A Bicyclist’s Sense Of Hearing: How Important?

by John S. Allen

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Other than warning about loose parts on the bicycle, what can the sense of hearing do for a bicyclist, and what can it not do?

There’s a lot of confusion on this subject. It’s often said that hearing is the bicyclist’s second most important sense, after sight. Well, not exactly. This statement neglects the sense of balance, the sense of touch and the kinesthetic, proprioceptive sense (sense of body positioning), which actually make it possible to ride a bicycle — even with your eyes closed. (See note 1 below). After these senses comes sight, which makes it practical to ride where there are things you might run into. But how far behind sight does hearing come?

In order to answer these questions, I’m temporarily going to trade my bicycle helmet for an engineer’s propeller beanie. (See my curriculum vitae if you wish to review my qualifications.)

Hearing: Sometimes Helpful, But Unreliable

In quiet (typically, rural) surroundings, the sense of hearing can sometimes alert a bicyclist to a motor vehicle, a charging dog or another potential hazard before the bicyclist can see it. Usually, the unseen hazard is either behind the bicyclist, or obscured by vegetation or another obstacle. A bicyclist may sometimes hear a car a mile away under ideal, quiet conditions, upwind and on level terrain or across a valley. But especially when riding into the wind, bicyclists are often surprised by motor vehicles overtaking them, and even more often by other bicyclists overtaking them. The refraction of sound waves by moving air works against the bicyclist in this situation, and so does wind noise.

The sense of hearing has a resolution of about +- 3 degrees for sound sources directly to the front or rear. At other angles, the resolution is poorer, since the timing difference between the two ears changes less rapidly with the angle of the sound source. At 50 feet (15 m), less than 2 seconds before the car reaches the bicyclist at a speed difference of 20 mph (30 km/h), +- 3 degrees amounts to a 6 foot (2 m) range of possible positions. This is in addition to the uncertainty as to whether the major noise source, the exhaust pipe, for example, is on the right or the left side of a vehicle.

Even under quiet conditions, then, the best that the sense of hearing can do is to provide an unreliable warning of a vehicle’s presence, and an inaccurate idea of its position. And while the sense of hearing can indicate that something is there, it can not indicate that nothing is there. Bicyclists learn very quickly not to trust their sense hearing to warn them before turning or changing lane position.

Under noisy urban conditions, the sense of hearing can not often provide an early warning, though often it does provide information about nearby vehicles. On a crowded street, only especially loud sounds such as car horns can provide an early warning.

It is not surprising, then, that the right-of-way rules in the traffic law are based on the sense of sight rather than hearing. A vehicle operator’s only hearing-related duty under the traffic law is to respond to special warning devices: horn, siren or bell. Despite this duty, no laws prohibit deaf persons from operating either a motor vehicle or a bicycle. Not only this, the only laws restricting sound systems on or in a vehicle are intended to reduce disturbance to people outside the vehicle. That is, except for except for laws which prohibit headphones. More about headphones later.

Contrast the facts I have just recited with the distorted, popular view of the role of the sense of hearing for bicyclists. This view is based on several assumptions, namely:

1) The incorrect assumption that bicycling is inherently very dangerous, and the related assumption that safety always outweighs all other considerations, for example bicyclists’ enjoyment of their sport or their need to communicate.

2) The assumption that a bicyclist can and should be held responsible for actively avoiding accidents for which the sense of hearing provides a warning;

3) The assumption that the sense of hearing is useful and reliable enough that it is essential to safe bicycle operation.

These assumptions most commonly are expressed as condemnations of headphone use while bicycling. Let’s turn to the headphone issue now.

Types of Headphones

There are three major types of headphones. They differ greatly in their effect on hearing of sound from outside:

1) Circumaural or “sealed” headphones. These form an airtight seal against the sides of the head, and greatly attenuate sound from outside. They are preferred in noisy environments such as an airplane cockpit. They once were popular for high-fidelity sound reproduction, but they are heavy, bulky, uncomfortable and sweaty, and with better options available, they are much less widely used now. They are very rarely used with portable sound equipment.

2) Supraaural or “open-air” headphones. These rest on the ear but form no seal. The conventional telephone earpiece is a common example, and so are most headphones used with portable sound equipment.

When supraaural headphones are used for high-fidelity sound reproduction, precise spacing of the headphone transducer from the ear is essential for predictable low-frequency response. The spacing is controlled by an open-cell foam pad which is transparent to sound from the headphone transducer and also to sound from outside. Sound from outside the headphone is attenuated slightly by the bulk of the transducer assembly and much less by the foam which surrounds it.

Small supraaural headphones 2 or 3 cm across, the most common type used with portable stereos, have very little effect on sound from outside — about as much as if you hold up two fingers next to each ear but not touching it. (Try this.) Such headphones produce essentially no hearing impairment, if silent, and increasing impairment the louder they are played –just as with a loudspeaker.

