Resonant room modes
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Resonant room modes affect the low frequency response of a sound system at the listening position. They are actually one of the biggest obstacles to high fidelity reproduction with modern equipment as they exist to varying degrees in all rooms and can only be reduced by the use of very big and bulky absorbent materials — like the foam wedges used in anechoic chambers. Most rooms have several main resonant modes in the 20 Hz to 200 Hz region, each related to a room dimension like length, breadth and height. Some relate to corner to corner reflection, and are stimulated more if loudspeakers are placed in the corners. The effect on music reproduction is heard as 'muddy' bass, with some notes, especially on bass guitar standing out and persisting longer than they should and some notes 'disappearing' (at antinodes). Most people are surprised when they first walk around a room while listening to low frequency test tones, the changes in level from almost nothing to loud often being quite startling.
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[edit] The mechanism of room modes
Room modes are the result of standing waves which result from sound being reflected back off the walls of a room and interfering with the original travelling wave radiated by the source. At certain frequencies there will be some points in the room where the waveforms will add to produce a node or loud spot, and others where complete or partial cancellation occurs to produce an antinode. Where the radiated wave can bounce back and forth, causing repeated reinforcement and cancellation strong nodes and antinodes will arise, the simplest example being the longitudinal mode along the length of a room in which a speaker is placed at one end. Since there can be no air movement perpendicular to a wall, pressure nodes will always arise there.
[edit] Minimising the effect of room modes
A rectangular room with hard surfaces and no soft furnishing will exhibit strong resonant modes with high Q, giving rise to sharply tuned resonances. To lower the Q, it is necessary that energy be lost rather than reflected repeatedly, and this can be achieved by adding absorbent material. Curtains and carpets are very effective at high frequencies (say 5kHz and above), but in order to be effective a layer of absorbant material has to be of the order of a quarter wavelenght thick. Absorbtion occurs through friction of the air motion against individual fibres, with kinetic energy converted to heat, and so the material must be of just the right 'density' in terms of fibre packing. Too loose, and sound will pass through, but too firm and reflection will occur. Technically it is a matter of impedance matching. Glass fibre, as used for thermal insulation, is very effective, but needs to be four to six inches thick if the result is not to be a room that sounds unnaturally 'dead' at high frequencies but remains 'boomy' at lower frequencies. As a rule of thumb, sound travels at one foot per millisecond, so the wavelength of notes at 1kHz is about a foot, and at 10kHz about an inch. Even six inches of glass fibre has little effect at 100Hz, where a quarter wavelength is over 2 feet, and so adding absorbent material has virtually no effect in the lower bass region 20 to 50Hz, though it can bring about great improvement in the upper bass region above 100Hz.
Open apertures, and irregular room shapes are another way of absorbing energy and breaking up resonant modes. In this case, as with the large foam wedges seen in anechoic chambers, the loss occurs ultimately through turbulance, with colliding air molecules converting kinetic energy into heat. Damped panels, typically consisting of sheets of hardboard between battens packed with glass fibre behind, have been used to absorb bass, in this case by allowing movement of the surface and energy absorbtion by friction.
Attempting to 'equalise' the bass response of a listening room can only work on steady state signals for one listening position, and even then, it fails on music because of sluggishness of the modes to build and die away.
The advice commonly given to position speakers, especially sub-woofers, in such a way as to stimulate the maximum number of room modes, is also of doubtful value. Sub woofers are often placed near corners, in order to stimulate diagonal and longitudinal modes in all directions, but while this may produce a more 'balanced' sound, it is really just balancing out a series of peaks and dips in response at the listening position. Unless energy can be absorbed (Q lowered), the resonant modes will still slow the rise and fall of bass notes causing sluggish bass and missing notes.
While loudspeaker placement cannot satisfactorily compensate for resonant modes, it can be used to advantage to eliminate some modes without detriment. For example, if a bass speaker is placed on the floor it will exite vertical modes, but raising the speaker to half way up the wall will eliminate the first order vertical resonance while stimulating higher order modes. Since ceilings are often very reflective this can be useful. Placing a sub-woofer in the middle of a wall, rather than a corner, will eliminate the first-order resonance across the room, if this is a problem, again stimulating the higher order resonance.
Dipole loudspeakers, such as electrostatic speakers are generally considered to be be better than conventional speakers with respect to room modes. Because they do not radiate sideways at all, they do not stimulate vertical room modes. In other words, they send less sound to bounce off the ceiling or floor.
All attempts at evening out or equalising resonant modes ultimately fail to produce 'solid' bass notes. This is because in an ideal listening environment, the note from a bass guitar for example, starts suddenly at full loudness and then continues at that level. When a system is equalised to reduce the effect of a peak in the bass response by attenuating the level at the frequency of the peak, the initial loudness of the guitar note is also reduced, with loss of impact. Only as successive cycles of reflection build up, does the level reach its correct value, and then, if the note stops suddenly, the resonance causes it to die away slowly. Loudspeakers are often wrongly blamed for this effect. While it is true that speakers, especially bass reflex designs, can contribute to sluggish bass, a good speaker will make little difference in a bad listening room. Recently, attempts have been made to cancel the reverberant field, using digital techniques that create inverse signals corresponding to the room reverberation, but they only work well for one listening position.
[edit] Concert halls
Very large rooms like concert halls, television studios, or outdoor stadia do not suffer the problem of resonant modes, since the dimensions of such rooms are usually very much longer than the wavelength of even the lowest audible bass notes. Instead of the reflections appearing almost immediately on top of the sound, as resonance, they arive after hundreds of milliseconds and are perceived by the ear as a diffuse reverberant field rather than a peaky response. While a recording studio might aim to have a reverberation time of around 100 milliseconds, a concert hall or cathedral can have a reverberation time of seconds. Provided that the reflections are very numerous and varied, the effect is a pleasing form of sound reinforcement on music, though speech intelligibility requires short reverberation.
