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Charlie Hughes: What sort of power output & freq for the levitation, and what mass were you supporting?

Tom: Most of the levitation sources that I made ran at 21 KHz - a typical source that could produce a sound pressure of 165 dB @ 12 inches. An array of six can produce in excess of 175 dB at the center - which is enough to light a cigarette with acoustic friction.

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Under controlled conditions, in an acoustical laboratory, the trained healthy human ear is able to discern changes in sound levels of 1 dB, when exposed to steady, single frequency ("pure tone") signals in the mid-frequency range. It is widely accepted that the average healthy ear, however, can barely perceive noise level changes of 3 dB.

On this scale, the normal range of human hearing extends from about 0 dB to about 140 dB. 0 dB is the threshold of hearing in healthy, undamaged human ears; 0 dB is not an absence of sound, and it is possible for people with exceptionally good hearing to hear sounds at -10 dB. A 10 dB increase in the level of a continuous noise represents a perceived doubling of loudness; a 5 dB increase is a readily noticeable change, while a 3 dB increase is barely noticeable to most people.

Sound pressure levels are applicable to the specific position at which they are measured. The levels change with the distance from the source of the sound; in general, the level decreases as the distance from the source increases. If the distance from the source is unknown, it is difficult to estimate the sound pressure level at the source.

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Since the human ear is not equally sensitive to all the frequencies of sound within the entire spectrum, noise levels at maximum human sensitivity?middle A and its higher harmonics (between 2,000 and 4,000 hertz)?are factored more heavily into sound descriptions using a process called frequency weighting.

The most widely used frequency weighting is the "A-weighting", which roughly corresponds to the inverse of the 40 dB (at 1 kHz) equal-loudness curve. Using this filter, the sound level meter is less sensitive to very high and very low frequencies. The A weighting parallels the sensitivity of the human ear when it is exposed to normal levels, and frequency weighting C is suitable for use when the ear is exposed to higher sound levels. Other defined frequency weightings, such as B and Z, are rarely used.

Frequency weighted sound levels are still expressed in decibels (with unit symbol dB), although it is common to see the incorrect unit symbols dBA or dB(A) used for A-weighted sound levels.

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A level difference of ?3 dB is roughly double/half power (equal to a ratio of 1.995). That is why it is commonly used as a marking on sound equipment and the like.

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