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Still Going Deaf? (Part 2)

Article from One Two Testing, January 1985

earstrain part two


Can volume kill your ears? Ben Duncan, who has stood before plenty of 20k stacks and can still hear the dicky birds, concludes his study on loud science.

Last month we introduced the odious decibel, and explained its relevance to the perceived loudness — and inferred danger — of a sound. Table 1 (below) puts sonic decibels (dB) into perspective — or does it?

The simplest sound — an unwavering note or tone — can be accurately described as so many decibels. But the arabesque of sounds in music, with pitch, level and harmonic content all varying in time, aren't so easily pinned down. The second snag is the (inevitable) difference between "objective" meter readings and the ears' own soundings.

To account for these disparities, people in white coats have evolved the following variants on the basic decibel thang: dBA, dBB, dBC, dBD, (etc.). These are not abbreviations for various line-ups of The dub Band, but weighted decibels. They're needed because our hearing doesn't have a 'flat' response, and is much more sensitive to midrange sounds than deep barse, or shimmering top-end. Worse still, the differences in our sensitivity across the frequency band varies with the sound level.

dBA means "Decibel weighting, A", and here, the low and high frequencies are filtered out to correspond with our perception at low sound levels, below 40dB. Similarly dBB was the weighting used for intermediate levels up to 70dB, while dBC means 'no filtering in use' because above 70dB, our response to highs, lows and mids evens-out.

Many readers will have spotted dBA (SPL) figures bandied about in articles on live music — "I put on my skin-tight white coat, took aim with my decibel meter, and feverishly studied the scale as the needle hovered around 108dB(A) (SPL)... aarrhhhh!" — but less often seen the dBC which is surely the one to use for Rock's mega loudness... er... SPLs.

The answer, believe it or not, is that the dBA, intended for sound levels beneath 40dB, was observed long ago to best correlate with the subjective (sic) irritation of an (unspecified) noise. For this reason, 'A' weighted decibels are routinely used for environmental measurements, and are unthinkingly applied to Rock as well. Not only does this wildly alter the actual dB readings (eg: for a bass-heavy sound, 35dBA = 60dBC. This represents a 300-fold change in energy at the level being discussed!) but also and most unfairly identifies Rock as a noise, because it is being measured in terms of 'nuisance level'. This applies even if we are not discussing outraged residents living near a venue (where nuisance value is relevant), but evaluating the much higher sound levels that may endanger hearing.

This is specious, because when we perceive a set of sounds as music the subjective loudness falls. In other words, we tolerate a higher SPL, and downrate the 'annoyance' of loud music in contrast to an irksome cacophony which has a similar dBA rating. The dBA is therefore arguably out-of-order when applied to music and statements such as, "A sound level of 96dBA can be safely tolerated for about an hour a day without risk of hearing damage", are highly dubious when the psychological subtleties can so readily swamp-out this fake 'scientific' precision.

IN YE TIME DOMAIN



'Pure' noise, the white or pink type used in synths, sounds rough and 'edgy'. Unlike a continuous tone, its level at any particular instant cannot be predicted — it has a gaussian randomness. Nevertheless, the SPL of pink noise (say) averaged out over a minute or so reveals a pretty stable mean value — it just goes shooooshhhhhh, on and on. Music, though, isn't like this — its peaks and perturbations are non-gaussian and uncorrelated — and even if they mean a lot emotionally, they defy meaningful statistical analysis. In short, you can't simply average-out the sound level of a 90 minute concert, and go on to compare it to another band or another gig, especially if the music and duration are different.

In practice these objections rather put the boot into any meaningful data on "how loud?", "who got hurt?", and "whose concerts represent the greatest danger to human hearing?", although hearing-loss researchers make much use of an esoteric, average SPL measurement called LEQ.

