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 the mechanical form into the electrical form, the electrical impulses reach the brain through nerve endings. Here they get elaborated, allowing sound perception to take place, and finally music 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 we'll know what sound-parameters we'd 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 every now and again appears from in between the other instruments. If we want it to be an ethereal presence, enveloping, indefinite, we can manipulate the sound and remove its high frequencies. All at once we'd see how one of the most relevant factors in knowing the direction of sound is its high-frequency content. In other words it is easier to know the direction of a sound with high high-frequency content, rather than one which has only low frequencies. So, if we want the little flute to be nice and 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 well. The sound gets reflected by the auricle and chanelled towards the ear canal whose length is approximately 3 cm.
Resonance frequency of an ear canal - there is an empirical formula that gives us the resonance frequency [Characteristics of a loudspeaker: Resonance frequency of a loudspeaker ] of a tube, with which we will compare our ear canal. Yes, numbers again! Yes, formulae! But the result of this calculation is important, so don't give up and keep reading!
The formula says that a tube of length d filled up with air, has a resonance frequency of approx:
Equation 2.1. Calculation of the resonance frequency of the ear canal
From the wave length we find the resonance frequency:
Still alive? If you are, you have just discovered that the resonance frequency of a human ear is approximately 3KHz. This means that when a group of frequencies of about 3KHz reach the ear, the ear canal resonates and so the frequencies undergo a natural amplification. We'll now see in how many cases this value in the audio field is put into action and you'll be able to happily say that you've overcome yet another obstacle to get to the core of this small yet important concept.
The ear canal terminates in a membrane, the ear-drum, that vibrates in accord 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 transmit it to the cochlea, another little bone whose function we'll shortly explain. This amplification is necessary seeing that whilst the ear-drum is a very thin, suspended membrane, the cochlea is filled with a dense fluid and is thus far more difficult to make vibrate. The three tiny bones are kept together by a series of small ligaments that have another function other than the one just mentioned, and this is to impede exeedingly large vibrations to take place and therefore to avoid the risk of damage in cases when the ear is exposed to overly high pressure levels. An opening within the middle ear takes us to the so-called Eustachian tube which is a tube connected to the oral cavity. Its function is to give vent towards the outside in order to balance out the atmospheric pressure on both sides of the ear-drum (that's why underwater, in order to compensate the external pressure, you should block your nose and blow hard, thus increasing the internal pressure).
This part of the ear converts mechanical energy into electric impulses to send to the brain for the elaboration of 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 the brain uses for the perception of balance, but this goes beyond what we're analysing here). This fluid receives the stapes' vibrations through the oval window and transports it inside where the main organ that converts mechanical energy into electric impulses is: the organ of Corti. Inside the organ of Corti we find the basilar membrane that has innumerable lashes on its surface: circa 4000 of them that vibrate in unison with the vibration of fluid. Every group of lashes is connected to a nervous termination that converts the vibration received from the fluid into electric impulses to send to the brain to be elaborated and perceived as sounds. The reason why the human ear perceives frequencies logarithmically is because of the main membrane's nature. The groups of lashes called critical bands are infact each sensitive to a segment of 1/3 of an octave frequency. In other words the main membrane is divided into sectors each of which is sensitive to a certain frequency-band (each 1/3 of an octave in frequency) and as such behaves as a kind of spectrum analyzer. Every time the sound increases by an octave a membrane that is equidistant to the previous one gets stimulated, consequently acting in a logarithmic manner [Decibels ] .
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Human ear