3) Intraaural or “in-the-ear” headphones. Hearing aids commonly use these. The effect of “in the ear” headphones on sound from outside to depends on their construction. Some intraaural headphones plug the ear canal, while others leave it partially open to the outside. Even a small opening will let most sound from outside pass.

A few headphone models electronically cancel out some of the outside noise, mostly in the low-frequency range. These headphones are expensive and uncommon, and they all have a switch to turn off the noise cancellation.

Headphone laws

Several states have laws prohibiting headphone use by motor vehicle operators and/or bicyclists.

Some of these laws permit headphones which cover one ear. The idea behind these laws is that the other ear will then be able to receive sounds from outside. To be consistent, these laws should in principle also allow the wearing of only one earmuff in cold weather, though somehow, nobody has thought of banning earmuffs. One-ear laws don’t make scientific sense, since a single headphone can actually have worse effects on hearing than binaural (two-ear) headphones. The desensitization of one ear by a single headphone played loud enough to cut through background noise changes the apparent location of sound sources. This problem is much less likely with binaural (two-ear) headphones.

Except when very unusual recording techniques are used, all sound sources reproduced through headphones appear inside the head or at the ear(s), where they are difficult to confuse with other sounds. This effect is even more pronounced with binaural (two-ear) headphones, and allows the programming they convey to be intelligible at a lower volume.

The wording “covers the ear(s)” usually found in headphone laws is supposed to distinguish between headphones and loudspeakers, but it does so poorly. To “cover the ear(s)” is a visual concept, but the ears do not see, they hear. Open-air headphones do not cover the ears, impairing hearing, any more than goggles cover the eyes impairing sight or a scarf covers the nose, impairing the sense of smell.

It is also fair to point out that headphones have practical advantages and legitimate uses for bicyclists, more so than for other vehicle operators. This is, after all, precisely why headphones are popular with bicyclists. Headphones are lightweight and require very little electrical power to operate, important advantages on a human-powered vehicle. Headphones deliver sound to the bicyclist without disturbing other people. Headphones may be used for entertainment or to gather information unrelated to bicycling — listening to a news broadcast, holding a conversation via ham radio, auditing a correspondence course — but they may also be used for bicyclist-to-bicyclist communication. In this context, headphones make it possible to teach safe riding, give route directions or relay vital safety messages over a far greater distance and more reliably than by mouth.

Did you ever wonder why television news correspondents always appear on camera with a little headphone plugged into one ear? It’s because this eliminates the problem of feedback from loudspeaker to microphone. For the same reason, headphones make it possible for a bicyclist using a two-way radio to conduct a normal conversation, rather than having to shut off the microphone when receiving.

Headphone laws are very rarely enforced. Many bicyclists ignore them. But enforcement is not the only way that the law affects people. One important reason not to wear headphones — even if they are not playing — is that they make it harder to collect on an insurance claim after a crash.

The first question a bicyclist’s attorney should raise when faced with this problem is whether the bicyclist had any duty to act differently if alerted by sound. Only if this is true is it important under the law whether the bicyclist actually heard the sound. For example, if an overtaking vehicle strikes a bicyclist riding in the normal position on the road, the overtaking driver had the duty under the law to avoid striking the bicyclist. The bicyclist had no duty to swerve out of the motorist’s way, and it is unlikely that hearing the car would have made it possible to determine whether swerving was necessary to avoid a collision. Therefore, the wearing of headphones should not be an issue in such a case. A judge ought to prohibit it from being discussed in the jury’s presence — but a judge may not do this. I have seen cases lost over this false issue.

Conclusions

As I hope that I have shown, laws banning headphone use by bicyclists are based on inaccurate ideas about headphone design. These laws outlaw the special advantages of headphones for bicyclists, particularly for two-way communication. Furthermore, the bicyclist is unusual among vehicle operators in having good use of the sense of hearing. If we held all drivers to the standard of being able to hear well, the only street-legal motor vehicles would be quiet, unenclosed ones such as golf carts. A bicyclist’s decision whether to wear headphones — particularly, open-air headphones — and of how loud to play them, ought to be of as little concern in the law as is the question of how loudly a motorist may play a radio inside a car.

I think that it is important for a bicyclist to think carefully about when to use or not to use headphones, and I certainly don’t encourage playing them loudly. Not only does loud playing of headphones shut out the outside world, it can damage hearing. I agree that headphones (or any other extraneous sound source) can sometimes affect the safety of bicycle operation. But the role of headphones in causing bicycle accidents is, in my opinion, deeply confused by faulty assumptions about the sense of hearing, and by ill-conceived laws which place headphones in a special category separate from other factors affecting hearing.

***

1) (Don’t try this at home unless you are Bill Gates. If you do try it, you will probably find that you can ride just as steadily with your eyes closed as with them open.)

c. 1997, John S. Allen
From John Allen’s Home Office Home Page. (Republished with permission.)

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