This is gonna be a straight up voice science post, just talking about phonation threshold pressure. Because science is fun, and voices are neat.
Phonation Threshold Pressure (PTP) is the amount of air pressure it takes to get the vocal folds vibrating in order to make sound. There are tons of things that can affect PTP, and several ways we can work with it in order to make it more efficient. For many clients, getting an adequate PTP without too much pressure or effort is worth the time and focus.
Air pressure can be so low in the vocal tract that the vocal folds either don't vibrate, or don't vibrate enough, to make sound. This is basically what's happening when you're breathing. There's air moving through there, but it's not pressurized enough to get the vocal folds to come together for sound. That's fine and dandy, and we don't need to change that.
When we do want to make sound, either through speech or singing, we need the vocal folds to get their act together. We pressurize the air column in the vocal tract by moving the breath faster or more forcefully, and that gets things moving. The exact moment that the air pressure moves the vocal folds together to make sound is the measurement for phonation threshold pressure.
Measuring PTP takes some fancy equipment and a bit of training using said equipment, so it's not something we can directly measure in the studio. However, we can identify some things that might be increasing PTP in our clients, and if we were to get those things operating more efficiently, their vocal folds will be much happier.
One thing that increases PTP, particularly in singers with less experience, is too much muscle involvement, whether from the vocal folds or false vocal folds being too squeezed, or from external laryngeal muscles putting pressure on the vocal folds. We've all heard it when singers try to reach for high notes, and they don't quite know how to do it yet, so they just muscle it on up there. That extra muscle work is going to be harder for the air pressure to overcome in order to make the vocal folds vibrate in a self-sustaining manner. Over time, helping clients to relieve that pressure and the feeling that they need to work so hard while singing can really help. SOVTEs can be a tool to use for training singers to feel phonation with less muscular effort. Specifically for PTP, however, one study suggested that using a straw in water actually slightly increased PTP for a small cohort of participants, though not significantly, so perhaps steer clear of that particular iteration for this purpose (Enflo, 2013).
Incomplete vocal fold closure can increase PTP, and this can be a tough one for our teens who are working through the adolescent voice change. It simply can take more air to get those notes to come out, and there's not much else to do except encourage as efficient a vocal technique as possible. However, when the voice change is nearing completion and the vocal folds are more likely closing completely (or nearly for those who retain the posterior glottal gap), it's a good idea to look at habits to make sure the breath is not needlessly over-pressurized.
Incomplete vocal closure can also be an issue for adult singers who experience muscle atrophy as a natural part of aging. When the vocal folds lose mass, they may bow in the middle, which makes phonation less efficient and increases PTP. Sometimes increased singing time, kind of like going to the gym for your voice, can improve vocal fold bowing. However, working with an SLP and a laryngologist is a good start to make sure that's what's going on and get specialized help. Sometimes injections into the vocal folds can be of benefit to some singers, but that is all going to be up to your client and his or her doc.
Dehydration is a big factor in increasing PTP. When the mucous on the vocal folds is thicker, it's more like velcro or rubber cement and it takes more to basically pry the folds apart to get phonation going. It is helpful for singers to be mindful of their daily water intake through food and beverages, but also anything that changes in their routines, like taking seasonal allergy medication or changes in the weather, that might affect their hydration levels. If singing feels more effortful, I'd look to hydration first.