This results in the requirement for very large objects at low frequencies, 1.25-2.5 m at 34 Hz, and very small objects at higher frequencies, 1.25-2.5 cm at 3.4 kHz. If the objects are too small, that is, less than one eight of a wavelength, they will not diffuse properly. If they are too big, that is, greater than about a half a wavelength, they will behave as acoustic mirrors in their own right and so will not diffuse effectively.

Clearly effective diffusion is a difficult thing to achieve in home cinema installations, in an ad hoc manner. Curved and angled structures found in many home cinema designs can help at mid and high frequencies, and at very high frequencies, greater than about 4 kHz, the natural rough textures of materials such as brick and rough cut stone are effective.

Because of the need to achieve well-defined diffusion characteristics, diffusion structures based on patterns of wells whose depths are formally defined by an appropriate mathematical sequence have been proposed and used. The design of these structures is quite involved and will not be mentioned here. However, home cinema designers would need to take into consideration such structures, if a high end dedicated home cinema installation is the case.

Home cinema designers can reduce the effect of the building structure by building a ‘floating room,’ which removes the effect of the building structure by floating the room on springs away from it. In practice, ensuring that no part of the building is touching the dedicated home cinema room by any means (plumbing pipes and electrical wiring conduits are popular offenders in this respect) is extremely difficult.

The effect of ventilation systems and air leaks are also a major source of flanking in many home cinema installation cases. In fact in the domestic situation the sound isolation is almost entirely dominated by air leaks and draught paths, and it is the removal of these that allow double glazing salesmen to advertise a dramatic improvement in sound isolation, despite having two 4 mm panes of glass in the double glazing.

So in order to have good sound isolation in home cinema installations, one needs good partitions and an air-tight, draught-free structure. Achieving this in practice, while still allowing the occupants in the home cinema to breathe is a challenge.

Most home cinema systems are designed and installed in acoustically small rooms; an example of an acoustically large room would be a concert hall, or even a large recording studio.

There are two different approaches in regards calculating the critical frequency of a dedicated home cinema, prior installation. The first is to assume that modal behaviour in the home cinema room dominates once the mean free path is equal to one and a half wavelengths. The expression (A) below is useful for making rapid assessments of the like hood of achieving a particular critical frequency in a given home cinema installation.

However, the real critical frequency may well be higher because a home cinema room can have significant modal behaviour at high frequencies if the absorption is low. Because of this the accepted definition of critical frequency is based on the modal bandwidth, hence equation (A). The main consequence of modal behaviour is the frequency and spatial variation caused by it. However, if a given frequency excites more than one mode in the home cinema room, both the spatial and frequency variation will be reduced.

The critical frequency is defined as when the modal overlap equals three, so at least three modes are excited by a given frequency, and is given by expression (B) below. Expression (B) clearly shows that larger home cinema installations will have their critical frequencies generally lower than smaller ones. That is why big cinema rooms are acoustically ‘large’ as well.

Reverberation in home cinema rooms also helps integrate all the sounds from an instrument so that a listener hears a sound which incorporates all the instruments’ sounds, including the directional parts. In fact we find home cinema installations in spaces which have very little reverberation, uncomfortable and generally unsuitable for listening to music in.

The time taken for reverberation to occur is a function of the size of the home cinema room and will be shorter for smaller spaces, due to the shorter time between reflections and the losses incurred on each impact with a surface. In fact the time gap between the direct sound and the reverberation in dedicated home cinemas is an important cue to the size of the room that the soundtrack is being played in. Because some of the sound is absorbed at each reflection on the cinema’s acoustic panels, it dies away eventually. The time that it takes for the sound to die away is called the reverberation time and is dependent on both the size of the home cinema room, and the amount of sound absorbed at each reflection.

In fact there are three aspects of the reverberant field that the home cinema room effects:

1. The increase of the cinema’s reverberant field level :
This is the initial portion of the reverberant field and is affected by the home cinema’s room size, which affects the time between reflections and therefore the time it takes the reverberant field to build up. The amount of absorption in the home cinema room also affects the time that it takes the sound to get to its steady-state level. In other words, the rate at which sound builds up in a dedicated home cinema, depends on the time between reflections and the absorption. That simply means that reverberant sound level will take more time to reach a louder level than a smaller home cinema room.

2. The steady state level of the home cinema’s reverberation field :
If a steady tone is played in a home cinema system, after a period of time the reverberant sound will reach a constant level because at that point the sound power input balances the power lost by absorption because of the acoustic wall panels. This means that the steady-state level will be higher in home cinema rooms that have a small amount of absorptive acoustic panels, compared to cinema rooms that have a lot of absorptive treatments.

3. The decay of the cinema’s reverberant field level :
When the tone in the home cinema stops, the reverberant sound level will not reduce immediately but will instead decay at a rate determined by the amount of sound energy that is absorbed at each reflection with the acoustic panels and other surfaces. Thus in home cinema installations with a small amount of absorption the reverberant field will take longer to decay.

In general the absorption coefficient of real materials used for acoustic treatments in home cinema systems will vary with frequency, but for the moment we shall assume they do not. The amount of energy or power removed by a given area of absorbing material will depend on the energy or power per unit area striking it. As the sound intensity is a measure of the power per unit area, this means that the intensity of the sound reflected is reduced in proportion to the absorption coefficient.

