HL (Hearing Level), SL (Sensation Level), and SPL (Sound Pressure Level)

The terms HL (Hearing Level), SL (Sensation Level), and SPL (Sound Pressure Level) are commonly used in audiology and neurophysiological monitoring to describe sound intensities in different reference frames. Here’s how they differ:

1. Hearing Level (HL)

• Definition: HL is a standardized scale used in audiometry, where 0 dB HL represents the average hearing threshold for normal-hearing individuals at a given frequency.

• Reference Point: Based on human hearing thresholds for each frequency.

• Usage: Commonly used in audiometry and evoked potential testing (e.g., Brainstem Auditory Evoked Potentials - BAEPs).

• Example: If a patient has a hearing threshold of 20 dB HL at 1000 Hz, a tone presented at 40 dB HL is 20 dB above their threshold.

2. Sensation Level (SL)

• Definition: SL represents the intensity of a sound relative to an individual's own hearing threshold.

• Reference Point: The person's specific threshold at a given frequency.

• Usage: Used when customizing sound levels for individuals (e.g., BAEP testing at 60 dB SL means 60 dB above the person's threshold).

• Example: If a person’s threshold at 2000 Hz is 25 dB HL, then presenting a stimulus at 40 dB SL means the actual sound is played at (25 dB HL + 40 dB SL = 65 dB HL).

3. Sound Pressure Level (SPL)

• Definition: SPL is a physical measurement of sound pressure in decibels, referenced to 20 µPa (micropascals), which is the lowest detectable sound pressure for the human ear.

• Reference Point: Absolute sound pressure (20 µPa in air).

• Usage: Used in physics and engineering to measure real-world sound intensity.

• Example: A whisper might be around 30 dB SPL, while a jet engine can exceed 130 dB SPL.

Key Relationships:

• dB HL to dB SPL: Conversion varies by frequency because human hearing sensitivity differs across frequencies (e.g., 0 dB HL ≠ 0 dB SPL).

• dB SL to dB HL: dB SL is relative to the individual's threshold, so dB SL = presentation level - threshold in dB HL.

Application of HL, SL, and SPL in IONM and BAEP Testing

1. Brainstem Auditory Evoked Potential (BAEP) Testing & IONM
BAEPs are commonly used during intraoperative neurophysiological monitoring (IONM) to assess auditory pathways, particularly the cochlear nerve and brainstem structures. The stimulus used in BAEP testing is usually a click sound delivered via earphones. Depending on the context, the intensity of this sound can be described in HL, SL, or SPL.

Example: HL vs. SL vs. SPL in BAEP Testing

Scenario: Setting up a BAEP Test in IONM

A patient undergoing posterior fossa surgery requires BAEP monitoring. You need to determine the stimulus intensity.

• Step 1: Determine the patient’s threshold (in dB HL)

  • The patient’s hearing threshold for the test click stimulus is 25 dB HL.

• Step 2: Choose a stimulus intensity

  • The standard BAEP testing level is 60 dB SL (sensation level).

  • To calculate the actual HL level, use:

Stimulus dB HL = Threshold dB HL + Stimulus dB SL

= 25 dB HL + 60 dB SL = 85 dB HL

o   So, you would set the stimulus intensity to 85 dB HL.

• Step 3: Convert to dB SPL if needed

  • The relationship between HL and SPL depends on the frequency and calibration.

  • Typically, 0 dB HL ≈ 30-40 dB SPL for clicks.

  • So, if 0 dB HL = 35 dB SPL, then:

85 dB HL=85+35=120 dB SPL

• In this case, the stimulus is 120 dB SPL.

Key Takeaways for IONM & BAEP

• BAEP stimulus is usually set in dB HL or dB SL, not dB SPL.

• dB HL is based on normative hearing thresholds, while dB SL is specific to the patient’s hearing ability.

• In intraoperative BAEP monitoring, it is common to use 85-100 dB HL stimulus levels.

• For patients with hearing loss, dB SL is crucial to ensure the stimulus is suprathreshold.