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Author: admin | 12.02.2014 | Category: Tinnitus Hearing Loss

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. 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. 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. 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.
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. 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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