The human ear acts as a transducer of acoustic energy, firstly into mechanical energy and secondly into electric energy. Once the energy has been converted by the ear from mechanical to electrical, the electrical impulses reach the brain through nerve endings. Here they get processed, allowing sound perception to take place, and finally sound is heard.
The hearing apparatus is divided into three areas: the external ear, the middle ear, and the internal ear.
Analysing the way these three areas work will allow us to understand the sound perception mechanism and to know what sound parameters we need to modify in order obtain the desired result. Speaking of which, let us now consider the following example. Let's suppose we're mixing a track where a flute pops up every now and again from in between the other instruments. If we want it to be an ethereal, enveloping, indefinite presence we can manipulate the sound so as to remove its high frequencies. In a moment we'll see how one of the most relevant factors in pinpointing the direction of sound is its high frequency content. In other words it is easier to know the direction of a sound with a high frequency content, rather than one which has only low frequencies. So, if we want the flute to be well present yet distant in our mix, we'll turn our pan-pot [Panpot ] to the right and we'll increase high frequencies (naturally paying attention to not denature the sound).
The first organ that sound meets when it reaches the ear is the pinna. The latter offers a large surface for sound and allows a wide portion of the wave front to be picked up. To create a wider surface just bring your hands to your ears as we all do instinctively when we try and listen to a sound we can't hear too well. The sound gets reflected by the pinna and chanelled towards the ear canal whose length is approximately 3 cm.
Resonance frequency of the ear canal - there is an empirical formula that gives us the resonance frequency [Resonance frequency of a loudspeaker ] of a tube, which in our case plays the role of the ear canal. Yes, numbers again, but the result of this calculation is important, so don't give up and keep reading!
The formula states that a tube of length l filled up with air, has a resonance frequency of approximately (considering that the lenght of the ear canal is around 3 cm):
Equation 2.1. Calculation of the resonance frequency of the ear canal
Knowing the wavelength we calculate the resonance frequency value:
Still alive? If you are, you have just discovered that the resonance frequency of a human ear is approximately 3 KHz. This means that when a group of frequencies of about 3 KHz reach the ear, the ear canal resonates and so the frequencies undergo a natural amplification. Next we'll see how often this value is employed in sound engineering.
The ear canal ends up in a membrane, the ear drum, that vibrates along with the sound that has reached the ear. On the opposite side of the ear drum there are three tiny bones called: incus (or anvil), stapes (or stirrup)and the malleus (or hammer). Their function is to amplify the vibration of the ear drum and to transfer it to the cochlea, another little bone whose function we'll shortly explain. This amplification is necessary seeing that whereas the ear drum is a very thin, suspended membrane, the cochlea is filled with a dense fluid and therefore vibrates far less easily. The three tiny bones are kept together by some small ligaments that have another function as well as amplifying. Namely they avoid the ear drum from following overly large vibrations and therefore limiting the risk of damaging it when it is exposed to excessively high pressure levels. An opening in the middle ear takes us to the so-called eustachian tube which reaches the oral cavity. Its function is to give outlet in order to balance out the atmospheric pressure on both sides of the ear drum (that's why when swimning underwater, you should block your nose and blow hard, thus increasing the internal pressure in order to compensate the external pressure).
This part of the ear converts mechanical energy into electric impulses which will be sent to the brain to be processed as sound. The last of the three little bones we mentioned, the stapes, is in contact with the cochlea through a membrane called the oval window. The cochlea is a bone shaped like a snail's shell containing fluid (it has three little circular canals directed towards the three space directions. Sense of balance comes from the cochlea). This fluid receives the stapes' vibrations through the oval window and carry them where the main organ that converts mechanical energy into electric impulses resides: the organ of Corti.
Inside the organ of Corti we find the basilar membrane that has thousands of hair-cells on its surface, about 4000 to be precise, all of them vibrating together with the vibration of fluid. Every group of hair-cells is connected to a nervous termination that converts the vibration received from the fluid into electric impulses.