[inPhase]

I realize that .707 (butterworth) is the most commonly recommended and design software usually targets this value as default. But when is it more suitable to use a low Q or high Q design? Is it better to use a certain alignment for a particular type/size of vehicle?

 

[mattldm]

Thats funny that u posted this poll. I was just about to ask a similar question regarding qtc!

I was wondering specifically about running an IDQ12 at QTC of .9 (in a .73ft^3 box)

 

[89grand]

I don't know for sure since I haven't tested my theory, but I would think the larger the vehicle, the lower the Q alignment you'd want. I base that theory on the fact that larger vehicles transfer function starts at a lower frequency than a small vehicle. A side effect of that though is that power handling goes down in a larger enclosure, while a larger vehicle needs more power to be equally as a loud as in a small one.

Either way though, if you have enough eq it really won't matter too much what you use, within reason. I suppose the .707 is a good all around performer, and you can eq the response that you want if you don't quite get it with that aligment.

 

[Lanson]

Definitely .707!

But...do you know why .707 is what it is?


There is mathematical perfection behind this alignment, which is why I prefer it over all others. .707 is the Golden Ratio. You can read about this stuff from other places later if that interests you, but to me it has great significance. Did you know that the entire universe and all of the things (large and small) relate to this ratio? The Phi spiral that makes up the galaxy is a mere example of how this ratio plays out. I think I could show you about 1000 examples given time to think deeply about it all. The octave is another example of it.

Because of the "perfect" alignment, I choose to get as close as possible to exactly .707 on most of my sealed box builds. Granted, I could easily be off by as much as 10% due to spec inaccuracy, temp differences, etc., but to me and my mild OCD, .707 or at least the attempt to get there does it for me.

Don't forget that the most important reasons to go for .707 are to achieve the most area under the curve of response, as well as to avoid sub "ringing" or overdamping / underdamping. I'm very certain that if you put a sub in 3 different boxes, one "perfect", one too large, and one too small, and tested them out in a listening room, you'd likely find the "perfect" alignment to be the most musical and more importantly, the most natural.

 

[mvw2]

It may depend more on desired frequency response, available power, and mechanical power handling. As you adjust the Qtc, the response and mechanical movements change. 0.707 does give the lowest F3 (-3dB roll off point). 0.5 is ideally dampened, although, relative dampening is non-linear through the excursion range. I personally tend to look more at frequency response. Most of the time, in a sealed setup, we almost need the lowest F3 we can get. We also generally prefer a smaller box and less excursion use. The higher Qtc design fits these goals. Manufacturers even push this by building higher Qts subs of 0.5-0.7 or even higher and then suggest relatively small sealed enclosure sizes. The suggested box generally gives a final Qtc of 1.0 and sometimes higher. The high Qts driver has a very extended low frequency response by design. Shoving it in a small box creates some loss but not a lot. It also creates a slightly peaky response but again not a lot. It also limits excursion use. Other then some lack of control via the low motor strength and low dampening, you get the low frequency output desired and a smallish box size. A high Qtc design using a high Qts driver can get you a F3 of 30Hz where as one would need a ported design at a lower Qtc to do the same.

In the end, it's a big mess to think too hard about this stuff. I personally tend to focus on what products give me the needed frequency response I need for in-car use. If sealed, I run 0.707. If ported, a flat response setup generally gives a Qtc of 0.55-0.6, not 0.707 like the sealed setup. Ported is different. 0.707 ported is peaky. 0.5 ported rolls off.

I say focus on enclosure size, frequency response, and then picking the best driver you can get that will work within that size and will give the desired response.

 

[Luke352]

Definitely .707!

But...do you know why .707 is what it is?


There is mathematical perfection behind this alignment, which is why I prefer it over all others. .707 is the Golden Ratio. You can read about this stuff from other places later if that interests you, but to me it has great significance. Did you know that the entire universe and all of the things (large and small) relate to this ratio? The Phi spiral that makes up the galaxy is a mere example of how this ratio plays out. I think I could show you about 1000 examples given time to think deeply about it all. The octave is another example of it.

Um I think you'll find your wrong there .707 has nothing to do with the Golden Ratio since the Golden Ratio is actually 1.61803399.

 

[bretti_kivi]

0.707 is root 2 over 2. That's kinda cool.

