Bandpass Subwoofers

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>Actually, most hi-power PA speakers are BPs.....>

most like in 5% ;)

i think a very good reason for little use of bp designs in real life is the fact that computer simulation programs like winisd , bassbox etc, simply cant calculate them right.

if you measure the box you simulated you will discover they have much less efficiency than calculated and thus are easily outperformed by reflex cabinets.

so you could say they are basically popular untill you try ;)

rgds and happy new year - karsten madsen -
8th order banpass


  • 8th order bandpass.gif
    8th order bandpass.gif
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BP boxes have a sealed box rate rolloff rate on the low end and a well-designed bandpass therefore has the transient response advantages of a sealed box. The upper rolloff obscures analysis and to really compare apples to apples you need to calculate step or impulse response of a sealed box with a HF rolloff at the same frequency as the bandpass box.

Playing around a number of years ago, I concluded that you frequently can get the F3 of a vented box and the rolloff of a sealed box in the same size box (4th order BP) as a typical vented box for that woofer.

One disadvantage not often stated for 4th order BP) is that max excursion occurs near the lower F3 and this is where minimum excursion happens for a vented box, so theoretically the vented box should have better output here.

Below F3, the 4th order bandpass has some protection from overexcursion due to the stiffening effects of the rear enclosure,.

6th order BP boxes have minimum excursion near the lower F3, but no protection below this point - similar to the vented box.
Quote Chris8 on bandpass boxes: "No superiority as compared to sealed OR ported designs, excpet when a significant amount of gain is designed in, to raise efficiency significantly. However, this narrows the already limited bandwidth and creates steeper cutoff rates, which in turn causes poor transient response, and peaking. Read: One-Noted bass! "

Beg to differ. First, a ported design is more efficient than a sealed one, so if you are of equal efficiency to a ported design, then you are ahead of the sealed.

The bandpass box consists of a sealed chamber feeding a ported chamber. If the sealed part of the bandpass has a Q of 1 or above, it will give superior bandwidth, at equal sensitivity, to a ported box.

The 3 dB down point of a ported box is given by the following equation:
F3 = [The square root of (Vas/Vb)] times Fs.

So in a ported speaker, when placed into a box equal to it's Vas, will be 3 dB down at Fs.

Here is an Audax woofer placed in a bandpass enclosure that is equal to it's Vas, but it's F3 is half an octave BELOW Fs.

Don't let those two humps fool you. This woofer is plus or minus 1.5 dB between cutoff points-a smooth response.

The woofer is the Audax AP210MO.
Qts = .72
Vas = 2.25 cu ft
Fs = 46 Hz.
SPL = 90 dB.

When the sealed box portion of a bandpass has a Qtc of 1 or above, it seems to outperform the ported box in terms of cutoff frequency. When the Qtc is lower, it does not.
Who can post the corresponding input impedance response for such an bandpass subwoofer?
I need this to design a compensation network for prevent damaged TDA7293V/TDA7294V again and again in a commercial powered subwoofer device.
By a power module from a friend I note the absense of any parts between speaker cable and power output pin 14.
Thank you very much therefore.
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