From the Bench

Big bass loudspeakers go well with large screens and home movies, and they o er good profi t margins, writes Alan Bennett

AS TV displays get bigger and better, and as movie

soundtracks and music recordings embrace wider range and fi delity 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 e ects such as explosions, earthquakes, gunfi re, 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 and small separate woofers. For

Subwoofer by DALI

the full e ect, and to enjoy Some are reinforced and the experience intended rigidised with secondary by the fi lm director, it’s “The sound from a materials. It has a very necessary to shift more subwoofer, because of fl exible butyl or rubber air, so to speak, by the longer wavelengths suspension ring around adding some muscle involved, is less directional it; and a voice coil, and ‘slam’ to the sound than that from the other generally of copper wire system in the form of speakers so that its and 1½ – 3 inches in a separate subwoofer positioning in the room diameter. This moves to incorporating its own is less critical. ” and fro in the annular gap internal amplifi er. These of a very powerful magnet can run to 16" diameter and to produce low frequency over 3kW drive power for a sound. Some woofers can reach large home cinema room and a down to 20Hz, the lower limit of dedicated (and rich!) enthusiast. For human hearing. At the upper end of the most home viewers a 12" woofer driven at up to range about 80Hz (adjustable) is appropriate for 500W is quite adequate, typically costing around home use. £700; bigger and more powerful models can run The enclosure of the woofer needs to be to £6500 or more, appropriate for a large home dense and rigid to avoid resonances and theatre. These systems seldom run at full rated ‘colouration’ of the sound. Generally, MDF power for more than a few moments at a time, or HDF is used for this box. Two types are but having such a reserve gives more punch and common: ported, in which one or more apertures potential to the sound system. alongside the speaker equalise the internal and CONSTRUCTION external air pressures to o er a gain in bass 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 sti material such generally somewhat less expensive. as paper, plastic, (e.g. polyester, polypropylene), The drive amplifi er is mains-powered and metal, (aluminium etc.), carbon fi bre or Kevlar. many of them work in Class D mode, in which the output transistors switch fully on and o 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 e cient and powerful.

CONTROLS AND POSITIONING

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 E ects) 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 refl ectance 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 fl oor 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 profi t 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 fulfi lling accessories?