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Old 19th September 2013, 02:42 PM   #11
sreten is offline sreten  United Kingdom
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Hi,

At one point I fitted a Fane 8" FR to the Decca cabinet. Not
particular efficient or particularly low Q. Pretty decent excursion
and fairly low Fs with a large rubber rolled surround.

Result : the worst one note bass I've ever heard .........
Full stuffing with fibreglass didn't help much .........

rgds, sreten.
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Old 19th September 2013, 06:55 PM   #12
DrBoar is offline DrBoar  Sweden
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Bjarne Börja recomend low Q driver for pipes.
Below are two 8" units with the same Fr and Bl but the left one has a much heavier cone (higherQ) than the Peerless on the right. JBL 2110 was excellent but was deemed to expensive in this setting. Note the peak in driver response just above the fundamental of the pipe and the frequency missmatch of driver dip and pipe output.


Click the image to open in full size.
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Old 19th September 2013, 06:58 PM   #13
xrk971 is online now xrk971  United States
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Uhh, I'm a little rusty on my Swedish... A translation would be nice
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Old 19th September 2013, 08:53 PM   #14
xrk971 is online now xrk971  United States
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Dr Boar,
Is there any chance I can get the CSA vs distance data from your sketch up drawings? I would like to model this horn and need detailed dimensions but am having a hard time pulling it off the plans since there are 3d contractions.
Thanks
X
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Old 20th September 2013, 03:30 AM   #15
DrBoar is offline DrBoar  Sweden
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xrk971
Anyone built the Decca Corner Horn (TQWP)? post 23 by Planet 10 has the original article as and PDF with all the measurements.

A more extended translation is on the way. The page is extracted from a series of articles about a Voigtpipe and I only translated what was pertinent to the task at hand, Decca drivers.
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Old 20th September 2013, 04:17 AM   #16
DrBoar is offline DrBoar  Sweden
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Picking suitable drivers for the Voigt-horn

Historically the drivers resonance frequency was used to select suitable drivers. At a time when compliance and moving mass for any given driver size was small that could be done. But now when you can get a 15" and 5" driver with the same resonance frequency, it is not useful.

Drives for tuned pipes should have the same characteristics as drivers for horns. A low moving mass and a high BL factor relative their mass, that is a low Q.

Fig 4 and 5 show the frequency response for two different drivers with similar Fr and Bl but the fig 4 is a high mass/low compliance (I think it refers to KEF B200 one of the high Q versions)

In fig 5 Peerless KP 825 WF is used. Several important factors can be read from the curves

1. The fundamental resonance of the pipe is unchanged by the different Mms and is at 50 Hz.

2. The Q of the pipe output (that does not coincide with the50 Hz dip) is higher for the high mass driver in fig 4A. At 50 Hz output is 10 dB below peak output. Compare with 5B where the output at 50 Hz has only dropped 2dB compared to peak output, due to better match between pipe and driver. In fig 4A the bad matching of radiation resistance is clear. Just above the fundamental there is peak in driver output and damping can only be seen at the harmonics. Compare with 5A where the pipe damp the driver all the way up to 200 Hz (suppress output by increasing radiation resistance up to 200 Hz).

The difference in sound quality is very clear at listening tests. With the loudspeaker fitted with the driver in fig 4 the bass is unclear with poor transient reproduction, high distortion and the driver tends to bottom out. On this star the KP825Wf was deemed suitable to be used to quite high standards in this construction. The driver in Fig4 was recommended in Seereo-HiFi nr 12 -75 and is a good driver but made for closed boxes.

JBL 2110 and LE8T were tried and gave far better results but due to cost reasons it was deemed less interesting for the general public so no further development was carried out. About 10 different drivers all with suitable characteristics were tried. finally the KP825WF was chosen due to low distortion, resistance to overload and low price.

