Unit 1: Acoustics For Musicians - The Mechanism of Human Hearing

Unit 1 – Acoustics For Musicians

The Mechanism of Human Hearing

Structure of our ear:

The Outer ear consists of outer ear itself, the pinna at the entrance to the ear canal, and the ear canal leading to the ear drum which is sometimes called the auditory canal. The outer ear picks up sound waves or vibrations, which are passed down the ear canal to the ear drum. The outer ear is larger than the ear canal so it can pick up more sounds than an ear with a smaller surface area would, and funnel them into the ear canal. As the ears are designed to hear sounds in the same frequency range as the human voice, these are amplified by the pinna as they travel towards the ear canal, and less audible ones like very high frequency sounds are filtered out as we cannot hear above a frequency of 20KHz anyway. The ear canal is the next place the sounds travel to, which has two main functions. The first is to create ear wax which contains chemicals that helps to prevent ear infections and catch dirt, and the other is to allow sound waves to pass to the ear drum at the end of the canal and the start of the middle ear. The middle ear is made of many small components, responsible for things like balance and the passing of vibrations to nerves connected to the brain to interpret the sound. Three small bones in the middle ear called the stirrup, anvil and hammer (collectively called ossicles) are responsible for converting and filtering the vibrations travelling down the ear canal to the cochlea in the inner ear. The cochlea contains fluid, so for it to interpret the sound it needs to be converted into amplified pressure waves, so this is what the ossicles do. Thy are linked to muscles which in turn are connected to nerves which contract and extend depending on the frequency of the sound so it can be safely amplified and not damage the inner ear. Once sounds have been converted and entered the cochlea they need to travel to the brain via the auditory nerve. Signals sent down nerves are electrical impulses or neural impulses, so the signals sent from the cochlea have to be able to travel down these nerves.
The Levels of human hearing applies to the range of frequencies we can hear and the sound pressure levels we can withstand. It's understood that a young human being can hear sounds between 20Hz and 20KHz, and as people age their ability to hear the higher frequencies lessens. Sounds below the human frequency range are called infra sounds and those above it are called ultra sounds, and although humans cannot hear these, other animals do use them to communicate. Sound pressure is the pressure exerted on air particles when a sound wave is produced. Changes in pressure are indicated by deviations away from the average pressure exerted in the air when a wave is produced. The average level is an ambient atmospheric pressure, and if a sound is made in a space without lots of space and particles the velocity of the sound passing between particles will be greater, so the stillness of the particles will be disrupted and the sound pressure will go up. This applies to the ear as there's a limit to how much pressure the components of the ear like the ear drum and cochlea can withstand without damage being caused to them. One way the ear helps prevent damage from loud sounds or high pressure sounds is for the muscles controlling the ossicles and ear drum to retract, both tightening the ear drum and pulling the ossicles away from the oval arch opening in the cochlea. This prevents loud sounds from damaging the sensitive hearing apparatus. 
Psychoacoustics is related to our perception of sounds, and how our brain interprets them and how we respond to them. Our brain can hear sounds and work out what they're produced by but also it can sometimes be tricked into thinking it's hearing something it isn't. For a sound to be processed by the brain it has to be converted from sound waves into neural signals, because of this the subtle differences between sounds with different frequencies may not be noticed by the brain, so when music is compressed from a lossless format to a smaller one like MP3 to take up less storage space, the frequency changes that are hard for the brain to spot when processing sound are removed, so the files can be smaller but not impact the enjoyment of the listener as the quality of the sound may not be such a noticeable change. This is one example of how we can use psychoacoustics to our advantage. Similarly, when music is being mixed and mastered it can be cleverly manipulated to give an impression of greater quality and inhance the sound we percieve, which is a great advantage during post production as otherwise, to achieve an equally good recording they would have to record the sounds with relentless attention to detail to achieve a similar sound to the one they could create using production techniques, which would be expensive and time consuming, which in a studio would mean extra costs. An example could be perceptual integration. When the same sound is played twice, but so close together that the ear cannot notice that the two sounds are seperate, they merge together and sound like one sound. This is why harmonic properties of notes make the notes differ from each other, as if we heard each small frequency variation that culminated to make one note played on an instrument, we'd not be able to hear the properties of the note like pitch, sustain, decay and timbre as we'd just be hearing the individual elements of that note. Because of this we can add to sounds or change them altogether with EQ and compression and trick our brain into thinking we're hearing something with different properties when really the changes are too close together in one note to spot, instead we just hear a sound that to us sounds enhanced. A technique used to make an instrument sound larger and richer in a stereo mix is to replicate it on another track, and play it one tenth of a second after the other (taking advantage of our ears not beinga able to seperate the two) and then panning one to the left and the other to the right. This short delay gives us the impression that the properties of the sound have been doubled in intensity creating a sound with more power, when infact this technique has just created an illusion if it.
It's very important to prevent damage to our ears if we want to maintain our ability to hear lots of frequencies as we get older. As I mentioned earlier, with age our hearing range lessens from 20KHz and depending on how well we look after our ears it can lessen naturally and safely or dramatically until hearing is impossible without the use of an aid. If a person exposes themselves to very loud sounds daily, their hearing will be far worse than someone who take preacutions protects their hearing and they could in extreme cases end up with a condition like tinnitus which is a ringing in the ears caused by damaged nerve endings, which can be annoying and painful. The Who guitarist Pete Townshend suffers from this from his exposure to loud noise playing in the band, but people who work in jobs at factories with loud machinery or on runways near airoplane engines can suffer from it. Ways of preventing these problems would be to wear protective ear muffs which prevent a lot of the loud sound waves getting to the ears and causing this damage. Often when musicians play live they where more discreat ear plugs which helps them to maintain good hearing despite their exposure to loud music playing shows almost every night on a tour. There are remedies for hearing loss both herbally and medically, but often they cannot restore the hearing to as good as it should be so the best option is to never allow your hearing to be damaged in the first place. The most common type of hearing damage is noise induced, but sometimes it can be genetic in a condition like tinnitus, and often the only solution provided by the National Health Service is a hearing aid.
Having two ears enables us to hear in stereo and enhances our ability to perceive the origins of sound or where the sound is being produced from 3 dimensionally. So having two ears placed either side of our head would allow us to detect where a predator was, and through evolution help us to survive in those circumstances.