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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. .

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what exactly is sound
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 and rarefactions
Compressions and Rarefactions.

Sound Pressure.

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.

common factors operating upon a sound before hitting your ear
Common Factors Operating Upon A Sound Before Hitting Your Ear
  • Echoes
    • A reflection off of a hard surface. A true echo is a wave that has been reflected by an obstruction in the medium.
  • Reverberation
    • A buildup of reflections in a space that persists after the original sound has passed. Differs from echoes based on the amount of reflected sound.
slide5
The Doppler Effect: An apparent change in frequency and wavelength perceived by a moving observer in relation to the sound waves. The sound’s pitch is higher as it approaches, and lower as it recedes from the listener.
how do ears work
How Do Ears Work?
  • While your sense of smell, taste and vision all involve chemical reactions, but your auditory system is based solely on mechanical, physical movements. (1)
  • Artificial additives to sound waves such as reverb, echoes, and the Doppler effect all occur before hitting the ear.
anatomy of the human ear
Anatomy of the Human Ear

The ear consists of: The outer ear, the middle ear, and the inner ear.

the outer ear
The Outer Ear
  • The outer ear consists of:
      • The Pinna
      • The Ear Canal
      • The Eardrum
the pinna
The Pinna

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
The Ear Canal

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
The Eardrum

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.

how does an eardrum transfer sound
How does an eardrum transfer sound?

Or… How does sound get from the outer ear to the middle ear?

  • When a pressure wave (compressions and rarefactions) reaches the ear, a series of high and low pressure regions hit the eardrum. The arrival of a compression or high pressure region pushes the eardrum inward; the arrival of a rarefaction pulls the eardrum outward. The continuous arrival of high and low pressure regions sets the eardrum into vibrational motion.

This vibration then proceeds on to the bone structure of the middle ear.

the middle ear
The Middle Ear
  • The middle ear consists of:
      • Auditory Ossicles
      • The Tympanic Cavity
      • The Eustachian Tube (Loosely Speaking)
auditory ossicles
Auditory Ossicles

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.

the tympanic cavity
The Tympanic Cavity
  • The tympanic cavity is an air chamber surrounding the ossicles within the middle ear.
      • When infected, the tympanic cavity can fill with fluids, and a procedure called tympanocentesis, in which a small puncture of the eardrum is made to allow fluids to escape, may be necessary.
        • “This practice, which fell by the wayside to such a degree that very few pediatricians have ever done the procedure, may be coming back into use again because of the dramatic rise in antibiotic resistance of common middle ear disease bacteria.”
the eustachian tube
The Eustachian Tube

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.

what is the eustachian tube
What is the Eustachian tube?
  • The Eustachian tube is a membrane lined tube (approximately 35 mm long) that connects the middle ear space to the back of the nose (the Pharynx).
what are the functions of the eustachian tube
What are the functions of the Eustachian tube?
  • Pressure equalization:
    • The Eustachian tube starts out closed, but can be opened to allowa small amount of air to enter the middle ear to allow pressure equalization with the atmosphere.
        • When this happens, we hear a small “pop.” Yawning and swallowing can cause muscles to tighten in the neck, causing the tube to open.
  • Mucus drainage:
    • The tube drains mucus, keeping ears from becoming “stuffy.” Mucus stuck in the tympanic cavity can develop a high level of pressure, and can lead to ear infections if bacteria becomes present.
the relation between ear infections and the eustachian tube
The Relation Between Ear Infections and the Eustachian Tube:
  • Most commonly by allergies and sickness, the Eustachian tube is prone to swelling and allowing germs into the middle ear, these germs can eventually lead to an infection.
  • In children, the Eustachian tube runs horizontally rather than sloping downward, causing foreign objects to enter the middle ear more easily. This is why chronic ear infections are more prominent in younger children.
  • Ear infections are the second most common diagnosed illness in children behind only the common cold. More than 75% children have had an ear infection by the time they have reached the age of three.
the inner ear
The Inner Ear
  • The inner ear consists of:
      • The Cochlea (Latin for snail.)
the cochlea
The Cochlea

The cochlea is a snail-like structure divided into three fluid-filled compartments.

the process
The Process
  • Inside the cochlea, the vibrational signal from the middle air passes through fluid until reaching the Organ of Corti, where it is turned into electrical impulses when coming in contact with “hair cells” and finally exits through the auditory nerve into the brain.
organ of corti
Organ of Corti
  • The Organ of Corti has highly specialized structures that respond to fluid-borne vibrations in the cochlea by movement of hair cells.
    • The Organ of Corti has “hair cells,” which are located atop a thin basilar membrane.
    • Once destroyed, these hair cells are not replaced. The hair cells responsible for higher frequencies are particularly sensitive and fragile (frequencies used when interpreting human speech!)
hair cells or auditory sensory cells
“Hair Cells” or auditory sensory cells.

Outer Hair Cells

  • Outer hair cells act as preamplifiers for higher frequencies, enhancing frequency selectivity.

