The sound radiated from loudspeakers depends not only on the quality of the signal coming form the amplifier, but is also largely influenced by the listening room.
Poor room acoustics can degrade sound quality even when very high quality equipment is used. And even when the listening room meets basic requirements, wrong placement of loudspeakers can cause many problems.
It would be hard to single out one optimum kind of room acoustics design and likewise it is hard to advise just one procedure for loudspeaker placement. In practical reality various solutions are used. Choice of the best one depends on the dimensions and functions of a given room, on the the speakers used, on the budget available and on other factors as well. But the problmes we need to solve are always the same.
Optimizing room acoustics and loudspeaker placement is performed in order to:
- smooth the frequency response in bass range, where standing waves are excited
- reduce the influence of early reflections on perceived sound.
- obtaind desired decay time characteristics
Low frequency balance
At low frequencies the response curves are largely affected by acoustic resonances of rooms. These resonances depend on the room shape and dimensions. The best solution, unfortunately not available for most of us, is to design a special, acoustically optimised room. This way we can choose the optimum ratios or even use inclined walls.
But usually we cannot change room dimensions and have to focus on best application of available spaces. Fortunately we can do quite a lot to improve sound quality, regardless of the room's shape. Employment of specialized bass traps helps to damp the most troublesome resonances, and at the same time to smooth bass frequency response. But in everyday practice optimizing loudspeaker placement and listener position is the most popular way to improve quality of bass reproduction.
There are several rules we should always obey. We should avoid the situation when all three distances from loudspeaker center to nearest walls (usually the floor, side wall and the wall behind the speaker) are the same. Minimum distances from the side and back walls should also be provided. These distances depend on the design of a particular speaker, so our recommendations given for each model should be observed.
More ambitious goals can be achieved as far as optimization of low frequency response is concerned. A classic solution is employment of the rule of the thirds. According to this procedure loudspeakers are placed at the crossings of lines dividing room into thirds along its width and its length. And the listener should be seated at the system axis, also at one third of the room's length. This setup is often hard to implement for practical reasons, but other setups providing similarely good excitiaions of room resonances can be found.
Early reflections control
Strong early reflection coming to the listener's ear shortly after direct sound are one of the acoustic phenomena which seriously degrade sound quality.
Theoretically one could hope that by increasing the distance between speakers and room walls we could delay the reflections and this way solve the problem. Unfortunately typical rooms - including fairly large professional studios - are too small to achieve that kind of effect.
Nevertheless quite a lot can be done to reduce the negative impact of early reflections. First of all the most basic mistake should be avoided - no hard surface reflecting sound should be present in the vidcinity of speakers. Secondly we should always take care of symmetry of our stereo setup. Both loudspeakers should have similar acoustic surroundings.
When such basic problems are solved, further optimization can be made. In general increasing the distance between speakers and walls is beneficial in this respect. Sometimes unconventional soutions are effective. In square rooms setting up the system so that its axis is directed along the diagonal is usually optimium. But acoustic treatment is the primary means for reduction of early reflections. The treatment may be based on sound absorption, diffusion or combined absorption and diffusion. In top quality audio systems such treatments should be considered a necessity.
Room decoration
While placement of loudspeakers can help a lot to even out low frequency response and minimize early reflections, the decay time characteristics depends almost entirely on room decor. The right balance should be found. Decay time should be kept within certain limits. When it is to large sound looses its clarity and becomes too aggressive. When decay time is too small sound become unnatural, dead and lacks dynamics. Both situations are fatiguing for the listener. It would be hard to support the practice when loudspeakers are chosen to compensate inappropriate room absorption, that is using bright sounding speakers in dead rooms, or warm sounding speakers in live rooms.
Quite often we can find two kinds of bad rooms. In the first case we have rooms with modern type of decor, with a lot of hard reflecting surfaces. Big windows withoud curtains, floors without carpets and sparsely used furniture are typical featurs of such rooms. Such style of often promoted by interior decoration press, but is definitely not recommended for listening rooms.
Rooms of the second type use acoustic treatment, but employed incorrectly. Somewhat surprisingly, many acoustic treatments rely solely on employment of sound absorbing materials. No doubt this is caused by economic reasons. Such treatments are often made from cheap foams. But their effectiveness and range of applications is limited. They do not work at low frequencies and do not provide sound diffusion. Installation of a dozen absorbing panels is not a universal remedy for acoustic problems, and results only in artificial damping of high frequencies.