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Installation Considerations for Loudspeakers in Home Theatres (30/7/2008)

By Christophe Anet, Genelec

When it comes to home theatre design, sound quality is often third on the list after the room aesthetics and video requirements. High picture quality and fabulous surroundings however, will fade into insignificance if the sound reproduction is poor, but with some attention to detail, the sound system can be designed for a truly enjoyable cinema experience.

When considering the design and installation of an audio system for home theatre, there are a number of factors that must be taken into account. These include the room size, the listening/viewing distance and the room acoustics. These will affect the type and size of speaker required, and its positioning.

The ideal loudspeaker placement is often compromised when working within a client's wishes and room design restrictions, but if we can work as closely to the ideal as possible, good sound can be achieved.

Main loudspeaker positioning

DVDs are normally mixed in studios that conform to the ITU standard 'ITU-R BS.775-1 Multichannel stereophonic sound systems with and without picture' (Geneva, 1992-94). The closer a home theatre can get to this standard, the better the audio result will be. A few of the more important recommendations found in this international standard are presented and discussed below:

For angles, the ITU recommendation is that left and right are positioned 30 degrees from the centre and the rears are positioned 110 degrees +/-10 degrees from the centre. The loudspeakers should then be angled so that they point towards the optimum listening position, as in the image below.

ITU-R BS.775-1 recommended loudspeaker positioning in recording studios.

In practice, these angles are generally not respected in home theatres. The centre loudspeaker is usually in the correct place but the left and right loudspeakers are positioned according to the size of the screen - a practice inherited from movie theatre 'standards'. Multiple loudspeakers are used for the rear channels to increase coverage in the listening area, and are positioned wherever is most convenient on the side and/or rear walls.

For distance, the ITU recommendation is that all loudspeakers should be equidistant from the 'sweet spot'.

In practice, it is often difficult to place the left, centre and right loudspeakers equidistant front the sweet spot as the loudspeaker enclosures are normally positioned in the same plane, i.e. against a wall or in some cabinetry. Although the resulting timing differences can be electronically compensated using a processor, this compromise is rarely referred to in most literature. The polar pattern of the loudspeaker is different at different listening distances, and this cannot be compensated for in a simple digital delay.

For height, the ITU recommendation is that the front loudspeakers should be placed at a height approximately equal to that of the listener's ears (1.2m/4ft from the floor). If the loudspeakers are to be placed higher or lower than the listener's ears, the loudspeakers should be angled vertically to point towards the listening position.

In practice, we recommend that our loudspeakers for example, be positioned at least 1.2m off the floor so that floor reflections do not dominate the low-mid and bass frequency response spectrum. If the loudspeaker is placed close to the floor below a screen for example, two things happen: a deep and wide notch occurs in the bass (typically around 100-200Hz), and the loudspeaker is loaded, which increases the entire bass output. Subjectively, the result is a muddy but thin sounding bass, and midrange masking, which makes speech less intelligible.

Conversely, be careful that the ceiling reflection does not start to dominate instead. The front loudspeakers can be placed such that the centre loudspeaker is a maximum of 7 degrees higher or lower than the left and right loudspeakers. This will not be audibly disturbing to the listeners, as the human ear is not good at localizing in the vertical direction (zenith plane). The rear loudspeakers can be positioned at a maximum of 15 degrees higher (lower is not practical) than the front loudspeakers as humans are less good at localising sounds coming from the rear.

Centre loudspeaker enclosure location

Ideally, the centre loudspeaker enclosure should be mounted at the same height as the picture so that the sound comes from the same place as the picture, but this is not always possible.

With perforated projection screens, the loudspeaker enclosure can be mounted in the centre of the picture, but there is a compromise to the frequency response. Some perforated projection screens are acoustically more transparent than others, but this should be weighed against corresponding compromises in the picture quality. And as the above graph shows, the loudspeaker enclosure should never be placed behind an unperforated projection screen!

The effect of projection screens on acoustic performance.

With unperforated projection screens, as well as large CRTs and rear projection displays, the loudspeaker enclosure should be placed above the screen. This will reduce the effects of the floor reflection as the reflection distance and angle are both increased. Doing this will cause the ceiling reflection to become significant but this can be easily treated by adding some damping material to the ceiling, such as 5cm/2" rock wool.

Mounting the centre loudspeaker above an unperforated projection screen.

