Resonant room modes
Encyclopedia
Room modes are the collection of resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

s that exist in a room when the room is excited by an acoustic source such as a loudspeaker. Most rooms have their fundamental resonances in the 20 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 to 200 Hz region, each frequency being related to one or more of the room's dimension's or a divisor thereof. These resonances affect the low-frequency low-mid-frequency response of a sound system in the room and are one of the biggest obstacles to accurate sound reproduction.

The mechanism of the room's resonances

The input of acoustic energy to the room at the modal frequencies and multiples thereof causes standing waves. The nodes
Node (physics)
A node is a point along a standing wave where the wave has minimal amplitude. For instance, in a vibrating guitar string, the ends of the string are nodes. By changing the position of the end node through frets, the guitarist changes the effective length of the vibrating string and thereby the...

 and antinodes of these standing waves result in the loudness of the particular resonant frequency being different at different locations of the room. These standing waves can be considered a temporary storage of acoustic energy as they take a finite time to build up and a finite time to dissipate once the sound energy source has been removed.

Minimizing the effect of room resonances

A room with generally hard surfaces will exhibit, high Q
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....

,sharply tuned resonances. Absorbent material can be added to the room to damp such resonances which work by more quickly dissipating the stored acoustic energy.

In order to be effective, a layer of absorbent material has to be of the order of a quarter-wavelength thick, which at low frequencies with their long wavelengths requires very thick absorbers. 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 matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....

 between 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, so that it provides absorption across a broad range of frequencies. Curtains and carpets are only effective at high frequencies (say 5 kHz and above).

As a rule of thumb, sound travels at one foot per millisecond (344m/s), so the wavelength of notes at 1 kHz is about a foot (344mm), and at 10 kHz about an inch (34mm). Even six inches of glass fibre has little effect at 100 Hz, where a quarter wavelength is over 2 feet (860mm), and so adding absorbent material has virtually no effect in the lower bass region in the 20–50 Hz region, 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 placed 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 chamber
Anechoic chamber
An anechoic chamber is a room designed to stop reflections of either sound or electromagnetic waves.They are also insulated from exterior sources of noise...

s, 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.

If a room is being constructed, it is possible to choose room dimensions for which it's resonances are less audible. This is done by ensuring that multiple room resonances are not at similar frequencies. For example a cubic room would exhibit three resonances at the same frequency.

Equalisation of the sound system to compensate for the uneven frequency response caused by room resonances is of very limited use as the equalisation only works for one specific listening position and will actually cause the response to be worse in other listening positions. Also large bass boosts by sound system EQ can severely reduce the headroom in the sound system itself. Some vendors are currently providing elaborate room tuning equipment which requires precision microphones, extensive data collection, and uses computerised 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.

Concert halls

Very large rooms like concert halls or large television studios have fundamental resonances which are much lower in frequency than small rooms. This means the closely spaced harmonic resonances are likely to lie in the low frequency region and thus the response tends to be more uniform.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK