Don't mind the size, this is actually uncomplicated
On the subject of subwoofer Qtc and tightness, I have decided to continue my posts from a peerless xls thread here, where I think they would be more appropriate. Although my application is discussed, the theory is important for anyone interested in the subject.
For those of you who haven’t been following, here’s where I’m at: I am designing a 3-way that uses dual, ported peerless xls 10”s in the bottom enclosure of each cabinet. I am using a single mid and tweet in the top enclosure, don’t plan to bi-wire, and will eventually read “the loudspeaker design cookbook” and acquire a computer where I can model graphs. Thanks to all who have been patient and helpful so far.
Here goes…
I've been reading a lot about Qtc and here’s what I understand:
-Calculated for a woofer in a closed box system
-Values of .5 - .707 seem to be sought after; .707 is maximally flat on paper
Bass Goal: Yes I will listen to a range of music and movies, but the most important attribute of this enclosure is that it produces tight, fast, deep, rich bass in dance music or rap. This is the music I listen to most. If after I am able to achieve this, I can modify the subwoofer to also perform well in other situations such as movies, fine, but this is my main priority.
Considering I am designing a ported system, I have anticipated two reactions. Please respond to the one you think is most appropriate.
A) Your ported box has a low tuning frequency (say, 25Hz), so you can pretend it’s sealed at higher Hz. Therefore what Qtc would people recommend to achieve this bass?
B) No, you are ported so you can’t think of Qtc. If so, how should I design my system to achieve the bass I seek?
Considerations for answering either question:
-At http://www.d-s-t.com/main/tech/appxls2.htm Peerless models a sealed xls 10 with Qtc=.34, but what would .5 and .707 be like?
-When designing to achieve my goal, do I want it so overall frequency response is linear (including room gain)?
This line of logic says that you can predict bad transient response to some degree by seeing where “bumps” in the FR occur.
-Related to this, on diysubwoofers.org its said avoid cutoff points much below 30Hz or the system gets boomy.
A quote from planet10 from “Subwoofer Qtc and tightness”:
With room gain a sub designed for flat anechoic response has way too much bass. You want your sub to roll off smoothly in an amount just opposite that of your room gain. A Bessel (Q=0.5) is probably the best choice in most rooms And as far as the actual box gives the best phase response -- critical Q. The downside is that this is often a pretty big box -- and if its your thing, won't give you the over exaggerated bass some like for their movies
I have put a lot of thought into this, and am really keen on getting it right. What do you guys think? Thanks for your help,
Philip
On the subject of subwoofer Qtc and tightness, I have decided to continue my posts from a peerless xls thread here, where I think they would be more appropriate. Although my application is discussed, the theory is important for anyone interested in the subject.
For those of you who haven’t been following, here’s where I’m at: I am designing a 3-way that uses dual, ported peerless xls 10”s in the bottom enclosure of each cabinet. I am using a single mid and tweet in the top enclosure, don’t plan to bi-wire, and will eventually read “the loudspeaker design cookbook” and acquire a computer where I can model graphs. Thanks to all who have been patient and helpful so far.
Here goes…
I've been reading a lot about Qtc and here’s what I understand:
-Calculated for a woofer in a closed box system
-Values of .5 - .707 seem to be sought after; .707 is maximally flat on paper
Bass Goal: Yes I will listen to a range of music and movies, but the most important attribute of this enclosure is that it produces tight, fast, deep, rich bass in dance music or rap. This is the music I listen to most. If after I am able to achieve this, I can modify the subwoofer to also perform well in other situations such as movies, fine, but this is my main priority.
Considering I am designing a ported system, I have anticipated two reactions. Please respond to the one you think is most appropriate.
A) Your ported box has a low tuning frequency (say, 25Hz), so you can pretend it’s sealed at higher Hz. Therefore what Qtc would people recommend to achieve this bass?
B) No, you are ported so you can’t think of Qtc. If so, how should I design my system to achieve the bass I seek?
