FROM THE BENCH
FROM THE BENCH the output transistors switch fully on and off at a very high frequency (way above the audible range) to produce full-amplitude pulses whose duty cycle/mark-space ratio govern the output power and hence the movement of the coil and cone. These are efficient and powerful.
CONTROLS AND POSITIONING
Big bass loudspeakers go well with large screens and home movies, and they offer good profit margins, writes Alan Bennett
AS
TV displays get bigger and better, and as movie soundtracks and music recordings embrace wider range and fidelity the sound stage has expanded and deepened, but many viewers and listeners miss out on it. A good subwoofer can bring a whole new dimension of sound into the lounge, and particularly a home cinema room.
DRAMATIC SOUND Action movies often feature sound effects such as explosions, earthquakes, gunfire, crashes, aircraft, drums etc. These cannot be properly reproduced by the speakers of the TV set or – to the full – by soundbars, even those which incorporate on-board Subwoofer by DALI and small separate woofers. For Some are reinforced and the full effect, and to enjoy rigidised with secondary the experience intended materials. It has a very by the film director, it’s “The sound from a flexible butyl or rubber necessary to shift more subwoofer, because of suspension ring around air, so to speak, by the longer wavelengths it; and a voice coil, adding some muscle involved, is less directional generally of copper wire and ‘slam’ to the sound than that from the other and 1½ – 3 inches in system in the form of speakers so that its diameter. This moves to a separate subwoofer positioning in the room and fro in the annular gap incorporating its own is less critical. ” of a very powerful magnet internal amplifier. These to produce low frequency can run to 16" diameter and sound. Some woofers can reach over 3kW drive power for a down to 20Hz, the lower limit of large home cinema room and a human hearing. At the upper end of the dedicated (and rich!) enthusiast. For range about 80Hz (adjustable) is appropriate for most home viewers a 12" woofer driven at up to home use. 500W is quite adequate, typically costing around The enclosure of the woofer needs to be £700; bigger and more powerful models can run dense and rigid to avoid resonances and to £6500 or more, appropriate for a large home ‘colouration’ of the sound. Generally, MDF theatre. These systems seldom run at full rated or HDF is used for this box. Two types are power for more than a few moments at a time, common: ported, in which one or more apertures but having such a reserve gives more punch and alongside the speaker equalise the internal and potential to the sound system. external air pressures to offer a gain in bass CONSTRUCTION output at the expense of size; and sealed, having A subwoofer is basically a beefed-up version of a faster transient response and more suitable, a conventional moving-coil loudspeaker, having perhaps, for smaller rooms. Sealed types are a long-travel cone made of a stiff material such generally somewhat less expensive. as paper, plastic, (e.g. polyester, polypropylene), The drive amplifier is mains-powered and metal, (aluminium etc.), carbon fibre or Kevlar. many of them work in Class D mode, in which
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GET CONNECTED
MAY/JUNE 2020
Subwoofers may be hard-wired or cordless (e.g. Bluetooth), apart from their mains connection, and the more elaborate ones are controlled by a remote handset or smartphone app. Most have pre-set adjustments on the rear panel, including volume, crossover frequency (the point at which the woofer takes over from the other speakers in the room) and phasing, which governs the relationship of the cone movement to the timing of the input signal: this delay, depending on the woofer’s placement relative to the other speakers in the room, can improve perceived bass response. Some ‘posh’ models have a little LCD panel to indicate the settings. The most common connections for wired subwoofers take the form of RCA phono sockets; in some types an LFE (Low-Frequency Effects) port is provided for Dolby-equipped gear with this special additional bass track. The sound from a subwoofer, because of the longer wavelengths involved, is less directional than that from the other speakers so that its positioning in the room is less critical. Even so, the room’s reflectance and absorption zones can lead to cancellation and/or reinforcement of bass sounds – ‘standing waves’ – so that even small changes in the positioning of the woofer may change the position of partial nulls and peaks in the sound and thus perceived bass level. Good positions, perhaps, are centrally on the floor between the main speakers or in the middle of the back wall of the room. Very often existing furniture and room shape largely dictate subwoofer placement in practice.
MODELS AND PRICES A rough rule of thumb, maybe, is that a subwoofer should cost about as much as the screen it partners. Typical prices of respectable models are 10" 300W £500-£600; 12" 500W £700-£800; 15 to 16" 800W £900 up. These all carry much better profit margins than TVs and the like! In recent reviews recommended products include the Fyne Audio model F3-12 at £600 and REL HT/1205 at £700, both with 12" drivers; and, higher up the scale, SVS model SB-3000 at about £1300. There is a wide range of products above and below these... If you don’t already sell this product line perhaps it may be worthwhile to talk to your new and existing big-screen high-end TV customers about these fulfilling accessories?
