Sound is transmitted as a wave through a medium such as air. The “cubic volume” of the air is not a critical matter. The distance from the stage to the back of the house is more critical because sound dissipates at an inverse ratio to distance from the source. But it is this characteristic of sound that can be reinforced by careful and cleaver acoustic design. A double parabolic reflector shaped building, such as in the Mormon Tabernacle will pass a mere whisper from one end of the building to the other if one stands in the correct location. A properly designed opera house will sustain sound energy through reflection and focus to make most of the seats in the opera house acceptable for acoustic singing.
High frequencies in the singing voice are of little value if they exist within the strongest frequency spectrum of the Orchestra because the orchestra has far greater amplitude (loudness) in that range, about 500 to 750 Hz. Properly trained acoustic singers, both male and female, produce a strong boost in amplitude in frequencies of about 2600 to 3200 Hz, well above the major power of the orchestra. This boost is now known as the Singer’s Formant. It was slowly discovered and gradually developed throughout the history of Opera and it grew in finesse of achievement as the Opera orchestra grew in size and power.
Singer’s Formant has little to do with “squillo” which is more a description of tonal quality than of vocal carrying ability. The human ear is very sensitive to the frequencies of the Singer’s Formant (2600-3200 Hz) so it hears it very well but it does not hear as a particular frequency but more as an intensity of tone.
Size of voice or richness of voice is less important in the ability of the voice to be heard. That is dependent on the presence of the Singer’s Formant. However, both of these qualities, size and richness of voice, are of primary importance in the appropriateness of a particular voice for a particular role. Most discussions about what roles a particular performer should sing are really discussions about size of voice and tonal qualities. It should be understood that all voices will carry if the singer produces the Singer’s Formant. Yet all voices, even if easily heard, must be placed in roles that best match their tonal characteristics and temperament.
In short, tonal characteristic, or Timbre of a voice is a different matter than the ability of the voice to be heard. Inversely, the ability of the voice to be heard is not a valid description of the Timbre of the voice. These two characteristics are created by increased amplitude of frequencies that occur at different frequency locations in the vocal spectrum of sound