But there's one useful, ballpark observation we can make — the loudest sounds in music tend to occur for short periods only. These are called transients and tend to exceed the nominal, average level (as measured over approx ½ second by an ordinary VU meter) by around 6 to 20dB, all depending on the music, of course. For Status Quo this 'peak-to-mean' ratio was around 6dB, whereas more percussive, grandiose or orchestral works peak about 12 to 20 dB above the ⅓rd second average. This leads on to why you need peak (of 'VU' or average) level metering to record decently. But even peak meters don't usually read actual instantaneous levels. Instead, the peak reading is really an average, taken over 5ms (5 thousands of a second), on the basis that a transient which lasts for less than a few milliseconds isn't relevant, however big, because the ear can't register such a short-lived sound.

The relevance of true peak SPL measurements is that they record what's actually out there, while any kind of average imposes a conceptual bias on the figures. Trouble is, most SPL's are quoted as mean or average levels, rendering them less than (i) truthful and (ii) useful. Awareness of peak versus average level arguments also highlights the differences between the professionals and the white-coats. When Doug Hall tells us there's 140dB at his kick drum mike, he means that peak level is really out there, ready to overload the desk and crunch-up the sound, and if it only happens for short periods at a time — albeit frequently — well to hell with the en'viron'mental nosey-parkers with their decibel meters.

The good news is that we can do a rough check on the average reading by simply subtracting approximately 10 to 20dB — so the mean kickdrum level is actually 120 to 130dB, still loud, but less worthy of hysteria. Conversely, you can convert the mean SPLs given in the table to peak values, but only for music.

The peak-to-mean ratio for all the other sounds listed (WW II battlefield included) are not necessarily the same. Incidentally, a battlefield sound level can easily reach 150dB+ peak levels, but whereas many WW II soldiers have undoubtedly suffered hearing loss, there's no evidence that these truly awesome levels are directly responsible for any deaths. On this basis you can now sleep safely at night, in the knowledge that 140dB (peak) of kick-drum is unlikely to strike you dead on stage.

TABLE THUMPING TIME



Getting dogmatic about the sound levels that represent a danger to hearing is naughty, not least because the 1930's, 40's and 50's psycho-acoustics data that's relied upon is itself an average and moreover, out-of-date. To wit, the limits of our aural perception recede just like the time taken to run a mile — in the 19th century doctors predicted that railway train speeds in excess of 25mph would be lethal.

But enough stirring, here's a fresh conjecture to mull over. Last month, I Implied that fatigue causes, or at least aggravates, potential hearing loss. So what makes us fatigued? Number one scapegoat is distorted sound. Here, I mean distortion in its broadest sense including bad colouration. And if it isn't rejected out-of-hand as noise, the sound of a heavily overdriven PA or monitoring system must somehow represent music, and our minds will work hard at synthesising the anticipated melody from the aural equivalent of a delicate watercolour painting that's been urinated upon.

The need for the rational, left-hand brain lobe to work hard at error-correction forbids any relaxation — presto, loud music winds us up when it's distorted. Overloaded, 'clipped' transistor amps also translate midband tones into high-frequency transients: smooth transitions turn into sudden, spikey energy (like WW II artillery), and these are very reasonably considered to be bad news.

This dovetails neatly with the experiences of many sound engineers: cleanly reproduced music doesn't sound 'loud', doesn't hurt peoples' ears, doesn't give rise to the tell-tale signs of excess (like ringing ears or temporary deafness), and doesn't apparently threaten the continued wholesome perception of music.

A sure sign that the music is happening properly is if you feel you could converse through it but when you open your mouth the music transcends mere words. I mean, you can't bloody well hear me, can you? (The men in tight white coats don ear-muffs, burst into the studio control room and put axes through the monitors — a scene from 'Decibel 451'.)

ACTIONS



In this polemic, no one is out to deny that very high SPLs can damage or curtail our hearing, but instead, make you aware that dangerous SPLs aren't so simply defined (eg "120dB will cause instantaneous death/deafness"), nor is it so easy to separate cause and effect. For example, more than a few studio engineers have developed defective hearing, but then the majority of studios are equipped with certain XYZ monitors which are emphatically PAINFUL when driven at high levels.

