What Is Pure Tone Audiometry?

The Speech Banana is a visual representation on an audiogram that outlines the area where the phonemes of human speech typically fall in terms of frequency (pitch) and intensity (loudness)

Why are pure tones important for understanding speech, even though we typically hear sounds composed of many different frequencies? The answer lies in understanding hearing loss.

Hearing loss usually does not impact all frequencies equally. The most common type, presbycusis, or age-related hearing loss, often affects high frequencies more than low frequencies.

Identifying the different frequencies in a complex sound, such as speech or music, can be challenging. These can make it difficult to distinguish one frequency from another.
Pure tones, consisting of just one frequency, pinpoint exactly where the hearing difficulties lie. Therefore, pure tones are essential for accurately diagnosing and understanding hearing loss.

Pure tones also have a direct relationship with speech sounds. Each phoneme, or speech sound, corresponds to a specific frequency range. Vowels are lower in frequency, while consonants are higher in frequency.

6. Threshold Search: Pure tone audiometry aims to determine hearing threshold levels. A hearing threshold is the lowest level of sound that a person can hear at a specific frequency. It represents the softest intensity at which a sound is detected at least 50% of the time.

Air conduction testing involves sending sounds through supra-aural or insert earphones into the ear canal to reach the inner ear.
When the patient hears the first tone, the clinician makes it quieter by 10 dB HL. If the patient still hears it, the tone gets quieter by another 10 dB HL. This process continues until the patient no longer responds.

Once the patient no longer responds, the clinician increases the pure tone by 5 dB HL. This back-and-forth process continues until the clinician finds the patient’s threshold, which is the softest sound they can hear at least half the time. According to ASHA, the clinician needs at least two out of three responses at a certain level to determine the hearing threshold.

Following ASHA guidelines, diagnostic pure tone testing should cover frequencies of 250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz. These frequencies are crucial for understanding speech.

7. Air Conduction Test Results: The clinician records hearing thresholds on a graph called an audiogram. The vertical axis shows decibels for hearing level, and the horizontal axis shows hertz for frequency.

Air conduction thresholds are recorded on the audiogram as circles for the right ear and x’s for the left ear.

8.Bone Conduction Testing (if needed): If air conduction thresholds are below normal levels, bone conduction (BC) testing must be completed to determine the type of hearing loss. A bone vibrator, or a bone conduction transducer, is placed behind the ear on the mastoid bone to test BC thresholds.

Bone conduction testing is critical in diagnosing the type of hearing loss, whether it be conductive, sensorineural, or mixed. Conductive hearing loss happens when sound waves are blocked or unable to pass efficiently through the ear canal and middle ear. Conductive loss is sometimes treatable with medical or surgical interventions.

Sensorineural hearing loss is a hearing impairment caused by damage or wear and tear to the inner ear (hair cells in the cochlea) or auditory nerve. It typically results in difficulty hearing soft sounds, understanding speech, and experiencing distorted or muffled sound quality.

Sensorineural hearing loss is permanent and cannot be corrected with surgery unless the patient qualifies for a cochlear implant.

Pure tones are presented through the bone vibrator, and the patient’s thresholds are determined similarly to air conduction testing. ASHA recommends that bone conduction thresholds be obtained at InterOctave intervals from 250 to 4000 Hz and 3000 Hz.

9. Bone Conduction Threshold Recorded: The bone thresholds are also recorded on the audiogram, next to the air conduction thresholds. Unmasked bone thresholds are denoted as triangular brackets. If bone thresholds are 10 dB HL better than the air conduction thresholds, referred to as an air-bone gap, masking must be performed.

10. Masking (if needed): Masking ensures that each ear is tested independently. A masker is any noise presented to one ear while the opposite ear is being tested.
During pure tone audiometry, a masker is usually a narrow band of noise centered around the test frequency. The audiometer produces this noise.

During air conduction testing, masking becomes necessary if the asymmetry between the ears exceeds the interaural attenuation. Interaural attenuation refers to the signal’s energy reduction as it travels from one side of the skull to the other.

If the opposite cochlea is sensitive enough to detect the signal that has crossed over, it can interfere with accurate audiological testing results. The general recommendation for minimum interaural attenuation during air conduction testing using supra-aural headphones is 40 dB HL.

However, when insert earphones are used the interaural attenuation is higher, ranging from 75 dB HL at lower frequencies to 50 dB HL at higher frequencies. A conservative interaural attenuation value of 50 dB HL for testing with insert earphones is typically used to ensure accurate assessment.