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Who’s listening?

Noise can be detrimental to our overall wellbeing and of course our hearing. Throughout the course of a day we can unwittingly expose ourselves to noise levels that are harmful to our hearing. This could be during the course of completing an exercise class at our local gym where the PA system plays or it could be whilst listening to a favourite album using headphones or attending a concert.

The Human Ear
Our ears are very sensitive and can incorporate huge audible range, in order to quantify this extremely large range it is necessary to use a logarithmic scale using decibels, this is not the only reason since our ears also respond logarithmically as every 10dB increase in level is perceived as double the loudness. There are some reference sound levels to consider, for example a pure tone at 1kHz can be heard by a young person with good hearing at 0dB, this is considered the threshold of hearing, with the level of a conversation at say a distance of 1 meter to the listener would be approximately 60dB and an airplane at take off power at a distance of 60 metres away would be 110dB, which is the average human threshold of pain. Sound levels measured on a decibel scale relative to the threshold of hearing are called SPL (Sound Pressure Levels). Not all frequencies are equal since lower frequency sounds require a greater sound level than equivalent higher frequencies in order to be audible, as a result frequency weightings are used when measuring sound levels to represent how the human ear weights different frequencies, which are known as ‘A’ weighting.

The human ear consists of three main parts:

 

The Outer Ear
Which includes the pinna, ear canal and the ear drum. For adults a consequence of the ear canals shape means that it amplifies frequencies in the region of speech, which is approximately 3-5 kHz, the amount of amplification for each individual will depend on the length and shape of the ear canal.

The Middle Ear
Includes what are known as the ossicles, these are the smallest bones in the body and are known as the hammer, anvil and stirrup.  These essentially take the great movement of the ear drum caused by the small force of sound and convert this into a small movement but greater force. These small bones are connected to the ear drum via what is known as the tensor tympani muscles. The ear drum has the ability to move to the point of bursting with excessive sound levels, however the tensor tympani muscles tense up and prevent this from happening, this is generally followed by a sensation of pain in the ear as a result.

The Inner Ear
One of the most important parts of the inner ear is the basilar membrane which is connected to the ossicles via what is known as the oval window. The basilar membrane is housed in the cochlea which is a snail like structure. Within the cochlea is a fluid and movement which starts at the ear drum and is passed to the oval window via the ossicles which causes a pressure change within the cochlea and also disturbs small hairs on the basilar membrane causing nerves at the base of the hairs to send electrical signals to the brain. The basilar membrane acts as a kind of frequency analyzer and due to the structure it resonates at different areas along its length which is dependent on the sound content, which is very stiff and has a low mass at the near end for high frequencies and not very stiff and high mass at the opposite end being more suited to low frequencies.

Noise Induced Hearing Loss
A rock concert can generate levels of approximately 100-120dB and after being exposed to these kind of levels without hearing protection for a certain period of time a person can experience what is known as a TTS (Temporary Threshold Shift), this is where the quietest sound that the individual could hear before the concert has to be louder now due to the over exposure of loud music. Most of the time the hearing returns to normal the next day, however permanent threshold shifts can also happen and the hearing never returns to normal. Current audiology research has also provided some evidence that even after the hearing has recovered from TTS, damage can occur which affects the way the hearing ages and can affect the way the small hairs on the basilar membrane communicate with the neurons of the cochlea which send electrical impulses to the brain. The unfortunate result of hearing damage is that the initial damage will happen within the range of frequencies responsible for speech. Another effect of hearing damage might be tinnitus, where the sufferer may be stuck with a ringing sound in their ears, but it can also be manifested as a rushing or hissing sound. The main problem with hearing loss is that it is not immediately noticeable and often when the individual realises that there is a problem there is no solution to cure their symptoms. Some people use the term ‘my ears are bleeding’ when describing having to listen to a song they hate or if a noise is too loud, it would be good if our ears did in fact bleed as this would give us an indication that there is a problem. However, not only excessive sound levels can cause hearing damage but also the combination of excessive sound levels and drugs, being legal or illegal, including alcohol. Hearing damage caused by drugs being known as ototoxic and can increase the damage further than loud music on its own. 