1/phi +1 = phi

and that jazz.

Bret

 

[Electrodynamic]

Manufacturers even push this by building higher Qts subs of 0.5-0.7 or even higher and then suggest relatively small sealed enclosure sizes. The suggested box generally gives a final Qtc of 1.0 and sometimes higher. The high Qts driver has a very extended low frequency response by design. Shoving it in a small box creates some loss but not a lot. It also creates a slightly peaky response but again not a lot. It also limits excursion use.

You need to take a look at the Vas also, not just Qts. If go on the notion of Qts is very close to Qtc, you'll end up with a lot of subwoofers that sound like a tin can because they require TINY enclosures but have next to nothing below 50 Hz. Take any high BL, low Qts driver and throw it in a small sealed enclosure and sit down for a listen. Model up a DD 9510 and see what size enclosure it calls for to reach a .707. And then look at the F3. Qts is only part of the equation when finding Qtc alignments.

The "peaky response" only comes into play with higher Qtc alignments past a 1.0. Below a 1.0 (such as a 0.9) you're talking about less than 1 dB of variance as compared to a .707. Less than 1 dB isn't anything to get worried about because you probably won't be able to distinguish that tiny variance...especially in the bandwidth that subwoofers play.

Other then some lack of control via the low motor strength

Motor strength has very little to do with control of the cone. If you're associating "control" with "speed", Le (inductance) has more to do with the control than anything else since that is the biggest factor in how quickly a speaker can react. If you wanted to associate Qts with anything, you can associate it with BL. But associating BL with cone control will lead you listening to more tin cans.

I feel I should elaborate on this topic. Higher BL means less power required for a given excursion level with everything else being constant (which never happens). However, it also means less bass. If you get your extra BL from extra L (length of wire) then you increase mass and loose your efficiency gain. High BL numbers also means you generate a lot more back EMF which causes higher impedance swings. Remember the Brahma "Extreme" subwoofers that Scotty Johnson ran? They were doing in the neighborhood of 160 dB and their BL was ~14 TM.



To answer the OP's original question - anything below a 1.0 is fine with me. I used to be consumed with the coveted .707, but in car audio there is such a small difference between a .90 and a .707 that the .707 isn't worth concerning yourself with to the point where you build an enclosure that is 3x larger than what it would be if you aimed for a .85. However, if you can get a Qtc of .707 out of 1.5 ft^3 and are willing to deal with the reduced power handling and larger enclosure as compared to a Qtc of .85 in 1 ft^3 with higher power handling and a much smaller enclosure, then by all means go for it.

 

[chad]

You guys :rolleyes" Sure your dear .707 sounds great in a home then you take the thinking that it sounds great to the car and guess what? it must sound great right?

Well superimpose that curve onto the curve that your car will do to to it's response and tell me what you find...... you find nothing, you find LESS THAN nothing actually because you will find that the same .707 enclosure will have radically different roll-offs in even cars that are similarly sized.

Answer IS, and no other. depends on the car and the listener, period, because all bets are off the second you open the boot to put that box in there.

 

[chad]

effing dupe feature. Makes me see dead people.

 

[Lanson]

You guys :rolleyes" Sure your dear .707 sounds great in a home then you take the thinking that it sounds great to the car and guess what? it must sound great right?

Well superimpose that curve onto the curve that your car will do to to it's response and tell me what you find...... you find nothing, you find LESS THAN nothing actually because you will find that the same .707 enclosure will have radically different roll-offs in even cars that are similarly sized.

Answer IS, and no other. depends on the car and the listener, period, because all bets are off the second you open the boot to put that box in there.

I respect your opinion and expertise on that Chad but in this case I'm talking about the relationship of performance that's audible due to over / under damping, not just roll-off. I agree the roll-off is totally different due to cabin gain and the "port-like" behavior that creates. But...the other part remains true.

 

[chad]

Pi Speaker Forum - Response curves of closed vs. vented systems - Wayne Parham, December 02, 2001 at 11:32:53

It has a TON to do with roll-off rate, that's what's audible.

 

[Electrodynamic]

Chad is right - wether a system is over/under/critically damped will play a huge roll in the roll off rate of the system. The driver used in Chad's example has some pretty radical changes in not only the F3 of the depicted alignments but also in the roll off rate of each system.

