Noise: A MO Investigation

It's everywhere you look!

..All the Who girls and boys
Would wake up bright and early. They'd rush for their toys!
And then! Oh, the noise! Oh, the noise! Noise! Noise! Noise!

- Theodor Seuss Geisel, How the Grinch Stole Christmas, by Dr. Seuss
New York: Random House, 1957.

The grinches at the National Institute of Health identify motorcycles, firearms and firecrackers as all sources of loud noise. Leave it to the NIH to write the prescription for fun. Perhaps as a motorcyclist you consider a hazard to your hearing trivial compared to the dangers presented by crack addled road clowns. Still, you can actually do something about this risk. I am sure you have been afflicted by the sneering self-righteous who examine your motorcycle and inquire: " you wear a helmet?" I just soak up their venom, duct tape them to a chair and state: "Thank you for your concern. I wear a helmet. I also wear earplugs. Let me explain..."

A Fiend's Delight: The Sears Catalog of 1897 also offered Peruvian Wine of Coca for a mere $1.00 per bottle. inquire: " you wear a helmet?" I just soak up their venom, duct tape them to a chair and state: "Thank you for your concern. I wear a helmet. I also wear earplugs. Let me explain..."

Part One: Sound - A Rustle In The Underbrush

Sound is a pressure wave. That pressure wave can be measured for intensity (watts/m2), and frequency (Hertz). For our purposes we will discuss sound intensity using decibels-which are in turn based on human hearing.

Human hearing has a surprising range to it. You can hear extremely faint sounds - from the threshold of hearing, or 0 decibels, all the merry way to the threshold of pain - 130dB. A sound at 130dB is ten trillion times more intense than a sound of 0dB. Unless you are Bill Gates or Carl Sagan, you are probably not comfortable flinging about billions, let alone trillions. Not a problem! The decibel scale is logarithmic and sops up all those pesky zeros.

Let's make this personal. Bob marinates in his cubicle, gazing lustfully at a bike review. Even with the background noise of the office (40dB) he becomes aware of an ominous sound. The sound in question comes from poorly fitted Italian loafers in the act of compressing carpet fibers (50dB). Suddenly Bob finds his cubicle besieged by the office annoyance. Cringing, Bob vainly pretends he is busy by wiggling his mouse. Too late, Bob. Too late.

"Oh, sorry Bob. Are you busy?"

Jumping from 40 Decibels to 50 Decibels represents 10 times the sound intensity- which means it is twice as loud! As with most blanket statements about sound, that is not truly accurate. Sound intensity is not the same as the perceived loudness of a sound. I am not done torturing Bob, though.

"Bob, you won't believe what happened to my cousin's neighbor. He had this motorcycle and..."

Bob tries to ignore the irrelevant story delivered at 60 dB. This is the level of "normal" speech. This means Bob is now being subjected to B.S. delivered with a million times more sound intensity than blessed silence (0 dB). At this point in the story a mythical motorcyclist has hit a mythical telephone pole. For a dramatic finale Bob's coworker rolls around on the carpet screaming out simulated death throes (120 dB). We will stop our murderous march up the decibel scale just south of the threshold of pain at 130dB. We certainly don't want to get to the Glock 19 (peak 150dB+) nestled in Bob's desk drawer, do we?

Bob's loud day continues when he gets home. The local garage band (Spankmonkey) is having a practice session. Bob puts his garbage can out on the curb and he watches as they close the garage door. This makes little difference. The low frequency of the poorly played bass and the frenetic drumbeats vibrate the walls of the garage and transmit right through. A window in the garage is opened to let smoke escape and that is when Bob is treated to frenzied shrieks and high pitched guitar wails.

The "music" follows Bob indoors. Bob knocks on his walls and discovers that they too vibrate at a low frequency, which is why he gets a free concert every third Thursday - at least the low frequency part. High frequency sound is more easily reflected - it is more "directional". This also explains why the neighborhood Harley acoustically saturates the neighborhood better than a weed whacker, even though they produce close to the same decibel level (and identical power on a dynamometer). Please be gentle with me - I bruise easily.


Too many zeros! Because of the human ear's sensitivity a logarithmic scale is used to measure sound intensity = Decibels.

Sound intensity is not the same as the perceived loudness of a sound, but look the other way for a moment while I state: If you increase the sound intensity by 100 times, or 10 decibels, then the sound will be twice as loud. Lower frequency sound is less "directional" than higher frequency sound. Lower frequency sounds usually transmit more efficiently into convents, churches, and daycare centers.

Part Two: Friends, MOrons, Countrymen, Lend Me Your Ears!

Your ears are designed to amplify faint sounds. What about really loud sounds - like motorcycles, for instance? The inner ear has a protective mechanism to handle loud sounds - it tightens muscles that limit the amount of eardrum vibration as well as the action of the three bones in the middle ear. This process does not work for sudden impulsive noises - like gunshots or a Bette Midler concert. When you shamefully ride your motorcycle without hearing protection you will probably experience this tightening effect in your ears. It is called a temporary threshold shift. I have forgotten to wear my earplugs on occasion. When the people in the office foolishly attempted to speak to me, the muting effect was obvious. Don't scamper off thinking this is a harmless thing, temporary at worst.

