(Q) or qtc value vs frequency response - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 6 Old 04-16-13, 12:02 AM Thread Starter
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(Q) or qtc value vs frequency response

I've done a fair amount of research on the effects of damping, and its impact on the characteristics of a system. I understand, to some degree, the effect of altering rise and fall times by overdamped or underdamped designs. I have built an enclosure for (1) Pi-18 (now known as UXL-18) with an approximate qtc of .607, and I have experimented with stuffing to further lower the q toward critical damping. I am extremely pleased, so far, with the low-end extension and transient response of the first subwoofer, but I have a second driver in need of a box, and before I proceed, I have a few concerns to address.

The consensus seems to claim that a higher (q) system has more "punch" or "slam" than a lower one. I have modeled smaller enclosures with a qtc closer to .707, and have noticed the slight peak in mid-bass response, which would emphasize those frequencies. My main question is: Other than the slightly higher response in the "chest kicking" frequencies, are there sonic differences between a .577 and a .707 application at the same output for a given frequency? More specifically: Will the impact of a kick drum be lessened in a critically damped subwoofer when volume level is taken out of the equation? My last question is: If I were to supplement the in-room FR with a parametric EQ, such as a BFD, what will the (q) value of the equalizer do to affect the final result?

Last edited by badmodemfinger; 04-16-13 at 12:19 AM.
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post #2 of 6 Old 04-16-13, 01:55 AM
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Re: (Q) or qtc value vs frequency response

SO smaller design which increases the Q would also have a faster roll off in a sealed box towards the low end. It would also become less efficient at the low end also. But I have played with so many different designs that I use Q as a guideline. With the different types of EQ that changes everything. When you have a speaker that likes huge boxes but you dont have the room you just use the box size you want. Then lets say the final alignment becomes .9. That I very high and not wanted by HT standards. But with an L/T circuit this changes the electrical Q design so that the speakers acts like it is in a .7q if you chose. Mechanically though it is still a .9q. Similar concept in adding stuffing to a box. It will lower Q but not you still treat the woofer with wattage like it is in a box with no stuffing.

Then after all this is done you then add EQ for your desired room and wants. Then listen. it is hard to tell what Q design you have unless you listen before EQ and L/T.

In your .5 vs .7 it is how the woofer is behaving in a said enclosure before any other adjustments are made electronically or other. So you would notice the difference if playing music and drums slightly. Only say slightly because it is dependent what type of drums and what kind of kick was recorded.

If you are augmenting a L/R speaker that are capable of enough spl at 60hz and up then adding your subwoofer wont lessen the impact no matter what design unless no EQ at all is applied. EQ is the key here. A lot of bad deigns can be made to sound VERY good with EQ.

For my subs there is no slow sound or fast sound it is just what is recorded that comes out. But my subs are in a .85q design. They sound great and when adding EQ they sound even better.

Now all this said and done a Mic and your ears will also be the best judge. I have designed so many subwoofers and become caught up a lot with q Fb and other things. It is always best to try a design, build it and have a listen. Then after some EQ listen again. Then after measurements see what your results were and how to improve.

A lot of others have said that you should design a subwoofer for a .5-.6q because after room placement and everything else you will have a subwoofer with a .7q anyways. So different philosophies but they all seem to work to some extent.

Now if you are using a UXL driver and wanting to put it into a .q of 8 or so that would be a very small enclosure. What were you thinking of doing?

Hope all this made sense. It did in my head.

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post #3 of 6 Old 04-17-13, 07:44 PM Thread Starter
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Re: (Q) or qtc value vs frequency response

Thanks for the response. That definitely sheds some light. The main reason for my inquiry is that I purchased 2 UXL's for two sealed enclosures for my listening room. I use it for HT and music, about 70/30. I built one enclosure first, to make sure I was pleased with the results before I started on the next. I was blown away with the outcome of the first, and I got lazy, and left the other driver out because I was enjoying the system.

My current measured in-room frequency response with the single box at .607 Q is flat to 7Hz after Audyssey MultEQ XT, via my Onkyo TX-NR3007. I'm driving it with a single Behringer EP4000 with 2,400w RMS at 4 ohms. My L/R are Klipsch Chorus II, and are pretty capable in the mid-bass territory. The only things I still feel are lacking are even response throughout the room, which will be helped by the second sub, and an apparent lack of "punch" when listening to music. The room also has some opportunity with decay times in the waterfall graphs I've measured, but I think room problems are the culprit there. I am planning on purchasing either a MiniDSP or BFD to flatten the peaks, and I'm considering adding a second EP4000 to maintain the headroom with the second sub.

To further illustrate my confusion, let's assume that hypothetically, two different sealed subwoofers share an identical frequency response along the whole spectrum, so that loudness plays no factor. One is .577 and one is .707, or even higher, like in your designs. If I'm sampling a track with a tight snappy kick drum in the 60Hz region, will the higher Q sub produce a more tactful punch, or is the reason for that notion simply because of the higher peak in response at those frequencies that are exhibited by high Q systems?

