Just for future reference, I will give you a quick education on bass response and driver size.
what controls bass response in woofers (disregarding excursion, i.e. woofer movement) is both the compliance of the suspension and the weight of the driver, along with enclosure type (ported or sealed). The heavier a driver is, and the more compliant the suspension, the lower the resonant frequency, and consequently frequency response, of the driver. This is why 15” drivers in pa speakers fail to reach below 60hz, while that 6.5” speaker you’re referring to digs down to 44hz.
That’s not to say driver size isn’t important, because it certainly is. The cone’s job in a woofer is to couple to the air and move it back and forth to recreate certain frequencies, hz is how many full back and forth movements per second, so 40hz requires the driver to move back and forth 40x a second. As frequencies decrease, these wavelengths become longer and longer. The effectiveness of a speaker coupling to the air across a particular frequency range is related to the diameter of the driver in relation to the wavelength. A 3000hz wave is about 4.5 inches. With a typical 1” tweeter, that gives about a 1:4.5 ratio of diameter of driver to wavelength. A 40hz wave is is about 28 FEET! Since it is incredibly impractical, let alone presenting huge challenges from an engineering standpoint, to design a subwoofer with say, a 72” driver, we have to deal with sizes ranging from 8”-18”. Even a 12” sub gives you a ratio of 1:28 diameter of driver to wavelength ratio at 40hz. This means even big speakers have to move quite a lot of air to reproduce these frequencies with any usefulness. A typical 12” subwoofer might have a sensitivity rating of 83dB at 1w, while a dome tweeter will often be 92dB at 1w. Since the decibel scale is logarithmic this means that the tweeter is almost 10x more effective at coupling to the air within its operating range.
When it comes to smaller drivers, the amount of excursion increases enormously, a 12” subwoofer would only need to move about 5mm one way to reproduce a 40hz wave at 100dB/1m, while a 6.5” driver would need to move nearly 17mm. This kind of excursion would destroy most small woofers, which likely have an xmax of 6mm. A ported box halves the excursion requirement at frequencies near the tuning, but that’s still about 8.5”.
Since woofers in two way speakers also must cover mid bass and midrange frequencies, they can’t be made super heavy with a very compliant suspension like a sub, because the driver has to move back and forth quickly to reproduce higher frequencies linearly. To do this effectively, the driver must be light, which also means thin. A thin small driver moving 4x a much a a thick, large subwoofer is going to flex, leading to lots of distortion at lower frequencies.
This is why people use subwoofers. I would be less concerned with bass response of the speakers (so long as it can cover down to about 60-70hz). The largest benefit of a bigger speaker is higher output with lower distortion at all frequencies. Use a sub for the bass.
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