QTS/OB explanation needed

[InclinedPlane]

I'd like to use the Eminence Alpha15A in pairs to go with my AER MD2Bs that will soon be arriving. I must confess I don't understand what 'Q' is really and why a high 'Q' driver will give me more bass in OB. I've found that generally it's the woofers with small magnets that have a high Q. Does this mean that the small motor exerts less control over the moving mass, allowing it to flop around more or something? Is a high Q driver merely a high-distortion driver? I would think a driver with a massive motor applied to the same mass would give far better transients, no? What am I really giving up by using a high Q driver?

Also, the AERMD2B has an insanely strong motor and low moving mass yet has a high .7 Q. I also don't understand this.

[v-bro]

You can either choose between a high Q driver or eq the bass up, or both....

A high Q (meaning less mechanical and electrical dampening) driver will (for what I know about it) distort more in an enclosure. But as any driver will have much less distortion in an open baffle it will mostly result in a system well within distortion limits....

Qts is electrical Q (Qes) and mechanical Q (Qms) together and therefore looking only at the motor of the driver won't tell everything about the Qts...

I hope this helps...

[DougL]

Target Qtc:
Best transient response Q = .5
Critically Damped Q= .7
Over-damped Q < .5 Base will roll off before F3
Under-damped Q > .7

Since Low frequency driver design balances efficiency, low F3 and Qts, most drivers choose to allow the enclosure to raise Qtc, usually by base reflex loading.

Basically, smaller magnets required to raise Qts result in either lower efficiency, higher F3, or both. it does not directly affect distortion, as a first order effect.

Open baffle speakers Qts = Qtc, so the choices are either choose a driver in the .5 to .7 range and accept the penalties, or use equalization to restore the frequency response lost when a driver of less than Qts of .5 is used. The equalization has its own penalties.

HTH.

Doug

[DrDeville]

FWIW, I'm fairly novice, but will try to distill what I think I've learned into something useful.

You are correct that q goes down with larger motor, lighter cone, and greater control over the speaker mass. Q is the ratio of stored energy to damping, and the magnet contributes to damping, so bigger magnet means higher damping and lower q. But no, higher q does not imply greater distortion, just a different tuning of the resonant system.

Oh, and a small sealed box adds resonance and increases q. As a sealed box get larger, it's resonance becomes less, its damping greater. and the system's q greater. An infinitely large box would add nothing to Qts, and act just like an infinite baffle! Isn't life grand?

There are two crucial consequences of Q (no extra charge for the alliteration), and Qts in indeed the useful measure in an open baffel system.

The consequences are frequency response, and stored energy--the amount of resonance.

To expand on Doug's useful distinctions:

Over-damped: Q < .7, Bass will slowly roll off before F3, and continue to slowly roll off below F3

Critically Damped: Q = .7, flattest response down to F3, then moderate rolloff

Under-damped: Q > .7, Bass begins to *rise* before F3, peaks at F3, then rolls off quickly.

The frequency-response upshot is underdamped systems have a peak, a boom, at F3, and little bass below. Critically damped systems have flat bass down to F3, with moderate rolloff below. Overdamped systems start to roll off before F3, but do so very slowly, so they will actually have more bass at frequencies significantly below F3.

The other factor is resonance, which, as it increases, tends to store energy and bleed if off gradually, smearing transient response. This effect leads to the "one note bass" complaint about tuned systems--whenever there is a note anywhere near resonance, the system hums for a while at its resonant frequency. And this "hum" smears transients. Think of the resonant systems you know--a tuning fork or a wine glass--they would make lousy speakers. Efficient, but lousy.

One of the things people like about open baffle bass is this lack of enclosure-induced resonance and overhang, and the corresponding ability to follow the bass clearly as it changes frequency. A resonant driver would degrade that.

Some people think that resonance affects the perception of "fast" or "slow" bass. Fast bass being damped and dying out quickly, and slow bass bring resonant with a lot of overhang.

So underdamped systems produce more bass with less power, but roll off quickly and have a resonant peak at F3. Hum hum hum--and equalization can flatten the total amount of energy, but can't stop the overhang/humming.

( servo-control or dynamic computer-controlled equalization might)

Critically damped systems have flat frequency response, store a moderate amount of energy, and roll off moderately.

Overdamped systems have bass that dips early but ultimately goes lower, and hold little energy. For this reason, some people like to pick overdamped bass drivers and then use equalization to give themselves flat bass that goes deeper, with less smearing (faster). And this requires more power from the amp.

All of these are fairly new thoughts to me, and some may be wrong. I welcome all comments and corrections, and hope this is useful.

Best,

George

[DougL]

George;

Very clear post.

Doug

[poptart]

This was posted in another thread recently and I found it interesting.

http://www.geocities.com/kreskovs/Box-Q.html

If you don't have time to read it I guess the short story is Q= 0.7 probably has the flattest frequency response in an anechoic chamber, not your room. Unless your room is a giant theater. I'd also note that a Q of 1.0 doesn't mean the driver rings on and on after a note ends, it's more like a muted half cycle bump, so at 40hz it moves for something like 1/100th of a second longer than it should.

With OB your baffle size is causing a bass roll off so a well placed bump in the bass response could counteract some of that.

[MJK]

If you don't have time to read it I guess the short story is Q= 0.7 probably has the flattest frequency response in an anechoic chamber, not your room. Unless your room is a giant theater. I'd also note that a Q of 1.0 doesn't mean the driver rings on and on after a note ends, it's more like a muted half cycle bump, so at 40hz it moves for something like 1/100th of a second longer than it should.

With OB your baffle size is causing a bass roll off so a well placed bump in the bass response could counteract some of that.

Poptart,

EXACTLY! I use the Eminence Alpha A15 with my Lowthers in an OB and there is no bloated ringing bass at all. I used a lower Qts driver and some EQ boost before getting the Alphas's and found the Alpha's to be a better solution for me in my room. The bass is tight and reaches 40 Hz easily, great for acoustic jazz. No boom in my room.

[InclinedPlane]

This is fantastic, guys. You all seem so willing to help. The answers I read here raise even more questions, so maybe I should cut to the chase and ask: what Q is best for a bass driver in an open baffle OR what Q do you prefer for a bass driver in an open baffle.

[MJK]

There is no one answer to your question. The bass response is detemined by the size of the baffle and the Qts of the driver. A higher Qts driver will allow a little bit smaller baffle for the same low frequency roll-off. I would look for a driver with a Qts between 0.7 and 1.2 and then design the baffle with the driver or drivers to see what bass you can expect for particular baffle sizes. It is a system with a number of trade-offs. If you go to my MathCad models page, there are two pdf files showing two different approaches to OB design with full range drivers and the Eminence Alpha A15 woofers.

Hope that helps,

[InclinedPlane]

I have not heard bass below 70hz in an open baffle only because the one time I tried it was with the only bass drivers I had sitting around which were the JBL 2020h. These seemed to be far less than ideal for bass given that I didn't hear anything lower than about 70hz but the midbass was remarkable, if terribly meagre in output. That midbass was so clean and tight I had the impression that nothing had been added to the source and nothing had been taken away - I was thinking 'is this what bass really is?' So now I'm headlong chasing that sound. I'm more than curious what 40hz open baffle bass could sound like...