TLS Guy mentioned Q in regard to woofers. It is an important feature of speakers, but it can be a difficult topic to understand. So I thought I would add to what he said. For anything that vibrates or resonates, I think of Q as a shape or slope of a response curve.
An electrical filter will oscillate or ring, to some extent, after the signal stops. The steeper the slope of this filter, the longer it will ring. Similarly, the higher the Q value of this filter, the more it will ring. Mechanical filters work the same way.
Speaker cabinets act as mechanical high-pass filters for woofers or subwoofers mounted in them. It passes frequencies above the cut-off or low frequency limit of the design, and the signal rolls off below this point at a rate determined by the design. The cabinet dimensions affect the characteristics of the high-pass filter, letting a designer tune the woofer’s bass response. A sealed cabinet acts essentially as a 2nd order high-pass filter, and a vented enclosure typically resembles a 4th order high-pass filter.
This figure shows the low end of the frequency response curves of a woofer in various sized boxes with different system QTC.
Ideally, a woofer will respond to a single bass tone by starting and stopping soon afterwards. A vented design will continue to respond (ringing or oscillating) about twice as long after the signal stops, as a sealed design. This transient response can be visualized in an impulse response curve, where a single pulse of sound is introduced, and the response is graphed vs. time.
The total system Q of the woofer combined with cabinet is a ratio of values (with no units) that defines both the shape of the frequency response curve roll-off. It also determines the amount of damping to oscillation or ringing after the signal stops. This is also spoken of as “transient response”. QTC is the term for sealed cabinets, and the similar QTS term is for ported reflex cabinets.
A sealed enclosure with a QTC of 0.5 is considered a “critically damped alignment”. An impulse response curve shows that it responds to the initial pulse and has little or no overshoot. For a given driver, a QTC of 0.5 requires the largest box. This low Q alignment has a downward-sloping response curve, but offers the best possible transient performance and the lowest frequency extension at -10dB.
A system QTC of 0.577 (also known as a Bessel alignment) has the most linear phase response and offers slightly less damping than Q = 0.5.
When QTC = 0.707 (a Butterworth alignment) there will be the flattest frequency vs. amplitude response. This is a common alignment for woofers because it offers a fuller sound, while still being reasonably well damped.
System QTC near 1.0 delivers a peaked response, but allows the smallest box size still considered by some to be acceptable. A woofer with a QTC larger than 1.0 is a boom box with a peaked response curve, and an impulse response that rings and rings. Guess where most less expensive home theater subwoofers fall? Woofers and subwoofers that play on after the signal has stopped (due to a high Q and the resulting ringing) sound slow and muddy.
This next figure (sorry about the size, it was the best I could find) shows the amount of ringing in the impulse response curves where QTC ranges from 0.5 to 2.0.
Looking at the first figure, it is easy to say, “Go for that big response where Q is 2 and get the most bass response for your money” High Q drivers usually do have less expensive small magnets, so this certainly will cost less. But along with the illusion of great bass at low cost is the hidden cost of lack of control over the movement of the woofer cone. It allows a big response around the woofer’s resonance frequency, but furnishes poor control over transients, causing muddy sounding or ringing bass. The second figure shows the response of a impulse response graphs of speaker several different QTC values. QTC of 2 hangs on the longest after the pulse has ended, much like the ringing of a bell. As QTC gets lower, the ringing ends sooner.
