Another notion of 'efficiency' has to do with long-term vocal health and short-term vocal endurance. If the vocalist wishes his/her voice to perform well for a lifetime, it is important to use the voice in a manner which will not harm it. In the short term, people need to use their voices in such a way that it will not 'wear out' or tire excessively, so that they can complete whatever vocal tasks they need to perform: a long speech or other public speaking, a business meeting, or an opera performance. Singing or speaking in a healthy, sustainable way is another kind of 'efficiency'.

Finally, there is the technical question of the degree to which a vocalist efficiently converts aerodynamic power from breathing into sonic energy radiated from the mouth without unnecessary "losses" of power in the larynx or vocal tract.

Given this background, we can ask several relevant questions about efficient voice usage:

1. Of the total aerodynamic power available from the lungs, how much of it is converted into sound energy radiated out of the mouth, and how much is 'lost'?
The maximum power output from the lungs, in the form of a quickly-moving stream of air, is approximately 10 watts, assuming that the glottis is fully open, and all of the air is allowed to rush out as fast as possible. Of course, in singing, the glottis is closed part of the time, and partially closed for a good portion of the rest of the time during phonation, so this limits the actual airflow to about 2-10% of the maximum. Assuming the high end of this range (10%), this means that the maximum radiated power from the glottis is approximately 1 watt (10 watts x 10%).

By way of comparison, the energy produced by the entire human body, on average, is about 100 watts on a sustained basis, but it can generate much more power for very short periods of time, such as in weightlifting or other strenuous sports that feature bursts of energy.

Acoustic power and efficiency - Acoustic power radiated by the glottis varies according to the square of two factors:

1. The amount of airflow used to produce the voice can vary by a factor of approximately 10: from 50 to 500 cm³/s.
2. Fundamental frequency (or Fo) also varies by a factor of about 10: from 50 Hz up to 500 Hz.

2. Where does the 'lost' energy go?
Only a tiny portion of the aerodynamic power supplied to the vocal folds is converted into radiated sound energy. What happens to the rest of the energy? Main sources of energy loss from phonation are as follows:

• Some of the power is transferred from the airstream to the vocal folds, in order to move the vocal folds back and forth; this is known as 'driving pressure' and consumes about 0.1 watts out of the total of 1 watt. The higher the pitch and volume being produced, the larger this loss becomes.
• Local air turbulence caused by airflow near the various surfaces in the glottis and at its exit causes further power losses; the extent of these losses has yet to be determined.
• Lastly, some of the wind power does get converted into sound, but is then lost in collisions with the soft walls of the vocal tract before it can be radiated into the surrounding environment. Again, this power loss has yet to be measured precisely.

3. Is the voice being used in a way which will not cause it to suffer excessive wear and tear, or fatigue?
Unlike an automobile engine, the vocal system cannot simply be replaced or have parts swapped in and out. An individual gets only get one larynx, and if it is damaged, there may be long-term implications for voice use. Thus, preventative measures which avoid damage are important for vocal performers and those who use their voices for more utilitarian means.

Anyone who has heard an opera singer struggling to make it through the last act of a difficult role or listened to Bill Clinton giving speeches during his presidential campaigns understands the need to care for the voice in the proper way, in order to avoid excessive fatigue.

How can this be accomplished? As in other physical activities, singing and speech involve the use of various muscles and connective tissue. These body parts will always function better and more efficiently if they are properly trained, conditioned, and warmed up prior to use. The principles behind proper vocal training are quite similar to athletic training for other physical activity:

• The voice must be gradually conditioned, building up slowly from short periods of voice use to longer periods, just as a distance runner must begin with shorter runs in order to 'get in shape' before running long distances at once.
• Voice training can help the vocalist to become more coordinated in the use of the various laryngeal and breathing muscles, so that their actions become more skilled and efficient, and excessive use of muscles which are not needed can be curtailed, leaving only the muscular effort which is actually required to accomplish the vocal tasks at hand.
• Proper nutrition is crucial; regular voice use causes small amounts of wear and tear on the vocal fold layers, and in order to rebuild these tissues and recover from exertion, the body needs a good diet.
• Water is very important as well. The vocal mucosa will vibrate much more freely if the tissue is properly hydrated. Trying to use the voice while 'dried out' will make the vocalist tired much more quickly, and can even result in damage to the vocal folds since the stresses on the vocal folds are greater if they are not flexible and wet. The laryngeal muscles also need a good supply of water to work properly.
• Exercise of the whole body is also necessary; if the body is well-conditioned, circulation improves, which allows the voice to recover more quickly from exertion. Good cardiovascular health will also help the vocalist to breathe better.