Inner Hair Cells

  • Due to the type of fluid [endolymph, a positive-ion rich fluid] surrounding the Organ of Corti, vibrations from the sound open a flow of ions to the cell which results in an electric signal being sent to the auditory cortex when deflected (moved).
the dynamics of hearing pt 1
The Dynamics of Hearing Pt. 1
  • The audible sound range is generally defined as 20 Hz to 20,000 Hz (20 kHz), though some can reportedly hear up to 22 kHz. Scientists define this range as “audio.”
  • Middle aged people tend to lose the ability to hear well from 10-20 kHz, partly due to natural aging, but also due to hearing damage.
what is frequency
What is Frequency?

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

Or…

440 Hz

short long who cares
Short / Long, who cares?
  • Practically speaking, this is why high frequencies come through more clearly through headphones.
  • There is less distance between the sound source and your ears, meaning shorter wavelengths are diffused far less by air, and the sound is ultimately more accurate.
slide33
It is generally considered advantageous to have “flat” hearing. That is, where all frequencies are distributed equally across a sound.
slide34

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.”

loudness is measured in phons
“Loudness” is measured in Phons.
  • Two 60 dB sounds do not necessarily have the same “loudness” due to the inequalities in frequency response.
  • 1000 Hz is generally the reference point for measuring Phons. 60 dB above 1000 Hz is said to be 60 Phon.
slide37

Back to the Equal-Loudness Contour…

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?

slide38

“Ideal” / Flat Hearing.

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!

what does the loudness button in my car do
What does the “Loudness” button in my car do?

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.

testing your hearing
Testing Your Hearing
  • Although humans are generally given the benefit of the doubt in having a dynamic range of 20Hz – 20 kHz, not all people can hear the entire range.
  • The following is a test of your own hearing dynamic range.
keep sound levels low
Keep Sound Levels Low!
  • 10,000 Hz:
  • 12,000 Hz:
  • 13,000 Hz:
  • 11,000 Hz:
slide42
14,000 Hz:
  • 16,000 Hz:
  • 17,000 Hz:
  • 15,000 Hz:
slide43
18,000 Hz:
  • 20,000 Hz:
  • 19,000 Hz:
  • 21,000 Hz:
if you were unable to hear up to 20 khz
If you were unable to hear up to 20 kHz…

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.

compressed music
Compressed 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.

the dynamics of hearing pt 2
The Dynamics of Hearing Pt. 2
  • Humans can typically hear from 10dB up to 140 dB before hitting a Threshold of Pain (the precise dB level of threshold is variable for all people).
  • It has been experimentally shown that a 10dB increase corresponds to a two fold increase in LOUDNESS. Therefore, 40 dB is 2^3 (eight times) louder than 10 dB, quite a significant difference.
what is a decibel anyway
What is a Decibel anyway?
  • The Decibel (dB) is a description of a ratio between two quantities. These measured quantities are generally of intensity or power, although the quantities may also be voltage or sound pressure.
how big is one db
How big is one dB?
  • One dB has been measured to be very close to a “Just noticeable difference” (JND).
  • Here are samples of a sound clip, falling 1 dB at a time. When played sequentially, the difference between the audio seems negligible, but when contrasting the first and last of the series, the difference is evident.
scale of intensity
Scale of Intensity
  • Threshold of Hearing: 0 dB
  • Rustle of Leaves: 10 dB
  • Public Library: 40 dB
  • Normal conversation: 50 dB
  • Alarm Clock: 80 dB
  • Lawn Mower: 90dB
  • Rock Concert: 110-130 dB
  • Gunshot: 140 dB
  • Rocket Launching (from the launch pad): 180dB
what this means
What this Means:
  • A rock concert can be anywhere from 128-512 times as loud as a normal conversation.
  • Though the human pain threshold is at 130 dB, that does not mean that sound below that level is not doing damage to the eardrums. Threshold levels for damage vary over time based on the intensity of the sound.
hearing damage
Hearing Damage
  • Sound at 90 dB (Lawnmower) can damage hearing after 8 hours of exposure per day.
  • Sounds at 100dB (Jackhammer) can damage hearing after 2 hours of exposure per day.
  • Sounds at 110-140 dB (Rock Concert) can damage hearing after anywhere from 3.75 – 30 minutes of exposure per day.
what does this do to ears
What does this do to ears?
  • Prolonged exposure to loud sound can cause a ruptured ear drum.
  • The ear drum helps not solely to produce sound, but also protects against infection of the middle ear. Those with ruptured ear drums must be careful of infection to not risk permanent hearing loss.
  • Hair cells in the cochlea do not get replaced if they are damaged or destroyed. If this happens, the next best thing is a cochlear implant designed to approximate electrical signals.
precautions it is best to avoid hearing damage before it becomes a serious problem
Precautions! It is best to avoid hearing damage before it becomes a serious problem.
  • Hearing loss is permanent but avoidable!
    • Wear hearing protection whenever possible over 90-100 dB.
    • Hearing damage becomes a much more serious concern when loud noises are extended over a long period of time.
    • Do not listen to music at an extensively loud volume, especially over an extended period of time. IPod type ear buds are particularly dangerous because they are inserted directly inside the ear, leaving no time for intensity to be diffused by air.