Electronic "Q" Adjustment on Woofer Tower

[Ron Resnick]

In amateur radio "Q" refers to how narrow or wide is the frequency range at which an antenna is resonant. "High Q" refers to an antenna which is resonant over only a narrow frequency range.

In loudspeakers I think that "Q" is used as a proxy for how well or poorly damped is a speaker driver. A loudspeaker's Q is the result of the mechanical and operational characteristics of the dynamic driver (surround, magnet, voice coil) and the size of the enclosure in which it is mounted and amplifier output impedance.

How does an electronic Q adjustment work on a woofer tower? Some actively powered woofer towers (with built-in amplifiers) have an electronic Q adjustment intended to provide flexibility and adjustability in integrating the woofer tower into the listening room to optimize bass performance in a particular room. But how, exactly, does an electronic circuit do that?

Does such a Q control likely just change the output impedance of the amplifier built into the woofer tower so the user can vary the damping on the woofer cones?

 

[amirm]

If it is a powered woofer which sounds like it is, then there is a low pass filter and that Q sets the parameters for that. Best to measure the in room response to know what it really does as opposed to what it says it does. Pretty sure it has nothing to do with amp impedance, damping factor, etc. It is the definition that you already know .

 

[LL21]

Not a techie, Ron, but i can only share with you what the Q adjustment on the Velodyne is supposed to do. You might already know this, but this is verbatim from the manual.

"The bandwidth determines how narrow the parametric filter is. The narrower the filter, the higher the Q value and the faster the roll off on either side of its center frequency. The default Q setting for the bandwidth is 4.3. The Q can be set from 0.1 to 20. The Q or bandwidth of the filter is usually adjusted last so that as little around the dip or peak being corrected is affected."

Like an eq boost or depression, it seems to widen/narrow the effect of this boost or depression around the specific target frequency.

 

[LL21]

As you can see in the graph below, we have attenuated some of the frequencies using the EQ (specifically EQ band #2)...the Q is at 4.3 but i could have widened that curve to affect more of the frequency range or tightened it up to affect less of it...both of which had differing effects on the flatness of the curve (recorded above). The current setting seemed to keep it the flattest.

 

[Mike Lavigne]

in the Evolution Acoustics MM7's (and MM2 and MM3) the 'Q' adjustment either tightens or loosens (softens) the response. and subjectively that is what you hear. what is crazy is how significant this issue becomes when you have everything dialed in.

of course, adjusting 'gain', 'crossover frequency', 'extension' and 'Q' they are all dynamically interrelated. adjusting one thing means all might need to now again be adjusted.

the process takes time and multiple sessions. for me it happened over a month or so to find the best combination. 4 sets of two 15" woofers, each set with 4 different adjustments. for me I took the approach to set each subwoofer set the same. I felt like trying to vary each set would be a step too far for my abilities. if I got the speaker designer to fly back up to do it, he would not be so restricted.

I am really happy with the results. and feel strongly that anything less agile as a speaker tuning tool is lacking.

 

[Ron Resnick]

Thank you Amir, Lloyd and Mike!

I understand Amir to be confirming what Q means in the context of an equalizer, such as that in Lloyd's Velodyne.

But, Mike, are you saying that your Q adjustment is, in fact, related to damping (as is suggested to me by "tightens or loosens (softens) the response"?

 

[Mike Lavigne]

Thank you Amir, Lloyd and Mike!

I understand Amir to be confirming what Q means in the context of an equalizer, such as that in Lloyd's Velodyne.

But, Mike, are you saying that your Q adjustment is, in fact, related to damping (as is suggested to me by "tightens or loosens (softens) the response"?

yes. exactly.

what processes occur to make that happen I cannot say. I don't know crossovers or speaker designs.

 

[DonH50]

The damping factor (DF) and Q factor (QF) are related: Q = 1 / (2 * DF). Fundamentally Q describes energy transfer in a system, i.e. energy stored vs. energy lost. Also note bandwidth BW = Fc/Q where Fc is the center frequency.

