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Things that go Wump...About Mic-Handling Noise

by Chris Woolf

If there is one complaint that I hear more often than any other when location microphones are being discussed it is "handling noise". And although almost every recordist has suffered it and would love to have it cured, whenever you pin a patient down to describe the symptoms they become strangely vague.

The explanation is that "handling noise" is not a single malady but an unsubtle mixture of half-a-dozen or more and, while lying down in a dark room might be appropriate first aid for some sufferers, it is often better to get them to go into a quiet one and listen carefully. With a pair of headphones and a step by step approach each element of the "handling noise" can be analysed, its source found and its transmission route recognised. 20 minutes of logical listening can produce some reliable cures.

Wumps

Directional microphones sense the pressure gradient across their capsule ports, a difference that is so small at low frequencies that the diaphragm must be stretched very loosely. Although this makes the device sensitive to minute pressure differences next to the static microphone body the reverse also applies. If the body of the microphone is moved the diaphragm will flap briefly because of its inertia (and that of the lumps of air on either side of it). The accelerations often met during fast-moving location work can produce enormous peaks in the infrasonic range - a few Hz - which are themselves, by definition, inaudible but which will overload almost any audio path. Typical effects when wagging the microphone are a flat "wump" followed by a brief silence, violent VU needle wagging (but not PPMs which should be immune to out-of-band signals) and apparently spurious peak LED warnings.

The cure is a bass cut filter as near to the capsule as possible. Many microphones have a fixed one, often with a rather high cut-off point to counter other defects, and that may be the explanation why these microphones are never accused of handling noise but sound rather thin. Some of the better ones have a switchable filter. Others, including small remote capsule types, have no room for this to be fitted and so expect to see a high-pass filter (HPF) in the mixer but this must be located before any transformers or active circuitry. Some older mixers, non-location versions and direct input recorders like Betacams or DATs may not be able to come up with the goods in which case you will need an add-on accessory HPF. The shape of the filter curve is a matter of taste - sharp cut-off third-orders preserve the flattest response but can sound hard, gentle first-orders have to have a higher corner frequency but can be less obtrusive. For recordists who cringe at the thought of losing any audio information I will point out that a bass droop can be re-equalised to what it should have been but an infrasonic overload is a total loss...

Rattling and Clacking

If you take your microphone out of its mount and shake it gently on each axis while still listening to its output you may well find it rattles. This may come as a bit of a shock if you have paid £1000 or more for it but it is very common. Circuit boards and rubber bushes may be a poor fit and flop about, and XLR connector pins and their mating sockets are notorious clackers, as are the spring catches. These noises may be quite faint and only apparent when the microphone is held at a particular angle. They add up to noisy handling but may be surprisingly difficult to single out. I would not advocate rebuilding microphones but low-temperature hot-melt glue inside the XLR connector to hold the socket inserts in place works well and for the catch, sticky tape. Not ghastly gaffer, please, which oozes everywhere after a week - try black plastic silage bag tape.

Squeaks and Twangs

The transmission paths for the noises so far have been directly through the body of the microphone but once it is fixed in an elastic suspension system everything becomes more complex. Although the suspension is designed to isolate the body of the microphone mechanically it will not have the same efficiency at all frequencies and along all axes. At very low frequencies the mount will have a resonance which will transmit and accentuate unwanted vibration rather than reduce it. For M/S addicts whose microphone diaphragms are at 90† to each other the resonance will be at quite different frequencies - higher for the S in most mounts - which is a partial explanation for the increased "handling noise" that often comes from the fig-8. At frequencies where the mechanical isolation might be expected to be quite good its performance will often be handicapped by an acoustic transmission path that short-circuits the structural one.

If the path of the noise cannot be interrupted easily then the source needs to be tackled . Fit the microphone into its cradle and test the mount with its shockcord, o-rings, plastic springs, rubber bushings or whatever by leaning the microphone as far as it will go in every direction. Then tap each part lightly with a pencil. Listen for rasping or squeaking noises as the parts of the suspension move against each other, twanging as stretched parts vibrate or ringing from metal parts (including the microphone body tube). You may recognise some interesting coloration sounds that you have heard in recordings but not realised where they came from. You cannot eradicate all these but replacement of the elastic parts, which inevitably change with age, may well improve things. And don't forget that "other" part of the suspension.

Wiggly Wires

The cable. If you thought that this was merely a means of electrically connecting the microphone, think again. It does do that but it also acts as a (rather poor) elastic element in its own right (particularly with lighter microphones). It acts, with varying degrees of efficiency, as a conductor of sound from the handle or boom-pole to the microphone and it acts both as an electrical and an acoustic noise generator. The first attribute is necessary, the second is bearable but the rest are very much unwanted.