Subwoofer positioning

The subwoofer should be positioned close enough to the front wall and slightly offset from the middle of the room by 30cm (1ft) to avoid the first pressure minima position. It can also be positioned in a corner, close to both front and side walls. This position will maximise the system efficiency due to the corner loading. A second subwoofer in the opposite corner may be required to avoid localising a single subwoofer. Alternatively, a lower bass management crossover frequency such as 60 or 40Hz could be selected, but one should be careful that the main loudspeakers can handle the remaining upper bandwidth.

These locations are contrary to the common belief that the best position for the subwoofer is in the front, on the floor and in the middle of the room, equidistant from the side walls. This location can be a serious compromise since the subwoofer sits in the pressure minimum of the lateral standing wave. Also, it should be remembered that adjustment of the gain (input sensitivity) and frequency response (bass roll-off) of the subwoofer is necessary to balance the subwoofer to the main loudspeakers. The subwoofer can also be flush-mounted into the front wall or some cabinetry but the discussion of the position of the source relative to the room remains valid. The phase adjustment on the subwoofer at the crossover frequency is also important, because if it is not correctly set, then adding subwoofers can actually result in less bass in the listening area.

Bass management

The bass response of a home theatre can be even more important than the visual image, right from the first note. Client satisfaction depends on the quality and quantity of bass energy, therefore careful attention is required for the lower frequencies. Unfortunately, simply putting many big subwoofers into the room will not work. Overblown and distorted bass can sound impressive at first, but the quality and quantity can quickly become tiring.

The most important aspect to consider for a good bass response is the room shape and dimensions. Low-frequency absorption will help reduce the effect of standing waves, but this is usually difficult to fully accomplish due to the large depth required to absorb long wavelengths at low frequencies. If a room is reverberant at longer wavelengths, less energy will be required for the low frequency reproduction. However, this has to be balanced against the listening position, as a standing wave in either the length, height or width dimension will have a negative effect, especially if the listener is in the middle of the room. The general rule is to specify the right size devices into the room volume with additional consideration given to the listening distance.

Cabinet mounting

Loudspeakers can often be mounted into cabinetry. Experience has shown that the void to the sides and top of the loudspeaker should be filled as much as possible with insulation or dense open foam. This usually helps keep the cabinetry resonances under control and inaudible, which can be further enhanced if the speakers offer room response controls. Mounting subwoofers inside cabinetry can also give good results.

Home theatre front wall

Home theatre front walls consist usually of a structural hard front wall construction with a combination of standard or custom-made cabinetry mounted in front, in which all loudspeakers, subwoofers, electronics and cabling are housed. A thin structure, often cloth material over wooden frames, covers the cabinetry to hide all equipment except the projection screen.

This construction method is not what is referred to as the 'flush mounting' technique. For loudspeakers to be acoustically flush mounted, they should be mounted into a solid and heavy wall structure which limits the space that the bass can spread into. Low-frequency sound can then only travel in that limited hemispherical space. However, if the main loudspeakers are mounted so that their front baffles are 1.3 - 1.7m (4 - 5ft) away from the hard wall behind them, a back wall cancellation will occur in the 50 - 70Hz region, thereby seriously compromising the low frequency response of these loudspeakers. One solution is to choose cabinetry constructions that are not too deep, for example, where the loudspeaker is placed as close to the wall as possible thereby avoiding the effects of the cancellation.

Correct flush mounting of a loudspeaker. Note the short distance to the hard wall, use of damping material and provision of amplifier ventilation.

Incorrect flush mounting of a loudspeaker. Unnecessarily long distance to the hard wall and lack of damping material will cause cancellation problems.

Also, when large front loudspeakers are positioned too close to the floor, their responses will have notches in the 80 to 120Hz frequency region, causing deterioration of the bass reproduction. Loudspeakers should be placed at a reasonable height, and if necessary, slightly tilted down, which will minimise such floor reflections. Avoid positioning the loudspeakers too high, i.e. >20 degrees, as this may become disturbing to listeners.

Conclusion

Ideally, in order to achieve the best results for the client, the services of a well-qualified room acoustician and a contractor with experience in designing home theatres or similar type rooms, such as music studios, should be employed. They will be able to ascertain what acoustic treatment the room needs, what are the best speakers to use, as well as the appropriate electrical and cable requirements. With the right speakers and technical expertise, there is no reason why a home theatre cannot sound like a truly professional installation and provide complete client satisfaction.

This article is based on extracts from the Genelec Active Speaker Systems for Custom Install Design and Installation Guide.

Christophe Anet is Technical Editor at Genelec, manufacturer of reliable, neutral-sounding audio monitoring systems for professional and home theatre applications of any size.

www.genelec-ht.com