I believe the tone of the sound has an effect as well. Given any two sounds with the same SPL (Sound Pressure Level; loudness) the one with the higher pitch (frequency, measured in hZ) would do more hearing damage.
Also, higher pitched noises require much less power to make, as whatever is vibrating (loudspeaker, tuning fork, drum skin) requires faster movement, but much less overall excursion. (That's why tweeters are so small; they don't have to move the air that big subwoofers do.)
That said, high tones, while technically being worse for your ears, don't have the same immediate discomfort deeper tones provide, simply because more air is moving in your ear canal with deeper tones.
A buddy and myself were foolishly shooting a small handful of rounds in his backyard with no ear protection one day. (First, last, and only time) He shot his 9mm with no discomfort, several mags.
I handed him my 1911. He shot three rounds, and handed it back, saying "You can have this loud ear-ringing mother ****er back!"
So point: Although higher tones (read: 9mm) can have a worse and longer-lasting effect (tinnitus), deeper sounds moving more air physically provide more discomfort.
Which would you rather shoot without protection? A 9mm, with a puny powder charge, or a full-house .45LC? Math says the 9mm is almost quadrupally (izzat a word?) louder thant the .45LC, but we all know better.