Bowers & Wilkins HTM2D 2POSITIONING, Stray magnetic fields, Application, f Go to, fGo to section

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2POSITIONING

Stray magnetic fields

If you are using speakers in a home theatre set-up and you are using a CRT screen (a traditional tube television or back projector), make sure the picture is not going to be distorted by stray magnetic fields from the drive unit motor systems. The following dedicated centre speakers are magnetically shielded because their application requires them to be placed right next to the screen:

HTM1D

HTM2D

HTM3S

HTM4S

All other speakers in the Series should be placed at least 0.5m (20-in) from such screens. Some television brands are particularly sensitive to magnetic fields and may require extra spacing. Plasma and LCD screens are not affected and front projectors are usually well away from the speakers anyway.

Application	f Go to
Front left and right	2.1
Front centre	2.2
Surround	2.3

2.1FRONT LEFT AND RIGHT

If you're only interested in audio and not movies, space the speakers apart approximately the same distance as you sit from them. This means that the included angle is about 60º. This applies whether or not you are using a centre speaker.

Apart from the dedicated on-wall model SCMS, the balance of the speaker is more natural if the speakers are at least 0.5, (20-in) from the rear wall. This also helps to improve the impression of perspective. (figure 2)

If you are only concerned with 2-channel audio, having the speakers further apart can lead to what is known as the hole-in- the-middle effect, where it's difficult to generate a stable phantom central image. If you have a centre speaker, you can space the left and right speakers further apart. All that happens is that the image widens. It's just like being closer to the performance.

If you have to space the speakers further apart because of domestic constraints, the central image can often be improved if you toe the speakers in towards the centre of the listening area. This can also

help the perception of the central image for any listeners sitting away from the centre line of the installation. (figure 3)

If you are also using the speakers for movies, you should try to match the audio image to the size of the screen. That generally means that the speakers should be closer together. A good starting point is to put the speakers about 0.5m (20-in) from the edges of the screen. (figure 4)

Bookshelf or on-wall speakers should be placed at a height that brings the tweeters approximately to ear level. In the vertical plane, the dispersion narrows in the crossover region between the midrange and tweeter drive units, when both units are working together. To preserve the optimum sound balance, try to keep within ±5º of this.

Floor-standing speakers have the angle of their optimum listening window adjusted for the height of the speakers and the typical range of ear height of seated listeners.

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2.2FRONT CENTRE

If you have an acoustically transparent screen, place the speaker behind the centre of the screen. Angle it towards the listeners if the tweeter is more than 5º from ear height. (figure 5)

If you have a normal screen, place the speaker immediately above or below the screen, whichever is nearest ear height. Angle it towards the listeners if the tweeter is more than 5º from ear height. A stand with tilt adjustment is available for the HTM2D, HTM2S and HTM4S. Consult your dealer for details. (figure 6)

If you are just listening to audio, place the speakers centrally and mount bookshelf or wall mount speakers with the tweeters at ear height. (figure 7)

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2.3SURROUND

Surround speakers generally fall into two main types – those that one might describe as 'normal' speakers – so- called monopoles, where the sound comes from a set of drive units mounted on the front of the enclosure – and those that give a more diffuse sound field, such as dipoles. Each type has its advantages.

Most multi-channel music is recorded with home entertainment in mind and is monitored using monopole surround speakers, whatever the multi-channel recording format. This enables better location of side and rear images,

although the formation of such images is never quite as precise as it is between the front speakers.

Most films are originally balanced for cinemas, where a large number of speakers spread around the auditorium are used to create the surround sound field. In that case there are more surround speakers than there are discrete channels of information and a less precise image is created that gives an all-enveloping effect. Dipoles and similar diffuse speakers are better at recreating this type of sound field in the home, but using fewer speakers to do it. Image positioning with these types of speaker is never as precise as it can be with monopoles. However, they do have the advantage of making it easier to balance the system for a larger listening area.

You may well receive conflicting advice from different sources on the best type of surround speaker to use. The truth is that there is no one perfect solution for all situations and the final choice for any given application will be influenced by several criteria, some of which may have a degree of conflict.

DS8S only

Within the 800 series, the DS8S is the only speaker to offer dipole operation. In fact, this specialist surround speaker has the advantage of offering a choice of both monopole and dipole operation, either via a switch located on the front baffle, behind the removable grille, or remotely, using a 12V trigger from the surround processor. You may therefore choose whichever type of operation best suits the conditions of the listening room, the size of audience and the type of programme being played. Indeed, you may even change the characteristic for different types of programme and, as the total energy into the room is the same in both modes, no recalibration of the installation is necessary when switching between them.

In monopole mode, only the two drive units on the front face operate. In dipole mode, the front tweeter is disconnected; the side firing drivers are brought into operation and the crossover frequency to the bass unit is lowered. The drive units on opposing sides are connected out of phase with one another, which creates a wedge- shaped null zone, approximately 60º wide, at right angles to the wall. If the listeners sit within this zone, they become less aware of the location of the speakers and hear more reflected sound; hence the diffuse nature of the sound field.

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