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Old 18th March 2011, 02:44 PM   #701
tb46 is offline tb46  United States
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Default Post #691.

Hi Djim,

I think you're looking for a victim to draw your suggestion from Post #691, i'll see if I can fit it in in the next few days. :-)

You may want to take a look at how your design compares to the SS15 @ Xmax (9.6mm) in the pass band (the SS15 is shown with a reduced S1 too).

Regards,
Attached Images
File Type: jpg SS15_Xmax.jpg (38.7 KB, 441 views)
File Type: jpg Djim_691_Xmax.jpg (39.5 KB, 431 views)
File Type: jpg SS15_Djim691_SPL_Xmax.jpg (33.2 KB, 428 views)
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Last edited by tb46; 18th March 2011 at 02:46 PM.
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Old 18th March 2011, 03:16 PM   #702
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Quote:
Originally Posted by AndrewT View Post
surely it's the faster responding (closer) drivers that need to be delayed??
Correct, a front loaded top cabinet would need to be delayed by the TH path length, plus any filter time delay contribution, to properly time align.

It looks like my Keystone TH, a longer path length than the SS15, would require front loaded top cabinets to be delayed by 10.5 ms for proper time alignment.

Fortunately, my top cabinets are short horns a bit over 1 ms deep, so the DSP I use, which only has 9 ms delay capability, should be just barely adequate.

Setting crossovers seemed so much simpler before learning more about phase, polarity, filter latency, arrival time and what each does in the big picture...

Art
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Old 18th March 2011, 04:41 PM   #703
jbell is offline jbell  United States
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art:

a few construction details on your keystone th? is it functioning more like a bandpass, than tapped?
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Old 18th March 2011, 05:53 PM   #704
Djim is offline Djim  Netherlands
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Quote:
Originally Posted by tb46 View Post
Hi Djim,

I think you're looking for a victim to draw your suggestion from Post #691, i'll see if I can fit it in in the next few days. :-)

You may want to take a look at how your design compares to the SS15 @ Xmax (9.6mm) in the pass band (the SS15 is shown with a reduced S1 too).

Regards,
Oliver, I hope the victim feels less victimised when I say: I would be more than pleased and take your time ;-)

I know the Xmax will gets extended when the Fs of the system drops. But in this case 60Volts (450watt) is reaching the 12mm theoretical excursion. 12mm is still under [(Xmax + Xlim) : 2] so Iím not worried and it is equal to its continues power rating.

What Iím more worried about is the total path-length. I need that 280cm to hit 40Hz at 1/3 wavelength and to make sure the low-cut filter can be set at 32/33Hz. This will make a big difference in the 40Hz area.

If you need extra lengths you can extend S1 a little more.
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Old 18th March 2011, 06:33 PM   #705
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Quote:
Originally Posted by jbell View Post
art:

a few construction details on your keystone th? is it functioning more like a bandpass, than tapped?
The Keystone functions like any TH, though the fold pattern allows for changing mouth size and shape easily, which affects the frequency response.
The fold pattern also allows the exit to be on the front (as built, and shown in the diagram below) or the side or bottom, though a bottom exit would not benefit from the keystone shape.

My achievement is that I am perhaps the first to realize that the Keystone exit shape is needed for this type of fold to act more like a TH similar to the DSL TH-115 or 118, the Apache, or the SS15, all of which have a more "normal" horn exit.

If there is anyone that has used the keystone shaped exit on a TH before me, I am unaware of it. I thought of it by visualizing what shape would be needed to act like a square exit when looking down a long duct. The left picture below shows the Keystone exit from a high overhead angle.

The response curve of the Keystone is almost exactly the same in magnitude and phase as a DSL TH-118, which uses the same driver. My tests show the Keystone to be about the same sensitivity as the dual 18" JBL SRX 728, while Danley's charts show the TH-118 as almost 7.5 dB higher in sensitivity. Hmmm..

Closing the upper portion of the Keystone mouth makes the path length longer, exit delay increases accordingly.

Reducing the mouth size by covering a portion of it on the SS15 ("step down" mode) would make the low corner lower, but won't increase the path length, so efficiency would take a bigger hit than it does on the Keystone.

I agree with your assessment that the SS15 is optimized for what it can do. I think the posts showing longer path lengths and a lower corner are too optimistic in the amount of LF output level that could be achieved, trying to squeeze more LF out of a TH that size will waste the upper bass response.

Once the upper bass advantage is gone, one might as well use a BR, a pair of 4015LF in a box the size of the SS15 would put out more LF than an "extended path" SS15, which probably could no longer be made from a single sheet anyway.
Attached Images
File Type: jpg Keystone Above.jpg (17.9 KB, 417 views)
File Type: jpg Keystone fold.jpg (18.5 KB, 407 views)

Last edited by weltersys; 18th March 2011 at 06:37 PM.
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Old 18th March 2011, 07:54 PM   #706
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Any energy delivered to the driver will initially come from the back of the speaker (the part nearest the mouth), as sound. Any sound comming thru
the "horn" will arrive later.

