Pipe fold geometries and harmonic patterns - diyAudio
Go Back   Home > Forums > Loudspeakers > Multi-Way

Multi-Way Conventional loudspeakers with crossovers

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 25th August 2013, 12:31 PM   #1
DrBoar is offline DrBoar  Sweden
diyAudio Member
 
DrBoar's Avatar
 
Join Date: Apr 2003
Location: Stockholm
Default Pipe fold geometries and harmonic patterns

When folding a pipe TLS, horn or other quarter wave resonanators, folding can affect how higher harmonics is exited. If the aim of a folded pipe is to get maximum radiation resistance at the fundamental resonanse and as little as possible of the higher harmonics, fodling should work toward this aim. That is that the folding geometry should intrinsically reduce unwanted harmonics. Damping material in the pipe should then be icing on the cake.

I plan to use some scrap material to build some pipes of different geometries and measure the resonanse pattern of the pipes. Measuring output at pipe opening and also near field output from the cone (to see if I can detect dips in response corresponing to peaks at the pipe output)

I have started with two pipes each about 150 cm long 4:1 taper (20x20cm to 5x20cm) one folded once and the other twise similar to the IMF TLS. I intend to drive the pipes with a Peerless 5.5" driver.

Suggestions for other folding geometries?
From a scientific standpoint the reference should be a unfolded pipe with the same length and taper, but it hardly represent a practical TLS. If no interesting suggestions are made I might build a Baily type of TLS. Then the taper is different as well as the folding but is a used geometry at least.

measurements will be impedance plots (Woofer Tester 3) and frequency response (Omnimic) Measurement points are opening of the pipe and also nearfield of the driver to see if peaks of pipe output correlate with dips in driver output.

Suggestions about pipes and measuremet techniques are wellcome

Click the image to open in full size.
  Reply With Quote
Old 25th August 2013, 05:19 PM   #2
diyAudio Member
 
Greebster's Avatar
 
Join Date: Aug 2010
Location: South of the Skyway
The port affects Q of the system and position of the driver. Adjust the drivers position so that you null the second harmonic. Minimal stuffing of the line with most if not all dampening directly behind the driver. Modeling shows a small offset of a little more dampening material below than above is common. When doing this recheck the measurements as you can go too far one way or the other depending on density and placement. Also 1-1.5" heavy dampening (pad) at the bottom helps. Do not do anything to the top, we want this reflection. Adding more to the line will reduce the peaking up high but really only reduces output at the bottom end. Little can be done to reduce the third harmonic other than physical taper used.
  Reply With Quote
Old 25th August 2013, 05:26 PM   #3
diyAudio Member
 
Greebster's Avatar
 
Join Date: Aug 2010
Location: South of the Skyway
These shorter twists and turns actually increase the peaking up high due to the shorter distance between bends and the internal reflections they induce. They do help in reducing higher frequencies at the port, while increasing them internally. Prefer to make three folds with the port exiting near the driver. If this is floor loaded creates a tapped horn.
  Reply With Quote
Old 25th August 2013, 07:01 PM   #4
diyAudio Member
 
Join Date: Sep 2006
Location: New England
A guy named Tom Cox did a study similar to the OP's plan years ago. IIRC, it was published in Speaker Builder Mag. The bad news is, Audio amateur Publications has disolved with the death of Ed Dell. Dell's web site used to offer for sale, back issues of SB. Now, it's successor, audioXpress is even a shadow of it's former self and is buried in amongst other audio hobbiest rags at elektor int'l publications..... audioXpress | Combining great articles, projects, tips and techniques for producing the best in quality audio, audioXpress connects manufacturers and distributors with audiophiles eager for sound solutions.
__________________
"It's all about the music!"
  Reply With Quote
Old 30th August 2013, 05:58 AM   #5
DrBoar is offline DrBoar  Sweden
diyAudio Member
 
DrBoar's Avatar
 
Join Date: Apr 2003
Location: Stockholm
This graph is what started me with folds and harmonics. As you can see a single fold gives the best suppression of harmonics. Tyrland had both driver and pipe opening on the top surface.
Click the image to open in full size.

In my case I have the driver on the forward face and the opening at the top rear. Measurements are at the center of the pipe opening and 1-2 cm in front of the dustcap of the driver (NF, near field). I have different foldings, like IMF, folded once and a untapered one Daline.
I start with the IMF folded one. Drivers are two PP cone 5” woofers with Fr of about 60 Hz. The Peerless one has a Q of 0.5 and the Taiwan one a Q of 1.0. Pipe:22 cm wide and taper from 20 to 5 cm.
Click the image to open in full size.
Pretty much as I expected. 3-7th harmonic is clearly seen. Note that the peak output is at 40 Hz while the dip in driver response is above 45 Hz.

Click the image to open in full size.
Sam pipe but with the high Q driver
Here there are peaks all over 500-1500 Hz! Note that the peak output is below 40 Hz while the dip in driver response is above 45 Hz.

Now it is time to investigate that pipe folded once that is supposed to intrinsically tame that third harmonic. . Pipe:22 cm wide and taper from 20 to 5 cm.
Lets start with the Peerless again
Click the image to open in full size.
Not that the fundamental has more of a peak and it also matches the dip in driver response. The 3rd and 5th harmonic is the same and the 7th is worse. Not what I had hoped for.

Click the image to open in full size.
Very broad fundamental and harmonics that is no improvement.

