I don't do growls or gutturals or anything like that, but I can give advice on the control of airflow. Find pyramidalis, about four inches below you navel - this muscle:
What you want to do is to use this to press upwards, because, when you do it right, it will drag both the diaphragm and the belt muscles with it, basically turning you into a human bellow. The routine is as follows:
Breathing in - Don't breathe. Just drop pyramidalis. When I practice this, I open my mouth wide as if surprised (though with jaws relaxed - always relaxed) and let my whole body sag and the stomach go out like a bayern beer keg. The point of this is that you create an under pressure (or whatever the correct English word is) in your lungs and so air rushes in faster and easier than you could ever drag it in. When you have it, all you do when you need to breathe is relax pyramidalis, and your lungs are full in an instant. But it takes a while to get down!
Next - connect the support. This is done when you tense pyramidalis, working upwards. And you only do this once, full force right from the start, regardless of the volume and strength you use in the song and don't relax it until next time you want some more air. Important note - it is very easy to start tensing your shoulders or even your back and neck. Don't do this, for all practical purposes you are strangling yourself. I'm entering my second year of serious study in this and I have only just reached the stage where I don't need to constantly remind myself to RELAX.
Also, when you breathe in it is very important to not close your throat, as this will a) result in a lot less air to work with and b) screw up whatever sound you want to achieve. To practice this you can do like this:
Breathe in like described above and place two fingers lightly on your adam's apple. When you open your mouth and relax pyramidalis, it should go down a little bit.
This is some of the basic techniques I use to sing everything from lullabies to opera, and rest easily with a tasteful vibrato on a single note over three or four measures. There's quite a lot to this, give a shout if you want more.
Listening to Slipknot and cutting yourself is ridiculous
Listening to Cannibal Corpse and cutting trees with a chainsaw, now that's metal
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