Understanding Your Hearing Test

Human beings hear a wide range of frequencies. The lowest pitch, 20 Hertz, is one octave below the lowest note on a bass player (though this is more “felt” than “heard”) and the highest pitch is 20,000 Hertz.

When your Audiologist tests your hearing, they can’t test all of the frequencies- that would take hours- so they test the most important frequencies for speech understanding. They start at 250 Hertz and then test each octave (double of the previous frequency) up to 8000 Hertz. Sometimes, inter-octaves (halfway between two octaves) are tested when there is a big difference in hearing between two adjacent octaves. To get an idea of the speech sounds heard at each frequency, take a look at the graph below called a “speech banana”.

Audiologists want to know the softest level you can hear for each frequency. This is measured in decibels (dB) and is called the hearing “threshold”. It is a sound that you hear only 50% of the time. Because this sound is so soft, many people feel they imagined hearing it or weren’t quite sure they heard it. But never fear, we have special techniques to make sure that we only ever get accurate results. In fact, it is impossible to “fake” a hearing loss thanks to our air-tight checks and balances.

The Audiologist will search for your threshold at each frequency and then record that result on the graph, called an “audiogram”. The symbols in red or “circles” indicate the right ear, and the symbols in blue or “x’s” indicate the left ear results.

We actually test thresholds twice during the hearing test. The first time, we test with headphones or earphones so that the sound has to pass through the entire hearing pathway from the external ear to the inner ear. These are called air conduction thresholds. They are indicated on the audiogram with our “circle” (right ear) and “x” (left ear) symbols. Air conduction thresholds determine the category of hearing loss you have. (See below for categories of hearing loss).

The second time we test thresholds, we place a vibrating piece of equipment on the bone behind the ear. This method directly stimulates the structures of the inner ear and bypasses the external and middle ear areas. This is called bone conduction hearing. Bone conduction thresholds are indicated on the audiogram with symbols that look like half triangles or half squares. Bone conduction results help us define the type of hearing loss it is. (See below for types of hearing loss).

Categories of Hearing Loss

Despite the usual question of “what percentage of hearing loss do I have?”, Audiologists don’t define hearing loss in terms of percentages. We define hearing loss in terms of expected difficulty of communication compared to a person with normal hearing thresholds.

• A threshold of normal hearing is an established standard of 25 decibels or better.
• A threshold of 30-40 decibels is a mild hearing loss. This means that a person would have an expected mild difficulty and be expected to feel a mild effect in their daily life as a result of their hearing loss.
• A threshold of 45-55 dB is a moderate hearing loss.
• A threshold of 60-70 decibels is a moderately-severe hearing loss.
• A threshold of 75-85 decibels is a severe hearing loss.
• A threshold of 90 decibels or worse is a profound hearing loss.

Types of Hearing Loss

 

• Conductive hearing loss is when there is a difference between the air and bone thresholds. Hearing is better when the inner ear is directly stimulated (bone thresholds), and worse when sound has to travel all the way through the entire hearing system (air thresholds). This hearing loss is often medically treatable or temporary.
• Sensorineural hearing loss is when the hearing for the air and bone thresholds is the same. This type of hearing loss is caused by a problem in the inner ear and is generally permanent.
• Mixed hearing loss is a combination of both conductive and sensorineural

Hearing losses are rarely flat (the same threshold for each frequency). In fact, the most common shape of hearing loss is sloping like a mountain. Bass frequencies are typically better than high frequencies and can even be normal or near normal, with hearing thresholds worsening as the frequency increases. Therefore, it is necessary to describe a hearing loss per frequency, rather than use a single category to define overall hearing.

In the audiogram below, this person’s hearing loss would be described as the following:

Hearing is within normal limits from 250-1000 Hertz, with a mild to moderately-severe hearing loss from 2000-4000 Hz, rising to a moderate hearing loss at 6000-8000 Hertz bilaterally (both ears).

Considerations for Children

These categories and standards for normal hearing are based on adult normative data. Our ability to fill in missing, unheard information, called “auditory closure”, requires experience with the language, which children lack, as well as development of higher processing in the auditory cortex, incomplete till about the age of 15. For these reasons, most Pediatric Audiologists agree that children need to have better hearing thresholds for proper speech and language development. Thresholds of 15 decibels or better is considered normal hearing for young children.

 

Speech Test Results

If you have had a hearing test, you probably remember being asked to repeat back words. Some of these were words like cowboy, baseball, and hotdog, and you had to repeat them over and over again as the decibel level was decreased. This gives us another threshold measurement called the Speech Reception Threshold (SRT). If you remember, threshold means the softest level of a sound you can hear 50% of the time. So the SRT is the softest level of speech you can hear, in decibels, 50% of the time. The SRT should be similar to the average of your air thresholds at 500, 1000, and 2000 Hz.

The other section on the test which involves speech sounds is called a Word Recognition score. Another name for this is a Speech Discrimination score. A longer list of words is read either through live voice or a recording, and the words are repeated back. This is the only part of the test done at a comfortable level, meaning above threshold. It is also the only part of the test to be scored in percentages. For each word repeated back correctly, the score improves.

The Word Recognition test must be performed at a significantly louder level than the SRT. Therefore, the test provides a good indication of speech understanding ability at a comfortable listening level.

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