BACKGROUND: Army scientists have found that some combat helmets are better than others in helping soldiers determine the location of sounds in their environment. Certain older helmet designs can degrade a soldier's ability to tell where a sound is coming from, which can be dangerous in combat.
HOW WE HEAR: Sound is a mechanical wave, meaning it produces vibrational energy in the medium through which it moves, such as air. The ear detects sound waves and sends those signals to the brain for processing. The incoming sound waves cause the eardrum to vibrate, and this sends mechanical waves vibrating through a fluid inside a narrow tube called the cochlea. The cochlea vibrates tiny hairs tuned to specific pitches or frequencies of the sound. The hair vibrations stimulate nerves, which in turn send signals to the brain. It's similar to how a microphone works, in which sound vibrates a membrane, causing electrical signals to travel through a wire to an electronic circuit card for processing.
DETERMINING DIRECTION: Because humans have an ear on either side of our heads, we can distinguish whether a sound is coming from the left or right by measuring the difference in arrival times of sounds at each ear. Sound coming from one direction will reach the ear furthest away about 1/500 of a second later than the closer ear, and our brains can pick up on this tiny time lag. Humans can also tell to some extent whether a sound is coming from above of below, and some can tell if a sound is in front of or behind them. With a little training, we can also tell how far away a sound might be, depending on how loud it is.
HELMET DIFFERENCES: On the battlefield, a soldier must balance his or her need for full head coverage in a helmet with the need to determine distance and direction of incoming sound. The older Personnel Armor System for Ground Troops helmet provides full coverage of the head, including the ears, but limits a soldier's ability to determine sound direction. The newer Advanced Combat Helmet has been cut away around the eyes and ears, reducing the overall weight of the helmet and improving the soldier's use of his key senses of sight and sound. But there is some debate as to whether it makes soldiers more vulnerable to head wounds.
BETTER EARPLUGS: Scientists at the same laboratory are also designing improved earplugs for troops to protect them from hearing loss that frequently results from being in close proximity to artillery fire.
The Acoustical Society of America contributed to the information contained in the TV portion of this report.