In the field of audiology, we are often presented with different quantities expressed in units of 'decibel'.
This brief guide offers you an overview on the most important details to know about the decibel.
The decibel (dB) takes its name from Alexander Graham Bell, the inventor of the telephone. The dB (a 10th of a Bel) was derived from the attenuation of a signal transmitted along a mile of telephone cable. The dB was linked with audiology from the beginning because this mile of attenuation was considered the smallest amount of signal change that the average listener could detect.
In audiology, many different suffixes are appended to the unit of the dB. The most used are dB SPL and dB HL (Figure 1).
Figure 1: Difference between dB SPL and dB HL.
dB SPL is the measured pressure relative to 20 micropascals.
This 20-micropascal reference was selected because it was the quietest sound pressure level that a group of normal hearing test subjects could detect.
dB SPL is an absolute and frequency-independent unit.
It is the unit most often used in the calibration of signals in hearing testing equipment.
All other suffixes used in acoustics to describe loudness are calculated from the SPL value.
dB HL refers to the hearing ability of a person and gives a statement about the severity of the hearing loss.
Hearing levels are measured with pure tones at different frequencies and the hearing level of an individual will vary depending on the frequency chosen.
Normal hearing is defined as 0 dB HL.
If a person has a hearing loss of 60 dB HL at 1 kHz, then he or she cannot hear a pure tone that is presented below 60 dB HL.
The notation of nHL is a reference to the frequency-specific threshold of normal hearing subjects.
We use a correction factor added on top of the SPL value to calculate the nHL value.