Considerations for answering either question:
-At http://www.d-s-t.com/main/tech/appxls2.htm Peerless models a sealed xls 10 with Qtc=.34, but what would .5 and .707 be like?
-When designing to achieve my goal, do I want it so overall frequency response is linear (including room gain)?
This line of logic says that you can predict bad transient response to some degree by seeing where “bumps” in the FR occur.
-Related to this, on diysubwoofers.org its said avoid cutoff points much below 30Hz or the system gets boomy.
A quote from planet10 from “Subwoofer Qtc and tightness”:
With room gain a sub designed for flat anechoic response has way too much bass. You want your sub to roll off smoothly in an amount just opposite that of your room gain. A Bessel (Q=0.5) is probably the best choice in most rooms And as far as the actual box gives the best phase response -- critical Q. The downside is that this is often a pretty big box -- and if its your thing, won't give you the over exaggerated bass some like for their movies
I have put a lot of thought into this, and am really keen on getting it right. What do you guys think? Thanks for your help,
Philip
Re: Don't mind the size, this is actually uncomplicated
Sorry about the large quote above, folks. Need it for context.
Anyway, Bessel Q is actually something like 0.577 for a second order system. Vented boxes are fourth order at least so Q doesn't exactly map here. Nonetheless, Bessel alignments do have some nice properties, such as somewhat better transient response than Butterworth or Chebyshev, and a more gradual albeit earlier rolloff, which in most rooms is actually useful since it better matches the low frequency increase in room gain. Those can be seen above in:
http://www.diyaudio.com/forums/showthread.php?postid=89556#post89556
For vented boxes, fourth order Bessel alignment is about 2/3 the size of a B4 box, at Vas/Vab = 2.2 for Be4 as opposed to Vas/Vab = 1.4 for B4. The speaker Qts needs to be lower for Be4, at 0.32 compared with 0.4 for B4. The tuning frequency is Fs, of course.
The XLS10 Qts=0.17 is entirely too low for this alignment, but perhaps this information might be useful for folks using different drivers.
Cheers,
Francois.
philip said:
Sorry about the large quote above, folks. Need it for context.
Anyway, Bessel Q is actually something like 0.577 for a second order system. Vented boxes are fourth order at least so Q doesn't exactly map here. Nonetheless, Bessel alignments do have some nice properties, such as somewhat better transient response than Butterworth or Chebyshev, and a more gradual albeit earlier rolloff, which in most rooms is actually useful since it better matches the low frequency increase in room gain. Those can be seen above in:
http://www.diyaudio.com/forums/showthread.php?postid=89556#post89556
For vented boxes, fourth order Bessel alignment is about 2/3 the size of a B4 box, at Vas/Vab = 2.2 for Be4 as opposed to Vas/Vab = 1.4 for B4. The speaker Qts needs to be lower for Be4, at 0.32 compared with 0.4 for B4. The tuning frequency is Fs, of course.
The XLS10 Qts=0.17 is entirely too low for this alignment, but perhaps this information might be useful for folks using different drivers.
Cheers,
Francois.
Keltic's graph showing the frequency response of woofers with different Q is actually quite deceptive. The problem is the X axis is F/Fb, not F. What that neglects is that Fb increases as you increase Q by making the box smaller.
Here's the real-world anechoic response of a Shiva driver as you make the box smaller to increase Q. Combine these response curves with the room gain below the lowest room mode, as Dave mentioned, and it's quite clear that boxes in the Q = .5 to .6 range will give the best in-room response.
Here's the real-world anechoic response of a Shiva driver as you make the box smaller to increase Q. Combine these response curves with the room gain below the lowest room mode, as Dave mentioned, and it's quite clear that boxes in the Q = .5 to .6 range will give the best in-room response.
An externally hosted image should be here but it was not working when we last tested it.
Since this seems to be about room gain/ Q stuff, I've attatched a couple pics.
