by Neil Bauman, Ph.D.
A student asked,
What is the relationship between the SPL dB scale and HL dB scale?
Good question. I’ll bet there are lots of hard of hearing people that are unclear about the differences between those two scales, and often treat these two terms as though they are interchangeable and mean the same thing—if they even wonder about those acronyms on their audiograms.
When audiologists measure your hearing, they measure your hearing in units called decibels (dB). The catch is that there are several decibel scales. Thus, in order to be meaningful, your audiologist indicates which decibel scale she used. The two most commonly used scales are the SPL (Sound Pressure Level) and the HL (Hearing Level) scales.
Sound meters are calibrated in dB SPL. This makes total sense because the condenser microphones used in sound meters are sensitive to changes in sound pressure in the air, just as our ears are. In contrast, audiometers are calibrated in dB HL, not in dB SPL like you would think would be done. This begs the question, “Why not calibrate audiometers using the SPL scale and forget about the HL scale?”
Here’s the reason why. Our ears do not hear equally well at all frequencies. If our ears heard all
frequencies of sound equally well, then we wouldn’t need the HL scale.
Our ears do not perceive low- and high-frequency sounds as well as they do sounds between 500 and 4,000 Hz. For example, the faintest sound a young person with normal hearing can hear at 2,500 Hz is 0 dB SPL. In contrast, at 20 Hz (a very low frequency sound), the sound needs to be much louder at 72 dB SPL in order to just faintly hear it. At the other end of the frequency spectrum, a very high-pitched sound at 15,000 Hz needs to be increased to 20 dB SPL in order for you to just detect it.
Thus, normal hearing, if plotted on an audiogram using the SPL scale, would be a curved, wavy line and look like the bottom line in Fig. 1. Since this line is both curved and somewhat wavy, it would be difficult to readily tell on an audiogram how much hearing loss a person has by frequency.
It would be ever so much easier to visualize the degree of hearing loss if normal hearing showed as a flat, straight line set at 0 dB on the audiogram. Then, any deviation from this line would indicate the degree of hearing loss.
This is the reason why they developed the HL scale. The curved SPL scale is normalized so that it becomes a flat, straight line at 0 dB. (We call this normalized SPL scale the HL scale.)
Using the HL scale, normal (“perfect”) hearing is a straight line across the top of an audiogram. When your audiologist tests you, any deviation from the 0 dB HL line indicates a hearing loss if it falls below the 0 dB line. (By the same token, if your hearing deviates above the 0 dB line, you have better than normal hearing at that frequency.)
To convert SPL readings to HL readings, audiometers are calibrated to add a specific amount to each frequency tested. This amount varies by frequency. For example, at 125 Hz, it adds 45 dB, while at 1,000 Hz it only adds 7 dB. Likewise, at 4,000 Hz it adds in 9.5 dB, while at 8,000 Hz it adds in 13 dB.
The result is that now your audiogram readily shows your hearing loss graphically in dB HL, rather than you trying to mentally visualize the degree of hearing loss if it were plotted in dB using the SPL scale.