All About Ears. What Exactly is Sound?. Sound is defined as a mechanical wave that propagates through a medium via local regions of compressions and rarefactions, stemming from the kinetic energy of a sound source. Particles in the medium are displaced by the wave and oscillate. .
Compressions are regions of increased particle density.
Rarefactions are regions of decreased particle density.
Sound moves by displacing molecules in the air, creating high, and low pressure pockets (Compressions and Rarefactions, respectively). Sound pressure is measured in Pascals (Pa).
Perceived loudness correlates roughly logarithmically to its sound pressure.
The ear consists of: The outer ear, the middle ear, and the inner ear.
The Pinna collects sound, acting as as a funnel to amplify sound and directing sound toward the ear canal.
In addition, the pinna doubles by adding directional information to the sound (thus the inherent ability to know which direction a sound is coming from.)
The ear canal is a tube running from the pinna to the eardrum, roughly 26 mm long and 7 mm in diameter.
The outer edge of the ear canal contains hair and wax to help prevent harmful items from entering the ear canal.
Acoustically, the ear canal provides 10 dB of boost to the frequencies 2,000-4,000 Hz. Because of the sensitivity of the ear canal to this range, prolonged exposure of high intensity can lead to hearing damage.
The eardrum is a membrane that transfers sound (coming from the air) into the ossicles of the middle ear.
If the eardrum is damaged / ruptured / infected, it can lead to hearing loss because of sound not reaching the middle ear – conductive hearing loss.
Or… How does sound get from the outer ear to the middle ear?
This vibration then proceeds on to the bone structure of the middle ear.
The function of the auditory ossicles is to transmit sound from the air striking the eardrum to a fluid-filled labyrinth inside the inner ear (Cochlea).
The bones are connected by small ligaments and transmit the vibratory motions of the eardrum to the inner ear.
The bones work in a mechanical way such that the area is ultimately decreased so that less pressure is needed for a larger sound. The resulting vibrations would be much smaller without the levering action provided by the bones.
Strictly speaking, the Eustachian tube does not directly relate to the mechanical process of hearing. However, it is important to consider that the ability to hear is largely contingent upon a correctly functioning Eustachian tube.
The cochlea is a snail-like structure divided into three fluid-filled compartments.
Outer Hair Cells
Inner Hair Cells
Frequency is the measurement of the number of repetitions of an event per unit of time.
Mathematically, frequency = (1/T) where T = the period (time between two consecutive incidences of the same event.)
The frequency of a sound wave holds an inverse relationship to the wavelength.
The frequency of a wave is equal to speed (v) over wavelength (λ / lamda):
(f) = (v/λ)
The Longer the Wave, the Lower the Frequency!
The Shorter the Wave, the Higher the Frequency!
For practical applications, this means that the shorter the wavelength, the higher the frequency, and the longer the wavelength, the lower the frequency. This is why bass notes travel further than treble notes…
For example, imagine a sound wave .7723 M long. Keep in mind that v = 340 M/s (the speed of sound).
f = v/λ
Where v = 340 M/s (Speed of Sound in Air)
And λ = .7723 M (given)
So f = 340 / .7723
Ever notice that music usually sounds fuller, and often times better, when the volume is turned up?
This is because humans do not hear all frequencies at the same level of “Loudness.”
The Equal-Loudness Contour (a continuation and improvement of the “Fletcher-Munson Curve”), shows the differences between sensitivity to sound pressure levels at particular frequencies. There are certain notable discrepancies to the chart, including poor bass and high-end response to audio.
To hear the same amount of Phons of a 20 Hz frequency as a 1,000 Hz frequency, you would need to have to increase the intensity level by nearly 70 dB in the low frequency range.
What does all of this mean?
At higher SPL (sound pressure level / dB), the bisection line splits the human frequency response rate relatively equally.
As the dB level increases, except for the slight dip still present near 4kHz, the level of Phons is fairly flat (small variation) across all frequencies – the discrepancy is most noticable when compared to frequency response at low volumes!
The ear is MOST sensitive from 1kHz to 5 kHz, with a dip at 4 kHz… At low levels, these frequencies are heard well, however, some treble and bass will appear to be lacking. The Loudness button significantly boosts bass and treble at low volumes, to “help” the ear hear the music at a more flat rate.
You are not alone! Fortunately, there are very few sounds not made by a signal generator that approach above 15 kHz.
The frequency range above 10 kHz is generally referred to as “air.” The sounds included here are harmonics of high-pitched sounds such as cymbal crashes in music.
MP3s compress and eliminate much of the “air” frequencies, sometimes leading to a cramped, smaller feel. The worse the bit-rate, the more compression, and more sound left behind.
This is the general difference between “CD Quality” and MP3 Quality. Listen to purchased CDs and compare them to MP3s of 128 bit-rate, and listen for differences.