Pic 1 is the 'winisd' prediction of my mains response, although in reality the Q will be lower as the cabs are well stuffed. I haven't measured these outside yet as it's too cold and too wet
Pic 2 is the response at seating position (pink noise run through the dcx x/o -crossed 24LR @ 80 Hz) No eq applied.
Cheers
Rob
Pic 1 is the 'winisd' prediction of my mains response, although in reality the Q will be lower as the cabs are well stuffed. I haven't measured these outside yet as it's too cold and too wet
Pic 2 is the response at seating position (pink noise run through the dcx x/o -crossed 24LR @ 80 Hz) No eq applied.
Cheers
Rob
Attachments
pic 2
This speaker btw
http://www.diyaudio.com/forums/showthread.php?s=&threadid=5868&perpage=15&highlight=&pagenumber=10
This speaker btw
http://www.diyaudio.com/forums/showthread.php?s=&threadid=5868&perpage=15&highlight=&pagenumber=10
Attachments
Thanks for all the posts guys - I am seeing how that although you pick a .707 Q, it actually falls off faster. .5 seems to be the best choice considering achieving a flat response and extended bass response.
Could someone explain DSP_Geek's post where he says:
"For vented boxes, fourth order Bessel alignment is about 2/3 the size of a B4 box, at Vas/Vab = 2.2 for Be4 as opposed to Vas/Vab = 1.4 for B4. The speaker Qts needs to be lower for Be4, at 0.32 compared with 0.4 for B4. The tuning frequency is Fs, of course."
What is this Vas/Vab ratio, and what is Be4 and B4? Also, I was looking at the peerless site and from what I gather the Qts is actually .38-.4, unless I have misunderstood something.
Philip
Could someone explain DSP_Geek's post where he says:
"For vented boxes, fourth order Bessel alignment is about 2/3 the size of a B4 box, at Vas/Vab = 2.2 for Be4 as opposed to Vas/Vab = 1.4 for B4. The speaker Qts needs to be lower for Be4, at 0.32 compared with 0.4 for B4. The tuning frequency is Fs, of course."
What is this Vas/Vab ratio, and what is Be4 and B4? Also, I was looking at the peerless site and from what I gather the Qts is actually .38-.4, unless I have misunderstood something.
Philip
philip said:
Well, yes and no. For one thing, Rob's listening room seems to give a nice, gradual, smooth boost of leading up +10 dB at 20 Hz. That is a great response-but every room might not have that.
Second, assuming your listening room is like Rob's, then the Qtc of .5 would appear to have advantages. However, the box size to achieve that Qtc of 0.5 is much larger than that to achieve 0.7. With Rob's woofer, the box necessary to achieve a Qtc of 0.5 is three times larger than the box size to achieve 0.7. Something to consider.
Of course, if you select a very low Qts woofer to begin with-such as the Peerless XLS series with it's Qts under 0.2-then the box size to achieve a final Qtc of 0.5 in the box will not be too large.
One more thing to bring up in favor of a low Qts woofer. From what I can gather, for two speakers with identical Vas and Fs, (these are out-of-box, free air measurements), the speaker with the lower Qts will be more sensitive. It will require a bigger magnet to achieve that, but the point is, with equal Vas and equal Fs, the lower Qts speaker will give you higher SPL's per watt in the midband.
So assuming your room has a response like Rob's, your speaker will be putting out higher SPL's per watt if you start out with a low Qts speaker to begin with.
Putting aside in-room responsed for the omement, here are two charts for the step response of sealed and ported box alignments. Both are taken from Richard Small's articles. This will give some idea of the comparison of the two different systems.
First, the closed box chart.
First, the closed box chart.
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Next, here are the vented box charts. For reference, a, or a , equals Vas/Vb. It is used in many speaker formulas. So if you see a formula where a =3, then you know that the speaker is in a box that is one third the size of the speaker's Vas. If a =2, then the speaker is in a box half it's Vas.
The red printing is added by me, to try to make some sense out of the numbers in Small's chart in relation to the Thiele-Small parameters. I could not figure out all the alignments, so some are blank.
