Please note that a bass bin with rather flat response to the 45-50 Hz range and some usefulness at the 41 Hz "low E" is quite adequate. Improving the lower bass response beyond this is generally not worth the larger size (or lower efficiency) needed of the bass bins. There is indeed a general relationship of bass cutoff frequency, efficiency, size, and response shape. To increase the efficiency by 3 dB or to extend the bass response by 1/3 octave, you need to double the box volume. A sharper cutoff makes the cutoff happen at a slightly lower frequency, given a particular efficiency and box volume.
You can make a more expensive 15 inch driver "cheaper" by adding a resistor in series with it to increase its Qts. This is better with speakers with a very high Qms, such as EV speakers. Although the impedance will be made higher, the efficiency will not decrease to less than that of a cheap speaker with the same Thiel-Small parameters as you are getting.
Now for some MH1540 box general designs:
This info is mainly good to those who already know how to design and build speaker cabinets, especially tuned port (bass reflex) bass bins. However, I like saying that the speaker cones move less at the port frequency than at nearby frequencies, especially if you add resistance in series with the speakers. It is a good idea to confirm the actual port frequency and adjust it as necessary.
1. A quad-15" large box:
Use a large box about 4 feet high and 4 feet wide and 29 inches deep. The box volume will be about 30 cubic feet (maybe as much as 32) after allowing for the braces (necessary), lip, and space taken up by the speakers and half the air in the ducts. (Half the air in the duct contributes to box volume, half does not.) The port frequency should be 48 Hz.
Predicted results, in the far field, on-axis, with the bass bin on a hard, reflective floor in an otherwise anechoic environment:
Sensitivity should be a good solid 106 dB at 1 watt, 1 meter for the general lower bass range (should be close to this or higher at all frequencies from 48 Hz up). The sensitivity will be a bit higher at frequencies around 63 Hz. The sensitivity theoretically also runs higher above 200 Hz, but this may not occur well off-axis or if floor reflection does not work well at frequancies this high. The frequency at which the sensitivity dips to 3 dB below some average bass sensitivity is 45 Hz. Usable response extends to low E. It is recommended to filter out or avoid frequencies below 40 Hz. Power handling is not too impressive but should be adequate, around 350 watts. Sorry, you can't get this in a 4-ohm version.
2. A dual-15 box:
Go for something about 4 feet wide, 2.5 or so feet high, and 29 inches deep, with a box volume around 18 cubic feet, and with a port frequency of 48 Hz.
This supposedly has a sensitivity of 104 dB 1 watt, 1 meter (in the far field, on axis, with the box on a hard, reflective floor in an otherwise anechoic environment). The 3dB down point below average bass sensitivity of something like 104.5 is about 43 Hz. The rolloff is fairly rapid below this, but gives you usability at low E. Power handling is about 150 watts, not too high.
3. A single 15 box to rock your house:
Use a large box volume of 11 cubic feet and a highish port frequency of 46 Hz. This makes a quite pleasant bit of a hump peaking around 49 Hz. The response runs high down to low E, and is 3 dB down at 37 Hz, the really low D. The sensitivity is a good solid 97 dB 1 watt, 1 meter. The hump peaks at about +4 dB. All this is with the box in standard half-space, recessed into a wall in an otherwise anechoic environment. If the box is taller and wider than it is deep and it is against a wall, you will get the better part of this impressive performance.
My Best Determination of Thiel-Small parameters of the Pyle MH1540:
Fs = 51 Hz
Qts = .7
Vas = 4.8 cubic feet
One source of MH1540's:
MCM Electronics, 1-800-543-4330. Their catalog number for the MH1540 is 55-190.
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