I want to follow up on my experience with canal phones over the past three days and their effect on my tinitus:
Because I enjoy portable audio at work, I bought Shure E2 canal phones in the hope that I could use them at a lower volume level due to their isolating nature.
SPL Level Question:
My first question was whether the advantage of the lower volume levels needed to drive canal phones is balanced out by their sealing the ear. The answer, simply enough, was stated in the instruction manual that comes with the E2s -- on their hearing protection Warning Page. Their literature states that SPL are directly related to the quality of the seal. Thus, although your amp - my iPod in this instance - may be driven at a lower volume to achieve the sound level you want, the relative sound pressure levels, due to the canal phone seal, are equivalent to driving your player to a higher level with a non-sealing headphone.
Therefore, I think it's something of a misnomer to say you will be listening at lower volume levels by using an isolating canal phone.
My experience with Tinitus and canal phones:
When using regular "open" headphones such as Sennheiser HD600s I can listen to music at low levels for perhaps 2-3 hours without my tinutus being exacerbated. When using regular "earbuds", namely the earbuds which came with my I pod I can listen for 4-5 hours without my tinitus worsening. Of course, this is not really "full range" sound, with the design and ill fit of most earbuds providing limited low frequency response. Unfortunately, I've recently learned that less than two hours with canal phones worsens my tinitus. This is with the very snug seal that many people strive for to improve bass response., eg wetting them before insertion and modifications. I modded the Shure E2s myself. I used the soft silicone tips from Sony EX70 earbuds on the Shure E2s to improve their seal, per Brokhen's mod. The result was excellent sound with a warmer midrange and improved low frequency response. Sadly, the result was an aggravation of my tinitus, more so than with regular earbuds or open headphones.
I want to clearly state that using the Shure E2s with the foam plugs and their silicon plugs allowed me to listen to them for 4-5 hours without worsening my tinitus - this, of course, was at the expense of low frequency response. Their bass response using them this way, to return to my finicky audiophile ways for the moment and forget being a researcher, being what I would call very unsatisfying. I also want to say that, using the modified E2, I do not believe that I ever pushed their limits to "excessive" bass, but rather to a natural satisfying level.
I've come to believe that bass impact and their subsequent SPLs seems to worsen my tintus more so than treble response. As bright equipment is so noticeably unpleasant to listen to, I had always assumed it was the treble causing my tinitus to flair up, but I now suspect it is the SPL of low frequencies, or bass impact, which aggravates my tinitus. I cannot state that this is the same case with all tinitus suffers.
But it is my initial observation that the bass impact created by in-ear or "canal phones" is more aggravating to tinitus than the impact of "open" phones - even when the open phones are providing a what I would consider a satisfying level of bass performance.
So, anyway, these are my personal findings. I hope they help others who may be experiencing similar problems. And if anyone can support my experiences with the aggravation of tinitus and the effect of low frequency impact, I'd appreciate their input. It might help me to find the best equipment to continue my love of headphone listening.