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Richard Cranium
 
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February 1, 2004, 06:50 AM

http://hope4hearing.org/motorcycle1.htm

Motorcycles; Helmet Noise
Motorcyclist magazine, November 1991
HELMET NOISE
A good helmet is a quiet helmet
by Andy Saunders


The body reacts to noise as a danger signal -- our nervous systems haven't had time to evolve from the time when the loudest noise you'd hear would be a mammoth charging it you. When we're exposed to loud noise, blood pressure rises, heart rate and breathing speed up, muscles tense, perspiration increases and hormones are released into the bloodstream. It's in your best interest to keep the noise in your life to a minimum.

Everyone has felt ringing in their years after a loud concert or a party; continued abuse of your ears makes that ringing -- called tinnitus -- permanent. Although loud noise can damage your ears "usually irreparably", it affects the heart most. Blood vessels constrict, pressure rises, and cholesterol and triglycerides also rise. The increased hormonal activity steps up the secretion of acid in the stomach, possibly causing ulcers.

Loud sounds also affect the reproductive system in a manner similar to alcohol: increasing sexual drive while decreasing potency. In addition, loud sounds affect the level of white blood cells and gamma globulin in the bloodstream, lowering the efficiency of the immune system.

Sound is measured two ways: by level and by the frequencies that make up that level. The adolescent human ear can hear sound waves at levels between a low 20 cycles per second, or 20 hertz (Hz), and high of 20,000 CPS, or 20 kilohertz (kHz). Low frequencies are a dull rumble, high frequencies a shrill whine. As we get older, frequency response drops. A middle-aged person may not hear beyond 10,000 Hz (10 kHz). The human ear is most sensitive to sounds in the 1 to 4 kHz range, the usual frequencies of human speech, and damage occurs first in these areas.

Hearing loss is a gradual process unnoticed by the loser. Noise doesn't have to hurt your ears to hurt your hearing: the Occupational Safety and Health Administration advises that habitual exposures to 85 dB for more than eight hours cause hearing loss. If you are regularly exposed to noise at the 95 dB level (10 times as loud), one hour is about the limit. Above 105 dB, your hearing can be damaged by repeated 15-minute journeys.

Motorcyclists risk hearing loss. Noise is the constant companion of a motorcyclist, no matter how quiet the bike. Pushing any projectile through the air at speed creates turbulence and noise, whether that projectile is a rifle bullet, an airplane or a human head. Riding with a bucket on your head creates more turbulence at the mouth of the bucket, and more noise. Some -- though very few -- full-face helmets can be noisier than no helmet at all.

Even the best helmets can't close out all the wind noise, so if you ride long distances frequently, you need to wear ear protection. The most convenient, the foam earplugs of the E.A.R. type, cut sound-pressure levels by 30 dB or so when properly inserted, and nearly every California motojjournalist uses them. Unfortunately, they are all breaking the law.

California's vehicle code states "No person operating a motor vehicle or bicycle shall wear any headset covering, or earplugs in both ears. The prohibition does not apply to any person wearing hearing protectors in the form of custom earplugs or molds that are designed in a manner so as to not inhibit the wearer's ability to hear a siren or horn from an emergency vehicle or a horn from another motor vehicle".

Of course, you can buy custom ear plugs, but they are uncomfortable inside a well-fitting full-face helmet and expensive at around $100 a set.

Avoiding damage to your brain is more important than avoiding damage to your hearing, but by selecting the right helmet, you can do both.
The results are interesting. The quietest helmet at 60 mph is much quieter than the loudest at 30, and the loudest helmet has the potential to damage hearing after quite a short exposure. At 30 mph, the padding of the open-face Bell was springy enough to effectively insulate the ears. At 60, the padding had deflected enough to make it one of the noisiest helmets.

Due to the difference between the human ear and electronic microphones, we expected the noise test results to disagree with tester's subjective evaluation -- but we were pleasantly surprised. With a couple of notable exceptions, what we heard is what the machine got.

The exceptions: the Kiwi K22 seems quieter than it is, the Sure louder. The Kiwi seems quieter because all its noise is low in frequency, and even at 60 mph the frequency level doesn't intrude much into the speech band of 1- 4 kHz. The Sure has exactly the same dB level at 60 mph and in fact is quieter at 30 than the Kiwi, but it seems louder and more unpleasant because a significant component of its noise occurs in the speech band.

We expected that our favorite, high-priced helmets would do a well on this test, and we were right. As a rule, the higher the price, the better padding and noise protection. The exceptions were AGV's Phantom and Thema, where price seems directly proportional to noise level. (But these helmets are also light, with large eye ports)

Shoei's X8, touted as the quietest, most aerodynamic helmet on the market, is quiet because it has more sound deadening padding in the comfort liner in this sized to fit very tightly. Our tester, who is on the borderline between large and extra large in Shoei sizes, found he was more comfortable in a large model RF200 than an extra large X-8; the RF200 was quieter too.

Would riding without a helmet do more damage to your hearing? Yes. Research shows that almost any open- or full-face helmet attenuates enough sound to afford some hearing protection. Shorty helmets are a real hearing risk, and motorcycle policemen may suffer significant hearing loss after years of wearing one (which should give cops who want better helmets a strong case)

A Helmet that has less sound insulation will not necessarily allow you to hear more at highway speeds. For a specific sound be audible, the signal-to-noise ratio would theoretically have to be greater than 1-to-1. In other words, if you are experiencing a 110 dB roar in your helmet, any siren or horn will have to be louder than 110 dB for you to hear it (in practice, sirens operate at high frequencies, so you may hear one even when the signal to noise ratio is less than 1-to-1). Earplugs effectively dampen wind noise, yet allow shrill sirens to be heard clearly. However, if you drive in very slow traffic most of the time, where you need to notice external environmental sounds, a quiet helmet with thick padding may mask some of these essential sounds.

