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The human auditory system is usually divided into 3 parts-- the inner, middle and outer ear.
The middle ear starts at the eardrum and comprises also the ossicles, which are connected to the oval window of the cochlea. Another function of the eardrum is to separate air in the auditory canal from that in the middle ear, which is connected to the larynx via the eustachian tube.
It's worth spending some time examining the function of the cochlea, which is depicted schematically below.
The cochlea is ultimately responsible for converting sounds to electrical signals that encode both the frequency content and spectral amplitude. The cochlea is a somewhat spiral-shaped organ comprising two fluid-filled regions, the scala vestibuli and the scala timpani. The shape of the scala media, bounded on one side the Reissner's membrane and the other by the tectorial membrane is specialized such that the tectorial membrane vibrates maximally at locations determined by the frequency content of the sound signal.
The concept of a correlation between fluid pressure wave maxima and the frequency content of a sound signal is called the Place Theory of Hearing. The critical band describes the zones of the basilar membrane that respond maximally to particular frequencies of pure tones. We can first test what happens as two identical frequencies are separated by slowly changing the frequency (pitch) of one of the function generators.
The textbook speaks of a 'courseness of tone' just when the beats get fast and just before we can distinguish two tones.
We can also set up one function generator to frequency modulate the other about some central frequency. Earlier we mentioned a neural processing technique called sharpening that allows us to hear very small changes in frequency.
As mentioned earlier, the amplitude response (dynamic range) of the human ear is very broad.
Technically speaking, sound intensity levels are expressed in Bels (named after Alexander Graham Bell), and a decibel is one tenth of a Bel.
Further, a 30 dB decrease in sound intensity is a reduction by a factor of 1,000 (divide original intensity by 1,000). The decibel scale of SIL corresponds to what we hear, in that a 10dB increase in SIL is heard as the same change in loudness, irrespective of the starting sound level. There are other tidbits in the textbook, including an interesting discussion of fundamental tracking.
The reduced pressure between the vocal folds causes them to pull together and nearly close, causing air pressure to build up in the trachea below the folds.
The lips are generally used to set the frequencies of the lowest formants, while the tongue, soft palate and other parts are used to shape the 'vocal tube' to emphasize or deemphasize higher frequency formants. It would be helpful to see some wave shapes, Fourier spectra and spectragraphs for various formants, best simulated by singing single note vowel sounds such as 'oo,' 'ah,' and 'ee'.
If there is a 'guest soprano' in class on Tuesday, we can record and examine what happens when that individual sings high notes. A cholesteatoma is a skin growth that occurs in an abnormal location, the middle ear behind the eardrum. Cholesteatoma is a serious but treatable ear condition which can only be diagnosed by medical examination. Otitis media is the inflammation of the middle ear and occurs most commonly during the winter and early spring months. Remember, without proper treatment, damage from an ear infection can cause chronic or permanent hearing loss. Earwax is healthy in normal amounts and serves to coat the skin of the ear canal where it acts as a temporary water repellent. Most of the time the ear canals are self-cleaning; that is, there is a slow and orderly migration of ear canal skin from the eardrum to the ear opening. When cerumen is removed from the ear, the physician uses sucion, a cerumen spoon, or delicate forceps to do so.
A perforated eardrum is a hole or rupture in the eardrum, a thin membrane that separates the ear canal and the middle ear.
Middle ear infections may cause pain, hearing loss, and spontaneous rupture (tear) of the ear-drum resulting in a perforation. On rare occasions a small hole may remain in the eardrum after a previously placed PE tube (pressure equalizing) either falls out or is removed by the physician. Most eardrum perforations heal spontaneously within weeks after rupture, although some may take up to several months. The most common symptoms of swimmer’s ear are mild to moderate pain that is aggravated by tugging on the auricle and an itchy ear. WARNING: If you already have an ear infection, or if you have ever had a perforated or otherwise injured eardrum, or ear surgery, you should consult an ear, nose, and throat specialist before you go swimming and before you use any type of ear drops. The tympanogram is a test that measures how easily the eardrum vibrates back and forth and and what pressure the vibration is the easiest. The ABR is a special hearing test that can be used to track the nerve signals arising in the inner ear as they travel through the hearing nerve (called the auditory nerve) to the region of the brain responsible for hearing.
The ABR can also be used on small infants since it requires no conscious response from the person being tested.
