Pure Tone Audiometry (PTA) is the standard behavioral hearing test used by audiologists to determine a person’s hearing sensitivity. This assessment identifies the quietest sound a person can detect, known as the hearing threshold, across a range of different sound frequencies. The results provide a precise picture of an individual’s hearing ability, which is then used to diagnose the presence, degree, and type of any hearing loss. This information is fundamental for recommending appropriate management or intervention.
Fundamental Principles of Pure Tone Testing
Pure Tone Audiometry uses specific sound signals called pure tones, which consist of a single frequency of vibration. Unlike the complex sounds encountered in daily life, these single-frequency sounds allow the audiologist to test specific areas along the auditory pathway. Different frequencies stimulate distinct locations within the cochlea.
The central goal is to establish the hearing threshold, which is the softest intensity level at which a person can detect a tone at least 50% of the time. This threshold is measured in decibels of Hearing Level (dB HL) for a selection of frequencies, typically ranging from 250 Hertz (Hz) to 8000 Hz. The testing procedure involves two main methods of sound delivery: air conduction and bone conduction.
Air conduction testing uses headphones or insert earphones to send sound through the outer, middle, and inner ear, evaluating the function of the entire auditory system. Bone conduction testing bypasses the outer and middle ear by using a small vibrator placed on the mastoid bone or forehead. This vibrator sends sound vibrations directly to the inner ear, stimulating the cochlea. Comparing the results of these two tests is crucial, as it allows the audiologist to determine the origin of any hearing loss.
What to Expect During the Procedure
The Pure Tone Audiometry test is conducted in a sound-treated booth or quiet room to ensure accurate measurement of the quietest sounds. This controlled environment minimizes background noise that could interfere with the detection of faint tones. The patient is fitted with either headphones or small insert earphones.
The audiologist operates an audiometer, which generates and controls the intensity and frequency of the pure tones. For air conduction testing, the audiologist begins by presenting a tone at an audible level and then systematically decreases the intensity in 10 dB steps until the patient no longer hears it. The intensity is then increased in smaller 5 dB steps until the tone is heard again, a process repeated to reliably determine the threshold.
The patient is instructed to indicate every time they hear a tone, no matter how quiet, usually by raising a hand or pressing a response button. This is a behavioral test, relying entirely on the patient’s voluntary response to the sounds. The test is repeated for a standardized set of frequencies in each ear independently.
Following air conduction, the audiologist places a small bone oscillator against the mastoid bone for bone conduction testing. The procedure for finding the threshold remains the same, ensuring a reliable and accurate threshold is found for each tested frequency in both ears.
Understanding Your Audiogram Results
The results of Pure Tone Audiometry are plotted on a graph called an audiogram, which serves as a visual map of hearing ability. The horizontal axis represents frequency in Hertz (Hz), ranging from low pitch (bass) on the left to high pitch (treble) on the right. The vertical axis represents intensity or loudness in decibels of Hearing Level (dB HL), with quiet sounds at the top and loud sounds toward the bottom.
Hearing thresholds are marked using standardized symbols. The air conduction threshold for the right ear is typically a red circle (O), and the left ear is a blue ‘X’. Bone conduction thresholds are marked using chevrons or brackets.
The distance between the air conduction and bone conduction symbols is the “air-bone gap.” A significant gap indicates a conductive hearing loss, caused by a problem in the outer or middle ear. When the air and bone conduction symbols are closely aligned, the hearing loss is sensorineural, meaning the problem lies within the inner ear or auditory nerve.
The severity of hearing loss is determined by where the threshold symbols fall on the vertical axis. Normal hearing is considered thresholds of 20 dB HL or better across all frequencies.
Severity Categories
- Mild hearing loss ranges from 26 to 40 dB HL, making soft speech and speech in noise difficult to hear.
- Moderate loss ranges from 41 to 55 dB HL, meaning a person will likely miss a significant amount of conversational speech without amplification.
- Moderate-to-severe loss ranges from 56 to 70 dB HL.
- Severe loss ranges from 71 to 90 dB HL.
- Profound loss is 91+ dB HL, requiring sounds to be very loud to be detected.
Beyond the degree, the configuration, or shape, of the plotted thresholds offers further diagnostic information. For example, a “sloping” audiogram, where thresholds are better for low frequencies and progressively worse for high frequencies, is commonly associated with noise exposure or age-related hearing loss. A “flat” configuration indicates similar hearing loss across all frequencies, while a “rising” configuration shows better hearing in the high frequencies than the low frequencies, though this is a less common pattern. The complete audiogram, including the degree, type, and configuration of loss, allows the audiologist to tailor an intervention plan, which often includes the recommendation for hearing aids or other assistive listening devices.