Decibels and acoustics 101
Different decibels.
Acoustics can be an incredibly challenging and complex area of physics to understand. For the sake of understanding audiology-based test results, the following section will discuss the different scales that may be used in the measurement of human hearing thresholds.
There are different variations and versions of decibel scales used in the measurement of the intensity of a sound.
dB SPL
Sound pressure level
As sound is a pressure wave traveling through air molecules, a common decibel scale used to measure sound intensity is the decibel sound pressure level or dB SPL. It's commonly used in acoustics to quantify how loud a sound is, relative to a reference sound pressure. In air, the reference sound pressure is typically 20 micropascals (μPa), which is roughly the quietest sound a human can hear.
Audiologists convert dB HL measurements into dB SPL measurements for the purpose of fitting hearing aids.
dB A
Industrial noise levels
A dB A is a weighted scale for judging the loudness of sounds that corresponds to the hearing threshold of the human ear. Humans do not perceive all frequencies at the same intensities. For example, in the low frequency end of the spectrum are reduced as the human ear is less sensitive at low audio frequencies than at high audio frequencies.
This scale is employed in multiple jurisdictions to evaluate the risks of occupational deafness and other auditory problems related to signals or speech intelligibility in noisy environments.
dB HL
Human hearing levels
dB HL is the only scale used for audiograms. While dB SPL is a useful scale for acoustical applications, audiograms use a different scale; hearing sensitivity is always represented to the reader in the decibels Hearing Level scale, or dB HL.
The dB HL scale has developed in a homogenized way, to represent an average ‘zero’ across frequencies that humans with normal hearing can detect. .
Human ears are specialized
The human ear does not detect all frequencies at similar sound pressure levels, and on average, there are more decibels sound pressure levels required for the detection of low and high frequency sounds. Less sound pressure is required for human ears to detect sounds across the mid frequencies of the audiogram – those typically associated with speech sounds. A normal audiogram represented in the dB HL scale may appear to have a flat X axis with all thresholds at 0 dB HL, but in fact, the same ‘normal average’ test result on the dB SPL scale would look like a U shape.
This background information is fundamental to the understanding that the human ear has evolved to be most efficient at capturing the range of frequencies that relate to human communication and environmental awareness. It is also fundamental to the understanding that not all hearing losses will have significant impacts to communication.