Seen and Heard: Testing Helmet Covers for a Night Ride

Designing a Helmet Cover for the Night Ride

1Every October, the bike-safety club at Pine Ferry Middle School leads a night ride for families on the paved loop behind the school. This year, club advisor Mr. Dashiell Quayle challenged members to design a helmet cover that helps drivers see a rider after dark but still lets the rider hear traffic. Eighth grader Emeka Fenwick led the tests. Light and sound are both waves. When a wave reaches a material, the material can reflect the wave, absorb its energy or transmit it, which means letting it pass through. Most materials do some of each.

2For the light test, Emeka turned off the lights in the school gym and set a bright flashlight 5 m behind a foam head wearing a helmet. The flashlight stood in for a car's headlight. He taped a patch of each material to the back of the helmet, and the flashlight beam hit each patch at an angle, the way headlights often hit a rider on a curve. A light meter measured the brightness at two spots: right beside the flashlight, where a driver's eyes would be, and 2 m to one side of the flashlight, in the direction a mirror on the helmet would bounce the beam. One sample was a gray safety tape covered with tiny glass beads. Another was a clear plastic rain cover stretched over black cotton. His readings are in Table 1.

3For the sound test, Emeka put a small microphone inside the helmet where a rider's ear would be. A speaker 3 m behind the head played two tones at the same loudness: a low tone like a car horn (400 Hz) and a high tone like a bike bell (3,000 Hz). He measured the sound level at the ear with each kind of ear cover on the helmet. A second microphone, just outside the ear cover on the side facing the speaker, measured the high tone there. Table 2 shows his results in decibels (dB). The club set one rule for safety: with the cover on, both tones must reach the ear at 60 dB or more, so a rider can still hear traffic.