The article series is about and update of an earlier construction. So "modern" driver is suggested, damping material and minor changes that could be introduced to an already built pipe and steps that does not require any measurement tools or more than two different kind of damping material.

As icing on the cake a TLS constructor well known in Sweden suggested that KEF B200 and filling the whole pipe with BAF was the way to go. The author B Börja disagreed and showd by measurements that that damping regime was not effective in reducing the 3rd harmonic and it also reduced the fundamental. Resulting in both a weak bass and boomy mid bass! A long exchange of letters to the magazine ensued…

Last edited by DrBoar; 20th September 2013 at 04:21 AM.
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Old 20th September 2013, 10:53 AM   #17
DrBoar is offline DrBoar  Sweden
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A new set of measurement of various pipe positions and parts of the pipe.
Black, brown and orange showes the Decca in a corner with the Microphone at the front, 10 cm inside the front and 10 cm inside the the side of the of the pipe. No difference between the microphone position. Green and Yellow is wall and floor position. +5dB at 3rd and 5th harmonics and +2-3 dB of 7th and 9th harmonic. So corner positions are good.
Click the image to open in full size.



I then measured the response at 1 m distance at driver hight. Wall looks pretty good both with driver facing front and wall.
Click the image to open in full size.

The corner measurement look the same but with a 5 dB lift below 300 Hz.
Click the image to open in full size.


I then wanted to explore the effect of damping material, So I took two rolls of BAF 60 cm long 82 grams each and stuffed down in the closed end of the pipe on opposite sides of the brace below the driver. Then I added 38 gBAF evenely spread out between the driver and the brace, finaly 82 gram BAF mat cut into 3 parts were used to cover the ceiling, sidewalls and the backwall opposite the driver. I expected this to kill the harmonics, but not so, it did smooth the response and took some few dB off so at least it does not seem to harm the pipe output.
Click the image to open in full size.


Bass is not everything, damping material will affect higher frequencies as well. So I measured near field response at the center of the small cone. Both with empty cabinet and filled as described above. I got different top end response between the two measurements, small changes in position is quite tricky here.

With an empty pipe there is a lot of ringing but evenly distributed both plotted in time domain and as periods.
Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.


Adding the BAF reduces the ringing substantially getting close to the resonances intrinsic to the driver.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

So conclusions so far
1. The Decca gives good response down to 40 Hz and even without any damping material the harmonics are benign.

2. For the sake of mid and high range resonances and reflections behind the driver some damping material will be good.

3. The closed end of the pipe can be filled with 200-300 g of BAF without any harm done to the fundamenal it seems but it does not eliminate the residual harmonics.

I will continue the investigation but at a slower pace
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Old 20th September 2013, 11:04 AM   #18
xrk971 is online now xrk971  United States
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Dr Boar,
Another excellent study of yet another BLH speaker, as usual from you. Very well done! This speaker looks on paper that it will sound good, and I think your listening impressions confirm this. Now if I can only get around to figuring out what the CSA's are from the drawings in that how to build a Decca horn article... Model to come as soon as I do that and we can hopefully see if effect of placement of damping material and wall vs corner positions agree with measurements. Thanks for doing this detailed study.
Regards,
X

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Old 20th September 2013, 12:34 PM   #19
DrBoar is offline DrBoar  Sweden
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Thanks for those kind words!
I plan to cut a hole for a 10" and then make a extra panel for 8" as the cutout for the 8.5" 9710 fit few other drivers. With 8" drivers I have collection of drivers so I can test how Q and Fr interact with the pipe.

I might try some smaller variations in panel damping but I will not bother with changing the taper or relocate the opening to the back panel as it works well as it is!
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Old 20th September 2013, 08:31 PM   #20
sreten is offline sreten  United Kingdom
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Hi,

FWIW with the Decca I went to (an old) 10" and managed
to swap the the magnet system to the bigger system
from the original 8" driver.

Going downhill at that point, but it sort of worked.

rgds, sreten.
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