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PostPosted: August 10th, 2017, 11:50 am 
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Cogito wrote:
Walt and J-ROB,

Roscoe was able to open the sealed enclosure back cover on my 9846-8A boxes. It was not easy, I would not been able to do it without his help. Thanks Roscoe.

A while ago, you both recommended using felt damper in 802-D compression drivers. Out of the four 802-Ds I have, three came with acoustical loading caps and one with felt damper. I want to try replacing the loading cap with damper. I cannot seem to find them on google. Any ideas where I can get them?

Rob, you wrote:
Quote:
Some pro models came with a black plastic loading cap over the back of the diaphragm, which won't make much difference at HF but will at 500hz. I made back covers with a much larger volume and really didn't think it was worth doing again but it did create a somewhat softer midrange.


What type of sonic characteristics does the loading cap induce?


Found the answer to my question at GPA.

Quote:
Altec, I had been told, recommended loading caps on its small format compression drivers which were crossed over at 500Hz or 800Hz to prevent diaphragm over-excursion at high output levels. I had also been told by another Altec home user that removing the cap would result in a sweeter and more natural sounding high frequency reproduction. The latter sounded very inviting to me, and, as I didn’t expect to run my A7s anywhere near theatre output levels, I removed the loading caps, re-secured the diaphragms with slightly shorter mounting screws and placed a thin layer of acoustically absorbent material inside the rear covers. It was good advice. The HF musical presentation was much more natural and relaxed sounding to my ears, and certain sounds, like brushes on cymbols, sounded fantastically accurate and real. That was over 20 years ago and I never used loading caps in home settings after that day.

https://greatplainsaudio.com/gpa-vintage-altec-ezine/altec-lansing-compression-driver-rear-loading-caps/

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PostPosted: August 10th, 2017, 2:29 pm 
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I was looking at one of Klippel's papers and found something you might find interesting about horns and compression drivers.

Tom


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Compression horn asymmetry _ Klippel.pdf [100.66 KiB]
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PostPosted: August 10th, 2017, 4:37 pm 
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What I did with my Altec horn drivers is NOT something you buy as a finished product. It is a DIY scheme to tune the horn driver enclosure like a you would a big box.

What I did was not arbitrary -- it was the product of trials with different materials. Also, I drilled two 1/16-inch holes in the back of the covers to relieve cavity resonance -- the holes act like restricted vents to stop the cavity resonance.

I ended with a combination of the original felt in the back together with fine fiberglass and carbon impregnated foam. The carbon foam works well because is not reactive.


Attachments:
00A Altec Horn Driver Mod 3.jpg
00A Altec Horn Driver Mod 3.jpg [ 427.08 KiB | Viewed 8056 times ]
00A Altec Horn Driver Mod 2.jpg
00A Altec Horn Driver Mod 2.jpg [ 1.63 MiB | Viewed 8056 times ]
00A Altec Horn Driver Mod 1.jpg
00A Altec Horn Driver Mod 1.jpg [ 1.66 MiB | Viewed 8056 times ]

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PostPosted: August 10th, 2017, 4:42 pm 
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Tom,

Without reading the entire paper, its not possible to understand the effect of wave steepening in Compression Drivers within "listening" areas. The pic in that link is very misleading. For example it shows, significant steepening within 1 cycle. it that were true,, we cannot tolerate listening to any sounds. Further the pic shows a pure sine wave which is not an audio wave we are interested in.

The highest frequency we are interested, 20kHz, amounts to only 200 cycles before the sound reaches us at 12ft. Are we to believe there would be any detectable steepening within 200 cycles?

While the theory is right, its application of audio frequencies in listening areas is not clear.

Added:

Wiki has a good analogy.
Quote:
Shock waves can form due to steepening of ordinary waves. The best-known example of this phenomenon is ocean waves that form breakers on the shore. In shallow water, the speed of surface waves is dependent on the depth of the water. An incoming ocean wave has a slightly higher wave speed near the crest of each wave than near the troughs between waves, because the wave height is not infinitesimal compared to the depth of the water. The crests overtake the troughs until the leading edge of the wave forms a vertical face and spills over to form a turbulent shock (a breaker) that dissipates the wave's energy as sound and heat.

