accent reduction

I'm tutoring a Japanese girl in pronunciation. She's an aspiring linguist, conscientious and sharp: thus, we've been working on some very fine distinctions. Today she asked me about the quality of her voice. She said that she thinks that Americans speak more from their chest and their throat. I agree: English is more guttural than Japanese, and English spoken by a Japanese person does strike native speakers as tense, spoken from the front of the mouth.

The problem is that I don't have a precise way to describe this, only vague generalizations. "English is more guttural; English is less tense; English has a lower pitch." It's easy to describe the difference between a dental and an alveolar fricative: for the one, the tongue touches the teeth, for the other it doesn't. Much harder to distinguish between a high-pitched and a guttural voice. "Your voice needs to be more in the back of your throat." But the voice is not an object, like the tongue or the teeth. When I say, "your voice is lower," what am I really saying?

It has something to do with tension. However, my student feels that many English sounds are more tense than Japanese sounds. This is because English requires her to use underdeveloped muscles: working those muscles makes her mouth feel tense. Likewise, Japanese may feel tense to me because I'm using muscles that haven't had much exercise. So to say, "in English, your mouth is less tense than it is in Japanese" may to some degree be true, but it isn't helpful.

It has something to do with pitch: constricting and relaxing the vocal folds. But there's more to it than just this, and besides this is a very relative judgment. An American soprano is going to have a higher-pitched voice than a Japanese baritone.

It has something to do with the tongue, but I can't quite describe what.