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Old 9th January 2013, 02:56 AM   #361
xrk971 is online now xrk971  United States
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Wow, that good huh? Can these still be bought somewhere?
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Old 9th January 2013, 05:01 AM   #362
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Originally Posted by xrk971 View Post
... I am still unclear how to specify the channel width as function of distance in Akabak for the true cornu. ...
Each horn component has an exponential profile. Simply specify the throat area, mouth area and length. AkAbak defaults to an exponential profile (T=1).
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Old 9th January 2013, 08:37 AM   #363
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Originally Posted by Don Hills View Post
Each horn component has an exponential profile. Simply specify the throat area, mouth area and length. AkAbak defaults to an exponential profile (T=1).
I would say that not all exponential curves are the same. In particular, for Cornu function, the expansion is very gentle at first near throat and rapidly expands near mouth. I don't think specifying just throat mouth and length will achieve same fidelity for the profile, which I have modeled previously using a linear conical for first 90% then switch to exponential at the end. This of course is only an approximation.
The Cornu function has the unique property that its curvature starts at zero (at mouth) and increases linearly with distance along the curve. This curve has the smoothest possible transition from straight flow to a curved flow with a radius of curvature defined by the driver chamber radius. Not sure if it matters much from acoustical modeling standpoint... but I would guess that it does.

Last edited by xrk971; 9th January 2013 at 08:40 AM.
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Old 9th January 2013, 10:00 AM   #364
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Try it youself - take one of the horn sections in your plan, measure the area at 5 different points (throat, mouth, 3 points in between) and enter a 4 segment horn in Hornresp. Now enter a single section exponential horn using the same throat, mouth and total length. You should find that they are within a couple of percent in terms of volume and response. The difference is swamped by the errors caused by actually building the horn - for example, the calculated curves ignore wall thickness, which is a significant source of error near the throat in a spiral horn.

I just tried the above myself - see the attached schematics. The "measured" horn was using dimensions off your original plotted horn, for the longer horn path (from the split point to the mouth).


One area where we may differ is in the calculation of effective mouth area in asymmetric horns. I think I'll start another thread to discuss it because it's not cornu specific.
Attached Images
File Type: png Cornu Measured.PNG (10.3 KB, 540 views)
File Type: png Cornu Exponential.PNG (10.4 KB, 537 views)
File Type: png Cornu Combined.PNG (10.4 KB, 491 views)
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Old 9th January 2013, 11:15 AM   #365
xrk971 is online now xrk971  United States
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Don,
I will check but those horn diagrams obviously do not flare out fast at the mouth like cornu's which go from 3 in to 12 in in last 12.
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Old 9th January 2013, 12:23 PM   #366
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I don't believe that severely asymmetric horn mouths are effectively as large as they appear. Draw a line from the tip of the "shorter" side of the mouth across to the nearest point on the "longer" side. That's how I measure effective mouth size, and it works out at about 6 inches, not 12. As soon as the wave is no longer confined by the shorter side, all bets are off. I've started another thread on this point, hopefully some useful discussion will occur. I'll also write some code to generate a maximum size ripple tank model. (The wavefront simulator in Hornresp only does 135x135 resolution, the original program from which it was derived goes up to 395x395.)
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Old 9th January 2013, 01:33 PM   #367
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Don,
OK, I see your reasoning but not sure if I would agree that the longer curved side has no effect. That's a good idea to put this to the test in the ripple tank sim. If the ripple tank shows no effect of the extended curve of the mouth, I will buy it. If so, it means the expansion ratio is only half of what I thought it should be (from 30 to 1 down to 15 to 1). From how much gain we are getting in the bass, I am thinking that a 15:1 gain would not be able to achieve those levels. I think much of it has to do with the fact that the wall constrains the wave and partially forces it to continue to utilize the longer side of the mouth. It would be a simple measurement to see if the bass freq peak moves with and without wall. I think Pano has the nicest setup and may be able to help out with this experiment if he is so inclined.
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Old 11th January 2013, 11:36 AM   #368
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I have run an endless number of sims for the FF125WK in the 70cm side cornu, assuming a straight taper at first and a pure exponential taper.
Simulations have been run on Martin's Double BLH worksheets, with mouth ports modeled above and below the box.
Results are below
Graph 1 is a straight taper with Throat Area= 2.23 Sd and horn length of 220cm
Graph 2 is a straight taper with Throat Area= 2.23 Sd and horn length of 280cm
Graph 3 is a conical taper with Thoat area = 1 Sd and horn length of 220cm
Graph 4 is a conical taper with Thoat area = 1 Sd and horn length of 280cm
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Old 11th January 2013, 12:16 PM   #369
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Hajj,
Great work! Thank you for running the sims in MJK. Can you clarify: the first two plots are a straight linear taper (conical) and the the second two are exponential taper? Did you achieve the 2.23 Sd by assuming depth of channel is increased - not by changing the channel profile? My impression from looking at the sims is that the bass gain is indeed very high (matches listener observations). 15 dB gain is quite remarkable... Have you put any stuffing in these sims and I wonder how that will affect the deep dip at 250 Hz for the 220 cm one?
Can you please post the whole Mathcad worksheet screen shots, especially the horn profile definition section, and the plot showing the horn profile as a function of distance. I am also interested in the impedance and cone excursion plots.
Thanks in advance.

On another note: I saw your post in the MJK BLH thread regarding use of Helmholtz resonators in BLH's. Would it be useful to add a Helmholtz chamber either directly underneath the main driver chamber or somewhere along the path of the main horn? This can easily be done by cutting a small hole in the back and gluing a small thin box over the hole, of course with the hole diameter and box volume appropriately sized. This may help to get rid of those main resonances beyond 300 Hz.

Last edited by xrk971; 11th January 2013 at 12:26 PM.
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Old 11th January 2013, 12:48 PM   #370
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Quote:
Originally Posted by xrk971 View Post
Hajj,
Can you clarify: the first two plots are a straight linear taper (conical) and the the second two are exponential taper?
Exponential, sorry for the confusion
Quote:
Did you achieve the 2.23 Sd by assuming depth of channel is increased - not by changing the channel profile?
The 2.23Sd was based on the cornu initial drawing accounting for channel width and enclosure depth
Quote:
Have you put any stuffing in these sims and I wonder how that will affect the deep dip at 250 Hz for the 220 cm one?
the whole horn is stuffed with 0.15lb/ft3 of stuffing, as for the 250hz dip, I wouldn't lose sleep over it as it's very sharp and shouldn't be a problem, especially if it's put into perspective with the rest of the horn's shortcomings.
Quote:
Can you please post the whole Mathcad worksheet screen shots, especially the horn profile definition section, and the plot showing the horn profile as a function of distance. I am also interested in the impedance and cone excursion plots.
Thanks in advance.
No prob

Quote:
On another note: I saw your post in the MJK BLH thread regarding use of Helmholtz resonators in BLH's. Would it be useful to add a Helmholtz chamber either directly underneath the main driver chamber or somewhere along the path of the main horn? This can easily be done by cutting a small hole in the back and gluing a small thin box over the hole, of course with the hole diameter and box volume appropriately sized. This may help to get rid of those main resonances beyond 300 Hz.
I have already mentioned this earlier in this thread
The resonator should help getting rid of the lower resonances should it be well designed, but will fail at taming the resonances higher in the freq range.
I already have an interesting resonator profile that should fit this application well, so if you feel like doing some mods to your enclosure, let me know
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