You are absolutely correct in being concerned. Research has shown that repeated exposure to loud noises can cause hearing loss. Let me go over some basic information about sound and hearing before giving you any advice.

The sound that we hear comes to us as pressure waves that is invisible to the eyes. These waves are collected by our ears and directed into the ear canals. At the end of the canal is the ear drum (also called tympanic membrane). The pressure waves cause the ear drum to vibrate. This vibration is conducted through three tiny bones behind the ear drum to an organ called cochlea, which has a spiral canal and looks very much like the shell of a snail.

The cochlea is lined with very sensitive and specialized hair cells that move when sound waves arrive from the ear drum. The movement of these hair cells produces electrical signals that are passed through nerve fibers to the brain where the vibration is recognized as sound.

There are two qualities of sound that we need to understand: pitch and loudness. Low pitch or tone produces low frequency waves (like 250 Hertz or cycles per second), while high pitch sounds produce high frequency waves, up to 8000 Hertz per second. The sound of a drum would be in the low frequency range, while a sopranoʼs voice has very high frequency.

Another quality of sound is the loudness, which is measured in decibels (dB). The louder the sound, the higher is the dB. Normal conversation is around 60 dB, while a vacuum cleaner often produces 85 dB sound. A motorcycleʼs engine is about 88 dB, while a riding lawnmower can be as loud as 94 dB. Believe it or not, a live band can go up to 97 dB, while a rock concert can be as loud as 112 dB. On the other hand, airplane engines, firecrackers, as well a guns can produce sounds up to 140 dB.

The louder the noise is, the greater the eardrum would vibrate, which in turn causes more disturbance to the sensitive hair cells, resulting in the brain recognizing that the noise is loud. We are all equipped with a fix number of hair cells when we are born. If we are exposed to loud noises for long periods of time, some of these hair cells will die. Unfortunately, no new hair cells will grow to replace them.

In United States, their National Institute for Occupational Safety and Health (NIIOSH) has established a national standard for occupational noise exposure. Workers exposed to 85 dB of sound at their worksite are allowed to be there for no more than 8 hours. At 94 dB level, which is found in riding lawnmowers, the recommended exposure time is no more than one hour. Live bands often reach 97 dB, and the exposure time is reduced to 1/2 hour. There is no safe exposure time for noises over 105 dB, but our kids are often exposed to 112 dB when they attend rock concerts!

If your son listens to his iPod when he mows the lawn with a riding lawnmower, he would have to turn the volume significantly above the noise produced by the lawnmower, which is 94 dB already. Therefore, listening to music while mowing the lawn is probably much more dangerous to his hearing than you can imagine. The noise level produced by the lawnmower already warrants one wearing ear muffs or ear plugs to reduce the noise reaching the cochlear. To turn the volume of music above that of the lawnmower would damage the ears at a much faster pace!

As you can expect, the louder the noise, or the longer exposure time, the greater is the damage to the ears. When you leave a noisy environment or stop a power tool, you would notice that everyoneʼs voices are muffled, there may be a ringing noise in your ears, and more than likely you will speak in a louder voice, because you cannot hear yourself as well.

If you are exposed to loud noise for a relatively short period of time, your hearing can recover (unless if the noise level is well over 100 dB). However, if this exposure happens regularly or for long periods of time, hearing loss will become permanent. The only remedy at that time would be hearing aids. That is the reason why NIOSH tries to protect workers from excessive exposure to loud noises at the workplace. However, there is no law that will protect your son if he cranks up the music and jam with his band members.

If you notice that your son already has hearing loss, the best thing that you can do is to get an urgent hearing test from an audiologist. There is a special exam called oto-acoustic emission test that is particularly effective in detecting hearing loss from loud noises.

In the meantime, you can buy some relatively inexpensive ear plugs that he can wear when he mows the lawn or jams with his friends. One such ear plug is ER-20 (produced by Etymotic Research). It can reduce sound level by 20 dB, potentially reducing harmful noise level to a relatively safe zone and preventing him from further hearing loss when he enjoys his music.