The Ear. The ear can be divided into three major parts: the outer ear, the middle ear and the inner ear.

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2 The Ear The ear can be divided into three major parts: the outer ear, the middle ear and the inner ear.

3 The Ear There are three components of the outer ear: Pinna: the fleshy outer part of the ear which directs sound inwards. Auditory Canal: the passage leading towards the eardrum. Resonance effects in the auditory canal amplify sound at frequencies in the range from 1000 to 6000 Hz. Eardrum (Tympanum): the membrane which vibrates following the pressure variations of the incoming sound waves.

4 The Ear The middle ear consists of three tiny bones called the ossicles. They are the malleus (hammer), the incus (anvil) and the stapes (stirrup). The ossicles transmit the vibration of the eardrum to the inner ear.

5 The Ear The ossicles amplify sound in two ways: The system of lever arms approximately doubles the output movement amplitude at the oval window with respect to the input amplitude at the eardrum. The area of the oval window on the cochlea is only 1/15 that of the vibrating area of the eardrum, thus the oval window sees a pressure amplification of 15 with respect to the eardrum even without the action of the lever arms.

6 The Ear The ossicles serve a second important purpose which is to protect the ear from intense sounds by selectively decreasing the amplification. When the movement amplitude is large, the geometry of the lever arms is such that the angle between the stapes and the oval window is reduced. The ear is also protected against intense sounds by a reflex which contracts a small muscle attached to the stapes and another attached to the eardrum, thus stiffening the membrane.

7 The Ear The ossicles are surrounded by air whose pressure is kept equal to that of the surroundings by the Eustachian tube which connects the middle ear to the back of the throat. If it were not for the Eustachian tubes a pressure difference could occur between one side and the other of the eardrum which would cause the membrane to bulge and stiffen, making it less responsive to incident sound waves.

8 The Ear The oval window which receives the vibration from the ossicles is at the end of one of the three canals which run the full length of the cochlea. The vestibular and the tympanic canals contain a fluid which resembles salt water, while the third canal, the cochlear duct, is filled with a more viscous fluid. Several structures are present in the cochlear duct and it is here that the transduction of mechanical energy to electro-chemical nerve energy takes place.

9 The Ear The most important structures of the cochlear duct are the basilar membrane and the organ of Corti. The organ of Corti rests on the basilar membrane along its entire length and consists of about 23,500 receptor cells which have protruding steriocilia. Mechanical stimulation of the steriocilia produces electro-chemical impulses which pass to the central nervous system through the auditory nerve.

10 The Ear The neurons of the auditory nerve perform much signal processing. There are cells which respond to complex noises but not to a pure tone. Others respond only to brief transient sounds such as clicks, while others respond to a variety of pure tones. The most common type of nerve cell is however called the tuned neuron because it responds maximally for only a very restricted range of frequencies, as shown in the case of the four tuned neurons presented above.

11 Decibel Scale Since the ear is a very sensitive organ special measures are used to represent the wide range of amplitudes that it can respond to. Sound is normally considered in terms of its power. Two sound levels can be compared by determining how many powers of 10 (the logarithm) one sound exceeds the other by. Each power of 10 is defined to be a unit of one bel. For example, if one power level is one million times greater than another (10 6 times greater) we say that it is 6 bels greater.

12 Decibel Scale The bel is a rather large unit when compared to normal hearing levels therefore it is common practice to work with units of decibels, which are one-tenth of a bel. db 10 Log10 power reference power where the reference power value is commonly taken to be Watts.

13 Decibel Scale Unfortunately, no instrument directly measures the power of a sound source. The power is calculated from the measured sound pressure. We therefore speak of the Sound Pressure Level (SPL) acting on the ear. The decibel value for a sound pressure measurement is db 20 Log10 pressure reference pressure The reference pressure is commonly taken to be 20 μ Pa, which is the average value of the perception threshold for sound measured in young adults at 1000 Hz.

14 Decibel Scale A word of caution regarding the use of decibel scales. Addition or subtraction of Sound Pressure Levels must be performed on the original pressure values, requiring the use of antilogs. Decibel values cannot simply be added together!

15 Decibel Scale Nomograms can be used to simplify the task of adding decibel values. They provide the number of decibels that must be added to the larger value given the difference in decibels between the two sources. As an example, two machines which each generate 100 db of noise would, if operated in phase, generate a total noise of 103 db (not 200 db).

16 Decibel Scale From the threshold of human perception to the threshold of human pain the range is about 140 decibels, i.e. 14 orders of magnitude of the sound power.

17 Range of Human Hearing The frequency range of adult human hearing is from 20 to 20,000 Hz. In adults frequencies below 20 Hz are sensed only as vibration (infrasound). Young children can hear higher frequencies than adults, reaching as much as 27,000 Hz in some cases. While the minimum frequency remains roughly constant, the maximum drops progressively with age. By age 60 the hearing loss for an 8,000 Hz pure tone is more than 40 db.