Exposure to loud sounds has a negative cumulative effect on your hearing. The damage done depends on a lot of factors - the level of sound intensity, time of exposure, personal physiology, hygiene....this means we get to play with one more type of math. Probabilities! Still, if you are likely to have caused damage, you are likely to have caused damage to the areas of the inner ear that allow you to figure out what the hell everyone else is prattling on about at the water cooler. Obscene gestures will only get you so far in this world and will not replace lost hearing. If I may be so bold to recommend the mohair Conversation Tube for a paltry $2.75?

Even if you can be helped by a hearing aid, there is a further penalty for damaging your hearing. You could develop tinnitus. Tinnitus is a "sound" only you can hear. It is essentially nerve damage. Constant ringing, buzzing, depends on the individual. It never stops and there is no cure.

Part Three - Chinchillas that rock and lesser methods of testing sound.

In the 1970s a generation of darn kids put themselves at risk attending rock

Dasypus novemcinctusconcerts. To save a generation from itself local ordinances were hastily drafted, and scientists were asked to quantify the hearing damage caused by infernal rackets. In one inspired test six chinchillas were subjected to a live rock concert (placed one meter away from the loud-speakers). These chinchillas were then examined for hearing damage, and the results were mixed. It appears humans and chinchillas share a very similar sense of hearing (and varying susceptibility to hearing damage).(1) Regardless, when I discovered that chinchillas are used in acoustic animal studies I immediately hatched Plan A.

Plan A

Strap a chinchilla to my motorcycle. Expose it to the sounds of motorcycling. Manipulate the data. Report the results.

Flaws of Plan A

I would need more than one chinchilla to make this a statistically meaningful test. Chinchillas are expensive (150 bucks a pop!) - maybe I could get a bulk deal? I would hug them and love them and name them all Gabe. This could add confusion. I don't know how to test hearing loss in a chinchilla. My wife said no.

Plan B

I buy a commercially available sound meter and figure out how to mount it to my motorcycle.

Flaws of Plan B

Sound meters are not soft and furry. I still want a chinchilla. My birthday is coming up and I know she hasn't bought me anything yet.

In the end I purchased a Galaxy Audio CM-140 Checkmate Sound Pressure Level Meter and mounted it on my 2003 Buell XB9R. The bike was purchased second-hand,

I was too cheap to buy a cell phone so I mounted a cordless phone instead.and it came with a Jardine brand exhaust system. I did install a "quiet insert" into the muffler, and this does minimally decrease the sound output. Regardless, I am in the process of returning the bike to stock so please postdate your hate mail for at least a few months. The sound meter mount was constructed of aluminum stock bent to shape. It was secured to the bike using the rearview mirror attachment bolts. I needed to be able to see the test meter while keeping my hands free for sudden obscene gestures. Also, the meter can swivel, so the sound level can be tested from behind the fairing (this did not prove to make a significant difference).


Sound filters - dBA and dBC

The test meter can switch between two sound filters. We hear "more" at certain frequencies. The dBA filter more closely matches human hearing. The dBC filter takes more frequencies into account (particularly lower). Both dBA and dBC are important to motorcyclists. dBA is the filter that is used for noise enforcement. Don't recommend they change the sound meter over to the dBC filter, as the sound intensity level will significantly increase. This is because your motorcycle pumps out a lot of sound at lower frequencies (particularly <500HZ). The reason the dBC is useful to us is because this is how Noise Reduction Ratings (NRR) are determined for hearing protection.

This illustrates the difference between dBA and dBC readings. Both are taken at idle:

dBA: A reasonable volume
dBC: Putting it up to eleven



A proviso is in order. This test is meant only to demonstrate that a motorcycle is loud enough to cause hearing damage at operating speeds. I didn't test your motorcycle, which you might declare is a stealthy steed indeed. Here is my challenge to you - prove it or wear earplugs. Hint: if you have experienced a temporary threshold shift in your ears after you ride - you lose.

Speed (MPH) Gear
Engine RPM
dBA dBC OSHA Max Exposure (dBA)
75 4th Gear 111 120+ <1/2 Hour
75 5th Gear
4000 rpm
105 116 1 Hour


5th Gear
100 113.3 2 Hours
65 4th Gear
103.5 115 1.5 Hours
65 3rd Gear
<1/2 Hour
65 4th Gear

1 Hour
55 4th Gear
102 113 1.5 Hours
55 4th Gear

2 Hours
40 3rd Gear
97 111 3 Hours
0 (idle) 1000rpm 79.2 93.5 >8 Hours
0 3000rpm 95.3 107.2 <4 Hours
0 4000rpm 101.4 110.2 Depends on Police Response Time

Chinchilla Reference: 1 "Am I Too Loud?", Jour Audio Engineering Soc, V25, p126, Mar 1977. 








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