For just about all practical setups, the ported box alignments have more "overshoot" than for all practical closed box setups. However, some ported box setups can begin to approach some closed box setups.
As you can see, the trend is for tighter bass the smaller that Vb is in relation to Vas. Of necessity, the smaller that Vb is in relaton to Vas, the higher F3 will be in relation to Fs.
So in a certain sense, the more bass extension you sacrifice, the tighter the ported box response becomes.
The red printing is added by me, to try to make some sense out of the numbers in Small's chart in relation to the Thiele-Small parameters. I could not figure out all the alignments, so some are blank.
For just about all practical setups, the ported box alignments have more "overshoot" than for all practical closed box setups. However, some ported box setups can begin to approach some closed box setups.
As you can see, the trend is for tighter bass the smaller that Vb is in relation to Vas. Of necessity, the smaller that Vb is in relaton to Vas, the higher F3 will be in relation to Fs.
So in a certain sense, the more bass extension you sacrifice, the tighter the ported box response becomes.
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Keep in mind that you can start with a small higher Q sealed box and then Linkwitz transform it to be a lower Q lower f box. It is a way to tailor the sub to the room and individual preferences.
The trade off is that this increases the requirements power handling requirements on the woofer for lower frequencies and corresponding subwoofer amplifier.
I am intrigued about being able to switch between a transform that is set for music (low q) highish f, and a low f higher q transform for HT rumble.
If you start with a low Q sub to begin with the power increase to get to flat anechoic for HT (if that is desired) is not that much of an increase.
The trade off is that this increases the requirements power handling requirements on the woofer for lower frequencies and corresponding subwoofer amplifier.
I am intrigued about being able to switch between a transform that is set for music (low q) highish f, and a low f higher q transform for HT rumble.
If you start with a low Q sub to begin with the power increase to get to flat anechoic for HT (if that is desired) is not that much of an increase.
philip said:
That would be moi, I suppose.
Let's take the points in reverse order:
1) The speaker has a Qts of 0.17 as indicated on the Peerless site. The application note at http://www.d-s-t.com/main/tech/appxls2.htm does describe a cabinet with Qtc=0.34, which might be where that number came from.
2) B4 is the standard Butterworth maximally flat alignment for vented boxes, while Be4 is my notation for the Bessel tuning.
3) Vas/Vab is the ratio between speaker compliance and cabinet compliance. Since both can be expressed as a volume in cubic feet (or litres), this is a dimensionless number. For example, the standard B4 alignment requires Vas/Vab to be 1.414, which means the speaker compliance is 1.414 that of the cabinet, so for a speaker of, say, 70 litres, the cabinet should be about 50 litres. For the Imperially inclined, a speaker with 7 cubic feet is B4 aligned in a 5 cubic foot cabinet. B4 alignment also requires the vent be tuned to the speaker resonance frequency.
4) Bessel alignments roll off earlier than Butterworth alignments, on top of having better transient response. It's the nature of the beast, but fortunately that's exactly what we need to deal with room gain. Bessel boxes are also smaller than Butterworth ones, as seen by Vas/Vab=2.2 beside Vas/Vab=1.4. For the above 70 litre example, the required volume for a Bessel cabinet is about 31 litres, compared with about 50 litres for Butterworth.
In other news, I suspect the Butterworth flat response followed by a quick rolloff is where vented boxes got a reputation for "lumpy" bass, since that characteristic would tend to produce, when coupled with room gain, a rise until the system cutoff frequency followed by a rapid fall in response. This could be perceived as a peak in the bass response close to the cutoff, where bass notes would be perceived as more noticeable or disappearing as the music progressed up or down the scale. A sealed box with its inherently higher cutoff followed by a 12db/octave rolloff would exhibit this effect to a lesser degree, which could be mitigated further by a judicious choice of Qtc as noted elsewhere in the thread. It looks as if others have also come to the same conclusion re vented box alignment, as high-end vendors such as Wilson and B&W have tuned their systems at or close to Bessel.