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Amazing all-natural Tinnitus breakthrough permanently eliminates Tinnitus issues without drugs or surgery. This can be compared to the human eye, which can detect light frequencies (colors) that vary by less than a factor of 2. As mentioned before, the eardrum and the ossicles provide preamplification of sound signals. Holes in the eardrum allow unwanted, rapid equilibration between canal and middle ear pressures, which ultimately reduces the preamplication of sounds, particularly for lower frequencies. It even detects phase differences between signals from the same source detected at each ear for purposes of location. Sounds signals transmitted by the ossicles to the oval window set up pressure waves within fluids in the scala vestibuli which travel down a narrowing chamber (the scala vestibuli) and continue along the scala tympani. These localized vibrations cause shearing of the stereocillia (or hair cells) which, in turn, excite electrical impulses in the organ of corti which ultimately drive signals along the auditory nerve. An experimental observation that two pure tones separated by an octave (a factor of 2 in frequency) excite equally-spaced regions along the sensor-studded basilar membrane. What do we hear when, instead, if we start with identical frequencies and slowly move them apart? This is useful for observing the minimal frequency variation where we can hear a difference in tones-- called the Just Noticeable Difference (or JND). This process evidently uses the higher frequency content (think overtones) of signals to improve (fundamental) frequency resolution. The ear can sense variations in pressure of 1 part in 10,000,000,000 (10^10th) of atmospheric pressure. The decibel is set up so that every increase in sound intensity level by a factor of ten corresponds to a 10dB increase.
In other words, we would interpret an increase of SIL from 10 to 20 dB as an equivalent change in loudness to that from 80 to 90 dB. This phenomenon is observed when we hear two successive overtones (but not the fundamental) of an overtone series. Air moving faster through constrictions in the vocal folds (gaps between them, etc.) creates a reduced pressure via the Bernoulli principle.
The speaker or singer controls the frequency of the source sound by tensioning the vocal flaps, and controls amplitude by varying the average air pressure available just below the vocal folds.
The latter causes serves to shorten the length of the resonant cavity-- opening ones mouth widely moves the position of the pressure node to just inside the mouth for example. Guitar players, think of how you change where you pick in order to emphasize higher overtones or not. These funnel through the ear opening, down the ear, canal, and strike your eardrum, causing it to vibrate. It is usually due to repeated infection, which causes an ingrowth of the skin of the eardrum. Persisting earache, ear drainage, ear pressure, hearing loss, dizziness, or facial muscle weakness signals the need for evaluation by an otolaryngologist-head and neck surgeon.
Old earwax is constantly being transported from the ear canal to the ear opening where it usually dries, flakes, and falls out.
However, if you want to clean your ears, you can wash the external ear with a cloth over a finger, but do not insert anything into the ear canal. It typically occurs in swimmers, but the since the cause of the infection is water trapped in the ear canal, bathing or showering may also cause this common infection.
If you do not know if you have or ever had a perforated, punctured, ruptured, or otherwise injured eardrum, ask your ear doctor.
This graph shows the amount of hearing loss expressed in units called decibels at different sound frequencies (also called Hertz).
The middle ear is normally filled with air at a pressure equal to the surrounding atmosphere. A special probe is placed up against the ear canal, like an ear plug, and the equipment automatically makes the measurements. The test is useful because it can tell us where along that path the hearing loss has occurred. The test involves running a cool liquid and then a warm liquid through the ear canal (it is usually done through a small tube so the ear itself remains dry). The dynamic range-- the difference between the quietest and loudest sounds we can hear-- is about 130dB (decibels, we'll get to these in a while) which corresponds to a factor of 10^12th (that's 10,000,000,000,000).


The eardrum is also equipped with a specialized muscle-- the tensor tympani muscle-- which can help dampen loud sounds and protect the inner ear.
The cochlea is where pressure waves in fluids, excited by movement of the stirrup bone against the oval window, are converted into electrical impulses that travel up the auditory nerve to the brain. The round window at the far end of the combined chambers is set up to minimize reflections of pressure waves which would travel backwards. The stereocillia and organ of corti rest upon the basilar membrane which is about 3.5 cm long and contains about 30,000 nerve endings (hair cells). It also corresponds to about 15 percent in frequency, slightly less than a minor third in musical terms. We can experiment by discerning what our frequency resolution is for square waves as opposed to sine waves (the latter have minimal overtone content when produced by our function generator). Loud sounds cause pain (the effective upper limit) at about 1 part in 10,000 (10^4th) of atmospheric pressure.
