An audiometer is an electro-acoustic instrument designed to measure hearing sensitivity. They are designed and manufactured to accepted international standards for audiometric testing which ensure that assessments of auditory sensitivity and processing capability are valid under controlled acoustic conditions. The American National Standards Institute (ANSI), the International Electrotechnical Commission (IEC) and the Insternational Standards Organization all have such standards. For example, ANSI-S3.6-1996 classifies audiometers into different types, according to intended usage, functional operation, portability and other factors.
- -Type I Audiometers- used for in-depth medical diagnostic evaluation
-Type IV – used for screening hearing impaired from the normal hearing sensitivity population
-Type HF- extended high frequency
-Type A, B & C- speech audiometers
The first commercial audiometer was created in the early 1920s by Bell Telephone Labs and was called the Western Electric 1A. However, it was not until the early 1950’s that audiometry became widely accepted in clinical practice.
An audiometer will contain one or more electronic oscillators that generate pure tones at various frequencies. It will also have an attenuating system, which is calibrated in decibels up to 100 dB, in 5 dB steps, to adjust the tone’s volume for testing. The audio output can be presented to the listener via various transducer types; these include loudspeakers, bone-conduction vibrators and earphones.
Routine procedures must be followed for checking and adjusting the instrument’s electroacoustic properties, in order to be certain that the audiometer performs in compliance with the appropriate international standard. Calibration of audiometers is typically done on-site on an annual or semi-annual basis, although the units can also be calibrated at an outside laboratory or manufacturer. Audiometer calibration is checked on a routine basis, using multimeters, spectrometers, oscilloscopes, sound level meters and other instruments. During checking, frequency, intensity and timing are verified for compliance with standards.
The most common test performed with an audiometer is a hearing threshold test, using the limits method. The procedure is for the testing technician to adjust the volume of different tone frequencies per a predetermined pattern, and the listener gives a response by hand signal or button pushing when the stimulus is heard. Tonal stimuli are given to the listener at sound levels below his audibility threshold and incrementally raised until a response is given. Each tone is usually repeated four times, two for the right ear and two for the left ear.
The results from audiometric tests are used as a basis for identifying treatment or intervention strategies for hearing impaired, such as selection of hearing aids, surgical procedures, cochlear implants, and appropriateness of vocational or educational placement.