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1969-74 Dynaco A-25 speakers compared to modern designs?

Dennis Murphy

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#21
Thanks for the great post. I had completely forgotten about the aperiodic tuning slot, which absolutely didn't do anything. I think it was Dynaudio that used to sell a stuffed circular stand-alone version that I tried out a couple of times. It didn't work either.
 

anmpr1

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#23
The double Advent thing was also curious and never really made sense, for reasons that are well understand by anyone who has ever dabbled in speaker design.
It made total sense once you found out that the guy who was pushing that configuration eventually stuffed his smallish room with a four module Infinity IRS monster, and proclaimed it to be 'the Absolute Sound'. Those who knew called it Harry's IRS in a water closet set up.

In any case, it's clear from his review that he just wanted something that played loud, and four Advents were certainly louder than two.
 

MrPeabody

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#24
Dynaudio (or someone) indeed sold aperiodic vents (or whatever one might wish to call them) for DIY use at least into the 1990s -- I remember seeing them in the PartsExpress catalog.

I am not sure that they don't do anything...?

https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/50s/Audio-Anthology-4-1957.pdf
(page 133)

View attachment 105494
My curiosity was piqued sufficiently for me to spend a little while reading the article. It is written by an electrical engineer for other electrical engineer, where the practice is to use electrical analogs for all mechanical aspects of a system, including acoustical aspects. My criticisms of the article would be much the same as with most amateurish articles encountered elsewhere, starting with the failure by the author to clearly state the goal at the beginning. Figure 8 on page 135 shows, at the right edge of the figure, a graph of "amplitude" vs. frequency. Except for the extremes it looks like an impedance graph. The text reads, "The resulting maxima of the induction voltage developed in the voice coil cause the well-known double hump of the voice coil impedance which is specific to the bass reflex system. The opposite polarities of the front and rear waves cause the lower resonance to be reduced in amplitude. A typical theoretical response is shown in Fig. 8." The author proceeds to discuss how his proposed enclosure design damps the double resonance. It was not clear to me whether he was talking about damping acoustic response peaks associated with the resonances, or damping the peaks in the impedance curve. Certainly the amplitude response curve in Fig. 8 is not representative of an acoustic response curve, even with the resonances. Possibly it would be for speakers that are driven by an amplifier having much higher output impedance than what is nowadays typical. He goes on to explain how the resonances that are evident in the electrical model can be suppressed with appropriate modifications to the analogous electrical circuit. After doing that, he then discusses the enclosure design by which the analogous electrical circuit is realizable mechanically. Crucially, it involves the enclosure being divided into two chambers, the smaller of the two housing the woofer and the slot opening with the acoustic impedance. The two chambers are separated by a cloth membrane that provides the optimal measure of acoustic impedance between the two chambers. At some point the author indicated that the front opening was large in order to improve the acoustic coupling. This in particular is bizarre, given that it is obvious that the acoustic wave exiting the opening will be 180 degrees out of phase with the direct acoustic wave from the woofer cone. The author certainly understood this. At the end of page 136, there is this: "The lower frequencies are somewhat attenuated by the cancelling action of the residual sound radiated by the slit. This defect is easily overcome by applying some bass boost in the amplifier." Given that he was well aware of this, why did he not realize that it wouldn't make sense to make the opening large toward increasing the acoustic coupling? I'm sorry but it just doesn't make sense, and the author ought to have realized that it didn't make sense.

The plain reason that the lower resonance is "attenuated" is that it does not exist because there is no Helmoltz radiator effect. As such, it is manifest that the two-chambered enclosure with the acoustic barrier between them is a means to damp the acoustic resonance that occurs in association with the driver's resonance Fs (shifted higher in frequency as a consequence of the stiffness of the air within the enclosure). Ditto for the ostensibly beneficial effect of the opening to the front. If the acoustic response peak associated with this specific resonance is dealt with in an optimal manner via either the well-established sealed enclosure principles or the well-established ported enclosure principles, then neither the extra internal chamber with the cloth barrier nor the front-facing opening will serve any real purpose. The only possible benefit of this opening is that it permits the enclosure to be small as compared to a sealed enclosure using the same driver. The penalty, which to me is not a small penalty, is that there won't be much bass to speak of, somewhere between a woofer properly mounted in a sealed enclosure and the same woofer suspended in free air.
 

MrPeabody

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#25
It made total sense once you found out that the guy who was pushing that configuration eventually stuffed his smallish room with a four module Infinity IRS monster, and proclaimed it to be 'the Absolute Sound'. Those who knew called it Harry's IRS in a water closet set up.

In any case, it's clear from his review that he just wanted something that played loud, and four Advents were certainly louder than two.
I was going to say that it still doesn't make technical sense, but then thanks to you it occurred to me that the real benefit may have been that if the pair was wired in parallel the halving of the impedance would mean that power would double, which would make the pair sound better for the same reason that turning up the volume control makes it sound better. But there would also be a shift in tonality toward bass, as I discussed.
 

Dennis Murphy

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#27
I recently obtained a single Large Advent Loudspeaker for Amir to test. I'll be sending it off to him next week. The woofer surround has been replaced professionally, and the TS parameters measure close to spec. It sounds like what I remember from the pair i owned 500 years ago, although what I have isn't the first generation. It's the "new" Advent Loudspeaker, which has the same tweeter but with fluid in the voice coil. This allowed them to switch from a 2nd order electrical high pass to a single capacitor. It's a super cheap NPE which I'll replace simply to make sure the value it's operating properly. I think the Spins are going to surprise you a little.
 

HiFidFan

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#28
Daedalus speakers out of WA uses aperiodic enclosures. They are the only ones I know of that currently build this type.

daedalusaudio
 

MrPeabody

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#29
I recently obtained a single Large Advent Loudspeaker for Amir to test. I'll be sending it off to him next week. The woofer surround has been replaced professionally, and the TS parameters measure close to spec. It sounds like what I remember from the pair i owned 500 years ago, although what I have isn't the first generation. It's the "new" Advent Loudspeaker, which has the same tweeter but with fluid in the voice coil. This allowed them to switch from a 2nd order electrical high pass to a single capacitor. It's a super cheap NPE which I'll replace simply to make sure the value it's operating properly. I think the Spins are going to surprise you a little.
Thanks for doing this! I'm very much looking forward to it. Since the surround replacement was done professionally and the T/S numbers are close to spec, this is likely going to be as close to the original as you could hope to get, as far into the future as are now. Yogi Berra once remarked that the future ain't what it used to be. Replacing the capacitor will help to insure that it will perform as it should. I'll be especially interested in seeing whether distortion in mid-bass is any lower than what it is nowadays common with ported speakers. The sealed enclosure potentially makes the restoring force more linear than what it can be with the driver suspension. Also, the off-axis response will be very interesting, because I've always wondered whether part of the reason for the concentric ripples in the cone is to allow just the inner part of the cone to move with higher frequency. I'm also anxious to see how much distortion that tweeter produces. I won't be surprised if it is significantly worse than most modern tweeters.
 
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