Because the dynamic range of human hearing is very large (factor of 1,000,000), and because the frequency response of the ear is non-linear (the same ratio between two frequency stimuli is always perceived to be the same musical interval), we use the logarithmic decibel (dB) to express sound intensity level (SIL). You will often see loudspeaker specifications indicating the frequencies where sound intensity is halved, or decreased by 3dB. The neural mechanism of the ear recognizes the two overtones as part of a harmonic series and will be heard as the fundamental. Those sound waves are filtered by changing the shape of the vocal tract comprising the larynx, the pharynx above it, the mouth and the nasal cavity. An analogy is a perfume atomizer, where air is blown across a small opening above a bottle of perfume and the low pressure at the orifice draws out very small beads of perfume into the air stream. Thus, the vocal folds are regularly closing and opening, creating sound by varying pressure in the larynx above.
The resonant frequencies of this chamber are controlled by altering the shape of these cavities and, thus, serve to filter the sounds produced. The vibrations are passed to the small bones of the middle ear, which transmit them to the hearing nerve in the inner ear. Cholesteatomas often take the form of a cyst or pouch that sheds layers of old skin that builds up inside the ear. The middle ear is connected to the nose by the eustachian tube, which equalizes pressure in the middle ear. When water is trapped in the ear canal, bacteria that normally inhabit the skin and ear canal multiply, causing infection and irritation of the ear canal. Simple tests, such as ones done in many schools, may be useful for screening, but a careful audiogram is necessary for accurate diagnosis of most hearing problems. If the middle ear is filled with fluid, the eardrum will not vibrate properly and the tympanogram will be flat. For example, the ABR is often used for individuals with a sensorineural (nerve) loss in just one ear. Special electrodes automatically record the nerve signal; the patient can even be asleep during the test. This change in temperature stimulates the inner ear which in turn causes rapid reflex movements of the eyes.
This mechanism isn't foolproof, there is a time lag between when loud sounds are sensed and when the muscle is completely activated.
Sound pressure waves within the scala vestibuli are propagated through the Reissner's membrane into the scala media, which contains a fluid with different properties than those in the other scala.
The neural mechanism has evolved to receive this broad range of pitch and narrow it down to a more precise frequency range-- a process called sharpening. The rate at which the folds open and closed is controlled by tension which is controlled by muscles.
Thinking about the boundary conditions of this chamber, we note that the bottom of the larynx, just above the vocal folds is a pressure antinode. The principal vocal formants are at these frequencies, but can be manipulated simply by manipulating the tube diameter at the position of nodes and antinodes for various overtones. Here, the vibrations become nerve impulses and go directly to the brain, which interprets the impulses as sound (music, voice, a car horn, etc.). Over time, the cholesteatoma can increase in size and destroy the surrounding delicate bones of the middle ear. If the middle ear is filled with air but at a higher or lower pressure than the surrounding atmosphere, the tympanogram will be shifted in its position. This movements are recorded, and from these we can get information about how well this balance mechanism is functioning. The customers feedback can engage you a great of the value and uppercase satisfaction of the product. Another page at (hyperphysics, Georgia State University {I presume this is what gsu stands for}) is also informative.
The pinna collects more sound that would normally be available at the opening of the auditory canal, thus providing a modicum of preamplification.
Dynamic range also varies according to frequency, with greater sensitivity between about 1000 and 6000 Hertz (approximately, this varies by individual). Instead of producing the fundamental with a very large pipe, the overtones for that fundamental frequency are played at the right proportions (using a smaller pipe) to trick the ear into hearing the fundamental (low) frequency. Some men can produce a falsetto voice-- producing nearly sinusoidal sound by highly tensioning the vocal folds. The other end, at the opening of the mouth, is a pressure node like the end of an open tube. Hearing loss, dizziness, and facial muscle paralysis are rare but can result from continued cholesteatoma growth. If the ABR is normal along that region of the path, the chances of having this tumor are quite small.
The area of the eardrum (tympanic membrane) is about 15 times that of the oval window dividing the inner and middle ear. We recall from earlier lectures that the overtone series for this type of chamber are odd harmonics. The overtone content (amplitudes of the odd harmonic overtones) can be adjusted by shaping the resonant cavities, forming vocal formants. The ossicles-- three bones called the hammer, anvil and stirrup, also provide about 3x preamplification.




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