*Just a note (yes, I've got my flame-resistant suit on)*: A lot of car audio drivers (drivers designed for and/or marketed towards car audio) that are targeted towards small sealed enclosures may not shift the F3's drastically in the different alignments. However, the main thing to keep in mind is this: is the juice worth the squeeze? Every time you increase your enclosure volume your power handling will go down. You benefit by receiving more bottom end out of the system, but your power handling goes down and your enclosure size is up - taking up more of your trunk space. And the same goes vice-a-versa. If you are decreasing your enclosure size to gain in the power handling department to where you're going significantly above the manufacturers recommended enclosure volume / Qtc alignment you'll run into a reduction in low end performance. With everything involving speakers, you have to walk a thin line. Everything is a give and take.

 

[Lanson]

Maybe I'm seeing this differently from you guys, but I'm not really talking about a graph like you've shown. Perhaps the difference would be better seen with a decay graph, like Zaph uses. I find that incorrectly (over/under) tuned sealed boxes far from .707 literally change the way the intended instrument or note sounds...and I'm talking not in FR but in overall timbre.

Hearing is believing...perhaps that's the only way to quantify this issue.

 

[chad]

I find that incorrectly (over/under) tuned sealed boxes far from .707 literally change the way the intended instrument or note sounds...and I'm talking not in FR but in overall timbre.

Which is truly a product of FR if overtones/harmonics are involved. Unless of course the driver rings like a bell, which in that case you should have chosen a better driver

 

[Electrodynamic]

Correlating woofer speed with a Qtc isn't really a viable option, as Qtc has more to do with how a subwoofer system will behave in an anechoic chamber in relation to F3 and rate of roll off. Woofer "speed" has to deal with inductance more than anything else. And not just inductance as a number published on the T/S list of every loudspeaker (actually, few manufacturers publish Le...I wonder why ) but also inductance as it relates to subwoofer performance. Klippel even has a hard time showing what happens with regards to inductance and position and power and frequency. Yes, all of the latter deal with inductance.

What I'm saying is that Qtc should not be directly related to how "fast" a subwoofer is going to be. Unless you're talking about the extremes (a Qtc of 3 compared to a .707) of the spectrum you're probably not going to hear that much of a difference as far as tonality and attack are concerned. What you will hear, however, is the addition of, or lack thereof, low-end, which will ultimately correlate to how you perceive the subwoofer to be performing. After all, bandwidth is a major player in how we perceive differences in sound.

 

[Luke352]

Whenever I think about this stuff I always refer to this thread. http://www.diymobileaudio.com/forum...ven-cabin-gain-reality-do-we-need-low-fs.html

 

[mattldm]

How does stuffing a box with polyfill affect the overall Qtc?

does it change the qtc at all?

 

[sqshoestring]

Pi Speaker Forum - Response curves of closed vs. vented systems - Wayne Parham, December 02, 2001 at 11:32:53

It has a TON to do with roll-off rate, that's what's audible.

I started in this when a lot of people did not have an EQ. If you wanted good sound, you ran away from those enclosures that show a peak or steeper roll off in those graphs. Roll off rate is exactly right, you always wanted a .7 or maybe less. Now you can EQ the snot out of something, so it matters less but it will be harder to tune. IMO it depends on the sound you want, if you like tight bass meaning big roll off then no problem a small box is for you. If you want full sub say 20-70Hz, then that will not work well. Big boxes suck, that is why I love IB. I get a flat roll off, use extra drivers to bump up output, and have an empty trunk. Beyond that, I look at response more than anything. My subs never go low enough, I build for that and put enough of them in to get the output I need.
Another issue is the driver you use, if I use say an 18 well then I will not worry as much about getting all the bottom I can because it will have more. With most 12s and smaller I always tune low as I can get away with for the install, but I like it capable of 20hz too and some don't care about that. WinISD shows qtc .68 for my 12s IB. However some dayton 15s I was looking at show more bottom at .70, just what I need.

 

[zGhost]

How does stuffing a box with polyfill affect the overall Qtc?

does it change the qtc at all?

In theory it is suppose to lower it slightly. Correct fill makes the box 'seem' slightly larger. I don't know the exact increase but I've heard the %10/%15 figure tossed around before.