One route would be to build a second box with a higher Q, and compare the difference, but my goal is to avoid the extra labor by either copying the first .607 design, or building two .707's. I'm hoping that with more helpful feedback, like yours, I can make a decision while staying on budget and avoiding unnecessary build time. As much as I love the design and construction steps, they take a backseat to the enjoyment I get from experiencing the final result.

I have more low end than I rightfully deserve, so I'd be willing to give up a tiny bit in favor of slightly more impact. Would you suggest smaller enclosures, or do you think that EQ or room treatments will achieve the results I'm looking for?
Many thanks for your time.

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post #4 of 6 Old 04-17-13, 08:30 PM
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Re: (Q) or qtc value vs frequency response

After 50hz there should not be much of a difference. But if you like a more tactful/dynamic sound then build a higher Q enclosure. My subs are in .84 I think and i dont have a problem with tactful sounds. BUT after hearing my dual 15's ported at 42hz and the subs together they sound much better. Simulations are one thing but hearing is another. I can only say design for your tastes then experiment. Proper amplifier for each sub helps out also.

SO while I may need the help of a better 40hz and up mains if I have more power that helps even more. Everyone feels different about the power provided per speaker but I have as much power as I can per speaker. I have 10x's the needed power for my l/R speaker and they sound a LOT MORE DYNAMIC. Wow. With just 150watts they are soft and suttle which is great. But after trying a bigger amp they sound so much better. And they are little midbass drivers. So with subs you get as much as you can. This will help with power transients and dynamic capabilities of your subwoofer. So double the power needed would be a good start.

I am still saving for my subwoofer amplifier. They are rated at 1200watts but I know they can take more than that and I will be getting an amp that will provide them with 4000watts to each of them. Then I wont have to worry about dynamics or low end.

Now I would design the woofer so that it is mechanically protected from over excursion. SO around .7 would be good or .8 also. But seeing the UXL likes smaller boxes .7 should be fine. Then get as much power as you can. If you can only afford 2400watts per sub then thats what you have. Room treatments would be a huge help. HUGE for getting rid of hanging bass and getting the sound you need and not boomy overhang.

Hope this made sense as I am trying to do too many things at once. And a pair of UXL's should be all you need for 80hz and down. After proper EQ and room treatments your should have what your wanting.

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post #5 of 6 Old 04-17-13, 11:07 PM
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Re: (Q) or qtc value vs frequency response

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chrapladm wrote: View Post
SO smaller design which increases the Q would also have a faster roll off in a sealed box towards the low end. It would also become less efficient at the low end also. But I have played with so many different designs that I use Q as a guideline. With the different types of EQ that changes everything. When you have a speaker that likes huge boxes but you dont have the room you just use the box size you want. Then lets say the final alignment becomes .9. That I very high and not wanted by HT standards. But with an L/T circuit this changes the electrical Q design so that the speakers acts like it is in a .7q if you chose. Mechanically though it is still a .9q. Similar concept in adding stuffing to a box. It will lower Q but not you still treat the woofer with wattage like it is in a box with no stuffing.
The part in bold is the most important sentence in this whole thread. EQ changes everything. EQ changes things in such a manner that most people look past. Boosting any frequency ruins your Qtc alignment. You are modifying the input signal, not simply modifying the physical and acoustical attributes of your speaker system. When you modify the input signal your group delay and Qtc goes out the window. However, that's not the most important piece of the puzzle. Impulse response overall is a direct correlation of bandwidth - period. No if's, and's, or but's about it. The speaker system with the largest overall bandwidth (without counting a crossover being involved) will have better transient response. This is specified in the Fourier Transform theory. If you listen to your system in an anechoic chamber you can follow what WinISD spits out and shoot for the lowest Qtc alignment possible for your personal preference desires. However, if you listen to your system in a room, let alone a car (which is basically an extreme acoustics class all together), you have room interaction and, more importantly, room gain. Said room gain adds natural boost to the low frequency response based on the size of the room. Where this boost comes in definitely modifies the overall LFE of your subwoofer system. Always in a positive manner.

I have designed and installed many "high Q" alignments (over 0.8) that have yielded F3's down to, or lower than, 20 Hz in a home theater environment. I know I'll get chastised for saying the latter but when you step back and listen to (let alone measure) the systems you have built over the years you realize that WinISD is not the God of your actual measured or audible sonic experience.

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post #6 of 6 Old 04-18-13, 01:15 AM
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Re: (Q) or qtc value vs frequency response

Well seeing that you know a little bit about speakers I think you are qualified to have your statements. And I feel the same way either way. Measure test measure test and thats the best results. We just use the tools we have to get us 1/2 way there.

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