And that's not necessarily a bad thing!
I'm talking specifically about air pressure that is happening at all times along the vocal tract, whether you are singing or not. Air pressure, for our purposes, is simply the force exerted by air molecules as they move through a given space. If the air pressure is greater, more force is being exerted on the surrounding tissues, and lower air pressure is less force on the tissues.
There are multiple places along the vocal tract where pressure can be measured. Each of these places can have an effect on phonation because they are all connected to one another. The areas don't exist in a vacuum (pardon the pun) and because they are all interconnected, any pressure changes in one area can change the pressures in other areas.
When we're talking about pressures in the vocal tract, these are the main ones we measure in voice science research:
intraoral pressure happens in the mouth
subglottic pressure (really better called "tracheal" pressure, see Hixon, Weismer, and Hoit) is the pressure that happens at the vocal fold level
transglottal pressure is what happens in the teeny tiny space in between the vocal folds
various pressures in the lungs, such as bronchial, alveolar, and pleural
pharyngeal pressure happens in the space between the mouth and the vocal folds
tracheal pressure is what happens in the trachea, of course
nasal pressure occurs as the air moves through the nose, such as in nasal vowels
Air pressure can be changed when the shape and size of the space is changed. Generally speaking, when a space gets smaller, the concentration of the air molecules increases, the the pressure those molecules exert also increases. The converse is also true, so expanding a space can reduce the pressure. In singing and speech, the goal is not always to reduce air pressure as low as possible. We need a certain amount of pressure to bring the vocal folds together and to create sound. Plus, the simple act of breathing has pressure associated with it. We can't get away from pressure, and we don't want to. But we do want it to be optimum.
SOVTEs can alter the air pressures along the vocal tract, often by increasing the pressures in the mouth. For example, using a small-diameter straw increases the air pressure at the point of the lips and in the mouth because the air has to bunch up to move through the smaller space. This can have the effect of also raising the pressure in the supraglottal space, or the area right above the vocal folds, but decreasing the pressure in the subglottal space, which also reduces the transglottal pressure, which has the overall effect of allowing the vocal folds to come together more gently.
Now granted, it is much, much more complicated than this, but I hope it also highlights how different SOVTEs can effect the vocal tract in different ways. A large straw will work differently than a small diameter straw, though perhaps similarly to a kazoo. Lip trills may be a bit similar to a hum, but different than a purse-lipped "ooh." What your client needs may be different than what you need, or even different day to day. We can't get so attached to one exercise that we neglect the benefits of others. It takes experimentation and a willingness to be wrong until you find the right thing. The more experience you have working with different SOVTEs will give you a better chance of predicting which ones are more likely to work.
Of course, SOVTEs are meant to be a tool that a singer can use to eventually be able to recreate the same or similar pressure situations along the vocal tract without the straw. After all, I've never seen a singer sing an aria through a straw in recital (though if you have that video, send it to me!). If you find that a client is not able to move from the straw to singing without it staying consistent and with a tone that the client wants, it might be worthwhile to refer to a medical professional to take a look at the whole system for any glitches along the way.
SOVTEs are a great tool, but they aren't magic. The real magic happens when a singer can sing with the freedom that allows him or her to enjoy their voice. When that happens, it's truly spectacular to watch.
As you are experimenting with SOVTEs and how they work in your clients' singing, I'm here as a resource for you! If you'd like to talk through a situation in your studio or invite me to observe and work with one of your clients, a Vocal Ped Party is perfect for that. You can book an hour with me, your Personal Pedagogy Professor, at the button below. I'd be honored to assist you!
Hixon, T. J., Weismer, G., and Hoit, J. (2014). Preclinical Speech Science: Anatomy, Physiology, Acoustics, Perception, Second Edition. Plural Publishing.
Hijleh, K. and Pinto, C. (2021). "Realizing the Benefits of SOVTEs: A Reflection on the Research." Journal of Singing, 77(3), 333-344.