Looking for something to give you a lift at the bottom end? A boom box could be the answer. Bob Dormon asks how low can you go?

We've all seen those belly-dancing Coke cans gyrating in response to sound, but have you ever seen your speakers pull the same trick? Well now you can with the new dbx 120XP Subharmonic Synthesiser. The 'P' indicates that it's a professional unit - the 120X having been around for some time now - designed to produce that low, low bass that gets complaints from residents five miles away from the club it's being used in.

In fact, dbx have been making subharmonic synthesisers - boom boxes to you and me - since the late '70s, identifying the need/trend for something throbbing around the bottom end. Their use isn't confined to just DJs and clubs: in recent years the boom box has begun to infiltrate film theatres and even recording studios, making low-end rumble an acceptable component of sound, rather than a symptom of a sick turntable.

With good neighbourly relations foremost in my thoughts, I gingerly unpacked this 1U black magic box, only partially reassured by the accompanying blurb that my speakers were not about to meet their maker. Not unless I was deliberately cruel anyway.

Turning up the Subharmonics control varies the intensity of the bass synthesis, and this is used together with the two Synthesised Frequency controls which alter the subharmonic mix within their respective ranges of 24Hz-36Hz and 36Hz-56Hz. Above each of these two knobs are three LEDs (green, yellow and red) which indicate the amount of new (subharmonic) signal being generated for that range.

It's all dependent on your source material, of course; if there's no bass, there's no pretty lights - and therefore no subharmonic activity. But you'd be surprised by what signals have a low enough content to bring the 120XP to life. Connecting up my video recorder, I discovered that not only did the predictable sonic booms and gut-wrenching growls in a movie like Die Hard elicit a response, but that even the effect on the human voice was quite marked (in this case the dulcet tones of Ian 'Lovejoy' McShane).

From a musical perspective, reggae has much to offer at the bass end, and as you might imagine produced lots of pretty lights on the front panel and a deep, dramatic boom from the speakers, in fact, the effect was a little too dramatic and I found a bit of tweaking of the two synthesised frequency pots was necessary so that instead of the exaggerated boom, you get punch with depth.

Of the remaining controls - LF Boost works rather like the 'loudness' switch on many domestic hi-fi amps. It's quite independent of the others, so you really can tailor the whole of this narrow bass/sub-bass band of frequencies to suit your needs. Sub-Woofer controls a separate output for feeding a (you guessed it...) sub-woofer system and switches at the rear allow you to select crossover frequencies of 80Hz or 120Hz. The main stereo output can also be switched - for full range or HF only.

The great thing about this unit is that the audio spectrum above the working range remains completely uncoloured by the processing. There's no muddying of the low-mid range: it's bass, the whole bass and nothing but the bass you're dealing with. This, apparently, is thanks to the 'Waveform Modelling' intelligent processing system employed by the dbx 120XP to produce bass notes below those in your source material.

Not having a sub-bass set-up, I fed the separate sub-woofer output into a bass amp and found this generally worked well. Having the opportunity to tap off this signal into equipment capable of operating at these frequencies is definitely the right approach as far as I'm concerned - it allows you to take full advantage of the audio range of the device.

Comparing the dry/treated signals by means of the Bypass switch reveals sounds that were once faint background whispers suddenly brought to fore, as if at last recovering from their shyness.

I was also impressed by the quietness of the 120XP in operation, and the degree of control it offers so that just the right amount of punch - or roundness - may be added without unduly affecting the overall mix.

I have to say, I haven't been a great fan of the mixing on a lot of dance music. On most desks the EQ doesn't deliver in the range that really cuts it. The dbx 120XP is, I'd say, the missing link between bass and bottom end, and I'm sure they'll all live very happily together.

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