Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Multi-Way Conventional loudspeakers with crossovers

Tweaking metal dome compression drivers and HF horns for hi-fi response. Case study.
Tweaking metal dome compression drivers and HF horns for hi-fi response. Case study.
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 7th November 2018, 07:43 AM   #1
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Default Tweaking metal dome compression drivers and HF horns for hi-fi response. Case study.

Case: both horn and compression driver are unknown model of unknown brand. These were found in 1980s Ecler TRO-120 Speakers. Re=10 ohm, 1.75" titanium diaphragm, dented suspension, axial slots and mixer, 1" exit. Oversized magnet. (fig.1, fig.2, fig.3)

(Measurement: Focusrite 2i2, Behringer ECM8000 on stand, SpecraPlus)

- Unmodified response: brown trace (fig.6): severe high Q dips in HF output (8Khz, 13Khz, 17Khz, 19Khz, 20Khz).

- What starts turbulent ends turbulent: Standing waves can ruin HF horn response (fig.7).

- Horn has sharp borders on cavity perimeter (fig.4). (I suppose the horn is an "eye copy" from another classic model, which includes a bullet piece. Precision in machining suggests China. In practice these borders are creating standing waves at HF.)

- Horn machined to remove borders (fig.5): orange trace (fig.6): 19Khz dip is removed, more low end output, less 14Khz peaking.
Attached Images
File Type: jpg metal_HF_motor_1.jpg (190.9 KB, 528 views)
File Type: jpg metal_HF_motor_2.jpg (96.6 KB, 538 views)
File Type: jpg metal_HF_motor_3.jpg (108.7 KB, 535 views)
File Type: jpg metal_HF_horn_1.jpg (70.4 KB, 532 views)
File Type: jpg metal_HF_horn_2.jpg (64.1 KB, 531 views)
File Type: png metal_HF_removing_motor_horn_borders_mismatch.png (31.8 KB, 168 views)
File Type: png what_starts_turbulent_ends_turbulent.png (8.8 KB, 150 views)
__________________
I use to feel like the small child in The Emperor's New Clothes tale

Last edited by Eva; 7th November 2018 at 07:49 AM.
  Reply With Quote
Old 7th November 2018, 07:58 AM   #2
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Increasing back chamber damping with poliester fiber lining (fig.1):
- Not lined: green trace (fig.2): Unmodified response
- Soft uniform lining: cyan trace (fig.2): reduces the 20Khz notches, but worsens the rest.
- Thick center lining: magenta trace (fig.2): splits the 20Khz notch in two.
Attached Images
File Type: jpg metal_HF_back_chamber_lining.jpg (85.2 KB, 115 views)
File Type: png metal_HF_back_chamber_lining.png (34.6 KB, 161 views)
__________________
I use to feel like the small child in The Emperor's New Clothes tale
  Reply With Quote
Old 7th November 2018, 08:56 AM   #3
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Adding damping material on throat slots and mixer cavity (stranded polyester fiber made from sheet):
- Fine lining of compression chamber (no back chamber lining): no picture: loss of 3dB efficiency at all frequencies, little improvement on dips.
- Lining on throat slots (no back chamber lining, fig.1,2): cyan trace (fig.4): lots of improvement at 13Khz and 17Khz.
- Lining on throat slots. Lining in back chamber, thick at center, soft around: magenta trace (fig.4): Less 20khz dip. Slightly less low end output.
- Lining on throat slots. Soft around lining in back chamber: pink trace (fig.5): more 20Khz dip.
- Lining on throat slots. Lining in back chamber, thick at center, soft around. Lining at transition in mixer cavity (moderate density, about 1/2 wave of 20khz length, fig.3,4): brown trace (fig.5): full improvement, no sharp 20Khz dip.

There is a sudden change of expansion law at the point where both slots are mixed. It is similar to a parabolic horn in shape (very reactive acoustical impedance), 1st half expands much faster than 2nd half. This already creates standing waves (turbulent pressure waves). The mechanism is very sensitive to phase mismatch between slots due to propagation delay of the dome. The acoustical output impedance of that thing is loaded with the relatively huge mass of air inside the horn.

But there is something else, there is a bend in the path of outer slot. The edges on the bend are all subject to diffraction. Unless all parts of wave arrive in phase, some energy of the wave is radiated in all directions, including backwards, standing waves are created.

Lining: It gradually turns pressure waves, following the laws of pressure propagation, into heat waves, following the laws of heat propagation, and back. Waves propagating in many directions transition to uniform direction as they pass lining material.
Attached Images
File Type: jpg metal_HF_slots_lining_1.jpg (232.3 KB, 170 views)
File Type: jpg metal_HF_slots_lining_2.jpg (132.4 KB, 136 views)
File Type: jpg metal_HF_mixer_lining_1.jpg (540.1 KB, 123 views)
File Type: jpg metal_HF_mixer_lining_2.jpg (135.9 KB, 114 views)
File Type: png metal_HF_slots_and_back_chamber_lining_1.png (35.0 KB, 124 views)
File Type: png metal_HF_slots_and_back_chamber_lining_2.png (35.3 KB, 122 views)
__________________
I use to feel like the small child in The Emperor's New Clothes tale

Last edited by Eva; 7th November 2018 at 09:21 AM.
  Reply With Quote
Old 21st January 2019, 09:21 PM   #4
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Incidentally, I got a discouraging email from an ex-engineer of a known national pro-audio brand, which probably is the one behind the no-brand no-model compression driver.

