Chapter 6: The Human Ear and Voice The study of the human ear and the process by which sound waves are received and transmitted to the nerve endings that convert mechanical vibrations into electrical impulses. Our study will relate the basic anatomy of the peripheral auditory system to the important features of the place theory of hearing. The frequency and amplitude responses of the ear. Vocal system and the production of speech and musical sounds. Analysis of speech patterns.

Peripheral Auditory System The human ear is one of the most amazing organs of the body. Possesses an incredible range of sensitivity in frequency and amplitude responses. Frequency response: 20 Hz  20 kHz. A factor of about 1000 or ten octaves Amplitude/Intensity response: Pressure variations of about 1,000,000 to 1. Intensity variations from 10-12 W/m2 – 1 W/m2.

Peripheral Auditory System Three Parts: Inner ear Middle ear Outer ear

Function of the Outer Ear

Eardrum

Middle Ear Consists primarily of the bone chain of the three ossicles: the hammer, the anvil, and the stirrup. These bones amplify the vibrations coming from the eardrum and transmit to the fluid of the inner ear. Muscles attached to the ossicles help to limit the vibrations of very large-amplitude continuous sounds. Sharp noises such as gunshots or loud music may occur too quickly for the protective mechanism to prevent damage to the middle ear.

Cochlea

Oval and Round Window

The organ of Corti

Inner Ear and Balance Inside the inner ear are three semicircular canals. Each canal contains tiny hairs, crystals, and fluid. These structures help the canals sense up-and-down, forward and backward, and side-to-side motion. Nerves carry the signals from the canals to the brain.

Place Theory of Hearing The place theory hearing correlates frequency with location of the response along the basilar membrane.

Place theory of hearing Place theory of hearing states that our perception of sound depends on where each component frequency produces vibrations along the basilar membrane. Therefore, the pitch of a pure tone would be determined by where the membrane vibrates. In technical terms, it states that frequency is encoded according to the tonotopic organization of the neurons. Place theory competes with the rate theory of hearing, which instead states that pitch is signaled by the rate at which the neurons fire.

Pitch sharpening The relative shortness of the basilar membrane contrasts with the large number of pitches which people can distinguish. Place theory is generally seen as incomplete, lacking a mechanism which would explain our large pitch resolution. Research using modern cochlear implants suggests that the perception of pitch may depend on both the neurons' location and rate at which they fire. http://hyperphysics.phy-astr.gsu.edu/hbase/sound/place.html

Anatomy of the Ear Sound enters the ear, travels through the auditory canal, and reaches the eardrum. The auditory canal is approximately a tube open at only one end. The other end is closed by the eardrum. A typical length for the auditory canal in an adult is about 3 cm. The speed of sound is 343 m/s. What is the fundamental frequency of the canal? (Interestingly, the fundamental frequency is in the frequency range where human hearing is most sensitive.)

Amplitude Response of the Ear

Human Ear and Sensitivity Audible frequency range: 20 Hz – 20,000 Hz Audible intensity range: 10–12 W/m2 - 1 w/m2 10–12 W/m2 = Threshold of hearing 1 W/m2 = Threshold of pain

Decibels The decibel (dB) is a measurement unit used when comparing two sound intensities. The intensity level b  (expressed in decibels) relative to the threshold of hearing, Io is defined as follows:

Typical Sound Intensities and Intensity Levels Relative to the Threshold of Hearing   Intensity I (W/m2) Intensity Level b (dB) Threshold of hearing 1.0 × 10-12 Rustling leaves 1.0 × 10-11 10 Whisper 1.0 × 10-10 20 Normal conversation (1 meter) 3.2 × 10-6 65 Inside car in city traffic 1.0 × 10-4 80 Car without muffler 1.0 × 10-2 100 Live rock concert 1 120 Threshold of pain

Periodicity If two or more tones whose frequencies are successive harmonics in some overtone series sounded simultaneously, an additional frequency will be “heard” by most listeners, particularly trained musicians. Periodicity: The neural mechanism of the ear is able to identify the two frequencies as being members of an overtone series and assigns to the total stimulus the fundamental frequency. This is called periodicity pitch, missing fundamental, or subjective fundamental.

Fundamental Tracking If one plays a sequence of pairs of tones that have frequency ratio 3 to 2 but have different fundamental frequencies, the ear will recognize the periodicity of each pair of notes and supply the fundamental tone for each. This is called fundamental tracking. Eg: 200 and 300 Hz followed by 300 and 450 Hz. Fundamental tracking is used in the production of very-low-frequency organ tones by sounding the 2nd and 3rd harmonics together.