The New Advent Loudspeaker Review (Vintage Speaker)

[TimmyO]

I always do my best to level match. I have conducted a speaker miced recording test to show the differences in the dampening products but I can't seem to attach the files. Have you got any advice on doing that here? thanks

 

[rdenney]

Did you control for bias? Meaning: Could you “see” what you were listening to? If so, your perceptions are not being fully controlled by your ears. That bit of psychoacoustics has been demonstrated repeatedly.

Rick “control for unknowns” Denney

 

[rdenney]

I always do my best to level match. I have conducted a speaker miced recording test to show the differences in the dampening products but I can't seem to attach the files. Have you got any advice on doing that here? thanks

Level matching has to be accurate—most here recommend using a voltmeter with a 1 KHz tone. Even a tenth or two of a dB can skew perceptions.

If you can park the file on the web somewhere, post a link.

Rick “try making the needledrop with and without the speakers playing to reveal acoustic feedback” Denney

 

[TimmyO]

Did you control for bias? Meaning: Could you “see” what you were listening to? If so, your perceptions are not being fully controlled by your ears. That bit of psychoacoustics has been demonstrated repeatedly.

Rick “control for unknowns” Denney

I get that. I could never conduct a blind test for myself at home. that would be impossible. I want to post the results here so they are blind for yourself and the forum members but I'm having problems uploading the files

 

[TimmyO]

Got it. Ok, I have conducted this recording test today to listen for the difference in dampening products. Equipment used was a U47 clone by NUDE Microphones into a Focusrite ISA one mic pre. I just used mono records for the test and close miced the one speaker about 700mm away at a level between the tweeter and driver. The other speaker was turned off. One test without and one with all of my dampening products. Vinyl weight by Project. Slip mat by Les Davis audio. RCA stop plugs. The better power supplies for my Project S1 phono preamp and the project X1 turntable. I couldn’t replace the speaker cables as I threw out my old ones. But they are Chord Company C Screen cables. 3 songs. 1 is the same setup for all songs just as is 2. For the Ramones song I added a 3rd track. Volume and mic placement was the same for all tracks. Nothing was changed.
Gloria 1
[/https://www.dropbox.com/s/1wge01rrdi7z2nv/GLORIA 1.wav?dl=0
Gloria 2
https://www.dropbox.com/s/b5vqzti25t9mnjv/GLORIA 2.wav?dl=0
Someday My Prince Will Come 1
https://www.dropbox.com/s/mv1xgfxz78q14l2/SOMEDAY 1.wav?dl=0
Someday My Prince Will Come 2
https://www.dropbox.com/s/b0k2o4w29rdfdx4/SOMEDAY 2.wav?dl=0
Ramones 1
https://www.dropbox.com/s/6w5q7lnf3iao1da/RAMONES 1.wav?dl=0
Ramones 2
https://www.dropbox.com/s/zj1gv2rldzvmeom/RAMONES 2.wav?dl=0
Ramones 3
https://www.dropbox.com/s/tcqw1ez3wtbyb0l/RAMONES 3.wav?dl=0

 

[TimmyO]

Level matching has to be accurate—most here recommend using a voltmeter with a 1 KHz tone. Even a tenth or two of a dB can skew perceptions.

If you can park the file on the web somewhere, post a link.

Rick “try making the needledrop with and without the speakers playing to reveal acoustic feedback” Denney

Results are posted

 

[Jim - where is The Abyss]

Weird. My home depot on 130 Ave SE Calgary cut them with a smile. But everyone wearing masks due to Covid.

I started at the Shaunessay Home Depot and they flatly said no way. Then I called the Crowfoot Lowes and he said he would but his saw was down. Then I went to the McLeod Tr. Lowes and they said go to the 130 Ave store. So off I go and at first the fellow resisted and then he got the head of the dept who then said no way citing health concerns. He also said his cutting table was out of alignment and would not cut long rips accurately. Of course, throughout my travels I am offering to pay per cut. This was in Feb. But I didn't try the 130 Ave Home Depot. Anyway, that was my experience.

 

[Keithdd]

My experience at Home Depot was a bit earlier now that I think about it. The wood sat in my basement some months before I used it earlier this year. So maybe it's a newer policy.

 

[Keith Conroy]

jkasch...............I wrote you a long response last night to answer your question. Then when I re-read it and I thought I deleted it. If you get a piece talking about mid range centering its just a piece of what I wrote. I think I have some valid comments, I promise they are coming!