Your "keystone" arguement is even less convincing. There is no advantage when the mouth is larger than the end of the flare. You can "look" at it
any way you want, but that's light ... not air, and we don't listen to light.
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Last edited by Don Snyder; 18th March 2011 at 07:59 PM.
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Old 18th March 2011, 08:23 PM   #707
Djim is offline Djim  Netherlands
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Art, I agree with all your criticism... As well with your statements in favour of my extension idea :-)



Quote:
Originally Posted by jbell View Post
Ok... since there has been much talk spent on 28v vs 2.83v measurements.... And I'm a fan of 'There is no substitute for doing...' I went and did something a bit crazy today. The crown was set for 40-300hz. (and yes, even a butterworth 48db/oct at 40hz cuts a couple db at the high pass freq.) The pictures speak for themselves.
Quote:
Originally Posted by weltersys View Post
Not sure what the pictures are saying, other than the 3 dB down point with the 40Hz filter is now about 55 Hz, and the 10dB down point 40 Hz.
Quote:
Originally Posted by weltersys View Post
As you said August 2008:

JBell quote: “If 40hz is your low frequency that you need for your PA, that efficiency at that freq sets the overall response you can achieve, as it's the hardest to reproduce and most power intensive frequency in the PA power band. (which we obviously know) A sub that's 13db more efficient at 100hz vs 40hz, helps little. If I need more 100hz out of a sub, I'll eq it in. However, If I need more 40hz... there's only one way to get that, big power and big excursion, and that limits the overall spl you can get out of your cabinet.”

Several years before that I had abandoned my Chorn design for Lab 12” in ported cabinets, like you I was willing to sacrifice the upper response for a gain in the lower response. The Chorn behaves like BFM’s and Jeff Permian's offerings.
Quote:
Originally Posted by weltersys View Post
Closing the upper portion of the Keystone mouth makes the path length longer. Reducing the mouth size by covering a portion of it on the SS15 ("step down" mode) would make the low corner lower, but won't increase the path length, so efficiency would take a bigger hit than it does on the Keystone.
Quote:
Originally Posted by weltersys View Post
Once the upper bass advantage is gone, one might as well use a BR, a pair of 4015LF in a box the size of the SS15 would put out more LF than an "extended path" SS15, which probably could no longer be made from a single sheet anyway.
Quote:
Originally Posted by weltersys View Post
In listening tests with both cabinets equalized to have the same frequency response and output, the primary difference was the port noise in the BR when pushed to the amp’s limit.
When the TH was pushed to the amp’s limit, it was basically noise-free, and 6 dB louder than the BR.

Last edited by Djim; 18th March 2011 at 08:28 PM.
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Old 18th March 2011, 08:23 PM   #708
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Quote:
Originally Posted by Don Snyder View Post
Any energy delivered to the driver will initially come from the back of the speaker (the part nearest the mouth), as sound. Any sound comming thru
the "horn" will arrive later.
.
You probably mean that any energy delivered at the listening position or test microphone from the back of the speaker will arrive first, and the horn output will come later, which I agree with.
The TH is an interesting arrangement where the front and back outputs combine "in phase" over a fairly wide bandwidth compared to a BR cabinet.
This allows around a 6 dB advantage in the case of the THs I have built and tested over a BR cabinet using the same speakers.

The "horn" output of a TH is doing the heavy lifting. The Smaart delay finder responds to HF, if the upper output of the TH was loud enough to register, the delay finder would deem the TH the same delay as a BR.

The TH output is delayed by the horn path length, as is clearly shown below.

Anyone using the dual FFT function of Smaart or similar systems can easily verify this fact.

If one wants to get a system to sound best at the crossover point, knowing the propagation delay of the low and high speakers is an important data point.

Perhaps Jbel will verify the path length of the SS15, with Auto Sm using his Smaart system.This would show the actual SS15 acoustic path length, which is not always the same as what a squiggly line through the middle of all the folds is.
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File Type: png BR, TH Delay.png (74.8 KB, 105 views)
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Old 18th March 2011, 08:35 PM   #709
kg77 is offline kg77  United Kingdom
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is it possible to mix box types. example, could u make a bandpass design with a TH/tline in one chamber and a ported design in the other.?

after reading about TH and how you can achieve a chord like effect from many freqencies layered.iam curoius what u can do. iam a sound engineer/music producer so have a good idea of what frequencies to layer.

phase shift ????????

when i make sub basses using synths hardware or software ,i often find the best way to get it from a sine wave to fat sub , is using a small amount of pitch adjustmentm using decay controls on an envelope.

also the key thing i find is the start phase/position of the wave. if u take 2 sine waves and shift the phase/position off one u can make the sound louder/fatter , it can also cause phase and canceling out of the sound at wrong positions.

how would u control the phase/position of the wave to create a fatter sounding box?.

could a band pass design have the 2 chambers then go in to one chamber combining the waves.

do i make any sense ? :}

Last edited by kg77; 18th March 2011 at 08:55 PM.
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Old 18th March 2011, 08:44 PM   #710
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Quote:
Originally Posted by Djim View Post
Art, I agree with all your criticism... As well with your statements in favour of my extension idea :-)
Sorry Djim, I am not in favor of your extension idea, I think the response extension you post is overly optimistic considering what the real SS15 LF corner is already.

I still agree with Jbel that the SS15 is pretty optimal for size/weight/cost/output. Reducing the LF corner will probably push the efficiency down to where there would be little advantage over a bass reflex cabinet.

That said, no one will know for sure unless they try cramming more path length in the same box size and test the results side by side.
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