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Old 30th November 2012, 03:39 AM   #21
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Quote:
Originally Posted by Matthew Morgan J View Post
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

Yep, it was set in "Rel" mode with the response at the mouth used as the reference. As a result the RTA will show a peak at the dip frequency when the actual response is flat at that frequency, because the RTA is comparing the measured response to the reference.

FWIW, my attempts at DFDing the tapped pipe were not successful. I'm not sure why.
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Old 30th November 2012, 06:01 AM   #22
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Ok, i think its starting to make sense to me now,

So Brian, let me make sure i understand this correctly.... The 1/3 in from the mouth point (the same thing as 2/3rds down from the throat) is where you got a dip-free response reading from the microphone correct? It was fairly flat in the midbass .... At that location you saw the best output/pressure at 150hz yet not necessarily a peak compared to the rest of the spectrum but it was a flattish response instead, however its a peak compared to your reference sample which was taken at the mouth right? So we could say that spot is a pressure maximum for 150hz ....
Then that was the same location where you installed the DFD constriction, correct? ....

I wonder why it didn't work for the other box?? Now thats interesting, since the design isn't truly that much different except for the scale of it, a little stuffing , and the lack of taper ...
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Old 30th November 2012, 06:44 AM   #23
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Originally Posted by Brian Steele View Post
FWIW, my attempts at DFDing the tapped pipe were not successful. I'm not sure why.

Im curious, was there even much of a dip left to fill in after you added the stuffing to the POC1 tapped pipe? Did you get any sort of effect at all when you tried to add a DFD to it?
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Old 30th November 2012, 07:26 AM   #24
GM is offline GM  United States
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Quote:
Originally Posted by Matthew Morgan J View Post
I wonder why it didn't work for the other box?? Now thats interesting, since the design isn't truly that much different except for the scale of it, a little stuffing , and the lack of taper ...
Taper makes a difference since it's a 1/2 WL resonator and the pipe is a 1/4 WL: Resonances of open air columns

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Old 1st December 2012, 04:20 AM   #25
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Originally Posted by Matthew Morgan J View Post
Ok, i think its starting to make sense to me now,

So Brian, let me make sure i understand this correctly.... The 1/3 in from the mouth point (the same thing as 2/3rds down from the throat) is where you got a dip-free response reading from the microphone correct? It was fairly flat in the midbass .... At that location you saw the best output/pressure at 150hz yet not necessarily a peak compared to the rest of the spectrum but it was a flattish response instead, however its a peak compared to your reference sample which was taken at the mouth right? So we could say that spot is a pressure maximum for 150hz ....
Then that was the same location where you installed the DFD constriction, correct? ....
That's correct.


Quote:
Originally Posted by Matthew Morgan J View Post
I wonder why it didn't work for the other box?? Now thats interesting, since the design isn't truly that much different except for the scale of it, a little stuffing , and the lack of taper ...
I'm not sure why. I experimented with it for several hours and couldn't get a result that made any positive difference. Stuffing part of the pipe produced much better audible results.
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Old 1st December 2012, 04:22 AM   #26
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Originally Posted by Matthew Morgan J View Post
Im curious, was there even much of a dip left to fill in after you added the stuffing to the POC1 tapped pipe? Did you get any sort of effect at all when you tried to add a DFD to it?
My attempt at DFDing the tapped pipe was done with it unstuffed. Once it's stuffed, a DFD isn't necessary as there's no dip to be concerned about
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Old 1st December 2012, 05:21 AM   #27
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Awesome! Thank you Brian,
You used poly fiberfill? and you stuffed the first 1/3rd to 1/2 right?

Also, according to the chart in addition to the smoothing effect it looks like you had the side benefit of lowering the fundamental resonance too, maybe 3-5hz ? Good stuff!!

It reminds me of the results that i have seen when stuffing the first half of some Quarter Wave Pipes that I built with a friend ..... We used Polyfill as well ..

Last edited by Matthew Morgan J; 1st December 2012 at 05:25 AM.
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Old 1st December 2012, 01:27 PM   #28
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Originally Posted by Matthew Morgan J View Post
Awesome! Thank you Brian,
You used poly fiberfill? and you stuffed the first 1/3rd to 1/2 right?
That's correct.
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Old 1st December 2012, 01:33 PM   #29
Forsman is offline Forsman  Sweden
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Thanks for all the kind and inspiring words! But remember that is us who do this together, not just me. Actually it is me who shall say thank you for all your fantastic contribution, both in this thread and in all the others I have read on this forum which are the main source of information that I build my understanding on. I appreciate this philosophy of sharing knowledge and information so much!

Today it is 10 degrees (C) below and my workshop isnít insulated so no new measurements today.

The other day played with the effects of a quarter wavelength resonator at the 150 Hz area (pressure minimum).

I tuned the pipe to 150 Hz, 344/150/4=0,57 m and attached the open end at 120 cm from S1, no damping, no paddles inside the horn.

I donít think this is the right spot to work with a quarter wave resonator but it can be interesting nevertheless.

Click the image to open in full size.

Click the image to open in full size.

As we can see it didnít make any change at all.

Just to experiment and see how things work I retuned the pipe to treat the peak at 170 Hz by cutting it a couple of cm shorter and got this result:

Click the image to open in full size.

Green Curve: Original horn
Red Curve: With resonator

It is interesting to how narrow the bandwidth that the tube affects is. So far the tube is without any damping.

Letís see what happens when we connect the tube as close to S2 as possible. In this case the closest I could get is 25 cm from S2.

Click the image to open in full size.

After some trials I found the right amount of stuffing, which turned out to be next to nothing. Just too much and the effect of the resonator was ruined, just too little and I got a sharp notch.

Click the image to open in full size.

Click the image to open in full size.

Green Curve: Original horn
Blue Curve: With ľ wavelength and no damping, tuned to 170 Hz
Red Curve: With ľ wavelength with damping, tuned to 170 Hz

SPL, phase and third tone distortion.

Isnít this pretty promising? One could take the volume for resonators in account in design rather easy and get a more controlled behavior above the crossover frequency.

/Forsman
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Last edited by Forsman; 1st December 2012 at 01:47 PM. Reason: spelling
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Old 1st December 2012, 04:35 PM   #30
tb46 is offline tb46  United States
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Hi Forsman,

Another "Thank You" for your effort here.

I assume you are aware of the use of closed pipe resonators in the DTS20: http://forums.klipsch.com/forums/t/7...x?PageIndex=13 ; there is also quite a bit of (acoustic foam?) lining in this one.

There has been quite a bit of theoretical work done on the subject of simulating resonators, etc. in, e.g.: the Collaborative Tapped horn project so it's great to see some experimental confirmation of the AkAbak work done by Mavo, jnb Cordraconis and others.

geitmans goes into the effect of filling (lining the horn path) in his thread: Dual 8" tapped horn = TH-SPUD (Post #28 (too much)
Post #47 (reduced)).

Changing the length of L45 in Hornresp will lower and raise the dips around the first big peak (if you do it in the Wizard it will maintain the overall horn length). I messed around a little with your Hornresp simulation, but didn't try to fit this into wood.

Keep up the good work!
Regards,
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