I have a pretty decent mastery of my range as a tenor, I can go from G2 all the way up to C above middle C pretty darn comfortably. But a lot of spots in my voice sound really thin and stringy, particularly my mix from Eb above middle C to A above middle C. I want to develop a much fuller, richer, meatier tone so my voice fills out rooms more. I'm thinking Sam Smith, whose voice sounds full regardless of where he is in his range. Are there some exercises I can do daily to help develop this?
Getting power on the high notes is a product of your whole body: it requires good breath support, diaphragm control, etc. One exercise that I like (and this is just me) is going up and down the scale on an 'L' and a neutral vowel up to the 5th, then take that up a half-step until you feel the edges of your range. Pay attention to keeping your diaphragm engaged and the breath flowing. Make sure your posture supports your breathing as well.
That being said, the best thing you can do is get a teacher. Voices are so individual that it's hard to give advice without hearing you sing.
There are exercises to address any desire you may have for your voice, although no amount of exercises will make you sound like a singer that has a markedly different instrument(vocal chords, throat and body).
If you want a meatier fleshier tone across your range, I would focus on doing exercises in the low end of your range. Working on your low end can help a lot with your top end, but crucially, it must be held together with compression.
Compression is how you use your vocal chords to resist the flow of air through your chords. If you are hitting good high notes, and if you think of yourself as a "tenor" you're probably already using a lot of it. There are compression swells that I would probably recommend for you.
If I were teaching you I would also do some experimentation with different vowel coordinations to see if any particular spaces in your voice get you closer to that rich sound you're looking for, and then expand out by training those coordinations into your other vowels and other parts of your range.
However, Talking about vocal exercises isn't really going to do you any good. In learning to sing we have to have exercises demonstrated to us, and then someone with some knowledge needs to put together a set of exercises to address your particular intentions, and to address any related techniques that are going to assist you toward those aims. This person can be a vocal coach or it can be you if you learn enough.
As a complete non singer I can still guide you to better singing by advising you to find a good singing teacher . There was a series on tv about singers who had a series of lessons over a few months. The transformation in all of them was startling. They changed from basically weak and insipid to vibrant and strong singers. I think that`s the best answer you could have here . Singing teachers get you to do all sorts of odd things but they understand the mysterious muscular mechanism involved .Mmm.
When you listen to Sam Smith — or any recording artist — you are hearing them through a dynamics compressor, equalization (EQ,) reverb, and likely auto-tune. When you are comparing your voice to theirs, are you also singing through that kind of processing? If you are singing acoustically, that is an unfair comparison. The dynamics compressor in particular adds a lot of power to a voice that you might not hear when listening to that voice acoustically. The softer notes are made louder, the louder notes made softer. If you imagine that performance is an acoustic performance coming only from the singer themselves, you may be setting up an ideal that even that particular recording artist could not reach. So you might be overly criticizing your voice.
Also, in a pop music recording session, the singer is not trying to “fill the room” — they are typically singing relatively quietly and letting the microphone do the work. Very much the opposite of what a theater singer might do, where they are projecting the voice. In pop music, the microphone is not sitting in the 20th row of a theater, it is between 0 to 30 centimeters from your mouth. The mental model is more like singing into somebody’s ear.
So if you want to develop a sound that is more like Sam Smith, vocal exercises — as important as they are, and continue to do them — may not be what you need to achieve that. You may want to do microphone exercises. You may want to get a microphone, dynamics compressor, EQ, and reverb (some small mixers have dynamics compressor, EQ, and reverb built-in, so you may just want a microphone, mixer, and headphones to hear your processed voice) and work with them as you practice so that you can develop good technique across the entire pop vocal instrument (which is the vocal instrument plus the microphone plus the processing.)
This is sort of like if you were an acoustic guitar player and you said you wanted to sound more like Jimi Hendrix — we wouldn’t say “do finger exercises,” we would say “get an electric guitar and a fuzzbox and an amp.”
You may find that once you stop trying to fill the room with your acoustic voice and sing in a more intimate style with a microphone, the concerns you have with some of your notes may go away. It may be that you are just oversinging and squeezing some of the life out of some of your notes. But at that point, at the very least you will be set up to sing a Sam Smith song and realistically criticize your performance compared to his and see what it is you really need to work on.