Growls designed to make bad things stop or go away may be deep, may involve bared teeth, and may be low and rumbling but pitch and volume depend on the size and breed of the dog and will vary from dog to dog. Take context and body language into account, don’t just consider the growl. A stiff body, a tail held high, maybe wagged but in a tense manner, wide eyes, ears back, lips pulled back, teeth bared – all these add up to a dog who really wants this situation to stop, now. Some dogs will not show all these signs though, a very small dog with hairy floppy ears may produce a higher-pitched growl, you may not see the ears back as they won’t go so far back as pricked ears, and the tail may be too hairy to see the movement properly or the muzzle may have such long hair you can’t see the lips drawn back or the bared teeth.
A dog’s happy growl sounds relaxed but most importantly, it’s relatively clear from the situation your dog is in if they are emitting a pleasure growl: if they are having their belly rubbed or something else, they really enjoy. This type of growl may or may not be accompanied by bared teeth, but look carefully, typically the lips will be soft, not drawn back hard, ears will not be pulled back, and body language will be soft and floppy.