Chapter 5: Normal Hearing

Objectives (1) Define threshold and minimum auditory sensitivity The normal hearing range for humans Define minimum audible field (MAF) and minimum audible pressure (MAP) Identify three primary psychophysical methods to determine threshold The effects of age on hearing List the major factors influencing auditory threshold The result of an audiogram The primary cues used in auditory localization The difference between air-conduction and bone- conduction

Stimulus Characteristics (1) Stimulus Frequency  Range of sensitivity: 20 to 20,000 Hz  Most sensitive frequency range: approximately 500-5,000 Hz-speech signals  Human audibility curve

Frequency: cycles/sec, or Hertz (Hz, from German physicist Heinrich Hertz) Middle C on piano ~ Hz

Comparative Hearing

Stimulus Characteristics (2) Stimulus Duration  Duration of a stimulus: 10 to about 30 msec.  Beyond about 300 msec, sensitivity is not affected by lengthening signal duration  Patients with hearing loss: some loss of sensitivity for higher frequencies, particularly 4,000 Hz.  Persons with cochlear damages: reduced temporal integration  Temporal integration function: a change in threshold as the duration of a stimulus changes

Stimulus Characteristics (3) Stimulus Intensity  10 to 100 dB SPL  Comfortable listening level (approximately 40 dB)  Uncomfortable loudness level (approximately 90 dB)  A 50 dB range of efficiently usable hearing

Amplitude Range

Methods of Stimulus Presentation (1) Earphones  Minimum audible pressure (MAP)  Not a natural method  Disadvantages: reduced natural amplification provided by the resonance of the outer ear, eliminating the slight increase in sensitivity, eliminating the effects of a reverberant acoustic environment.  Advantages: the increased ease of control of stimuli, isolation of ears, and reduction of ambient noise

Methods of Stimulus Presentation (2) Speakers  Sound field testing  Minimum audible field (MAF): more sensitive than MAP by 6 to 10 dB  Several other contributing acoustic effects:  Reverberation: incident sound-the sound coming directly from a speaker, reflected sound. Incident and reflected sound can interact  Head shadow: a reduction of sound level at the ear on the far side of the sound source  Boby baffle: the acoustic effect of the body’s presence in the sound field

Assessment of Auditory Sensitivity (1) Method of Limits  Changing one parameter of the auditory stimulus (intensity)  Descending approach: decreasing intensity between presentations until it is inaudible  Ascending approach: increasing intensity between presentations until it is audible  Descending approach is 3 to 4 dB more sensitive than ascending approach

Assessment of Auditory Sensitivity (2) Method of Adjustment  Having the subject adjust some parameters (intensity) to a point where it is barely audible.  Tracking threshold

Assessment of Auditory Sensitivity (3) Method of Constant Stimuli  Two-alternative forced-choice procedure  Percentage of time each level of stimulus was detected  Threshold: 50% response level

Assessment of Auditory Sensitivity (4) Listener (Subject) Variables  Subject: determine whether a stimulus is heard or not  The direction given by the examiner  False-positive response: an individual responds frequently when no stimulus is being presented

Assessment of Auditory Sensitivity (5) Age Variables  Presbycusis: age-related hearing loss  Socioacusis: hearing loss usually associated with the aging process  How is Noise-induced hearing loss related?

What is “Normal Hearing”? Audiogram  Graph or table showing thresholds in hearing level at different frequencies  Degree of hearing loss (0-110 dB)  Frequency information (250-8,000 Hz)  Types of hearing loss (Air conduction or bone conduction)

Localization of Sound (1) Intensity Cues  The near side and the far side  Head shadow effect  Azimuth: angle of incidence of a sound wave as it reaches the head. A zero-degree azimuth indicates that the sound is directly in front of the head.

Localization of Sound (2) Time Cues for Localization  Interaural time differences  Minimum audible angle: smallest separation of angles of incidence that can be perceived  Time cues serving as the important cues for the localization of lower frequencies  Intensity cues serving the localization of higher frequencies

Hearing by Bone Conduction Bone conduction  Transmission of sound waves to the inner ear through vibration of the bones of the skull  Three different modes  Inertial bone conduction: result from the initial of the ossicular chain  Compressional bone conduction: results from segmental vibration of the bones of the skull  Osseotympanic bone conduction: results from the production of air conducted sound in the external auditory canal by the vibration of the bony walls of the canal  Occlusion effect: production and enhancement of air-conducted sound in a plugged external auditory canal that was originally created by the vibration of the bony walls of the canal