... and how to fend it off. Ben Duncan plays the shifter in the search for an on-stage spot, without howlround.

WHENEVER two or three musicians are gathered together to make live music in a confined space, they leak. That is, some of their rampant sound energy finds its way into the mikes, whether that's intended or otherwise. Microseconds later it comes back out of the speakers. When it re-emerges it adds to the general field of sound energy in the room which is again picked up by the mikes to be recircled, this time a little stronger.

In effect, the music is chasing its own tail and will eventually lock into a troublesome circuit — mike to speaker to air to mike, etc — excluding the musicians.

If the sound level is high enough at any particular frequency, the circuit becomes self sustaining, signalled by the arrival of a giant howl, or shriek. It's unnerving because it doesn't always stop just because you've laid down your instruments.

Howlround is a positive feedback circuit, but it need never happen. It's nature's way of saying that you can only play with high energy sound if your PA is properly tuned up. The sound emanating directly from your PA speakers should have a reasonably smooth frequency response, only deviating by 3 or 4dB from a straight line graph.

If you were outdoors, and put a mike with a smooth, uncoloured response directly in line with the cabs ('on axis to'), you'd find that the mike level could go very high before howlround threatened. Back indoors though, the unholy combination of the typical PA speaker's irregular sound dispersion, a mike (or mikes) with a bright and peaky response, plus the machinations of room acoustics, all help produce a series of massive dips and peaks in the music's frequencies — some of them 10 or 15dB high. Of these, it's the biggest peaks which will first set the howlround into action. And the higher the peak, the less overall sound level we can get before screeching the house down.

Acoustic Tune-up tips

There are two ways of dealing with shriekbacks: before, or during. Fighting duels with howling mikes in the middle of a gig is a classic; we dealt with it under 'Mixing for the Small Gig, Part 2' (in MM's September '86 issue). Otherwise, we can tune-up the acoustics before the band play.

Once the PA is set up, but before the soundcheck, wind the mike level up on the mixer, so it's as loud as it's going to be when the gig's at full swing. Pick the vocalist's mike off the stand, and walk around the stage. Once you've found a squeak or boom, how are you holding the mike? If your hands are covering any vents or slots down the side, try clasping it so they're NOT obscured, and the howl could well vanish. If it's part of the vocalist's "thing" to lean over the front of the stage, or swirl the mike around, then do it, the idea being to find out ahead if there are any howling bad places for the vocalist to be.

Having found a position that's prone to feedback, you may be able to alleviate it by moving the PA speakers forwards, or pointing them away from a reflective surface — like a bare wall. Presuming it's not too much hassle, turning the speakers on their sides, raising or lowering them, even turning them upside down, can all be tried.

Unexpected villains of howlround include mouth organs and saxes. With their hard metal exterior acting as an efficient reflector of sound, they can easily precipitate a loud squeal when brought up close to the mike. And it's bad practice to push a brass instrument's bell over the mike, but who's to say that won't happen in a moment of euphoric enthusiasm? So test it out — at worst, you can forewarn the vocalist/saxophonist. If one of the band picks up a shiny acoustic instrument half way through the set, don't forget to try that out during the soundcheck as well.

The frequency (tone) of the howlround tells us about its origins. Is it a boom, howl or squeak? These signify low, midrange and high end feedback respectively, giving us a clue as to which EQ knob to reach for.

Before 'EQing' (ie adjusting any tone controls) on a full scale PA, think carefully about the likely causes of the howlround. If the problem is definitely confined to one instrument or mike, try changing the mike. Next, make full use of the EQ on the corresponding mixer channel, especially if the frequency is sweepable. By doing this, we're not affecting the tonality of all the instruments. If the channel EQ is limited to a pair of bass and treble knobs, it'll probably not cope with a bad midrange feedback. But say it's a mild case: then a small twist towards 'cut' (-dBs) is worth a try.

Whenever trying to EQ-out a howlround, the question is 'am I getting anywhere?' To test for effectiveness, raise the main fader very gently, and hold it to the point just below where the howl starts. If your colleague now speaks into the mike it'll have a curious 'singing' tone, or maybe slightly Dalekish. If your EQ adjustments make any sense, the sound will return to normal. Now see how far you can (gently!) push the fader up before the ringing tone builds up again. [Score: one mark for every extra dB; time allowed: 3½ minutes]. Of course, if you deviate, the PA will rise into a heavy screech. If this happens, be sure to protect your ears and the PA by bringing the fader(s) sharply down to the bottom. Then you can bring them gently up again.

If, on the other hand the PA just does its nut when turned up to a certain sound level, irrespective of which mike or instrument is turned up or down, this signifies that the overall acoustics — or the PA (or monitor) cabs — are guilty. So, this time, we want to EQ the PA's main outputs — the so called 'house EQ'.

Some of the better monitor and outfront mixers have sweep or parametric EQs built into their output sections. Otherwise, it's common enough to find a graphic equaliser sitting next to the mixer and wired in line with its outputs. Then we just carry on EQ'ing, as above — aiming to keep raising the main fader as we gradually 'tune out' the ringing edge. EXCEPT that to be at all useful, this needs to be a 27 band (⅓rd octave) model. Then the cure can be pretty specific, important because the acoustically generated peaks we're aiming to quell will be very narrow.

No free lunch

Despite having up to 15dB of boost or cut, any attempt to make full use of the fader travel is doomed to sound awful. You can always spot the work of a skilled sound engineer by the way his/her graphics are adjusted:

★ Always cut — never boost (Obvious, really?)
★ The majority of faders will be in a straight(ish) line.

If you have to 'cut' any fader below -10dB, something is wrong. Have you exhausted all the acoustic/miking possibilities? With these principles in mind, it's easier to suss why fully tunable parametric equalisers are favoured for howlround elimination; something you can plug into one of the mixer's channels if you have real bad problems. It might only have two or four bands, but it's a winner for cleaning up the worst of the howl-round at the usual one or two spot frequencies. Once you've learnt how to tune it, it'll have a less adverse effect on the overall sound than an over-adjusted graphic EQ. And maybe cost less.