Sir, Its a little late but I owe you a more detailed reply on speaker centering. Your question, Is it a problem you have to worry about over time? My examples would be based on speakers that are mounted in a speaker system. Then this system stands in the vertical. In general its been my experience that newer speakers say 1995 and later have better centering characteristics. Speaker soft parts designs started to improve around this time.

If we split things up and talk about mid-range speakers to start. I would say( IN VERY GENERAL TERMS) centering drift should not be an issue. This should be true even in systems that are say 15 years old or more. As I said before, there are so many design & parts choices that impact this issue. However I think its a safe bet to assume your mids will stay good over time. See below about other noises & why. These can happen in Mids also.

Now if we talk about woofers the parts make up and the size of the woofer can affect things more. Even after saying this, I don't think centering off-set will be the worst problem you could run into over time. You can just as likely run into a very slight rub caused by debris in the gap. You might ask how can I get debris in a sealed woofer? He is one example of what can can happen. When your designing with brittle magnet materials, very very small pieces can break off during the build process. They then get trapped inside the motor structure. Once the speaker gets magnetizes these small magnet particles seat themselves & stick to parts of the motor. As long as they stay put in a spot where they don't affect clearance. Then they might never cause a problem. This even after years later. However sometimes because of speaker movement over time they can move slightly and then create an interference rub. Another issue, can be small pieces of the metal plating breaking off. This happens over time because of bad plating. I have actually seen this in NEW/OLD stock Aura car audio speakers. When I was involved with Chinese speaker manufacturing, they routinely had brown outs in their power grids. So sometimes because of lower voltage the final plating was not ideal.

Of course large sub woofers definitely can have suspension sag over time. Its that gravity thing. Again not so much when the woofer is mounted in a system in the vertical orientation. If your into DIY and your storing large raw woofers in shipping boxes. Then flip the box orientation 180 degrees every 6 months. On your particular speakers, they were made by quality companies. This goes a long way to help make sure you don't have issues. The Aura Home stuff I saw was also pretty good.

Your could take the woofers out of the cabinet and inspect the spider for flatness. Lay them on a table & make sure the spider landing area under cone is well lighted. You should be able to look across the spider and see a flat profile across the tops of the spiders convolutions. You can lift up on the cone slightly to change the profile. Then judge for flatness. Make sure to seal things while when you put them back in the cabinet. If you have not heard any issues to date, I would not lose to much sleep over things!! I'm sure there going to throw me off this website with these LONG POSTS!!! I hope you get some value out what I wrote. It was meant to talk about what could happen and why to create noise over time in speakers This related to the motor....................

 

[richard12511]

That treble brightness may have been endemic in Henry Kloss' loudspeakers. Having just been fiddlin' around with three pairs of KLH Sixes, I noted that they sound pretty good but noticeably more piercing than I (at any rate) prefer.

DSC_0219 (2) by Mark Hardy, on Flickr

Although the KLH Six was, arguably, its direct predecessor, I never much cared for the OLA ("Original Large Advent") FWIW.
There is a pair of OLAs in the basement, and I even have replacement foams for them, but have never worked up the gumption to refoam them.

The reported LF response of the NLA in this review makes me wonder about the "professional reconing" (refoaming?) of the woofer. One thing that the New England cadre of acoustic suspension loudspeakers can do, in room, is astonishing bass response if fed with gobs of power.

Nice! How many vintage speakers do you have?

 

[mhardy6647]

Nice! How many vintage speakers do you have?

What, like I keep track?

That said, I'd wager that there's an odd number of loudspeakers here -- please don't make me count.

 

[Aperiodic]

What, like I keep track?

That said, I'd wager that there's an odd number of loudspeakers here -- please don't make me count.

One must be a center channel I guess....

 

[mhardy6647]

One must be a center channel I guess....

would that it were so...

 

[MrPeabody]

Back to that?!? Except for one poorly written paragraph on one third party website, all available sources specifically indicate that the stuffing in the port was adjusted to achieve the objective. I can see it now: Put the interior screen on the inside front panel before installing the front panel as the last part of cabinet assembly. Install drivers. Stuff some fiberglass in the port (I bet that after the first couple hundred thousand, they could probably get pretty close on the first try.) Connect up the system as described and feed it the stipulated signal as described at length in multiple sources. Staple the front screen on. Attach the grille cloth. Next!