Sound Level and Exposure Time
Most music lovers at some point like to sit down and listen to music at loud levels, although how loud is too loud and for how long? For example in the workplace the health and safety executive sets out sound level thresholds. If these levels are breached employers must take action to protect workers hearing. The daily or weekly upper limit is 85dB(A) or a peak limit of 137dB(C), if the levels are at 85dB(A) or higher employers must take action to protect their workforce. However, what if you are concerned about your sound level exposure outwith your job for example, well it is not quite straight forward because it depends on the type of exposure, for example listening to spoken word there are points when there is silence so the exposure time is reduced, however if listening to loud music this is generally more continuous so quite quickly you could use up your exposure budget.

The exposure budget generally works on the principle of a 3dB exchange rate, for example at 85dB(A) you can listen to music for 8 hours, but for every 3dB increase this halves the exposure time so for 88dB(A) this would be 4 hours and so on. This is very pertinent nowadays since many people listen to music via their mobile devices using in ear earphones, devices like the IPhone at maximum volume can deliver at least 100dB to the eardrum via ear buds so within approximately 15mins at full volume you could be susceptible to damaging your hearing. Sound engineers for example fulfil an occupation where they are at an increased risk from hearing damage and it is not always possible to accurately track their sound exposure over the day or week, however there are solutions for example the TC Electronic Clarity X (87-0201) monitor controller which allows the user to calibrate their speakers and headphones in order that they can accurately track their sound exposure level over the day or week. Canford also provides level limiting of headphones. This is a special process which involves the fitting of custom limiter circuits which are calibrated to the headphones in order to limit the maximum level the headphones can reproduce to assist employers in the management of workers sound exposure in line with the health and safety executive rules.

For non professionals, there are a number of apps available mainly for the IPhone that allow you to track your sound level exposure so that you can budget your personal listening time over headphones, for example the SoundLog Noise Dosimeter (itunes.apple.com/us/app/id1063941394). There are Android apps too, however IPhone apps provide the best functionality, due to the advanced audio processing of IPhones compared to the array of different Android component manufacturers and the open software platform. In general it is always better to start off listening to music at lower sound levels since we habituate quite quickly to sound levels and after sometime the level does not seem as loud and we have to turn it up, so starting lower is safer as you can save louder levels toward the end of your listening period. Most professionals in the audio, video and broadcast engineering are familiar with the sound level measurement metric SPL (Sound Pressure Level) which in general provides an instantaneous or momentary sound level measurement. However, a measurement metric which provides a more accurate representation of the sound level over a certain period of time is the SEL (Sound Exposure Level) which in short is an A weighted energy measurement squared and integrated over a certain period of time and normalised to an 8 hour working day or 40 hour week.

Of course it is a lot more difficult when out in bars and clubs or restaurants to monitor sound levels or get an indication as to the sound levels you are being exposed to, however there are a number of sound level measurement apps available, these include the Decibel X for IPhone and Android, Sound Meter for Android and Noise Hunter for IPhone. Of course these measurement apps provide approximate measurements using the phones built in microphone, however adding for example the micW i436 microphone creates a measurement system with greater accuracy.

When many people think of hearing protection straight away they think killjoy: ‘ I won’t be able to enjoy my favourite band at a live concert wearing hearing protection and it will sound rubbish’ however many good quality earplugs are worth the investment since they work more like filters providing attenuation of sound levels across a range of frequencies. Knowing the performance of an earplug is provided using SNR (Single Number Rating), like the Proguard Noizezz which allows the user to determine how effective the earplug will be, higher SNR numbers indicate greater attenuation. Attenuation values are also given across a range of frequencies and of course due to the wavelength low frequency attenuation is never as great as high frequency attenuation, however there is a limit to the level of protection provided by earplugs since incorrect fitting and bone conduction reduce the maximum level of protection.

Overall when considering the general term ‘loud sounds’ there is a difference between industrial noise and loud music, since loud industrial noise is something you would definitely not want to be exposed to, yet loud music is not so offensive. Many people get a lot of pleasure from listening to music at loud levels and studies carried out over a decade ago found that responses to loud rock and dance music of 90dB(A) and greater from within a part of the inner ear called the vestibular, which is a part that controls balance and spatial coordination, suggests that this response may indicate a minimum sound level for dance and rock music to work giving it the term ‘rock and roll threshold’.

Nowadays nearly everything we enjoy comes with a health warning. When it comes to our hearing however this isn’t always the case and there are many situations where we can be at a risk from over exposure. However there are some precautions that can be taken to help reduce the risks. As with everything in life moderation is the key and there are ways and means to protect your hearing but at the same time enjoy music to the fullest.

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