- Resonant room modes
Resonant room modes affect the low frequency response of a
sound systemat the listening position. They are actually one of the biggest obstacles to high fidelityreproduction 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 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.
The mechanism of room modes
Room modes (sometimes called nodes) are the result of
standing waveswhich result from sound being reflected back off the walls of a room and interfering with the original traveling waveradiated by the source. At certain frequencies there will be some points in the room where the waveforms will add to produce an antinode or loud spot, and others where complete or partial cancellation occurs to produce a node. 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.
Minimizing 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 5 kHz and above), but in order to be effective a layer of absorbent material has to be of the order of a quarter wavelength thick. Absorption 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 matchingbetween air motion and the individual fibres. Glass fibre, as used for thermal insulation, is very effective, but needs to be very thick (perhaps four to six inches) if the result is not to be a room that sounds unnaturally 'dead' at high frequencies but remains 'boomy' at lower frequencies, that is to provide absorption across a broad range of frequencies.
As a rule of thumb, sound travels at one foot per millisecond, so the wavelength of notes at 1 kHz is about a foot, and at 10 kHz about an inch. Even six inches of glass fibre has little effect at 100 Hz, where a quarter wavelength is over 2 feet, and so adding absorbent material has virtually no effect in the lower bass region 20 to 50 Hz, though it can bring about great improvement in the upper bass region above 100 Hz.
Open apertures, dispersion cylinders (large diameter and usually wall height), carefully sized and places panels, and irregular room shapes are another way of either absorbing energy or breaking up resonant modes. for absorption, as with large foam wedges seen in
anechoic chambers, the loss occurs ultimately through turbulence, as colliding air molecules convert some of their kinetic energy into heat. Damped panels, typically consisting of sheets of hardboard between glass fibre battens, have been used to absorb bass, by allowing movement of the surface panel and energy absorption by friction with the fire bat.
Attempting to 'equalise' the bass response of a listening room works only for relatively broad frequency bands (one eighth of an octave is common) and without expensive very high Q filters, cannot cope with narrow band modes. The effect on music is often unsatisfactory, as is the equalization because of this limitation. For rooms with broader modes, equalization can be helpful. Some vendors are currently providing elaborate room tuning equipment which requires precision microphones, extensive data collection, and uses computerized electronic filtering to implement the necessary compensation for the rooms modes. There is some controversy about the relative worth of the improvement in ordinary rooms, given the very high cost of these systems.
The common advice to position speakers, especially sub-woofers, in such a way as to stimulate the maximum number of room modes, is also potentially problematic. At best, it only distributes resonant energy across the room modes. If they're are fortunate in both frequency (depending on distances between parallel surfaces) and all different, the listening situation may improve. In principle, unless mode energy can be absorbed (ie, the Q of the resonances lowered), or prevented by dispersion, the resonant modes will still slow the rise and fall of bass notes causing sluggish bass and missing notes.
While loudspeaker placement cannot fully compensate for resonant room modes, it can be used to advantage to reduce or eliminate some modes. For example, if a bass speaker is placed on the floor it will excite vertical modes, but raising the speaker half way up the wall will eliminate the first order vertical resonance while stimulating higher order modes (usually less objectionable). Since ceilings are often very reflective this can be useful. Placing a sub-woofer in the middle of a wall, rather than a corner, similarly eliminates first-order resonance mode across the room, again stimulating the higher order resonance.
Dipole loudspeakers, such as
electrostatic speakersare generally considered to be better than conventional speakers with respect to room modes. Because they have much reduced radiation sideways, and because they are never placed against a surface, they will not stimulate some room modes.
Attempts at evening out or equalising resonant modes ultimately fail to produce 'solid' bass notes in some rooms. This is because in an ideal listening environment, the note from a bass guitar for example, begins suddenly (ie, high risetime) and has a relatively slow tail off (ie, decay time). When a system has been more or less improperly is equalised to reduce the effect of a peak in room bass response by attenuating the level at the frequency of the peak, the rise time of bass notes at that frequency will be affected, and the room mode will still store energy at that frequency, also interfering with those bass notes decay times. At the extreme, these effects cannot be avoided and will cause audible problems (eg, loss of impact). Loudspeakers are often wrongly blamed for this effect. While it is true that speakers, especially poorly designed
bass reflexsystems, contribute to sluggish bass, a better speaker can't really improve 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 work best for a single listening position, as the room modes are not really affected.
Very large rooms like concert halls, television studios, or outdoor stadia have much reduced resonant mode problems, especially at low frequencies, since the dimensions of most such rooms are usually 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 arrive after hundreds of milliseconds and are perceived by the ear as a diffuse reverberant field rather than a peaky response. In addition, because of the larger dimensions, the resonant modes in these spaces are much lower than any musically significant frequency. A problem some large spaces have is unpleasantly audible reverberation times (ie, echoes). These can be, and are, corrected with architectural modifications such as panels and diffusing structures. A famous case is Avery Fisher Hall in New York City; the New York Philharmonic found the new hall nearly impossible, and the hanging panels added after an acoustical survey were very effective.
A recording studio might aim to have a reverberation time of around 100 milliseconds (across the musical range), but a concert hall or cathedral can have a reverberation time of seconds, and with poor acoustics, unbalanced reverberation as well. Provided reflections are numerous, varied, and diffusing, the reverberation can be a pleasant addition to the sound of the hall for music, though speech intelligibility usually requires a different characteristic reverberation.
* [http://www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/Web-standing-rooms.htm Animated illustrations of room modes]
* [http://arts.ucsc.edu/ems/music/tech_background/TE-02/modes/Modes.html Room mode calculations and tables]
* [http://www.audiocheck.net/testtones_sinebursts20-200.php Test tones playable online: helps localizing resonant frequencies in your room.]
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