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The Variety of Decibel - Basics

In the field of audiology, we are often presented with different quantities expressed in units of “decibel”. This quick guide offers you an overview on the most important details to know about the decibel at a glance.

What is a decibel?
It takes its name from Alexander Graham Bell, the inventor of the telephone. The dB (a 10th of a Bel) originally 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 could be detected by the average listener.

The decibel suffixes 
In audiology, many different suffixes are appended to the unit of the dB. The most commonly used are dB SPL and dB HL. 

Sound Pressure Level (SPL)
dB SPL is the measured pressure relative to 20μPa (micropascals). Historically, this 20μPa reference was selected because ir was the quietest sound pressure level that could be detected by a group of normal hearing test subjects. 

dB SPL is an absolute and frequency independent unit. It is the unit 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.

Hearing Level (HL)
dB HL is used to refer 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.

0 dB HL is what normal hearing is defined as. If a person has a hearing loss of 60 dB HL at 1kHz, he or she can’t hear a pure tone that is presented below 60 dB HL.

Normalized HL (nHL)
The notation of nHL is a reference to the frequency specific threshold of normal hearing subjects and describes the intensity level of stimuli used in the field of electrophysiology (eg. ABR, ASSR etc.). We use a correction factor added on top of SPL value to calculate the nHL value.

March 2016
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