Cheers,
Francois.
Although not entirely on the subject of box Q etc but still regarding philips speakers. What other drivers do you intend to use with the XLS??, for your mid and tweeter.
I ask this because you want to use two XLS per cabinet. A standard XLS is 88dB two of them in parallel will yeild 94dB. Ignoring BSC because the XLS will already be 6dB down due to BSC and wont get any compensation because all of its frequency response will be affected thus no comp required. And the mid will need BSC so it brought 6dB dwn anyway so all in all the stock sens can be used (if thats confusing ignor it, it doesnt matter).
Basically the XLS will be 94dB, most mids tweets wont be 94dB sens, some will but these cannot be crossed over low, which will be required to meet the XLS. You most deffinately dont want to have the bass more sensative then the midrange, applying padding to a bass driver is one massive waste of power. Esp if your amp is driving a 4ohm load anyway, as per two XLS, you dont want half of that heating up a resistor.
I ask this because you want to use two XLS per cabinet. A standard XLS is 88dB two of them in parallel will yeild 94dB. Ignoring BSC because the XLS will already be 6dB down due to BSC and wont get any compensation because all of its frequency response will be affected thus no comp required. And the mid will need BSC so it brought 6dB dwn anyway so all in all the stock sens can be used (if thats confusing ignor it, it doesnt matter).
Basically the XLS will be 94dB, most mids tweets wont be 94dB sens, some will but these cannot be crossed over low, which will be required to meet the XLS. You most deffinately dont want to have the bass more sensative then the midrange, applying padding to a bass driver is one massive waste of power. Esp if your amp is driving a 4ohm load anyway, as per two XLS, you dont want half of that heating up a resistor.
Thanks for all the replies, guys - a lot of my questions are being answered. For me, there's so much information to digest that I think I'm going to wait until I get a chance to start modelling on a computer - that way I can more fully understand the relationship between all these parameter. Something tells me once I do this, it won't be that hard to see what works. Talk to you guys later,
Philip
Philip
No i have not he wants two XLS per cabinet! as in four XLS in total.
WIth regards to the baffle step I meant. The XLS will already be 6dB dwn in its passband as all of its range will be omni directional. So a stock sens of 88 will give you 88 again because the 6dB gain you get for having two drivers in one cabinet as per MTM will drop to 88 from 94 because of Bstep.
With regards to the midrange, i meant that the mid will need BSC so if your mid is 88 it will infact play as 82 which is 6dB down.
So if comparing the XLS sens to mids BSC can be ignored because in a real speaker the XLS will be 6dB dwn over the stock sens quoted by peerless, and mids will also be xovered to 6dB less with BSC.
So if for instance you want to see if your two XLS will be more sensative then a W15cy001.
W15cy = 86 dB this is without BS
Two XLS - 94dB so is this
Yes the XLS will be 8dB more sens.
Now if we include BS
W15cy001 = 80dB
Two XLS = 88dB
Still 8dB above.
I hope that clears it up.
he wants two XLS per cabinet! as in four XLS in total.WIth regards to the baffle step I meant. The XLS will already be 6dB dwn in its passband as all of its range will be omni directional. So a stock sens of 88 will give you 88 again because the 6dB gain you get for having two drivers in one cabinet as per MTM will drop to 88 from 94 because of Bstep.
With regards to the midrange, i meant that the mid will need BSC so if your mid is 88 it will infact play as 82 which is 6dB down.
So if comparing the XLS sens to mids BSC can be ignored because in a real speaker the XLS will be 6dB dwn over the stock sens quoted by peerless, and mids will also be xovered to 6dB less with BSC.
So if for instance you want to see if your two XLS will be more sensative then a W15cy001.
W15cy = 86 dB this is without BS
Two XLS - 94dB so is this
Yes the XLS will be 8dB more sens.
Now if we include BS
W15cy001 = 80dB
Two XLS = 88dB
Still 8dB above.
I hope that clears it up.