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Hearing Sensitivity and
How It Is Measured

In order to correctly gauge hearing, an audiometrical evaluation should be conducted. This evaluation should be done by a Doctor of Audiology in a test booth, which reduces environmental sounds that can cause errors in measurement and lead to a false diagnosis. This is why a hearing test should not be given in the home.

An audiometric evaluation consists of two parts - the ability to hear and understand speech and the ability to hear pure tones of various frequencies.

Speech

There are two aspects to speech audiometry - a speech reception threshold (SRT) and speech discrimination. The SRT is the softest familiar word a patient can repeat back 50% of the time. Special words, called spondee words, are used for this part of the test. These are words with two syllables with equal stress on both syllables - for example, “baseball” or “hotdog”. This is what is known as a threshold measure.

Speech discrimination is the ability to understand speech at a comfortable listening level. This is a supra- (above) threshold measure. The word list for this test is usually read at 40 decibels above the SRT measurement or at a comfortable level for the patient, whichever is greater. Word lists used for this test are balanced phonetically, meaning the percentage of time each sound appears on the list is equal to its occurrence in the English language. These are all one-syllable words. This is a very important measurement, and gauges how much difficulty a person has with simple communication. Reduced levels indicate distortion within the auditory pathways and reflect that speech is perceived as garbled by the listener.

Tones

Hearing is measured with an instrument called an audiometer. The audiometer can emit tones of varying frequencies and intensities to the patient via earphones (air conduction) or through a bone oscillator (bone conduction) that is placed on the mastoid bone behind the ear. Air conduction tests all three parts of the ear. Bone conduction directly tests the inner ear by vibrating the bones of the skull and bypassing the outer- and middle-ear mechanisms.

Human speech includes a wide variety of pitches, from low pitches with a frequency of 250 cycles per second (Hz) to high pitches with a frequency of 8,000 to 12,000 Hz. The aging process in people gradually reduces their ability to hear the higher-frequency sounds to the extent that, by age 70, most people lose usable hearing above 6,000 Hz.

Testing

One of our Audiologists will conduct the hearing evaluation using both air conduction and bone conduction. Both ears of the patient will be tested in order to obtain the thresholds (the softest tone, measured in decibels (dB), that a patient hears 50% of the time) for varying frequencies, measured in Hertz (Hz). The thresholds will indicate different pitches. The frequencies of 250 through 8,000 Hz are assessed for air conduction, while frequencies of 500 through 4,000 Hz that test bone conduction are typically tested, but high-frequency audiometry is important for some symptoms and to establish subjective history.

Uncomfortable loudness levels (UCLs) are obtained for each ear at specific frequencies.  These are obtained in order to ensure that making sounds loud enough, such as with hearing aids, will not be uncomfortable or painful to the patient.  This test also helps us to identify people who are sensitive to loud sounds, and can guide us as to which treatment may be most helpful to the patient.

QuickSIN is a test to determine how an individual performs in the presence of background noise.  For this test, a speech stimulus is delivered simultaneously with a group of people chatting in the background.  The patient is asked to repeat sentences spoken by a female speaker during this test, as the background chatting noise level is increased. This test is designed to identify people who will perform best in background noise without amplification, as well as those who will need special consideration for directional microphones with hearing aids or possibly an FM system to overcome the noisy surroundings.  Difficulty with this test with normal hearing warrants further diagnostic testing to allow treatment options to be developed.

Acceptable noise level test (ANLT) is a test performed to obtain the patients perceived subjective difficulty in background noise.  For this test, the patient does not repeat the sentences, but instead states when a background noise level is no longer comfortable.  This test is used in comparison with the objective QuickSIN results.   These test results are used to determine the best treatment for each patient.

Client oriented scale of improvement (COSI) is used to establish the subjective goals for hearing aid fittings.  The patient begins with prioritizing their expected outcomes or goals for amplification.  Over the course of the hearing aid use, the Audiologist will verify these goals with the patient to mark improvement or need for adjustment or training.

Test Booth

A test room is a sound-treated booth that keeps the patient from hearing sounds outside the room, ensuring accurate measure of the capabilities of the patient’s ears without the influence of ambient noise contamination. There is a window that allows the patient and the Audiologist to see each other during the test.

Test Chart - Audiogram

The horizontal axis represents frequency, and the vertical axis represents intensity, with normal results typically found at the top of the chart, and more severe hearing loss toward the bottom of the chart.  Larger numbers for the Intensity indicate greater hearing loss.

Patients’ thresholds are plotted using special symbols. For example, if a patient hears 1,000 Hz at 10dB 50% of the time via air conduction in the right ear, a red circle will be placed at the intersection point of 1,000 Hz and 10dB. The air conduction symbol for the left ear is a blue “X”. Although a complete audiogram includes both air and bone conduction, hearing sensitivity is classified on the basis of air conduction thresholds. Straight lines connect the air conduction symbols for the right ear and for the left ear.

It is important to note that the relationship between patients’ air and bone conduction helps the Audiologist differentially diagnose the type of hearing loss.

It is possible for patients diagnosed with a sensorineural hearing loss to enhance their listening quality with the use of hearing aids and/or assistive listening devices. However, patients with a conductive or mixed hearing loss must be referred for medical management to rule out a medically treatable condition. If a physician feels medical management is not appropriate, a hearing aid or implantable device may be an option.

What is considered normal hearing?

Normal hearing sensitivity is defined as thresholds that range from 10dB to +20dB. Thresholds falling out of this range are considered abnormal or consistent with some degree of hearing loss. Hearing loss has degrees of severity based on the dB range where air thresholds are found. The following degrees of hearing impairment are based on specific dB ranges:

Degree of Loss

Range in dB

Normal

-10 to 19

Mild

20 to 39

Moderate

40 to 59

Severe

60 to 79

Profound

80 to 110


What are the functional effects of hearing loss?

So far, we have considered the audiometric effects of hearing loss. Stach (1998) has determined the communication effects of different degrees of hearing loss as follows:        

Degree of Loss

Communication Problem

Minimal

Difficulty hearing faint speech in noise

Mild

Difficulty hearing faint or distant speech

Moderate

Hears conversational speech only at a close distance

Moderately
Severe

Hears loud conversational speech

Severe

Cannot hear conversational speech

Profound

May hear loud sounds; hearing is not the primary communication channel



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