The Pure Tone Average (PTA) is a single, numerical value that summarizes an individual’s hearing sensitivity in each ear. Audiologists use this standardized metric to quantify the degree of hearing ability following a comprehensive hearing evaluation. Derived from pure-tone audiometry, which tests the quietest sounds a person can hear across different pitches, the PTA provides a quick, reliable snapshot of hearing function, making it a foundational measurement in clinical settings.
Understanding Pure Tones and Thresholds
The process of determining the PTA begins with measuring an individual’s response to different pure tones. A pure tone is a sound wave that consists of only one frequency, or pitch, unlike the complex sounds encountered in daily life. Testing with these single-frequency sounds allows the hearing professional to pinpoint the exact pitches where a hearing difficulty may exist. During the test, the intensity of these pure tones is varied to find the softest level a person can detect.
This softest detectable level is called the hearing threshold. The hearing threshold is defined as the quietest sound intensity at a particular frequency that a person can hear at least 50% of the time. These thresholds are measured in decibels Hearing Level (dB HL), a scale unique to audiology. The dB HL scale is based on the average hearing sensitivity of young, healthy listeners, so that 0 dB HL represents the softest sound an average person can hear. A person with hearing loss will have a threshold that is higher than 0 dB HL, indicating the sound must be louder for them to perceive it. The individual thresholds collected across various frequencies form the raw data that will be used to calculate the summarizing PTA value.
Calculating the Pure Tone Average
The Pure Tone Average is calculated by taking the arithmetic mean of hearing thresholds obtained at a specific set of frequencies. The standard PTA calculation most commonly uses the thresholds measured at 500 Hertz (Hz), 1000 Hz, and 2000 Hz. These frequencies are selected because they cover the range of pitches most important for understanding conversational speech. The formula involves simply adding the three threshold values together and then dividing the sum by three. For instance, if a person’s thresholds for these three frequencies are 30 dB HL, 40 dB HL, and 50 dB HL, the PTA would be 40 dB HL. This process is performed separately for each ear, as hearing loss can be different between the left and right sides.
There are variations of the PTA calculation sometimes used, such as the four-frequency average, which also includes the threshold at 4000 Hz. The inclusion of 4000 Hz is often used when a person has a hearing loss that severely affects higher pitches, which is common in noise-induced hearing loss. The standard three-frequency PTA provides a reliable estimate that closely correlates with the Speech Reception Threshold (SRT), confirming its value as a predictor of a person’s ability to hear speech in a quiet environment.
Interpreting the Results
The final PTA number is highly significant because it translates a set of individual test results into a single value that defines the overall degree of hearing loss. This number is used to place the person’s hearing into one of the standard categories of hearing loss severity:
- Normal hearing: 25 dB HL or less.
- Mild hearing loss: 26 dB HL to 40 dB HL (soft speech and speech from a distance may be missed).
- Moderate hearing loss: 41 dB HL to 55 dB HL (difficult to hear conversational speech without amplification).
- Moderately severe hearing loss: 56 dB HL to 70 dB HL (most conversational speech is missed).
- Severe hearing loss: 71 dB HL to 90 dB HL (the person can only hear very loud sounds).
- Profound hearing loss: Greater than 90 dB HL (the person relies heavily on visual cues and non-auditory communication methods).
The calculated PTA is a foundational component for the audiologist when discussing diagnosis and recommending treatment options, such as hearing aids. This single number helps communicate the general impact of the hearing loss, particularly on the ability to perceive speech sounds.