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Key Points

• Within the inner ear are small hair cells which transmit sound information to the brain. Loud sounds sent through the system can cause these hair cells become damaged and can even die.
• Hazardous noise is defined as that which exceeds 85 dB if an individual is exposed for an 8-hour day.
• Noise-induced hearing loss can present as a decrease in a person’s ability to hear sounds at different frequencies, or as a difficulty understanding in noise.
• If your employees complain about sounds being too loud, have difficulty understanding one another, or complain of diminished hearing at the end of the day, it is time to invest in some noise conservation measures.

In order for us to hear, sound must travel through the complex system that is our ear. As described in the figure above, when sound enters the ear through our ear canal it moves to our ear drum. The ear drum, or tympanic membrane, is vibrated which sending sound waves through three tiny bones in our middle ear called ossicles (malleus, incus, stapes). These ossicles then send the sound wave through the inner ear, the organ of hearing. Within the inner ear, or cochlea live delicate hair cells which move and bend to send sound information to the brain. When an average sound is sent through, the hair cells move within incident, snapping back to their normal state.

If loud sounds are sent rippling through this system, however, these hair cells become damaged and can even die. This damage can occur all at once with an extremely loud sound, an acoustic trauma, or slowly over time (Fig. 1). Damage to the hair cells prevents speech and sound from traveling to the brain in a precise and accurate way. If cell death occurs, this can prevent individuals from hearing certain frequencies, or pitches, of sounds at all. Typically, this damage affects pitches in the range of human speech, making communication difficult. The inner ear is organized by frequency, or pitch. Higher pitches are more susceptible to damage because they are closest to the outside world (Fig. 2).

Figure 1

If damage to the hair cells occurs due to frequent daily exposure to loud sounds, individuals may not notice the effects until they become more pronounced. With continued exposure, the damage can cause hearing loss severe enough to warrant hearing aids or other forms of amplification. With extremely loud sounds, like explosions or fireworks at close range, physical damage to the ear can occur like ruptured eardrums or damage/dislocation of the middle ear bones, the ossicles.

The National Institute of Occupational Safety and Health (NIOSH) estimate that 30 million Americans are exposed to dangerous noise levels daily. Worldwide, 500 million individuals are exposed to this level of noise. Hazardous noise is defined as that which exceeds 85 dB if an individual is exposed for an 8-hour day. Exposure for 8 hours, however, is not necessary for damage related to noise to occur. In a basic sense, the louder the sound is, the less time it takes for permanent hearing damage to occur.

Some common examples to give you a sense of what exactly “85 dB” means can be seen below. The shaded region represents 85 dB – the cut-off for what qualifies as “hazardous noise” in relation to the intensity of each common sound.

We often measure this damage in “thresholds” or the level at each pitch an individual is ability to pick up sound. As noted before, noise can lower these thresholds causing individuals to have difficulty picking up certain pitches. Some individuals, however, may seem completely normal in terms of their ability to hear that even soft sounds are present, but may complain of difficulty understanding in background noise or other difficult listen situations like a reverberant room. This phenomena may occur after what is called a temporary threshold shift.

Noise Induced Hearing Loss

There are two broad categories of “Noise Induced Hearing Loss” (NIHL): temporary threshold shifts and permanent threshold shifts. If an individual is either exposed to an incredibly loud sound for even a second, like a gun firing, or a lower level sound like an electric drill for a longer period of time daily, they can present with a permanent threshold shift. This means that noise has damaged inner ear hair cells resulting in a permanent hearing loss.

In contrast, an individual may experience momentary hearing difficulty after a rock concert (~110 dB) that lasts 3 hours, for example, this being a temporary threshold shift. With a temporary threshold shift, the hearing loss at each pitch fully recovers to normal, or whatever the hearing was before exposure.

Although “temporary” threshold shift may seem benign since the perception that sound is present recovers, the damage from this exposure can still be lasting. When exposed to a moderately loud sound for a shorter amount of time the inner hair cell damage or death, as mentioned above, may not occur. What does happen, however, is a loss of cochlear nerve terminals or spiral ganglion cells, most simply defined as the connection between the inner ear hair cells to the brain. This loss or damage can occur very rapidly and is irreversible. This damage is what causes these individuals to have difficulty understanding speech, especially in background noise.

So, How Loud is Too Loud?

There are some simple ways to tell if the noise at your workplace is too loud without any special equipment. Do you need to speak louder when talking to co-workers? If your employees complain about sounds being too loud, have difficulty understanding one another, or complain of diminished hearing at the end of the day, it might be time to invest in some noise conservation measures. You might have an issue with noise if you work in a factory, on a construction site, in a concert hall, or if you use any power tools or loud machinery.

In Part Two of this series, we will discuss options for noise management, as well as the trouble with hearing in noise.

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