Disconnect the cable and simply bend a 20cm loop of it in your fingers. Feel how pliable it is - is it as dead as putty or does it kink easily and spring from one position to another? Now reconnect it and bend the same loop while holding the microphone absolutely still and listening to the output. You will certainly hear a lot of transmitted finger noise but also, quite possibly, the noise of the screening braid or the cores making a creaking or grating sound.

The internal structure of a cable varies enormously depending upon the desired function. For use as a "mic tail" ultra-flexible, ultra-quiet cables tend to have a lapped screen - a double wound Reussen type is the best - rather than a woven braid. Most are twin core but thin starquads have excellent electrical properties although the extra metal does make them stiffer. These cables usually have very fine stranded or tinsel cores but on some the stiffness of their PVC insulation detracts from the flexibility. The filler strands in the cable - those bits of string that seem to serve no purpose - are there to pack the cable so that the individual cores do not move in relation to each other and also to cushion the conductors. This is to prevent the cable itself behaving microphonically and generating a signal when moved. For the outer sheath most people pick extra-flexible PVC, polyurethane (better in cold weather) or soft woven fabric, and of course it will be thin - no more than 5mm OD. If what you have at the moment is a piece of foil-screen installation cable or a 20 year old stage lead with more kinks than the band it first heard please put it discretely in the bin.

The dressing of the cable is also important both to prevent it becoming too significant a part of the elastic suspension - a severe problem with light microphones and soft suspensions - but also to attenuate longitudinal transmission of sound waves. The ideal is a large swept curve - the limit is the point where there is a danger that the free loop will hit the suspension or part of any windshield. Form the loop, make sure the cable is clamped firmly at the base and then try tapping it downstream from the clamp. With a good, well-dressed, cable you will hear the tapping sound distantly through an acoustic path - a bad one will sound very close as the noise is transmitted mechanically through its almost rigid cores direct to the connector pins of the microphone.

Whistles, Whooshes and Rumblings

All recordists know to dress up a microphone in a windshield when they take it outside. The reduction of wind noise by up to 20-30dB can mean the difference between a good recording and none at all. They will also undress the microphone when they come back to work indoors - they want as little extra clobber as possible. In still-air situations this is the ideal but while you are in your quiet room try waving the microphone sideways and up and down. Use a filter to hide any inertial effects. Some microphones will generate a quiet whoosh or poorly tuned whistle, some may make a fluttering rumble. This is caused by air movement through or across the slots. The same sound will be produced when the microphone is held still and the air is being stirred by draughts. Like some of the rattle sounds it can be a low level signal, dependent upon direction, definitely a noise but not easily isolated amongst programme audio. The cure is a windshield, even indoors. A plain basket works well - if space is a problem and the air movement very gentle a foam one may be adequate. But never be tempted to leave the foam one on and put a basket over it: the wind suppression will be worse, not better.

Creaks and Rustles

Windshields can cause a few noises too - mostly creaking sounds when the shield structure is twisting during use. This will only happen if the section is under some stress and a possible cause is squashed or cracked shields that have been forced back into shape and are under permanent tension. These sounds, and the ones that could most accurately be described as true handling noise - those produced by the sound of fingers on the pistol grip or knocking the end of the boom-pole against the PA's clipboard - are both transmitted acoustically as well as mechanically.

One of the essential points of any practical microphone mount and windshield is that it should be light. On the end of several metres of boom-pole a few extra grams becomes an agony after a 5 minute take. This lightness makes mechanical isolation of the mount from its support very difficult. To isolate it adequately at low frequencies requires a soft coupling that can wobble an enormous distance - yet still hold the microphone steady. Various ingenious attempts have been tried and some work within limitations but the low frequencies are usually transmitted through the fixing point irrespective of the isolator. For higher ones the acoustic path can be more significant but the radiation may not be from the obvious source of the sound. If you hit the bottom of the pole the sound tends to be conducted up the pole and the acoustic source that the microphone hears may well be vibration of the bracket bolted to the top. Use your fingers to tap and rub the pistol grip or the boom-pole. If the rustling sounds are very distant then the isolation is good - you are hearing them from their real source. Further improvement can only be by suppression of the source - try a soft fabric grip on the pole. If they sound close - scratch closer parts of the mount to get a comparison - there is significant conduction. The cure is the same but the improvement will be greater. At least in this version of handling noise you have direct control over the generator.

This identification parade will not nail every culprit - some are too fly to be caught - and the facts of physics will always be a limitation but if handling noise has been a real problem for you it should have helped you minimise it. The only other important thing to remember is that having analysed your kit and cured its defects you then keep it quiet.

Written by Chris Woolf, technical consultant for Rycote Microphone Windshields.

This article was commissioned for Sound Pro magazine and published in the Oct 97 edition. Sound Pro is a now-extinct British audio journal.