In order to calculate the intensity of an early reflection from an acoustic wall panel in a home cinema installation, we can use the formula shown below.

The first thing to note is that just because a material is good absorber of sound doesn’t mean that it is a good isolator of sound. In fact most absorbing materials are terrible at sound isolation for home cinema installations. This is because, in the sound isolation case of a home cinema system, we are interested in the amount of sound that travels through a structure than the amount that is absorbed by it.

A poor value of sound isolation would be around 20 db yet it corresponds to only one hundredth of the sound being transmitted. A good absorptive acoustic panel with an absorptive coefficient of 0.9 would let one tenth of the sound through, which corresponds to a sound isolation of only 10 db! For hi-end home cinema installations we are more interested in sound isolations of 40 db as a minimum, so absorption is clearly not the answer.

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Colour Style is the ultimate colour changing light switch that can create bespoke lighting effects with the press of a button. The colour of the LCD screen can be customised to match the room’s interior decor and there are four different metal plates to choose from. Colour changing lighting, done simply.

1. After a short delay the listener in the cinema room will hear the sound of the starting speaker, which will have travelled the shortest distance between it and the listener. The delay will be a function of the distance, as sound travels 344 meters per second. The shortest path between the starting speaker and the listener is the direct path and therefore this is the first thing the listener hears. This component of the sound is called the direct sound. The direct component is important because it carries the information in the signal in an uncontaminated form. Therefore a high level of direct sound is required in home cinema installations, for a clear sound and good dialogue intelligibility. We need to point out that the intensity of the direct sound reduces as the square of the distance from the source, in the same way as a sound in free space.

2. A little time later the listener will then hear sounds which have been reflected off one or more surfaces (walls, floor, etc.). These sounds are called early reflections and they are separated in both time and direction from the direct sound. These sounds will vary as the home cinema speaker or the listener moves within the cinema room. We use these changes to give us information about both the size of the home cinema room and the position of the home cinema speakers in the space. If any of these reflections are much delayed, total path length difference longer than about 30 milliseconds, they will then be perceived as echoes. Early reflections can cause interference effects, and these can both reduce the intelligibility of dialogues of home cinema installations, and cause unwanted timbre change in music in the home cinema room. The intensity levels of the early reflections are affected by both the distance and the surface from which they are reflected. In general most surfaces in dedicated home cinemas absorb some of the sound energy and so the reflection is weakened by the absorption.

3. The absorption coefficient of acoustic wall treatments in home cinema installations defines the amount of energy, or power that is removed from the sound when it strikes them. In general the absorption coefficient of real acoustic treatments in a home cinema room will vary with frequency. The amount of energy, or power, removed by a given area of an acoustic wall panel will depend on the energy, or power, per unit area striking it. As the sound intensity is a measure of the power per unit area this means that the intensity of the sound reflected is reduced in proportion to the absorption coefficient.

High end home cinema designs will take early reflection analysis into consideration, in order to produce an acoustically correct home cinema environment.

This effect is commonly known as an ‘echo.’ But what outdoors usually appears as an interesting experience may be rather unpleasant in a dedicated home cinema room that it distracts the listener’s attention. In severe cases an echo may severely reduce our enjoyment of a movie, or impair the intelligibility of dialogues, since subsequent speech sounds or syllables are mixed up and the text is confused.

We will use the term ‘echo’ for any sound reflection in a home cinema room which is subjectively noticeable as a temporal or spatially separated repetition of the original sound signal, and we will discuss the conditions under which a repetition will become an echo.

From his outdoor experience the listener may know that the echo produced by sound reflection from a house front, etc., disappears when he approaches the reflecting wall and when his distance from it becomes less than about 10m, although the wall still reflects the sound. Obviously it is the reduction of the delay time between the primary sound and its repetition which makes the echo vanish. This shows that our hearing has only a restricted ability to resolve succeeding acoustical events, a fact which is sometimes attributed to some kind of ‘inertia’ of hearing. The echo disturbance depends not only on the delay of the repetition but also on its relative strength, its direction, on the type of sound signal, on the presence of additional components in the impulse response and other circumstances.

A very important point relevant to home cinema installations is that our hearing is less sensitive to echoes in music than speech. The reason for this is obviously the fact that music does not have to be ‘understood’ in the same sense as dialogue has, in a home cinema system. The annoyance of echoes in very slow music, as for example organ music, is particularly low.

We can summarise in the following statement: in home cinema acoustics the law of the first wave can be considered to be valid in general. Exceptions will occur only in special situations, as for example when most of the home cinema’s boundaries, except for a few remote portions of wall, are lined with absorptive acoustic treatments, or when certain portions of wall are concavely curved and hence produce reflections of more than average intensity by focusing the sound.

In a certain sense, the diffuse sound field in a home cinema room is the counterpart of a plane wave. Just as certain properties can be attributed to plane waves, so relationships describing the properties of diffuse sound can be established. They are of particular interest to the whole of home cinema acoustics, since, although the sound field in a home cinema is not completely diffuse, its directional structure resembles much more than that of a plane wave. Or, put in another way, the sound field in an actual home cinema design, which always contains some irregularities in shape, can be approximated fairly well by a sound field with uniform directional distribution on account of its great complexity. In contrast to this, a single plane wave is hardly ever encountered in a real situation.