Audiology Basics                  EN2.8 Domain K/S Level SH Core

Some of the common definitions used in audiology are explained here:

 

Sound:

 

It is defined as a vibration that typically propagates as an audible pressure wave through a trasmission medium like air, liquid or solid.  In human physiological sense reception of these waves and their perception by the brain is termed as sound.  Humans can hear sound waves as distinct pitches and differentiate them only when the frequency lies between 20 Hz and 20 kHz.  Sound waves above 20 kHz are known as ultrasound and is not perceived by humans.  Sound waves below 20 Hz are known as infrasound.  Hearing spectrum varies between different animal species.

 

Frequency:

 

This is the number of cycles per second.  The unit of frequency is Hertz which is named after the German scientist Heinrich Rudolf Hertz.  A sound of 10000 Hz indicates 10,000 cycles per second.

 

Puretone:

 

A single frequency is known as the pure tone.  This is a sine wave tone.  Hence a pure tone is described by a sine wave which is a mathematical function based on the circle.  The sine wave for each tone has a specific frequency (Hz), or cylces per second. In audiology pure tone is used to test the threshold of hearing.  In pure tone audiometry the hearing threshold is measured in decibels for various tones from 125 to 8000 Hz.

 

Complex sound:

 

Sound with more than one frequency is known as complex sound.  Human voice is a typical example of complex sound.

 

Overtones:

 

Complex sound has a fundamental frequency (it is the lowest frequency at which a source vibrates).  All frequencies above that to that lowest frequency are known as overtones.  The presence or absence of overtones determines the quality or the timbre of sound.

 

Intensity:

 

This is the strength of the sound which determines its loudness.  Intensity of sound is usually measured in decibels.  Given below are some of the common sound intensities:

Whisper  - 30 dB

Normal conversation - 60 dB

Shout  - 90 dB

Discomfort to the ear  - 120 dB

Pain in the ear  - 130 dB

Loudness is actually a subjective sensation produced by intensity of sound.  More intense the sound greater will be the loudness.

 

Decibel:

 

This is a logarithmic unit used to measure sound level.  It is a logarithmic way of describing a ratio.  The ratio could be power, sound pressure, voltage or intensity.

It is not an absolute figure but represents a logarithmic ratio between two sounds, namely the sound being described and the reference sound.  Sound can be measured as power (watts/cm2) or as pressure (dynes / cm2).  In audiology it is measured as sound pressure level which is indicated as SPL equivalent to 20 micropascals.  This actually corresponds roughly to the normal threshold of hearing in normal subjects at 1000 Hz.  Decibel is used in audiology to avoid dealing with large figures of sound pressure level.

 

Noise:

 

This is defined as an aperiodic complex sound.  The types of noise include:

 

1. White noise:

 

This contains all frequencies in the audible spectrum and can be compared to that of white light which contains all colors of the visibile spectrum.  It is a broad band noise and is used for masking

 

2. Pink noise:

This is the most common signals in the biological systems. In this type of noise each octave carries an equal amount of noise energy.  This name arises from the pink appearance of visible light.  The human auditory system which processes frequencies in a roughtly logarithmic fashion does not perceive different frequencies with equal sensitivity and siganls around 1-4 kHz sound the loudest for a given intensity.  Human ears can differentiate white noise from pink noise with relative ease.

Both white and pink noises contain all the frequencies that are within human audible range i.e. 20 - 20,000 Hz.  The difference between these two noises lie in the manner in which the signal power is distributed among those frequencies.  White noise has equal power per hertz throughout all frequencies while the power per hertz in pink noise decreases as the frequency increases.  The lower frequencies in pink noise are louder and have more power than the higher frequencies.  Most people perceive the sound of pink noise as been even / flat because it has equal power per octave.

 

Octave:

 

In acoustics an octave is a frequency band whose highest frequency is twice its lowest frequency.

 

Narrow band noise:

 

This is white noise with certain frequencies above and below the given noise filtered out.  It hence has a frequency range smaller than that of broad band white noise.  This type of noise is used to mask the test frequency in pure tone audiometry.

 

Speech noise:

It is a noise having frequencies in the speech range (300-3000 Hz).  All other frequencies are filtered out.

 

Masking:

 

This phenomenon produces inaudibility of one sound by the presentation of another.  This phenomenon is used in clinical audiometry by keeping one ear busy by a sound while the other is being tested.  Masking of non test ear is essential in all bone conduction tests and for air conduction studies it is needed only when the difference of hearing between two ears exceed 40 dB.

Normal frequency range in human hearing:

Normal humans can hear frequencies of 20-20,000 Hz but in routine audiometry testing only 125 - 8000 Hz alone are tested as they form the base speech frequency range.

 

Audiometric zero:

 

This is the mean value of minimal audible intensity in a group of normally hearing healthy young individuals.

 

Sensation level:

 

This is the level of sound above the threshold of hearing of an individual.  If a person is tested at 40 dB it indicates that the individual was tested at 40 dB above his threshold.  For normal person this would amount to a sound of 0+40 dB.  In a person with a hearing loss of 30 dB then the value would be 30+40 = 70dB.

 

Most comfortable level:

 

This is the sound intensity level that is most comfortable for a person.

 

Loudness discomfort level:

 

This is the level of sound which produces discomfort in the ear.  It is usually in the range between 90-105 dB.  It is very important to find the loudness discomfort level of a person while prescribing a hearing aid.

 

Dynamic range:

 

This is the difference between the most comfortable level and the loudness discomfort level.  The dynamic range is reduced in patients with positive recruitment phenomenon as is the case in cochlear type of hearing loss.