Thorn F1 - a learning experiance with Tapped Horn - Page 2 - diyAudio
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Old 29th November 2012, 02:24 PM   #11
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Originally Posted by Forsman View Post
The microphone wasent flush. I have inserted it about 7 cm.
I shall see if I can get the measurements from pos. 10 and 20 Cm from S1. Do You want me to take a measure at the cone center position too?
I am not sure what the "0 db" level is unfortenutley.

I did drill three holes in the back of the horn, placed 80 g of polyester fiber wool there and sealed it up properly.

Well that much damping eats a lot of the bass response and makes the dip at 150 Hz even worse!

I am a bit surprised of this result. I really thought that damping at the pressure minimum would have positive effects on the 150 Hz dip.
Fredrik,

A center cone measurement would be interesting compared to pos. 10 and 20 Cm from S1if you can get the mic in that far.
Do you have a dB meter you can compare the measurements to ?

Your cabinet response looks fairly smooth, and the rising response around 100 Hz can easily be equalized out, which ends up giving a lot more headroom in an frequency range that gets pounded on with vigor in most pop music.
I much prefer the extra headroom over the loss that damping always presents- if you want flat response, might as well just make a bass reflex cabinet .

Art
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Old 29th November 2012, 04:23 PM   #12
Forsman is offline Forsman  Sweden
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Originally Posted by weltersys View Post
- if you want flat response, might as well just make a bass reflex cabinet .
Art, no swearing in my thread please...

/FF
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Old 29th November 2012, 04:39 PM   #13
Forsman is offline Forsman  Sweden
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Quote:
Originally Posted by weltersys View Post
- if you want flat response, might as well just make a bass reflex cabinet .
Art, no swearing in my thread please...

/FF
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Old 29th November 2012, 04:48 PM   #14
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hehe, Forsman , i got that .....

So i have a question ... It looks like a Pseudo "DFD" was tried with this TH but its not like the one that Brian Steele came up with because it was made of damping material instead of a constriction right?
If i understood correctly the method for finding the areas to place this damping material was also different because Brian found his location by searching for a pressure maximum at the frequency of the dip he wanted to fill in ....
Have you considered trying that? It might fill in that first big dip and give you useable response right up to about 260hz , because as it is your second dip doesn't seem to be that bad at all really in the chart that you posted.. Not nearly as bad as in the simulation ..
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Old 29th November 2012, 05:54 PM   #15
Forsman is offline Forsman  Sweden
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I am very sorry for my multiple posts above. I got outsmarted by my phone and somehow posted it several times. Apologies.

/mvhff
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Old 29th November 2012, 09:43 PM   #16
Forsman is offline Forsman  Sweden
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Yes Matthew, it is as pseudo ďDFDĒ.

I havenít come that far yet that I have tried these ideas at the pressure maximum. I think it absolutely is worth trying. Looking at the phase response I donít think this horn is usable above 150 Hz as it looks like now. I am curious though what would happen with the phase response if I could tame the SPL. If I get a controlled SPL will that automatically give me a controlled phase response then?

Brian, your ďDog Food DuctĒ is a very interesting idea. I have tried something similar but I didnít get the same good result as you did.
I am still working at the pressure minimum for the 150 Hz dip, which might be all wrong.

Letís take the out the damping and construct a couple of paddles to fit in the holes, by doing so I obstruct approximately half of the horn area. The bends seam to help create standing waves in the horn. By cutting the cross section by half I hoped to induce the same effect as a bend tend to do.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Unfortunately my idea didnít work at all.
Have I cut the cross section down too little? Or am I trying to address the problem at the wrong place in the horn, should I maybe aim for th pressure maximum at 150 Hz instead?

/Forsman
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Old 29th November 2012, 10:03 PM   #17
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Greetings Forsman,
I haven't had a chance to go through all your data yet but I just wanted to thank you for your fantastic contribution to the diyAudio community. Your in depth investigation will no doubt help many individuals, myself included, refine their TH designs/builds.

Thank you for sharing the results of your time, energy, and resources spent on this.
Keep up the good work.

-Matt
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Old 29th November 2012, 10:55 PM   #18
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Forsman , As Mattlong8 said above, thank you ... I commend your experimental efforts and scientific spirit!

And yes , I would give the 150hz pressure maximum a shot .... If i understand Brian's work properly then i think he found the general location by sending a microphone into the horn while playing a sine wave at the target frequency (in this case 150hz) ..... Look for the point where 150hz is at it's loudest, which was roughly 1/3rd of the way up the pipe starting from the mouth of the horn...

Once you have found your location then you can adjust the panel back and forth a few centimeters until you get it perfectly dialed in.. You may find that the panel will end up being slightly deeper inside of the pipe than you initially measured since the constriction shifts the fundamental resonance down by a few hertz (4hz in Brian's POC2)..
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Old 29th November 2012, 11:58 PM   #19
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Originally Posted by Matthew Morgan J View Post
Forsman , As Mattlong8 said above, thank you ... I commend your experimental efforts and scientific spirit!

And yes , I would give the 150hz pressure maximum a shot .... If i understand Brian's work properly then i think he found the general location by sending a microphone into the horn while playing a sine wave at the target frequency (in this case 150hz) ..... Look for the point where 150hz is at it's loudest, which was roughly 1/3rd of the way up the pipe starting from the mouth of the horn...
It's close. I used an RTA and some pink noise, and snaked the mic up the horn until I found the point where the notch disappeared from the frequency response. I then messed around with various constrictions until I found one that seemed to produce a decent result at the horn's mouth. It's possible it can be optimized even further, or I was very lucky getting the result that I did .

The DFD can be simulated in AkaBak (check my thread), which can take some of the guesswork out. But yes, the position of the panel is very important. Shifting it back and forth just a few cm made a HUGE difference in the response around the notch area. Maybe I was very lucky that I got pretty good results just there's a bend in the horn.

The experimentation did leave me wondering what the effect of multiple DFDs might be .
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Old 30th November 2012, 12:59 AM   #20
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ahhh ok, pink noise ...... Cool ..... That makes sense.. I need to learn to pay closer attention to detail when i read

Brian, your DFD and Stuffed Tapped Pipes are awesome demonstrations of "out-of-the-box" thinking and i really admire that sort of pioneering bravery ... You've come up with some useful innovations that could be applied to other cabinets ..

I'm a little confused on the microphone part though, i thought you would look for a peak (pressure maximum) with the microphone as you are moving it down the pipe ....

I was going by something you said in the THAM15 discussion ...
" The effect would significantly reduce, if not disappear, if I moved the obstruction caused by the cards only a inch or two forwards or backwards in the pipe. I also placed the mic *in* the pipe in that location, and set TrueRTA to Rel mode, the graph started to show a peaked response around the same frequencies that the notch appeared in the tapped-pipe's response curve."

THAM15 - a compact 15" tapped horn
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