First of all, let's talk about what vocal resonance is. Resonance refers to the way that sound vibrations travel through the human body and are amplified in different areas. Male and female voices have different resonance patterns, which is why their voices sound distinct. In general, male voices tend to have a deeper, fuller sound, while female voices are often brighter and have more variation in their speech patterns.
Vocal resonance is dependent on the size and shape of the vocal tract as well as the size of the vocal folds themselves. However, we are able to change our resonance and as a result change the perceived gender of our voice. This can be achieved through modifying the position of the larynx, changing the amount of tension in the throat muscles and altering the overall space in the vocal tract.
If you're mtf and want to feminize your resonance through mtf voice training, one way to achieve a more feminine voice is to work on brightening your vocal resonance. This can be accomplished through vocal exercises such as humming  on “m” which I will explain below.
Humming to feminize vocal resonance:
1) Make a humming sound using the letter "m".
When humming it's important to make sure that we're never clenching the jaw or pressing our lips together, if you are you won't be able to feel the sensations. Our jaw should be relaxed and our lips should be softly resting against each other.
2) As you hum, close your eyes and pay attention to the buzzy vibrations that you feel being produced inside your body.
3) Now while continuing to hum "m", glide the pitch up and down to make "peaks" with the hum and through this you will be able to feel the "buzzy" sensation move.
You may notice it starting in your chest or throat and then rising into your mouth. What we are aiming for is to feel it in the front of the mouth, specifically the lips.
4) Once you feel a moderate to large sensation in the lips, attach a vowel sound from the list below, these are known as CV blends. Your aim is to keep the sound of each vowel bright while using the hum as a tool to get you there. Remember to take as long as you need to feel those sensations, never rush or force it.
5) Once you find the sensation that you're looking for, continue to hold the hum for 2 seconds and then once you release to do the vowel, hold the vowel for 3-4 seconds.
6) If you practice regularly, Over the next few weeks/ months you will notice the sensation feels easier and easier to isolate in the lips and the sound you can produce will become brighter as well.
It's worth noting that achieving a more masculine or feminine resonance will take time and practice. It’s also important to remember to never force the sound out through tightening your muscles or holding your breath as these are likely to cause damage to your vocal folds and create inauthentic sounding voices; A brighter or darker resonance is simply the byproduct of muscle development and is never something that we ourselves have to force out. Additionally, it's not uncommon for individuals to experience frustration or self-doubt during this process, especially if progress is slow. However, with patience and persistence, it is absolutely possible to achieve the voice that feels most authentic to you.
Vocal resonance is a complex topic, but understanding the basics can be a helpful step for individuals seeking to change their voice. Remember, mtf voice training takes time, effort and patience, but with dedication, you can achieve the voice that aligns with your gender identity and helps you feel more comfortable and confident in your own body.
1. Gallena, S. J. K., Stickels, B., & Stickels, E. (2018). Gender Perception After Raising Vowel Fundamental and Formant Frequencies: Considerations for Oral Resonance Research. Journal of voice : official journal of the Voice Foundation, 32(5), 592–601. https://doi.org/10.1016/j.jvoice.2017.06.023
2.Schneider, S., Courey, M. (2016). Transgender Voice and Communication - Vocal Health and Considerations | Gender Affirming Health Program. Available at: https://transcare.ucsf.edu/guidelines/vocal-health
3. Jackson Hearns, Liz, author. (2018). One weird trick : a user's guide to transgender voice. [Place of publication not identified] :CreateSpace Independent Publishing Platform