These tests were performed without engine or external traffic noise, so they represent the minimum noise present at those speeds. Engine noise and external traffic noise add to the racket inside your helmet, though you'll notice it more at 30 mph than at 60 mph. Helmets like the Sure S8-N, AGV Sukhoi, Bieffe B3R, C. T. S., Fulmer, Hondaline, MDS, Nolan N35 and Jeb's all sounded louder than their rankings would suggest. None of these are fiberglass lids, suggesting that the type of material used in construction may play a part or that the cost of materials has an effect. Certainly, the fit between the face shield and shell was important. The wind whistled through the gaps in a the Sure's shield, adding to the cacophony.

The plastic helmets were also at a relative disadvantage due to their single shell size. Helmets sized large or extra large were used exclusively for this test, and the polycarbonate lids have only one size shell for all head sizes. Thus, extra small wearers get much more EPS and comfort-liner - and more sound attenuation.

How much are you at risk? If you wear a well fitting, quiet helmet on short journeys to work every day, you'll probably never need ear protection. But motorcyclists who ride often or far -- couriers, policemen, motojournalists and high-mileage touring riders -- do need protection

You may find that a helmet-closure device, like Noj's chin shield or Lockhart's Apple Warmer, will cut down the level of sound reaching your ears, although the face shield will fog up more easily and you'll experience more heat in warm weather.

A future generation of helmets is likely to offer more active noise-reduction (ANR) technology, currently in use in airplane headsets. ANR works by generating mirror-image sound waves, waves of opposite phase, for every component of the environment's noise. The two sound waves cancel each other out. Since even the best current ANR system takes a few seconds to precisely analyze and duplicate the sound wave components of noise, ANR will work best on touring machines, where noise remains constant. Sport-bike owners, don't throw away your earplugs. Recording the sound
by Andy Saunders

We wanted to measure sound level at the ear in a full-face helmet, excluding all variables that could distort the test. Using a dummy head in a wind tunnel was considered but rejected as to unrealistic.

To make the results as objective as possible and eliminate mistake causing variables, we decided to test all the helmets on the same standard motorcycle without a fairing (a fairing creates turbulence and can increase wind noise). We didn't want the noise of the engine to be included. We thought of putting the motorcycle and rider in the back of a pickup truck at steady speed on the freeway, but then the relative wind would be affected by the aerodynamics of the truck and the highway patrol would get interested very fast.

We were running out of ideas, then thought, Why not use a downhill stretch of road, long enough to cruise at 30 mph and 60 mph with the engine off? Then the only noise would be wind, tires and brakes. Luckily, we found canyon road that fit our needs perfectly: a private road leading to a new housing development, with almost no traffic, it was a mile long and newly surfaced, with enough downhill grade (over 10 percent) to get up to 60 mph halfway down. A bonus was the line of trees on either side, which sheltered the road from wind and extraneous noise.

A Suzuki GSX1100G was the test bike of choice -- a simple standard with no wind blocking fairing and heavy enough together speed quickly downhill. The tester took pains to sit in exactly the same position, with his head aligned the same angle, for each downhill trip. For consistency, the same tester performed all tests.

The technique was to accelerate in the first gear at the top of the hill, hit 30 mph, cut the motor and cruise at 30 for an eighth of a mile. Then let off the brakes and accelerate downhill, still with a dead engine, to 60 mph, hold 60 for a quarter mile and let the recording equipment do the rest.

The recording gear, engineered by Patrick Moriarity of Rolling Sound Productions, was a Hollywood-standard Nagra 4.2L mono synk recorder, which uses huge 7-in. reels of tape but is still portable enough to be carried in a shoulder pack. It was connected through to preamps to a pair of Tram TR-50 lavalier microphones one in each ear. The omni-directional electret condenser lavalier microphones are shirt-button sized yet accurate with frequency response from 40 to 16,000 Hz. They were turned to face into the ear canal and were insulated from drafts within the helmet with open cell foam. Microphones are very sensitive to wind gusts, so great care was taken in mounting and no helmetless recordings were made. The microphone padding foam was taped to the tester's ear with surgical tape. Tests were completed in two afternoon sessions (the tester swearing he was never going to let anyone tape his ears again).

Each helmet was put on very carefully to leave the microphones undisturbed. Only one helmet, the Bieffe KeiBi8, was so tight at the ears that it could not be worn. We only had Fulmer HT 60 II and HT 70 helmets in medium and large sizes -- too small for our extra large tester. One open-faced helmet, a Bell Magnum, was included in the test to compare sound levels. (In one test in the late '70s, the Magnum was quieter than the contemporary full-face lids, but the best of today's full-coverage helmets are much quieter).

After each batch of runs, the tape was played back to ensure the sounds recorded jibed with the rider's recollections of the run. The tapes were analyzed on an lvie electronics spectrum analyzer, which shows a breakdown of the sound according to frequency and aggregate decibel reading

Sound pressure level (SPL) is measured in decibels (dB) and is an aggregate of sound over all discernible frequencies. The decibel scale is logarithmic, so 100 decibels is 10 times louder than 90 decibels and 110 decibels is a hundred times louder.

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February 9, 2004, 10:17 PM

Good article, thanks.


Ride your ride and be safe this season.
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