As a point of reference, a vented speaker of one cubic foot internal volume and 90 dB/1 watt/1 meter efficiency will be -3 dB at about 44 Hz, assuming good boundary reinforcement from one room surface (woofer close to floor, for example). Different loudspeaker modelling programs may give slightly different results, but this should be in the ballpark.
If we factor in moderate boundary reinforcement from the two closest walls and the ceiling, we can extend that to perhaps 35 Hz.
If we put the speaker up on a stand way out in the room, our -3 dB point could end up closer to the 55 Hz ballpark.
Here's how you can make use of this point of reference:
If we want to lower the -3 dB point by 1/3 octave, we'd have to either trade off 3 dB of efficiency or double the box size. Put another way, if we double the box size we gain 3 dB of efficiency or 1/3 octave of bass extension. Put yet a third way, in order to gain 3 dB of efficiency we'd have to either double the box size or trade off 1/3 octave of bass extension. Or some combination thereof.
To calculate the frequency that would be 1/3 of an octave lower you multiply by .793; and to calculate the frequency that would be 1/3 of an octave higher you divide by .793.
To estimate box size, assume 3/4" thick walls and then deduct 10% from the gross internal volume to allow for bracing, driver magnets, and ports.
If anyone sees significant errors in these numbers please feel free to make corrections.
Different speaker manufacturers make different assumptions about the room's contributions, and the information above will allow you to estimate how much room contribution the manufacturer is factoring in. Recognize that you probably are not comparing apples-to-apples when you rely exclusively on specs from different manufacturers. In general, a smaller speaker cannot be both more efficient and have deeper bass than a bigger one if they use the same type of enclosure and both are competently designed. There are exceptions where a designer trades off bass extension for transient response, but that's another topic for another day.
Duke
dealer/manufacturer
