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Sub-Harmonic Synthesizer By Paul White
Published March 1994

Such was the outcry when dbx discontinued their original 'Boom Box' that they were forced to introduce a new model. Paul White finds out whether it really plumbs new depths.

There's something very primal about deep bass — it doesn't matter if it's Stravinsky's Rite of Spring or a warehouse rave; bass energy seems to open direct channels to the emotions and instincts of the listener. This fact has been exploited in music since time began, but sometimes the natural sounds at our disposal just don't hit the spot. That's where a device like the 120XP, or 'Boom Box', as it's affectionately known, comes in very handy.

The Boom Box is easiest to understand if you imagine it as being like an Aural Exciter in reverse; instead of synthesizing new upper register harmonics based on the existing mid‑range program material, it synthesizes new sub‑bass information based on the existing bass end. This is achieved by taking two narrow audio bands (48‑72Hz and 72‑112Hz) and using these to generate new components exactly one octave below the original. These are then mixed with the original signal to give instant bass enhancement.

Visually, the 120XP is both unremarkable and uncomplicated, offering two audio channels packaged in a 1U rack case, with mains power supplied directly via a captive lead. All audio connections are on jack sockets; the inputs are balanced, and the controls are all ganged for stereo operation. There's no power switch, so you have to switch the unit off 'at the wall'.

Though the Boom Box might be perceived as a studio tool, it is also configured to work with PA systems, where it can provide a feed for a mono subwoofer. A rear‑panel switch also enables the unit to operate as an electronic crossover with a choice of either 80Hz or 120Hz crossover points. The main output may then be switched to full range or to the HF part of the spectrum (above the crossover point) only; the subwoofer output has its own level control.

The control setup of the BoomBox is fairly straightforward, with separate level controls for the two synthesized sub‑bass bands (24‑36Hz and 36‑56Hz). Sub‑harmonics acts as an overall level control for the bass enhancement, while an additional LF Boost control acts as an equaliser to bolster up any program material that falls between the existing mid/bass information and the newly added subharmonics. A bypass switch completes the control layout.

In Practice

The first thing to be aware of is that the sub‑harmonics produced by this device are well below the range of a typical near‑field monitor, so the only obvious difference you might notice is the sound of your speaker cones banging against their end stops. If you are going to add sub‑bass to your mix, you really need to be monitoring on speakers capable of working properly down to at least 45Hz, and preferably lower. For the purposes of this review I used a pair of high quality, mid‑sized monitors and found the result could be appreciated reasonably well, though to get the full 'lunch‑loosening' effect, you really need a club PA system!

I expected to get good results on bass and drum sounds, and in this area I wasn't disappointed, but what really surprised me is how well the process works on complete mixes. Even on smaller speakers, you can hear the bass end take on a new solidity — though, to be honest, the lower of the two frequency bands comes in at levels that only a monster PA or monitoring system could reproduce properly. Used carefully, the 120XP can be used to fine‑tune the spectral balance of a mix, and in this context it might be advantageous to use it in combination with an exciter of some kind. In fact, it surprises me that dbx haven't taken the opportunity presented by the redesign to add a high frequency enhancer.

As when using an exciter, it's easy to get used to the effect and then add too much, so it's advisable to bring yourself back to earth on a regular basis by using the Bypass button. It's vital to keep the end‑user's music replay system firmly in mind when mixing with this device; don't go adding oodles of sub‑bass to mixes that are likely to be played back on bedsit hi‑fi systems. On the other hand, a dance mix destined for the club circuit could really benefit from a spot of seismic punishment — the only uncertainty is whether the club might be using a dbx 120XP on their own PA, in which case your music could end up with a double dose!

So, do you need one? If you can be trusted not to over‑use it, then this device is a very useful ally when trying to beef up thin bass sounds from synths or samplers. It's also another weapon in the creative armoury if you're trying to remix somebody else's material. If you want your mixes to reach an all‑time 'low', this has to be the box to do the trick.

How It Works

The bass enhancement circuitry of the Boom Box is both simple and ingenious, operating entirely in the analogue domain. The two frequency bands from which the sub‑bass information is derived are extracted by means of bandpass filters, and the filtered signals used to operate 'bi‑stable' flip‑flops which produce a square wave one octave below the input frequency — rather like conventional octave divider pedals. The clever part is that the square waves aren't used as sound sources; instead they're used to switch the phase of the filtered sound, which results in a reconstructed waveform with the same amplitude characteristics as the original. Further filtering removes any undesirable harmonics added by the process, before the new waveforms are added back into the original signal. Because very low frequency sounds convey very little directional information, the added sub‑bass information is mono. I'm sure that the original Boom Box worked on four frequency bands, but the new model doesn't seem any less effective for being simplified.


  • Easy to use.
  • Effective on single sounds or mixes.


  • The full effect can't be properly evaluated on anything other than a full‑range monitoring system.


Though a good general‑purpose sound‑shaping tool, the dbx 120XP is probably of most benefit to those mixing for the clubs or those operating club PA systems.