The guy claims that no amount of fibrous material added nowhere in throat slots or compression chamber can improve sound performance.

I did further measurements and improvements to refutate that hypothesis.

A few conclussions can be derived from these measurements:
- The compression driver performs extremely good THD-wise without back chamber.
- More, overall or localized, acoustic loading is what causes more THD.
- Addition of back chamber increases lower octave THD.
- Back chamber padding with polyester fiber optimizes lower octave THD.
- The right amount of polyester fiber padding, installed just at the entry of throat slots, partially touching the dome, shifts THD lower in frequency, while correcting upper octave high-Q dips (that do not correct properly with conventional DSP, due to non "minimum-phase" nature), as already shown in previous posts.

Note: in all cases, the piece of fibrous material across the zone in slot mixer where sound waves have to bend was installed.

The following measurements were made all at the same microphone gain adjustment, generator set for approx. 90dB SPL, and all at same measurement distance, approx. equal to the width of the mouth of the horn, approx. 30cm.

- fig.1: No throat slot padding. No back chamber.
- fig.2: No throat slot padding. Back chamber: cover, no gasket.
- fig.3: No throat slot padding. Back chamber: cover, gasket.
- fig.4: No throat slot padding. Back chamber: cover, gasket, moderate padding.
- fig.5: Throat slot: too much padding. Bach chamber: cover, gasket, moderate padding.
- fig.6: Throat slot: slightly too much padding. Bach chamber: cover, gasket, moderate padding.
- fig.7: Throat slot: optimum padding. Bach chamber: cover, gasket, moderate padding.
- fig.8: Back chamber cover came with no gasket originally. Adhesive gasket installed.
__________________
I use to feel like the small child in The Emperor's New Clothes tale

Last edited by Eva; 21st January 2019 at 09:30 PM.
  Reply With Quote
Old 21st January 2019, 09:34 PM   #5
Zvu is offline Zvu  Serbia
diyAudio Member
 
Zvu's Avatar
 
Join Date: Jan 2009
Location: Belgrade
Great stuff Eva.

Thanks for sharing.

That phase plug looks problematic too. It's quite low tech.
  Reply With Quote
Old 21st January 2019, 10:04 PM   #6
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
The whole design is low tech hehe

But the addition of elementary fiber technology on top of elementary metal technology produces quite reasonable audio performance, as for "now and forever" good sounding low cost models.

(I did not tell which brand was, however I can give one hint, it has been the supplier of loudspeakers to Television Espaņola for many years.)
__________________
I use to feel like the small child in The Emperor's New Clothes tale
  Reply With Quote
Old 22nd January 2019, 06:13 AM   #7
Zvu is offline Zvu  Serbia
diyAudio Member
 
Zvu's Avatar
 
Join Date: Jan 2009
Location: Belgrade
It really doesn't matter what brand is it. If it is renowned now, it is renowned for a reason. In PRO-audio world - if quality isn't there you go down very fast.

If you have access to the 3D printer, there's whole thread here about making better phase plug that will distribute pressure more evenly.
  Reply With Quote
Old 22nd January 2019, 06:09 PM   #8
Joseph Crowe is offline Joseph Crowe  Canada
diyAudio Member
 
Join Date: Mar 2012
Location: Picton Ontario
Thanks for sharing your test. That's a lot of work to put all this together. I've done similar experiments to the Fane CD131. I actually cut a big 1.38" hole in the rear cover of the compression driver. I then stuffed insulation into the rear chamber. I noticed a significant improvement in SQ each time my modification went further. I didn't however look at the frequency response since it would tell me very little. I looked at the spectrogram images. This clearly showed reduced decay times within the first 5 milliseconds. I also looked very closely at the impedance curve using the DATS V2 tester. Cutting the big hole in the back cover reduced the FS from 1200 Hz to 900 Hz. Adding the insulation reduced the two large peaks in the impedance curve by about 50%. In the end I decided not to use this particular compression driver because it just didn't sound good enough subjectively, but it was a fun experiment.
  Reply With Quote
Old 22nd January 2019, 10:13 PM   #9
ErnieM is offline ErnieM  United States
diyAudio Member
 
Join Date: Dec 2009
Location: Fresno, CA
It seems that a vented or oversized back chamber is beneficial in many cases. I know the Radian 745Neo has a small vent on the back cover. Not sure if any other modern drivers have that.
  Reply With Quote
Old 22nd January 2019, 10:19 PM   #10
EarlK is offline EarlK  Canada
diyAudio Member
 
Join Date: Mar 2006
Location: Toronto (GTA), Ontario
Quote:
Originally Posted by ErnieM View Post
It seems that a vented or oversized back chamber is beneficial in many cases. I know the Radian 745Neo has a small vent on the back cover. Not sure if any other modern drivers have that.
Ahh, notice the vent;

Click the image to open in full size.

  Reply With Quote

Reply


Tweaking metal dome compression drivers and HF horns for hi-fi response. Case study.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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Domes on Horns vs Compression Drivers on Horns Patrick Bateman Multi-Way 92 8th September 2019 12:26 AM
FS: 18 Sound Compression Drivers and Horns sunandmoon Swap Meet 1 13th September 2011 04:05 PM
Compression Drivers without Horns? NV&H Multi-Way 17 15th August 2007 06:03 PM
WANTED: Horns & Compression Drivers Carl_Huff Swap Meet 5 14th May 2007 07:43 PM


New To Site? Need Help?

All times are GMT. The time now is 07:39 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki