http://stringplayer.websitetoolbox.com/post?id=2881564&goto=nextoldest said:
There are two ways to look at the loudness match, Peach.
One is in dB, the number that shows up on the audiometer gauge in front of the audiologist at the moment that you, sitting inside the booth, indicate that the sound you are hearing through the headphones (working slowly up from 0 dB) is as loud as your tinnitus at the tinnitus frequency (pitch). Lets say that Fred and Bill both have tinnitus that measures 43 dB using this method.
The second is in dB SL - dB sensory level.
OK. Fred has really good hearing at the tinnitus frequency, a threshold of 0 dB. But Bill, on the other hand, has a 40 dB threshold of hearing at the tinnitus frequency. That means that he does not hear he first 40 dB of sound introduced to the booth. Fred goes first, and at 43 dB he indicates that the sound he hears through the headphones is as loud as his tinnitus. Next comes Bill. The audiologist starts increasing the volume of the sound being introduced to the booth. 10 dB. 20 dB. 30 dB. What's Bill doing? He's still waiting for the audiologist to start the test! When the audiologist gets to 40 dB, Bill finally hears the faintest sound through the headphones. (Remember, at 40 dB Fred was already hearing a very loud sound through his headphones, but it still wasn't loud enough to match his tinnitus!) Back to Bill. At 43 dB (a mere 3 dB over his threshold of hearing) he indicates to the audiologist that the sound he is hearing though the headphones is as loud as his tinnitus.
So both Fred and Bill have a 43 dB tinnitus loudness match, right? But clearly Fred's tinnitus is much much louder than Bill's. And the only way we would know that from the loudness match would be by taking the threshold of hearing into account. That's what dB SL does. It subtracts the threshold of hearing from the loudness match in dB. So Fred's tinnitus is 43 dB SL, while Bill's is 3 dB SL.
Please note that I chose these figures for illustrative purposes only - as a tinnitus loudness match of 43 dB SL is almost unheard of. In fact in 70% of cases the loudness match is 6 dB SL or less.
Please also note that neither dB nor dB SL correlates well with how loud you judge your tinnitus to be on a 1 to 10 scale. But that's another story.
Hope this helps more than confuses.