Over the years, many marine engineers have suffered permanent hearing loss due to hours of exposure to the high noise levels in the engine room. You can prevent the loss of your own hearing by using proper hearing protection such as earplugs and earmuffs religiously whenever you are exposed to loud noises.
Your ear is a delicate instrument which contains thousands of fine hair cells called cilia to detect sounds. Prolonged exposure to loud noises can injure or break these cells or even tear off groups of these cells. Since the hair cells don’t ever repair themselves, when enough of them are damaged, a hearing loss results.
Short, intense, sounds such as an explosion can cause immediate and permanent hearing loss, but most hearing loss occurs gradually due to prolonged exposure to high noise levels. Most of the time this loss is so gradual that you are not even aware that it is occurring. Hearing loss is progressive and will continue to get worse as you continue to expose yourself to a noisy environment, but if you can avoid further exposure, the hearing loss won’t get any worse.
Permanent hearing loss is not the only hazard associated with noisy environments. Tinnitus is a condition where a ringing or roaring sound is heard continuously in one or both ears. Tinnitus occurs when the hair cells are damaged in such a way that they produce sound signals to the brain even in the absence of actual stimulation. This noise is sometimes described as like having a cricket permanently stuck in your ear, and is very annoying.
Sound intensity is measured in units of decibels (dB). The decibel scale is a logarithmic scale in which 0 dB indicates the quietest sound that can be heard by a normal person. A quiet library is about 20 dB. A normal conversation occurs at a sound level of about 60 dB. A lawnmower produces about 85 dB, and at this level you should be wearing hearing protection. Sandblasting or a rock concert has a sound intensity of about 115 dB. At this level hearing damage can occur after only about 15 minutes of unprotected exposure.
According to OSHA regulations, when the 8 hour average noise exposure is above 85 dB, employers are required to institute a noise monitoring program, provide adequate hearing protectors, and ensure employees wear them. Continual, unprotected exposure to noises louder than 85 decibels for 8 hours or more can be dangerous, and may result in hearing loss.
You can easily tell if you should be wearing hearing protection. If you find yourself in a situation where it is too noisy to carry on a conversation at arms length, or if you notice that your ears ring or feel plugged for a time after leaving a noisy environment, you can be sure that you should have been wearing your hearing protection.
Here are some commonly available hearing protection devices:
Figure1.3.3.Hearing Protection
Disposable foam plugs Foam plugs are inexpensive devices made of a formable material designed to expand to fit to the shape of each person’s ear canal. To use, roll the expandable plugs into a thin, crease-free cylinder, and insert it into the ear canal. Apply a slight pressure to the plug until the foam completely expands. These plugs can be reused a couple of times, but never reuse them if they have become dirty. Some people may find that the standard expandable plug is too large for their ear canal, and may find that the smaller plug model fits better.
Reusable, pre-molded plugs Pre-molded plugs are made from silicone, plastic or rubber and are manufactured as either as a custom fit, or are available in several standard sizes. Be sure to choose the proper size for your ear canals. In some cases, different size plugs may be required for each ear. The advantage of pre-molded plugs are that they are inexpensive, reusable, washable, convenient to carry, and come in a variety of sizes. Nearly everyone can find a plug that will be comfortable and effective. Another advantage is that in a dirty environment like the engine room, you don’t need to handle or roll the tips. You insert this type of plug by reaching over your head with one hand to pull up on your ear. Then use your other hand to insert the plug with a gentle rocking motion until you have sealed the ear canal.
Hearing Bands Hearing Bands, also called canal caps or semi-insert devices often resemble earplugs on a flexible plastic or metal band. The earplug tips of a canal cap may be a formable or pre-molded material. Some have headbands that can be worn over the head, behind the neck or under the chin. The main advantage of canal caps is their convenience They are easy to put on and take off . When it’s quiet, employees can leave the band hanging around their necks, but they can quickly insert the plug tips when hazardous noise starts again. These devices are ideal for intermittent use, but they provide less protection than either plugs or muffs. Some people find the pressure from the bands uncomfortable.
Over-the-Ear Earmuffs These are designed to cover the entire ear and provide a physical barrier to block out noise. They are often adjustable for a better fit.
What is a Decibel?
The Decibel (dB) is a logarithmic unit used to describe a ratio. The ratio may be power, or voltage or intensity or several other things. To get a taste for logarithmic units, first let’s look at some numbers: Suppose we have two loudspeakers, the first playing a sound with power \(P_1\text{,}\) and another playing a louder version of the same sound with power \(P_2\text{,}\) but everything else (how far away, frequency) is kept the same. The difference in decibels between the two is given by
This example shows one feature of decibel scales that is useful in discussing sound: they can describe very big ratios in power with numbers of modest size.
Sound is usually measured with microphones and they respond (approximately) proportionally to the sound pressure, \(p\text{.}\) The power in a sound wave is approximately equal to the square of the pressure. Since the log of the square of \(x\) is just \(2 \log x \text{,}\) this introduces a factor of 2 when we convert to decibels for pressures. Therefore the difference in sound pressure level between two sounds with \(p_1\) and \(p_2\) is defined as:
This formula involves a ratio of pressures, so to measure the sound level for a single sound, a reference level must be chosen. For sound intensity, the reference level (for air) is usually chosen as 20 micropascals, or 0.02 mPa. This is very low: it is 2 ten billionths of an atmosphere. Nevertheless, this is about the limit of sensitivity of the human ear, in its most sensitive range of frequency. Usually this sensitivity is only found in rather young people or in people who have not been exposed to loud music or other loud noises. Personal music systems with in-ear speakers are capable of very high sound levels in the ear, and are responsible for much of the hearing loss in young adults in developed countries. So if you read of a sound intensity level of 86 dB, it means that \(20 \log (p_2/p_1) = 86 \) dB where \(p_1\) is the sound pressure of the reference level, and \(p_2\) that of the sound in question. Divide both sides by 20: \(\log (p_2/p_1) = 4.3 \) dB. 4 is the log of 10 thousand, 0.3 is the log of 2, so this sound has a sound pressure 20 thousand times greater than that of the reference level. 86 dB is a loud but not dangerous level of sound, if it is not maintained for very long.
What does 0 dB mean? This level occurs when the measured intensity is equal to the reference level. i.e., it is the sound level corresponding to 0.02 mPa. In this case we have sound level
So 0 dB does not mean no sound, it means a sound level where the sound pressure is equal to that of the reference level. This is a small pressure, but not zero. It is also possible to have negative sound levels: - 20 dB would mean a sound with pressure 10 times smaller than the reference pressure, i.e. 2 micropascals.
