Introduction to Audiometry: Equipment, Thresholds, and the Audiogram

The Audiometer and Hearing Thresholds

• The Audiometer: This is the primary piece of equipment utilized for hearing testing. Sessions will involve learning the use of specific buttons, controlling various settings, and understanding the core mechanics of how the equipment functions.

• Definition of Hearing Threshold: In audiology, the hearing threshold refers to the softest level that a person can hear. This is the primary metric sought during testing to quantify an individual's hearing sensitivity.

• Shorthand Notation: Hearing thresholds are often represented in shorthand with the Greek letter theta ($\theta$). If this symbol appears in audiology reports or chart notes, it explicitly denotes the hearing threshold.

• Determining Thresholds: The process of quantifying hearing sensitivity involves going to individual pure tone frequencies and presenting a tone. The clinician then decreases the intensity of the tone until the softest level the patient can perceive is identified. This concept of thresholds serves as the foundation for nearly every aspect of the hearing test.

Behavioral Testing and Patient Instructions

• Behavioral Nature of Testing: Hearing testing is categorized as a behavioral test. This means the clinician relies on the patient to report what they are experiencing. It is not an objective test in the sense that a patient remains passive; rather, the patient must actively participate for data to be collected from the auditory system.

• The Importance of Specific Instructions: Because the test relies on patient participation, the instructions provided by the clinician are critical to success. Inaccurate data can result from "considerable biasing" if instructions are vague.

• Avoidance of Vague Instructions: A common example of non-specific instructions is the prompt: "Raise your hand when you hear the beep."

• Establishing the "Softest Level" Criteria: Since the goal is to find the absolute softest level (the threshold), the patient must be encouraged to respond even to sounds they are uncertain about. The specific phrasing recommended to avoid biasing is: "Even if you think you hear it, raise your hand as long as you think it is there, and then put your hand down when the beep stops."

• Threshold Perception vs. Certainty: A patient who only responds when they are 100% sure the sound is there ("Yep, I hear it definitely") is likely responding above their true threshold. The actual threshold is the point where the patient thinks: "I think I hear it."

The Auditory Response Area and Minimum Audibility Curve

• Auditory Response Area: This is a graph representing the "window of hearing." It identifies the difference between the softest levels humans can perceive and the loudest, most intense sounds they can tolerate across the frequency range.

• Frequency and Intensity Axes:

  • The bottom axis represents frequency, moving from low frequencies on the left to high frequencies on the right.

  • The side axis represents intensity measured in Decibels Sound Pressure Level ($\text{dB SPL}$). This scale ranges from soft (less intense) sounds at the bottom to very intense sounds at the top.

• The Minimum Audibility Curve: This curve represents average normal hearing sensitivity across the frequency range. It demonstrates that human hearing sensitivity is not uniform; we are more sensitive to mid-frequencies and less sensitive to very low and very high frequencies.

• Numerical Sensitivity Specifics (Minimum Audibility Curve):

  • At $125\,Hz$: Humans require approximately $50\,dB$ of intensity to perceive the sound.

  • At $1000\,Hz$: Humans only require approximately $8\,dB$ (specifically $7.5\,dB\,SPL$) to perceive the sound, indicating a high level of sensitivity.

  • At $16000\,Hz$: Sensitivity decreases again, requiring nearly $40\,dB$ of intensity to perceive the sound.

• Range Comparisons:

  • At $125\,Hz$, there is a much smaller range (the speckled "window") between the softest perceivable sound and the loudest tolerable sound.

  • At $1000\,Hz$, humans experience the most sensitive spot with the largest range between the softest level and the loudest tolerable intensity.

Transitioning from dB SPL to dB HL

• Normalization of the Audiogram: The developers of the audiogram wanted to simplify the interpretation of thresholds. Instead of using the wavy "Minimum Audibility Curve," they "flatlined" the average softest levels perceivable by humans and set that reference point to zero ($0$).

• Comparison of Decibel References:

  • dB SPL (Sound Pressure Level): A physical measurement of sound. It involves a microphone where sound physically hits a diaphragm. It measures the intensity of vibrational patterns moving through the air.

  • dB HL (Hearing Level): A clinical reference where $0\,dB\,HL$ is defined as "audiometric zero," representing the softest sound humans can hear at any given frequency.

• The Underlying Values of Audiometric Zero: While the audiogram displays a flat line at $0\,dB\,HL$, different physical intensities ($dB\,SPL$) exist beneath those points:

  • At $250\,Hz$: $0\,dB\,HL = 26.5\,dB\,SPL$.

  • At $500\,Hz$: $0\,dB\,HL = 13.5\,dB\,SPL$.

  • At $1000\,Hz$: $0\,dB\,HL = 7.5\,dB\,SPL$.

• The Audiogram Layout: The audiogram is effectively the normalized graph flipped upside down. This places $0\,dB\,HL$ (normal/average hearing) at the top of the graph. Numbers increasing downward indicate the degree of deviation from normal hearing.

• Interpreting Deviation on the Audiogram:

  • A threshold of $40\,dB\,HL$ indicates the sound had to be increased by 40 units or steps away from audiometric zero for the patient to hear it.

  • Comparison example: A patient with a $20\,dB$ threshold at $250\,Hz$ and a $110\,dB$ threshold at $500\,Hz$ has a much more significant hearing loss at $500\,Hz$ because they are 110 steps away from average hearing compared to only 20 steps.

• Conversion Rule: The $dB\,SPL$ value will always be larger than the $dB\,HL$ value at threshold. The calculation is generally: $\text{dB SPL} = \text{dB HL} + \text{Average Threshold (Minimum Audibility Curve value at that frequency)}$.

Clinical Transmission Modes: Air and Bone Conduction

• Air Conduction (AC) Testing:

  • Uses headphones to deliver sound.

  • Sound travels through the entire auditory system: outer ear, through the middle ear, into the inner ear (cochlea), and up the auditory nerve to the brain.

  • This is modeled by a tuning fork vibrating air molecules via alternating compression and rarefaction that enter the ear canal.

• Bone Conduction (BC) Testing:

  • Uses a special device placed on the mastoid bone (the bone behind the ear) or any part of the skull.

  • The device vibrates the skull, which directly stimulates the cochlea.

  • This method bypasses the outer ear and the middle ear entirely.

• Differential Diagnosis via Comparisons: Clinicians compare air conduction thresholds to bone conduction thresholds to identify the location of the hearing loss within the auditory system:

  • If AC and BC thresholds are the same, it indicates one type of hearing loss.

  • If AC and BC thresholds differ, it indicates a different kind of hearing loss (e.g., a problem specifically in the outer or middle ear).