For geeks, for a second-order filter function the transfer function H(s) is given by

H(s) = numerator / [s^2 + (w0/Q)*s + w0^2] = numerator / [s^2 + (2*DF*w0)*s + w0^2]

where the numerator depends upon the type of filter (lowpass, highpass, etc.) and w0 is the center frequency in radians/second (w0 = 2*pi*f0 where f0 is the frequency in Hz).

As already stated, the circuit almost certainly adjusts filter parameters without actually affecting the amplifier's output impedance or anything like that. Where it gets tricky is that another definition of damping factor as related to amplifiers is related to the output impedance: DF = Zload / Zout where Zload is the speaker and Zout is the amplifier's output impedance.

Wiki link (though not one of the better sources; I used an old textbook): https://en.wikipedia.org/wiki/Q_factor
Another link to damping ratio that includes the relationship to Q: https://en.wikipedia.org/wiki/Damping_ratio

HTH - Don

 

[Ron Resnick]

Thank you very much, Don!

 

[DaveC]

Right, and going from Don's post we can see the Zload directly affect Q and would be a fairly simple parameter to adjust via the crossover.

For speaker damping it's more useful to think of Q as describing the response as underdamped, overdamped or critically damped. Overdamped response means overshoot and ringing, underdamped takes longer to hit target and critically damped is the shortest time to hit target without overshoot. This behavior does make a big difference in the overall response of the speaker as music is not a simple impulse or steady tone. This only really matters at relatively low frequencies...

Here's a link, scroll down to "shaping low frequency response" to see a graph and explanation... it's basic control system/2nd order differential equation math...

 

[NorthStar]

• http://www.diyaudio.com/forums/subwoofers/8114-subwoofer-qtc-tightness.html#post88693
_________

"The efficiency of a speaker driver is given by:

? 0 = ( 4 ? 2 F s 3 V a s c 3 Q e s ) × 100 % {\displaystyle \eta _{0}=\left({\frac {4\pi ^{2}F_{s}^{3}V_{as}}{c^{3}Q_{es}}}\right)\times 100\%} \eta_0 = \left(\frac{4 \pi^2 F_s^3 V_{as}}{c^3 Q_{es}}\right)\times100\%
Where the variables are Thiele/Small parameters. Deep low frequency extension is a common goal for a subwoofer and small box volumes are also considered desirable. Hofmann's Iron Law therefore mandates low efficiency under those constraints, and indeed most subwoofers require considerable power, much more than other individual drivers.

So for the example of a sealed speaker box, the box volume to achieve a given Qts is proportional to Vas:

V b = V a s ? {\displaystyle V_{b}={\frac {V_{as}}{\alpha }}} V_b = \frac{V_{as}}{\alpha} Where: ? = Q t c 2 Q t s 2 ? 1 {\displaystyle \alpha ={\frac {Q_{tc}^{2}}{Q_{ts}^{2}}}-1} \alpha = \frac{Q_{tc}^2}{Q_{ts}^2}-1
Therefore, a decrease in box volume and the same F3 will decrease the efficiency of the sub woofer. Similarly the F3 of a speaker is proportional to Fs:

F c = ( Q t c F s ) Q t s {\displaystyle F_{c}={\frac {(Q_{tc}F_{s})}{Q_{ts}}}} F_c = \frac{(Q_{tc}F_s)}{Q_{ts}}
As the efficiency is proportional to Fs3, small improvements in low frequency extension with the same driver and box volume will result in very significant reductions in efficiency. For these reasons, subwoofers are typically very inefficient at converting electrical energy into sound energy. This combination of factors accounts for the higher power output of subwoofer amplifiers, and the requirement for greater power handling for subwoofer drivers. Enclosure variations (e.g., bass reflex designs) are sometimes used for subwoofers to increase the efficiency of the driver/enclosure system, helping to reduce the amplifier power requirement."