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dB (Decibel) - a term used to describe sound pressure level. 3dB - the smallest amount of sound pressure change average people can hear. To hear a 1dB change in sound, you need to be in a recording studio or similar room.
An increase of 3dB in sound pressure level is twice the power. A decrease of 3dB in sound pressure level is cutting the power in half. To double the (perceived) loudness of sound, you need to increase the power 3 times, or 9dB.*
When you double the distance from a given point, the sound level decreases 6dB. * When you half the distance from a given point, the sound level increases 6dB.* Sound travels through water faster than through the air. Sound travels faster as humidity increases.
Sound travels at 1130 feet per second. Frequency is "vibrations per second", which in turn is called "Hertz" (Hz).

The human voice has a range of 80Hz to 1200Hz.

The tone of a sound is called "frequency". Harmonics are multiple vibrations of the fundamentals. Harmonics are what make each voice and instrument different sounding.
The human voice has both fundamental vibrations and harmonic vibrations. ESSS sounds in speech are often caused by the spaces in a person's teeth. ESSS sounds are from 5,000Hz to 9,000Hz.
Harmonics extend the range of the human voice up to 6,000Hz. The wavelength of 200Hz is 5.63 feet. The wavelength of 2,000Hz is 6.75 inches.
The wavelength of 80Hz (the low end of a persons voice) is 14.06 feet. For a surface to control a frequency, it has to be as thick as 1/4 the wavelength. (Carpet 3/4-inch thick with 1/2-inch underpad begins to absorb sound above 2,000Hz.) Carpet absorbs sound above the speech range, makeing carpet a poor controller of speech in a large room. This also kills the musical aspect of the room.
To create a sound, an object has to vibrate a full cycle of movement which involves a push and pull motion. Building acoustical control into a church when it's being built often doesn't cost the church anything extra except for the knowledge of how to do it. What is not absorbed by a surface (like carpet) is reflected back into the room.

*In a closed room, these rules don't always apply.

This information adapted with permission from the Church Sound Network & EdB Sound, Acoustics, 1997 edition.