Echoes in Home Cinema Installations - November 13th, 2008
A reflection which is perceived at all in a home cinema room does not necessarily reach the consciousness of a listener. At low levels it manifests itself only by an increase of loudness of the total sound signal, by a change in timbre, or by an increase of the apparent size of the sound source. But at higher levels in a home cinema system, a reflection can be heard as a separate event, i.e. as a repetition of the original sound signal.
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.