. The responses of Genelec monitors have 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.

Sometimes it is not possible to place the loudspeakers at the same height with the listeners’ ears, although this is desirable. If you have to place the loudspeakers high, you should aim them towards the listener. This removes the effects of the increasing 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 program. 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 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 at least the rear wall of the control room at low frequencies with sufficient absorption material.