Building the bass

Ask someone who runs a sound how loud it is and they will usually look slightly pained. Loudness is not the target. What they are after is a particular relationship between the bass and everything above it, and a particular way the low end loads a room. Getting there is a building project as much as an audio one.

Split the range, use the right box

The defining choice is to split the frequency range and give each part its own enclosure, each shaped for the job.

The bottom belongs to the scoop: a large folded horn bin, usually built in pairs, that loads a driver into a long tapering path. It is inefficient in the sense that it is enormous, and extremely efficient in the sense that it converts amplifier power into moving air far better than a sealed box of the same size. This is where the frequencies you feel in your chest come from.

Above that sit mid range bins and horns, often mounted separately and aimed. Because each box only has to handle a narrow slice, none of them are asked to do something they are bad at. That is the whole trick, and it is why a well built sound can be very loud and still comfortable to stand in front of for six hours.

Amplification and the pre-amp

Power gets split the same way, with separate amplifiers for each band. Plenty of crews keep valve amplifiers on the bass, not out of nostalgia but because of how they behave when pushed: the distortion arrives gradually and sounds warm rather than brittle.

The pre-amp is the sound’s personality. These are frequently custom built, one of a kind units with wide, dramatic controls designed for live use, not for careful studio balance. When a selector sweeps the bass out of a tune and slams it back in, that is the pre-amp doing what it was built for. Two sounds playing the same record through different pre-amps do not sound like the same record.

Tuning is done in the room, on the night

None of this is set and forget. Every hall behaves differently. Corners load the bass, low ceilings smear the mids, a full room absorbs high end that an empty one throws straight back. Crews arrive hours early, put a familiar record on, and walk the room listening. Adjustments get made by ear, at the amps and the pre-amp, until it sits right.

If you are thinking of building

Three pieces of advice come up again and again from people who have done it.

• Build fewer boxes and build them properly. Two well made scoops beat four bad ones.
• Buy the best drivers you can and treat the cabinets as the cheap part. The cabinet is woodwork; the driver is the instrument.
• Spend real time on power distribution and cable. Most disasters at a dance are electrical, not acoustic, and they are all avoidable.

Above all, go and stand in front of other people’s sounds first, and ask questions. This is a culture that has always taught by showing, and almost nobody who has built a sound is unwilling to talk about it.