ABSTRACT
At low audio frequencies, normal rooms (i.e., not anechoic chambers with enormous sound absorption) have resonances at a series of frequencies where one dimension of the space corresponds to a multiple number of half wavelengths of the sound being radiated. The halfwavelength is the basic unit because there must be a node, that is a point of zero amplitude, at each end. Sound travels at about 345metres/second, so a room with a maximum dimension of 5 metres will have resonances from 34.5 Hz upwards. This is simply calculated from velocity/ frequency = wavelength, bearing in mind that it is the half-wavelength that we are interested in. We might therefore expect a resonance at 34.5 Hz, and another at about 69 Hz; this is twice the frequency because we now need to fit in two half-wavelengths between the two reflecting surfaces. This continues for three and four times the lowest frequency, and so on. These “resonant modes” cause large peaks and dips in response, the height of which depends on the amount of absorbing material. A room with big soft sofas, thick carpeting, and heavy curtains will be acoustically fairly “dead,” and the peaks and dips of the frequency response will typically vary by something like 5-10 dB. A bare room with hard walls and an uncarpeted floor will be much more acoustically “live,” and the peaks and dips are more likely to be in the range of 10 to 20 dB, though larger excursions are possible. Resonant modes at low frequencies cause the greatest problems, because they cannot be effectively damped by convenient absorption material such as curtains or wall hangings. Room equalisation that attempts to deal with this situation is a very different subject from active crossover design and is not dealt with further here.