18 Range of Human Hearing The maximum pressure variation the human ear can tolerate is roughly 280 dynes/cm 2 above or below atmospheric pressure, where 1 dyne is the force needed to accelerate 1 gram at 1 cm/s 2. At the hearing threshold level of dynes/cm 2 the air molecules are displaced about cm, which is only one-tenth their diameter.

19 Loudness Loudness is a complex subjective experience which depends on both the intensity and the frequency of the sound. Several numerical loudness indices have been developed, two early ones were the phon and the sone.

20 Loudness The phon was developed from experiments which used pure tone sounds of fixed frequency and amplitude. In each test the participant was presented a 1000 Hz pure tone as a reference, then the frequency was changed and the participant was asked to adjust the amplitude of the new sound until it was of equal loudness. By testing many frequencies and many people it was possible to define equal-loudness curves.

21 Loudness From the equal loudness curves it can be seen that human perception varies as a function of frequency. Humans are particularly sensitive to frequencies from 1000 to 6000 Hz.

22 Loudness The phon was designated the unit of loudness and was set equal to the decibel level of the 1000 Hz reference tone. All tones judged to be of equal loudness to the 60 db reference tone were designated as having a loudness of 60 phons.

23 Loudness The frequency weighting networks used in sound level meters are based on the phon curves developed by Fletcher and Munson. The A and B frequency weightings are the 40 and 70 phon contours, but with some minor modifications to simply the required electrical filter network.

24 db(a) Loudness The A-weighted Sound Pressure Level L A is defined as L A 10 Log 10 p p ( t) referecne db Where p A (t) is the instantaneous sound pressure measured using the standard A scale frequency weighting shown below. A 2

25 Loudness Phon curves provide information about the equivalence of sounds, but not about the absolute level of perceived loudness. We cannot say, for example, how many times louder a 40 phon sound is with respect to a 20 phon sound. Fletcher and Munson therefore performed further tests with a rating scale which was later named the sone. One sone is defined as the loudness of a 1000 Hz tone of 40 db (40 phons). A sound which is judged to be twice as loud as the 1000 Hz standard reference tone has a loudness value of 2 sones, a sound judged three times as loud is 3 sones, etc..

26 Loudness The graph presents the relationship between the level in phons and the perceived loudness in sones for pure tone sounds. The perceived loudness grows rapidly with increasing sound pressure, particularly at lower levels.

27 Loudness The graph above gives an approximate indication of the sone values of some typical sounds from everyday life.

28 Annoyance One term that is sometimes used to describe the effects of unwanted sound is annoyance. Annoyance is a subjective quantity associated with the inappropriateness or unwantedness of the sound. It is important to note that the loudness value of a given sound is only weakly correlated with its annoyance.

29 Design Classic: Harley-Davidson Sound Harley-Davidson attempted to register as a trademark the distinctive engine "chug". In February 1994 the company filed its application with the following description: "The mark consists of the exhaust sound of applicant's motorcycles, produced by V-twin, common crankpin motorcycle engines when the goods are in use". Nine of Harley Davidson's competitors filed oppositions against the application, arguing that cruiser-style motorcycles of various brands use the same crankpin V-twin engine which produces the same sound. After six years of litigation Harley Davidson withdrew the application.

30 Auditory Communication Auditory communication is defined as the transmission of information from a source to a receiver by means of sound. It can be either verbal or non-verbal in nature. Auditory communication requires environmental noise levels which are sufficiently low to permit the detection of the information carrying sounds.

31 Articulation Index The hearing threshold, the overload region and the typical speech region for a male raised voice at 1 metre distance are presented below as a function of frequency.

32 Articulation Index An articulation index of 100% is a situation where the spectrum levels of speech at the listener s ear lie above the threshold of hearing and below hearing overload. The speech levels must however also be above those of the background noise. If the noise spectrum covers part of the speech region, or if part of the speech region falls below the threshold of hearing, the articulation index is less than 100%

33 Articulation Index For an AI of 0.6 or more the communication will be satisfactory, while for an AI of 0.3 or less the communication will be unsatisfactory. There are however other factors which influence intelligibility. An example is provided by the NASA test results.

34 Design Classic: Aliph Jawbone The Jawbone uses a voice-activity sensor that rests on the cheek to determine when the person is talking. When not, a pair of microphones samples the sound to create a noise profile of the environment. The system subtracts the ambient noise when speaking IDSA Design of the Decade Gold Award 2010 Mobile Choice consumer awards - Best Bluetooth headset 2008 Engadget's Wearable Device of the year award 2008 International Design Excellence Awards Finalist 2008 Mobile Choice consumer awards - Best Bluetooth headset 2008 TIME Magazine - '50 Best Inventions of the Year' 2008 Spark Award - Gold Award 2007 International CES Innovations Design and Engineering Award 2007 BusinessWeek/IDSA International Design Excellence Award 2007 International Design Awards 2007 Spark! Award 2006 if Product Design Award 2005 CES Innovations Award 2004 BusinessWeek Industrial Design Excellence Award 2004 Fortune - Best Products of the Year: Electronics 2004 Chicago Athenaeum Museum of Architecture and Design Award

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