So does colouration and intermodulation distortion damage the hearing directly or is it a byproduct of the stress caused by distorted, coloured, 'shouty' monitors? And if you already have trouble hearing 6kHz? Well, that's easy: let's boost the 6K on the graphics by 12dB, and drive the monitoring heavily into overload. Result: more fatigue and strained looks from all the people who've managed to retain their hearing. So here, to conclude is a holistic approach that sidesteps 'scientific' dogma and reactionary hysteria.

1. If loud music coincides with fatigue or irritation (they needn't be causally connected), then try to skip the extra decibels until you're feeling more in control or more laid back.

2. If loud music hurts you or causes buzzing, rasping or other untoward sounds in your ears, avoid it or take steps to clean up distortion or colouration in the PA or monitoring which is more-than-likely responsible for these unpleasant effects. Ironically, our ears alone introduce considerable distortion at levels over 90dB(C!), and this effect is commonly felt to be pleasurable. This may explain a craving for loud music, but the internal 'distortion' is primarily a strong dose of the 2nd harmonic not the random and dissonant sounds that issue from poorly executed and overloaded sound systems.

3. When music as a vocation hots up, and you're naturally keen to push yourself to the limits try to balance this by treating yours ears sympathetically. Broadly, this means curtailing irksome and irrelevant environmental noise as much as possible — it's like saying, "dear ears, I'm going to make you work damn hard tonight on stage — it's gonna be 120dB all the way, OK? Then afterwards I'm going to treat you to a dead quiet night in the countryside or the hush of a studio control room/heavily carpeted hotel so bear with me!"

4. If you're unlucky enough to suffer 'Threshold Shift' (mild, temporary deafness) (i) be prepared to give your ears a long-term rest — several weeks or months in quieter than usual surroundings may be needed, and (ii) try to identify the cause of dislocation — the person who owns the sound system may be too far gone to realise the damage(s) he's caused, or (iii) perhaps I should shut up.

DISTORTION FX-1

With harmonic distortion the equipment produces harmonically related garbage, akin to a musical instrument. In small doses this is pleasant (eg: tube amps), but still deviates from the intended music. Also, odd-numbered high order harmonics (eg: 9th harmonic = 9x the fundamental frequency) are very energetic and apt to be unpleasant in anything other than tiny doses.

Women are very sensitive to intermodulation distortion. Equipment guilty of this throws out a random assortment of thousands of sum 'n' difference frequencies. Few of these are harmonically related to the original tone or note — most are dissonant and rapidly bring on fatigue or wind you up. Interestingly enough, intermodulation distortion is primarily something that gets out of hand as equipment approaches its limits. It would seem that researchers have confused high SPLs with the intermodulation distortion that frequently accompanies high level sound reproduction, whenever the equipment can't cope.

Colouration is the vice of sound systems which, like our ears, don't reproduce all frequencies equally and thereby place an unnatural focus or emphasis on some of the harmonics in the music, radically distorting its direction and meaning.

The brain can readily correct for the ears' idiosyncracies, whereas correcting the uneven response of external things like speakers and mikes is hard work, if not impossible. Ever had your pix taken under a lime tree in summer? So was your face really green? In real-time our mind can correct for the green light (equivalent to severe midrange colouration!) but the film records what's 'out there', and once set down, our vision is unable to compensate for the effect. Speakers which bite, screech, splash or boof-boof are akin to bright yellow, blue, purple and crimson lighting respectively, and, are similarly fatiguing even if the effect is novel and exciting in the short run.

NB: All ratings are plus or minus 20dB, depending on whether levels are Peak or Average; dB(A) or dB(C); etc


Series - "Deaf Defying"

This is the last part in this series. The first article in this series is:

Deaf Defying
(12T Dec 84)


All parts in this series:

Part 1 | Part 2 (Viewing)


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Heroes

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Who's That Man?


Publisher: One Two Testing - IPC Magazines Ltd, Northern & Shell Ltd.

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One Two Testing - Jan 1985

Topic:

Live


Series:

Deaf Defying

Part 1 | Part 2 (Viewing)


Feature by Ben Duncan

Previous article in this issue:

> Heroes

Next article in this issue:

> Who's That Man?


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