Similar phenomena affect strong sound waves in gas or plasma, due to the dependence of the sound speed on temperature and pressure. Strong waves heat the medium near each pressure front, due to adiabatic compression of the air itself, so that high pressure fronts outrun the corresponding pressure troughs. There is a theory that the sound pressure levels in brass instruments such as the trombone become high enough for steepening to occur, forming an essential part of the bright timbre of the instruments.[4] While shock formation by this process does not normally happen to unenclosed sound waves in Earth's atmosphere, it is thought to be one mechanism by which the solar chromosphere and corona are heated, via waves that propagate up from the solar interior.


I guess I don't understand how it effects our listening rooms.

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PostPosted: August 10th, 2017, 7:35 pm 
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Here is the complete article but what I sent you is the only thing I found on horns. It is a paper on non-linearities of drivers in general and is a good read. I did an article in audioXpress in 2003 comparing the DUMAX measurement with the Klippel system of that time. Since then he has gone way beyond those early stages. In my opinion, Wolfgang Klippel knows more about dynamic drivers than anyone else. I was fortunate to attend a seminar given by him in at his facilities in Dresden, Germany and it was totally over the top.


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Loudspeaker_Nonlinearities–Causes_Parameters_Symptoms_Klippel.pdf [1.38 MiB]
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PostPosted: December 20th, 2017, 10:04 pm 
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J-ROB wrote:
Starting a new thread to present some measurements of the 511B provided by Bjorn Kolbrek, taken in anechoic chamber at NTNU, Trodheim, Norway. 902b driver.

Frequency response shows broad 5dB peak in presence region, 12dB rolloff below~800hz

Impedance plot shows serious dropoff in loading below 800hz, Various major discontinuities through frequency range. Blue is Resistive component of Z, Red is reactive Z.

FFT shows various nasty hangovers and delays. Check it out.

Hope this gives those who like to ground evaluations in empirical measurements something to cogitate on. I foreground subjective experience over measurements, personally, but I'll grant that a basis in physical phenomena exists and sometimes gross measurements do provide some insight.


Joe,

Point taken. Tractrix is far superior to 511B and every other exponential horn I listened to.

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PostPosted: December 25th, 2017, 1:17 pm 
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Shashi, watch the generalizations, bro!

Some tractrix horns are really good but I don't know if it can be said that all tractrix horns are better than all exponentials.

BTW, which tractrix are you liking? 1" or 2"?

Note: I have a fair bit of 1/4" F9 (I think) wool felt if anybody needs some.


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PostPosted: December 25th, 2017, 7:49 pm 
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J-ROB wrote:
Some tractrix horns are really good but I don't know if it can be said that all tractrix horns are better than all exponentials.

BTW, which tractrix are you liking? 1" or 2"?


Right now I am using 2" 500Hz tractrix with 1" 802-D Driver.

Tractrix have a sonic purity which is missing in the exponentials I tried. But then, I have'nt tried circular expos.

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PostPosted: December 25th, 2017, 10:21 pm 
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Who makes the 500hz tractrix you are using, to restate my question?

Gotta get specific or else it is all buzzwords. Also we need to scavenge for gear tips and reactions where they arise.

I am a fan of some tractrix horns but others were out the door before long.


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PostPosted: December 26th, 2017, 3:53 pm 
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J-ROB wrote:
Who makes the 500hz tractrix you are using, to restate my question?

Gotta get specific or else it is all buzzwords. Also we need to scavenge for gear tips and reactions where they arise.
.


It is made by someone who is not a member of this forum.

Unlike rectangular exponential horns I tested, circular tractrix horns do not muck with the signal. Rectangular exponential horns are designed to limit the dispersion on the vertical axis and expand the dispersion on the horizontal taxis. In doing so, they "come in way" (like lenses) of the sound, so they do alter the sonics. Circular tractrixs are pure acoustic amplifiers, they just get out of the way of the signal. So, as long as horn is properly sized, throat and loading frequency , there should be very little difference between the horns of different makes. Material of construction might make a small difference.

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