Audiometry (Pure Tone and Impedance Audiometry)

Yumnam Ranbir Singh *

A diagram of the anatomy of the human ear :: Pix - Wikipedia/Chittka L, Brockmann

An audiometry evaluation is a painless, non-invasive hearing test that measures a person’s ability to hear different sounds, pitches, or frequencies. Patients who have a tumour in or around the ear or those on certain medications may undergo audiometry testing to determine whether hearing loss has occurred. It is also used to evaluate whether hearing aids or surgery may improve one’s hearing.

The prevalence of hearing loss varies with age, affecting at least 25 percent of patients older than 50 years and more than 50 percent of those older than 80 years. Adolescence and young adults represent groups in which the prevalence of hearing loss is increasing and may therefore benefit from screening.

WHAT IS AUDIOMETRY USED FOR?

Audiometry is used in a number of different circumstances:

o To evaluate possible hearing loss in anyone who has noticed a persistent hearing problem in one or both ears or who has had difficulty understanding words in conversation.

o When determining the type and severity of hearing loss (conductive, sensorineural and mixed).

o Screening babies and young children for hearing problems that might interfere with their ability to learn, speak, or understand language.

o To screen for hearing loss in people who are repeatedly exposed to loud noises or who are taking certain antibiotics.

COMMON CAUSES OF HEARING LOSS

Some common causes include:

o Acoustic trauma.

o Chronic ear infections.

o Diseases of the inner ear.

o Head injury.

o Ruptured eardrum.

o Medications that can harm the inner ear, including certain antibiotics, diuretics, and large doses of salicylates.

o Occupational hearing loss.

o Inherited conditions.

HOW DOES HEARING TEST WORK?

An audiometry examination tests one’s ability to hear sounds. Sounds vary based on their loudness (intensity) and the speed of sound wave vibrations (tone).

Hearing occurs when sound waves stimulate the nerves of the inner ear. Eventually, the sound travels along nerve pathways to the brain.

Sound waves can travel to the inner ear through the ear canal, ear drum and bones of the middle ear (air conduction), or through the bones around and behind the ear (bone conduction).

The INTENSITY of sound is measured in decibels (dB):

o A whisper is about 20 dB.

o Loud music (a concert) is around 80 – 120 dB.

o A jet engine is about 140 – 180 dB.

Sounds greater than 85 dB can cause hearing loss after a few hours. Louder sounds can cause immediate pain in the ear, and hearing loss can develop in a short time.

The TONE of sound is measured in cycles per second (cps) or Hertz (Hz):

o Low bass tones range around 50 – 60 Hz.

o Shrill, high-pitched tones range around 10,000 Hz or higher.

The normal range of human hearing is about 20 – 20,000 Hz. Some animals can hear upto 50,000 Hz. Normal human speech is usually 500 – 3,000 Hz.

PURE TONE AND IMPEDANCE AUDIOMETRY

A PURE TONE ADIOMETRY test measures the softest, or least audible sound that a person can hear. During the test, one wears a headphone and hear a range of sounds directed to one ear at a time.

A word recognition test (Speech Discrimination Score and Speech Recognition Threshold) assesses a person’s ability to understand speech from background noise. If one’s speech discrimination is poor, speech may sound garbled. Word recognition scores can be helpful in predicting the usefulness of a hearing aid.

An IMPEDANCE AUDIOMETRY is done to determine the status of the tympanic membrane and middle ear. It allows the audiologist to measure how well the eardrum is vibrating when sound strikes and how well the tiny bones of the ears are functioning to transmit those vibrations. Abnormal finding on this test may suggest a conductive loss requiring referral for medical evaluation.

Another part of the impedance test is acoustic reflex measurement. Acoustic reflex measurement allows the audiologist to further examine the integrity of the middle ear system.

HOW IS THE TEST PERFORMED?

The specific procedures may vary, but they generally involve blocking one ear at a time and checking one’s ability to hear whispers, spoken words, or the sound of a ticking watch.

A tuning fork may be used. The tuning fork is tapped and held in the air on each side of the head to test the ability to hear by air conduction. It is tapped and placed against the mastoid bone behind each ear to test bone conduction.

To test air conduction, one wears headphones attached to the audiometer. Pure tones of controlled intensity are delivered to one ear at a time. One is asked to raise a hand, press a button, or otherwise indicate when one hears as sound.

The minimum intensity (volume) required to hear each tone is graphed. An attachment called a bone oscillator is placed against the bone behind each (mastoid bone) to test bone conduction.

PREPARATION FOR THE TEST

No special preparation is needed for this test.

HOW WILL THE TEST FEEL?

There is no discomfort. The length of time varies.

ABNORMAL RESULTS

There are many different kinds and degrees of hearing loss. In some types, one only loses the ability to hear high or low tones, or one loses only air or bone conduction.

PREVENTION OF HEARING LOSS

o Avoid excessive noise.

o Limit exposure to loud sounds.

o Avoid tobacco use and exposure to tobacco smoke.

o Remove ear-wax properly.

o Check medications for hearing risks.

o Have hearing tested when there is unexplained change in hearing.