Please explain "Q"

[squidbait]

I am confused on what Q is, and how it affects the final product. I believe this affects the Qtc of a box, but am not sure what Qtc value is optimal.

Can someone explain this in "laymens" terms, or give me a link to a site that isn't too techincal.

Thanks!

[SY]

It's the inverse of damping; the higher the Q, the lower the damping. Which value is "best" depends on a lot of things, including your taste. A Q of 0.707 is maximally flat. A Q of 0.577 rolls off sooner, but has better phase characteristics. A Q of 1.0 has a bump up in the bass before rolloff and can sound punchier. And so on. There's nothing "magic" about these particular numbers.

As an aside, Qtc refers to the Q of a closed-box speaker- not just the box, but the box/driver combination.

Vance Dickason's Loudspeaker Design Cookbook has some illuminating charts. You ought to own that book.

[Ron E]

Q = (energy stored)/(energy dissipated)

Qts is the free-air Q of the woofer, Qtc is the closed box Q of the speaker/box system. Qtc of between 0.5 and 1 are best for speakers, and 0.707 gives the lowest F3 and flattest 2pi response.

Qtc=Qts*sqrt(Vas/Vb+1)
Vb=Vas/((Qtc/Qts)^2-1)

Higher Q gives more power handling because the smaller volume behind the driver acts like a stiffer spring.

Q of up to 1.5 is listenable, especially in a car. I have a friend with Qtc=1.3 woofers in his car and he thinks they are really tight.

People will try to tell you that 0.5 is better than 0.7 or 1 based on their half-understood notion of "critical damping", but the differences are very subtle and mostly not worth the increase in box size needed for the lower Q.

[squidbait]

Thanks for your quick response!

I purchased the Loudspeaker Design Cookbook, but found it is very technical and assumes some previous knowledge.

When you say a Q of 0.707 is maximally flat, are you referring to the Q of the speaker itself, or the Qtc?

Are there specific values that are better suited for different environments, such as music versus home theatre, or is it just personal taste?

Also, are these different values very distinct to the ear?

[squidbait]

Ron E

Than you for the formula. That is helpful.

[SY]

"Maximally flat" normally refers to the Q of a speaker-box system.

[Timn8ter]

Nobody is going to go for the 007 joke?

[Bose(o)]

Music Qtc will probably be from 0.7-1.0 or higher, as people usually prefer quick and tight bass response for music--holding onto the accuracy part. HT may prefer 0.5-0.7. Right now I have a Shiva in a 53L. enclosure yielding a Qtc=0.75 which is fine for an all around enclosure. Also, Power requirements increase as Qtc increases..and vice versa.

[squidbait]

Thanks for everyones help! This has been very helpfull.

[sreten]

There is some misinformation here :

The practical range of Qtc is 0.5 to 1.1 for a sealed box.

Q=0.7 gives a maximally flat Butterworth alignment.
Q=0.5 gives a critically damped alignment no overshoot.
Q=0.6 gives a maximally flat delay Bessel characteristic.
Q=1.0 gives a 1dB ripple/peak before roll-off, Chebyshev.

Q below 0.7 is generally regarded as a damped alignment,
the lower the Q the better the transient response.

Q above 0.7 is generally regarded as an under-damped
alignment, maximising the amount of bass and F3.

The lower Q values are suited to hifi sub-woofers.
The medium Q values are suited to speakers and subs.
The higher Q values are common in cheap speakers
and home theatre sub-woofers.

However I would note that very often a 4th order reflex
alignment is chosen nowadays over higher Q sealed
alignments, 0.9 to 1.1.

I'd regard 0.6 and 0.9 as the minimum and maximum
sensible values to use for a sealed box speaker.

sreten.