Hearing Chapter 5

Range of Hearing Sound intensity (pressure) range runs from watts to 50 watts. Frequency range is 20 Hz to 20,000 Hz, or a ratio of 10 3 between highest and lowest frequencies.

Intensity Power Rate of work (Work/Time) Pressure Force per unit area (F/A) Intensity Power per unit area carried by a wave, expressed in watts/square meter (W/m 2 ).

Frequency Unusual to find someone who can hear the entire “normal” range of 20 — 20,000 Hz.

Structure of Ear 3 Parts: Outer, Middle, Inner Outer: Pinna and auditory canal, terminated at ear drum. Middle: begins with ear drum, ossicles Ossicles function as mechanical transformer Eustachian tube Inner: semi-circular canals and cochlea.

Transduction Transduction is the process of transforming energy from one form to another. Middle Ear: changes in air pressure to mechanical energy Inner Ear: mechanical pressure to hydraulic variations to neural impulses (electrical)

The Ear

The Outer Ear Pinna Helps to collect sound and aids in the localization of high sounds (above 4 kHz). Auditory Canal Acts as a pipe resonator that boosts hearing sensitivity in the range of 2 kHz to 5 kHz.

Middle Ear Eardrum Variations in air pressure are changed to mechanical vibrations. Ossicles Act as a lever, changing small pressure exerted on eardrum into a much greater pressure on the oval window.

Inner Ear Semi-circular canals affect balance Cochlea Receives pressure variations from stapes onto Oval Window. Transforms pressure variations into mechanical vibrations of basilar membrane, and into properly coded neural impulses.

Cochlea Filled with two types of fluid Basilar membrane Organ of Corti Hair Cells Frequency detection, but not pitch.

Basilar Membrane Amplitude of displacement varies by distance (from stapes) as a function of frequency. Higher frequencies show displacement closer to stapes than lower frequencies.

Signal Processing and Critical Bands Critical Bands relate to overlap on the basilar membrane. From Rossing, when two pure tones are so close in frequency that there is considerable overlap in the amplitude envelopes on the basilar membrane, they are said to lie within the same critical band.

Critical Bands Approx. 24 Critical Bandwidth varies with center frequency. 1/3-octave for most of the hearing range

Sound Localization Binaural hearing (Two ears), aids in localizing sound. Below 1000 Hz, localization is due to inter- aural time differences and phase differences. Precedence effect (Haas effect) Above 4000 Hz, localization is due to intensity differences. Between 1 kHz and 4 kHz, our ability to localize sound takes a nose-dive.

Psychophysics The relation of sensation to perception. Multiplication of sensation leads to addition of perception. logarithmic

Logarithms A logarithm is a relationship between a base and that base raised to a power. 100 = 10 2 so log = 2 (log100 = 2) Helps in expressing a ratio relationship between sensation and perception. Related to exponents and scientific notation (discussed earlier)

Logarithmic Identities log AB = log A + log B log A/B = log A - log B log A n = n log A