Vocal Ring, or The Singer's Formant
The Swedish voice scientist Johann Sundberg studied recordings of the famous tenor Jussi Bjoerling in the 1970s, and found that the average frequency spectrum of his voice had a large 'hump' at around 3000 Hz, especially when Bjoerling was singing with loud orchestral accompaniment. That hump is visible in the dotted line in the figure below, and is known as the 'singer's formant':
Now, the reason that this is relevant lies in the 'Orchestra' (solid) line. As you can see, the orchestra produces lots of energy at around 500 Hz, but falls off steadily at higher frequencies, producing relatively little around 3000 Hz. The well-trained operatic voice produces quite a bit of energy around 3000 Hz, however, and so it can be heard even above the orchestra.
What is responsible for this strong vocal resonance? Sundberg also studied this question by modeling a small resonator inside the vocal tract, just above the vocal folds. For optimum resonance, this resonator must be about one-sixth as long as the entire vocal tract, and also have about one-sixth of the cross-sectional area of the vocal tract:
The closed end of this resonator is the glottis, and the open end is the rim of the epiglottis.
It turns out that introducing this small contricted resonator into the vocal tract causes the vocal spectrum to contain much more energy at around 3000 Hz, as you can see in this animation. Note that as the left end of the tube (representing the vocal tract) is gradually constricted, the hump in the spectrum around 3000 Hz appears:
Research conceived and conducted by Dr. Brad Story (formerly of the NCVS) uses imaging technology to better understand the shape and subsequent behavior of the vocal tract. The CT images shown below represent the air spaces produced by an individual producing various vowel sounds.