Then I have the Daline fold. That is a top chamber and then 3 equally wide pipe section of 5.5cm with the opening at the very bottom of the rear panel.
Click the image to open in full size.
Here the Q of the fundamental is lower, harmonics similar to the IMF fold. Here the peak output of pipe and dip in NF is matched.

Click the image to open in full size.
Very low Q fundamental and some peak suppression in the 500-800 Hz range.

I do have impedance measurements as well but then I will overload the post.

It does seem that the harmonics is not only due to pipe geometry but that features of the driver comes into play.

There is obviously a lot to learn. More to come…

Last edited by DrBoar; 30th August 2013 at 06:00 AM.
  Reply With Quote
Old 30th August 2013, 01:14 PM   #6
DrBoar is offline DrBoar  Sweden
diyAudio Member
 
DrBoar's Avatar
 
Join Date: Apr 2003
Location: Stockholm
Skip the previous post!
I have made more carefully matched measurements and then the harmonics follows the pipe and is not affected by the drivers. (The Taiwan drivers is tricky to seal that might have been the cause).The finding that the pipe folded once is no better or worse than the IMF stands. In a way that is good news.
  Reply With Quote
Old 1st September 2013, 10:47 AM   #7
DrBoar is offline DrBoar  Sweden
diyAudio Member
 
DrBoar's Avatar
 
Join Date: Apr 2003
Location: Stockholm
I have Stridbeck/Tyrland speakers that are very similar to the IMF TLS80. However, Tyrland published a graph that folding the TLS only once reduced the third harmonic by 10 dB compared to the IMF way of folding.
Post 5 top image

I build the two variants in smaller scale as well as a untapered TLS in 3 sections the DaLine.
Measurements and layout are drawn with a felt pen on the side. Internal width is 20 cm. I have tried to make the pipes in all 3 of equal length and having the taper the same in the IMF and the pipe folded once. But there was some limitations due to the scrap material I had at my disposition.
Click the image to open in full size.

Measurements were in sets of six.
1. Peerless driver near field of driver open pipe
2. Peerless driver near field of driver closed pipe
3. Peerless driver mid of pipe opening of open pipe
4. Taiwan driver mid of pipe opening of open pipe
5. Taiwan driver near field of driver open pipe
6. Taiwan driver near field of driver closed pipe
This to minimize microphone movement between for the same driver with open and closed pipe and also minimize the movement of microphone in the pipe when measuring output from two different drivers. The stuffing is about 250 g of BAF roll that was inserted into the pipe opening the end of the roll was folded once and tightly jammed into the opening, the impedance curves suggest that it behaves as a closed box.

I measured the Thiele Small parameters and also for the driver in the closed pipe.
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.


To compare the output of the pipe I have adjusted the peak to the same level as driver output. To do a proper addition radiating surface and phase angles has to be taken into account, so this is just for comparative reasons.

Click the image to open in full size.

Click the image to open in full size.

The result is that the 3rd harmonic is +10dB the 5th +6 dB and 7th at about 0 dB compared to the fundamental for both the Peerless and the Taiwan driver. The pipe suppress the driver output by 5 dB or more between 40 and 50 Hz but below that the driver is less protected than in a closed box.

So to conclude this first part the harmonics are substantial as measured from the pipe opening as well as indicated by the near field measurements of the cone with dips and peaks in output correlating with the harmonics.

The signal sweep in Omnimic sound very hollow, like talking trough a metal duct.
  Reply With Quote
Old 1st September 2013, 12:39 PM   #8
xrk971 is online now xrk971  United States
diyAudio Member
 
xrk971's Avatar
 
Join Date: Oct 2012
Location: Virginia
Dr Boar,
Very nice work and very meticulous measurement - as always from you. I see these harmonics in my AkAbak sims all the time and wonder how much makes it out the opening but you have confirmed they indeed make it out. Can you share your exact build geometry and driver specs? I will model in AkAbak and we can compare simulations and measurements. I will need to know exact speaker placement relative to back wall and mic position.
Regards,
X

Btw, OT... but I saw your antique corner horn project and was going to sim that but the 3d geometry makes it tricky. Maybe you have a detailed plan of CSA vs distance that you have from your drawing that includes the 180 deg turn?
  Reply With Quote
Old 1st September 2013, 12:47 PM   #9
diyAudio Member
 
Join Date: Sep 2006
Location: New England
Stuffing is a key component used in TL's to suppress harmonic peaks. Your studies do not include this. Am I to assume then, you are trying to find the optimum TL configuration INDEPENDENT of what further improvements could be had with stuffing?
__________________
"It's all about the music!"
  Reply With Quote
Old 1st September 2013, 12:57 PM   #10
xrk971 is online now xrk971  United States
diyAudio Member
 
xrk971's Avatar
 
Join Date: Oct 2012
Location: Virginia
Some designs use as many 90 deg sharp turns as possible to act as low pass filters to suppress the harmonics. We should see some effect here as more turns are introduced. Stuffing is normally only used in the driver chamber and should suppress some of the higher frequencies but probably not the 3rd harmonic - still too low for fibers to affect. If one puts stuffing in actual horn passages it really attenuates the bass gain of the LF in addition to the HF. This is an interesting study though.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Radiation Patterns Borat Multi-Way 27 13th January 2010 07:19 PM
are all functions patterns? Chris8sirhC Everything Else 25 22nd March 2009 06:09 AM
Optimising Heatsink Geometries EUVL Pass Labs 11 11th November 2008 03:52 AM
Distortion in various core materials and geometries capslock Multi-Way 6 9th August 2002 10:46 PM


New To Site? Need Help?

All times are GMT. The time now is 06:21 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2