"....Maximum Efficiency of Direct-Radiator Loudspeakers
The Thiele/Small method of low frequency direct-radiator loudspeaker system analysis neglects the radiation impedance components in the equivalent electric network model. When these components are included some surprising results are evident. Due to the definition of efficiency widely used in direct-radiator loudspeaker analysis, the absolute maximum efficiency is limited to 25%. With voice-coil inductance neglected, inclusion of the radiation impedance components transform all responses from high-pass into band-pass functions. Fow low frequencies, the maximum achievable nominal power transfer efficiency is found to be proportional to cone diameter. For a specific diameter, the maximum efficiency depends only on the moving mass to air-load mass ratio. Relationships and graphs are presented which relate the true nominal power-transfer efficiency to the Thiele/Small derived efficiency.
Author: Keele, Jr., D. B. (Don)..."
Un paper che puo chiarire molti dubbi, (e forse crearne altri) ma purtroppo a pagamento se non si é AES member...
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Forse era questa la formuletta che ricordavi di aver visto:
3. Efficiency & Loudness
The efficiency of a driver is given in decibels of sound pressure
level (SPL). 0 dB SPL is defined as 2.0E-10 bar (2.0E-5 N/m^2), which
is the lowest level of 1 kHz tone the average person can detect. A 10
dB increase in SPL results in an apparent doubling of the loudness and
requires 10 times as much power. Accordingly, a 10 dB decrease halves
the loudness and reduces the power requirement by a factor of 10.
Most driver manufacturers specify the SPL of the driver with a one
watt input measured at a distance of one meter. To calculate the SPL
at other power levels, add the following number to the SPL rating:
10*log(POWER), where POWER is in watts, and the log is base 10. This
equation is derived from the fact that a doubling of electrical power
produces an doubling of acoustic power. To calculate the SPL at other
distances, subtract the following number from the SPL rating:
20*log(DISTANCE), where DISTANCE is in meters. This equation is
derived from the inverse square law of wave propagation.
One watt of acoustic power is equal to 112 dB SPL at one meter. To
calculate the efficiency of the speaker in percent, use the
following: %EFFICIENCY = 100*(10^((SPL - 112)/10)), where SPL is the
driver's SPL rating in dB, at one watt, measured at one meter. For
example, a driver with a 92 dB SPL rating @ 1W/1m is 1% efficient.
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