An audiometer is the specialized instrument audiologists use to measure a person’s hearing sensitivity. This device systematically determines the quietest sound an individual can perceive across various pitches, or frequencies. Testing involves presenting calibrated tones through headphones or speakers to plot a detailed map of the person’s hearing ability. The primary function of the audiometer is to find the lowest intensity level, known as the threshold, at which a patient consistently detects the sound. This process determines if a hearing impairment exists and its specific degree and pattern.
Understanding the Decibel Hearing Level Scale
The intensity of sound is measured in decibels (dB), but the audiometer utilizes Decibel Hearing Level (dB HL). This specialized scale is the standard for clinical hearing tests because it relates sound intensity directly to human hearing ability. Unlike Decibel Sound Pressure Level (dB SPL), which is an absolute physical measure of sound pressure, the dB HL scale is normalized to the average human ear.
The crucial difference is that 0 dB HL does not mean the complete absence of sound energy. Instead, 0 dB HL represents the softest sound the average young, healthy adult can hear at a given frequency. Because the human ear is not equally sensitive to all pitches, the physical sound pressure (dB SPL) required to reach 0 dB HL varies significantly across frequencies. For example, a low-pitch tone at 250 Hz needs a higher dB SPL to be heard than a mid-pitch tone at 1000 Hz, yet both are represented as 0 dB HL on the audiometer. This standardization allows audiologists to use the 0 dB HL reference on the audiogram to represent normal hearing sensitivity across the entire frequency range being tested.
The Minimum and Maximum Standard Test Range
The standard audiometric test range covers the full spectrum of hearing ability, from acute hearing to profound hearing loss. At the quiet end of the scale, clinical audiometers typically test sounds as low as \(-10\) dB HL. Some high-precision devices can present tones down to \(-15\) or \(-20\) dB HL, identifying sensitivity better than the established average. These negative decibel values indicate the person can hear a sound quieter than what the average young adult population can detect.
The maximum output of a clinical audiometer is necessary to measure the hearing thresholds of individuals with severe or profound hearing loss. For air conduction testing using standard headphones, this upper limit often extends up to 110 dB HL or 120 dB HL. The highest volume the machine can produce varies depending on the specific frequency being tested and the type of transducer used, such as supra-aural headphones versus bone conduction vibrators. This extensive range ensures that those with significant hearing impairment can have their residual hearing measured accurately.
Measuring Loudness Discomfort Levels
Beyond finding the quietest sound a person can hear, audiologists sometimes measure the Loudness Discomfort Level (LDL), also known as the Uncomfortable Loudness Level (UCL). This testing identifies the intensity at which sounds become physically or psychologically uncomfortable for the patient. The LDL is a measure of tolerance, not detection.
For individuals with typical hearing, the average UCL for pure tones usually falls between 86 and 100 dB HL. The UCL for speech sounds is often slightly higher, typically between 100 and 110 dB HL. This measurement is relevant when fitting hearing aids, as it helps establish the maximum safe output level for the device. Setting a hearing aid’s output volume above a person’s UCL would cause distress and potential rejection of the amplification.
What the Test Ranges Reveal About Hearing Loss
The threshold measurements taken across the audiometer’s range are organized into standardized categories to diagnose and classify the degree of hearing loss. A measured threshold of 25 dB HL or lower is considered within the normal hearing range for adults.
As the measured threshold increases above 25 dB HL, the degree of hearing loss is categorized:
- Thresholds between 26 and 40 dB HL signify a mild hearing loss, meaning quiet speech and soft sounds are missed.
- A moderate hearing loss is indicated by thresholds ranging from 41 to 55 dB HL, which makes following a normal conversation difficult without assistance.
- Severe hearing loss is diagnosed when thresholds fall between 71 and 90 dB HL, meaning the person can only hear very loud sounds.
- Any measured threshold greater than 90 dB HL is classified as a profound hearing loss, where a person relies heavily on visual cues or specialized devices.