Cabinets for Amplifiers
Speaker Cabinets Fast Facts Info
To improve power handling, similar speakers can be connected together in the same cabinet and driven by one amplifier. This allows you to use a powerful amplifier to drive into speakers which individually have a moderate capacity. Another solution to the problem of handling different frequencies is to use passive frequency splitting. The amplifier’s output is split by a crossover network into high and low frequency components. Low frequencies are channeled into a bass speaker and high frequencies into a treble speaker. Three-way passive crossovers are also used, with the bass speaker handling frequencies up to around 850 Hz, treble speaker from 1,500 Hz-20,000 Hz, and a mid-range speaker being brought in to cover frequencies from 800-2,000 Hz.
A Few Tips on Speaker Cabinets
Speaker cabinets (or “enclosures”) play an important role in the reproduction of sound. As air in front of the cone is compressed when the cone moves forwards, so air behind is rarefied. The reverse happens as the cone moves backwards. If the air in front of the cone reaches the air behind quickly, then the difference in pressure is cancelled out, resulting in loss of sound output. The pressures on the back and front of the cone are then said to be out of phase with each other. A function of the enclosure is to prevent or reduce phase cancellation. The cabinet also “loads” the cone in such a way as to optimize its coupling with the surrounding air. Ideally, it must not color the sound in any way by adding its own resonance.
The materials from which enclosures are constructed should be thick and heavy, to reduce vibration of the cabinet walls. Sound-absorbing materials such as heavy felt are often used to cover all interior surfaces that could potentially reflect sound back onto the diaphragm or set up “standing waves”. Curtains of sound-absorbing material are also hung within the cabinet to damp out internal reflections.
There are four basic types of enclosure. An infinite baffle speaker is a sealed, air-tight box, filled with sound absorbers to soak up all internal energy. Since the air inside the cabinet is completely separated from that outside, there is no problem with phase cancellation. However, such speakers are comparatively inefficient, since only about 0.5-2 % of the electrical signal delivered to the speaker is actually converted into sound.
Open-backed (or finite baffle) speakers are more efficient than infinite baffle models (approximately 5%). Since sound is projected from both the front and rear of the speaker, any wavelength longer than the minimum distance from the front of the speaker around the cabinet to the rear will be attenuated by phase cancellation.
A third type of speaker is the bass reflex (or tuned port) cabinet. This has an opening, called a port, in the enclosure, through which bass frequencies from the rear of the cone are channeled. The port is “tuned” to cause low frequencies from the port to be added, in phase, to the front-of-cone output. Efficiency is again around 5%, but the cabinet has to be relatively large if it is to handle low notes well.
The most efficient speakers currently available are the horn-loaded type. Though often bulky, such speakers are capable of converting up to 50 percent of the input energy into sound. The flared shape and the size of the horn are computed to regulate the dispersion and frequency range of the sound they project, and to give good coupling between the cone and the air. Many PA speakers are a combination of horn and bass reflex designs.
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