I'm just in the process of learning how to do some kind of that "gravelly" sound, i.e. distortion. I was in a masterclass for singers this weekend and I got some tips on how to do it in a way that's not harming your voice. (Yes, anyone can do it. If Tom Waits can do it and have done it for decades, you can too). I got to practice this a bit and managed to find that sound, I just need to "polish" it a bit so I'm completely sure I do it right every time. Here's a short explanation on it.
First, you need to be able to make a clean, healthy tone onto which you you want to put the distortion effect. When you have that sound, leave it for a moment, and try to "whisper" a distorted sound only. It is very important that you don't form the distorted sound in your vocal cords in your larynx, as that may be extremely harmful. To get the distortion, you need to "over-twang" the sound. To first find the twanged sound, push your tongue backwards and make it as wide as possible, or just try to sound like a duck or laugh like a mean witch. There you have the twanged sound. Just saying "twang" with a south-state American accent should also give you a similar twanged tongue position. Now, exaggerate the twang sound as much as possible. When you twang the sound, what happens is that the epiglottis tunnel, which the sound passes through and usually is more or less round, gets bent (think of the way a plastic hose gets when you bend it). When the tunnel gets too small (bent) for the sound to pass cleanly, the sound gradually gets more and more distorted. Don't vocalize the sound before you have found the distorted sound. Warning: if it feels strange or hurts somewhere when you try to find the sound, you have done something wrong. The error isn't necessarily in the throat position, it might as well be the support that's failing, you have some constrictions in your throat that you shouldn't have, or something else. Just don't repeat what you just did if it didn't work. Instead, try a slightly different approach after a short rest.
When you have found the (whispered) distortion effect, memorize how you did it by repeating it a few times. Then go back to the original sound and start singing it (on a pitch that is easy for you to sing) and make sure you have everything under control with this sound. Then start to twang the sound until you reach the position where you got the distortion. It is not possible to get this distortion without twanging the sound so don't even try that. When you got it, you can put it on any vocal mode from soft and breathy, even falsetto, to very powerful, full-edged sound, without hurting yourself. Just don't let any excessive amounts of air pass through the vocal cords in any other mode than the softest, breathy one. (That is btw a general rule: as soon as the sound starts to get an edge to it you cannot anymore add air to the sound without hurting yourself). Still a couple of words about twang: It is used in all kinds of music, even classical. Classical tenors twang their voice when they need to reach the high C, for instance.
So... instead of finding someone else to do this vocal part for you, why don't try to learn it yourself?
When correctly done, it's completely harmless to your voice and you can sing completely normally afterwards. I realize you might need to get a sound clip on how to do this, and if you're interested, PM me and I can see if I can record something....
If you're interested, this info is pretty much straight from the complete vocal technique book that I warmly recommend to any singer, just in a very simplified form. Check my posts in the sticky in the Band Management and Performance forum to read about it....
Btw, that Marge Simpson imitation advice is actually really good.
