Articles and whitepapers

What to Look For When Purchasing a Sub-bass System (21/10/2003)

By Stuart Wright

Just in case you are unsure about the value of adding a sub-bass system to your system, let us consider why you might need one.

What is the point of a sub-bass system? Most speaker systems produce sound from an upper limit of about 20kHz, and a lower limit of between 70Hz and 30Hz depending on the style of speaker. Floor-standing speakers usually have deeper bass than bookshelf speakers, for example, but music and movie soundtracks contain sound which goes lower than 30Hz. Usually the lower level is quoted as 20Hz for movies, but I'm certain many soundtracks go lower even than that.

Sub-bass systems usually produce sound between anywhere from 300-100Hz upper limit, down to 20Hz and often lower. The upper limit can usually be adjusted so that it does not overlap too much with your main speakers (see Figure 1).

Figure 1 - Frequency range of main speakers and sub-bass system

The point is to fill the gap between your main speakers and the lowest frequencies produced by the music/movies. Despite the fact that the human ear cannot hear below about 60Hz, we can certainly 'feel' sound that low.

One point worth mentioning, is that there is still some resistance to sub-bass systems in the audiophile market. This is mostly based on bad experiences with poorly-designed sub-bass systems which muddied the sound and did not integrate with the rest of the speaker system. Today's sub-bass systems are a completely different kettle of fish. Excellent integration and true deep bass can produce fantastic results. More and more audiophiles are being convinced that a sub-bass system can improve a sound system.

I would like to quote an audiophile friend of mine. He has quite an extravagant hi-fi including a £1500 CD player, a record deck with a £1000 stylus and tri-amped, tri-wired speakers on granite plinths. He tried a second-hand 40-watt sub-bass system costing just £200 and exclaimed, 'It's made the biggest single improvement to my sound than any other component - it's brilliant!'

Dolby Digital and DTS - a stronger case for sub-bass

With the advent of digital sound in the home in the form of Dolby Digital and DTS, the case for adding a sub-bass system has been made even stronger. Dolby Digital 5.1 and DTS specify five amplified channels (stereo pair up front, centre speaker and stereo surrounds) plus an unamplified sub-bass system channel (the '.1' of '5.1').

Both standards require a sub-bass system. In reality you can still get away without a sub-bass system as many home cinema amplifiers can be switched to send the bass frequencies to your other speakers. But you would be short-changing yourself. For the best audio experience, both in movies and music, a sub-bass system is an essential addition.

Active or Passive?

Put simply, an active sub-bass system has its own amplifier built-in. A passive sub-bass system relies on external amplification. Most current sub-bass systems are active - the advantage being that the amplifier and driver (speaker) can be matched perfectly by the manufacturer. Some people also say that active sub-bass systems apply the characteristics of their own amplifier to the sound rather than being 'transparent.' Others would argue that the effect is negligible or non-existent.

What a lovely pair

It is widely believed that two sub-bass systems are better than one - adding an evenness to the soundfield and extending the depth of the sub-bass even further. Some subs have even been reviewed in pairs. It depends on budget as always, but if it is possible, try to audition a pair.

How do you tell a good sub-bass system from a bad one?

Remember that your speakers produce sound down to anywhere between 70 and 30Hz. Your sub-bass system must produce bass deeper than this. Look for a sub-bass system which goes down to 20Hz at least, with a variable crossover so you can integrate the upper roll-off frequency with your main speakers.

As far as power is concerned, do not get too hung up on getting the most powerful sub-bass system. It is the quality of the sub-bass which is important, and this is what you should listen for when auditioning.

Set 'em up

Getting a decent demo is not always as easy as it should be. Try to pick a dealer who has some experience of setting up sub-bass systems. Ask the salesperson if they have their own sub-bass system at home. Chances are that if they have, they will know how to set one up properly. It takes time to properly set up a sub-bass system. Do not think for a minute that you can just plug it in and switch it on. A good salesperson will take time to integrate the sub-bass system with the speaker system, and this is important, since a poorly setup sub-bass system can sound awful.

What do you listen for?

Do not go for the most 'boomy' box. For music, the sub-bass system should have a subtle effect. Try to pick a familiar track which you know has deep bass in it. The music should quite simply be 'deeper.' My favourite is the bass guitar line in 'How About That' from the Bad Company 'Here Comes Trouble' album. The music comes alive with the addition of a sub-bass system.

For movies, do not just listen - try to 'feel' the bass through the pressurised air waves, or the floor. Pick a movie with a bass-filled scene (on DVD is best). The battle scenes from the Saving Private Ryan DVD have bass in copious quantities.

'With and without' comparisons are useful. Try disconnecting the sub-bass system for a scene and then repeat the scene with the sub reconnected. With a well-integrated, good sub-bass system, the sound should be richer and fuller.

Without doubt, the best audition happens in your own home. Connecting the sub-bass system to your system is probably the only way that you will get a true idea of what it will sound like before you buy.

Stuart Wright owns a REL sub-bass system.

REL Acoustics is a manufacturer of quality sub-bass systems for use in hi-fi and home theatre.

www.rel.net