Why are you so resistant to the idea that Dyna actually did what they said they did? The whole procedure probably took less than two minutes after you'd done it a few times. It probably took less time than most builders put in on final QC checklists. Back then, people- on a lot of things- actually took time to get things right rather than trying to squeeze the profit margin another 0.03%.

To me, it does not seem entirely unreasonable that this sort of fine tuning on a unit-by-unit basis was actually done, by Dynaco. I am a little skeptical, but I think it is not entirely unreasonable. Regardless, I think it is pertinent to have a correct understanding of what the aperiodic enclosure actually was (and is). The comment from the late Jeff Bagby, quoted for us in this thread by Dennis Murphy, sums it up pretty nicely:

There's no question that aperiodic loading can reduce the box tuning peak, and I guess that would help out some tube amplifiers. But there's always a price to be paid. The late and lamented Jeff Bagby summed it all up:

"The way it works though is very simple -it just introduces a lot of leakage to a sealed box. You essentially end up with a sealed box (I guess misnomer in this case) with a Q of around 3. You can model this in something like Unibox and see what the effect is. It will reduce the impedance peak and also decrease the low frequency output by lowering the Qtc of the system through the introduction of a large leak that changes the air compliance in the box. Are they worth using? Generally, no. The only time they have a beneficial application is in flattening the response of higher Qtc systems, and/or with high Qts woofers. It will allow them to be used in smaller enclosures without as much peaking, but it is at the expense of a higher F3 as well. There's always a trade-off."

My guess is that the Seas woofer in the A25 has a high QTc [this should be Qts] that would have required a larger box than Seas and Dynaco wanted to use. The aperiodic loading probably helped in allowing a smaller box, but it also reduced the bass reach. Anyhow, I'll be able to measure all of that when I get the speakers.

To be clear, the so-called "aperiodic enclosure" (I dislike this silly name) is equivalent to an acoustic suspension (sealed) speaker with a large opening added for the express purpose of leakage, in order to obtain an acceptable Qtc using an enclosure that is much smaller than what the woofer otherwise requires. Beyond this, there is only the fact that the driver is more efficient than a driver that would work well in that smaller enclosure would otherwise be. The advantage in efficiency, however, does not equate to improved sensitivity at bass frequencies, because of the leakage through the large opening.

If the amount of acoustic impedance placed at the big opening was in fact fine-tuned at the factory, what this would actually amount to is the fine-tuning of the effective enclosure volume, i.e., adjusting the effective enclosure volume to make it a better match to the Qts and Vas of the driver. Whether this would or wouldn't be especially beneficial would depend on the statistical unit-to-unit variance in the Qts and Vas of the driver. Vas is actually an expression of the stiffness of the driver suspension, i.e., the spring effect within the suspension. Qts is an expression of the severity of the primary resonance of the driver, which depends partly on the spring effect within the suspension, and also on the damping, which derives from the mechanical loss of the suspension and also the electrical loss due to Ohmic heating of the voice coil. These driver parameters are known to vary significantly from unit to unit, for any given model of driver, even with modern drivers.

It is thus reasonable that a speaker manufacturer would seek to achieve better quality control, by adjusting the effective size of the enclosure to accommodate the variation in the driver's Qts and Vas. However, if this was in fact done, I would not regard this as any vindication of the aperiodic design, given that equivalent unit-by-unit optimization may be done with any true acoustic suspension speaker by adjusting the amount of internal damping. It would probably be more difficult to do with a sealed speaker, i.e., it would likely require removing the woofer, but it is nevertheless possible to do this to the same end. (And other than this potential, uncertain advantage of the aperiodic design, there is no genuine benefit in using a driver that inherently requires an enclosure very much larger than the enclosure that is intended to be used.)

I am generally skeptical of the marketing claims that Dynaco made, which probably aren't any worse than what was common then and common now, but nevertheless lacking in believability. For example, I recall seeing a marketing brochure where two graphs of system impedance were shown, to illustrate the effect that the opening has for the severity of the impedance peak. In that brochure, what they showed was the difference for the completed speaker with the big opening, compared to what the impedance peak would be using that same driver in the same size enclosure without the opening. The impedance peak was not made less severe than it is for a typical acoustic suspension speaker without a big opening but using a driver that is better suited to the size of the enclosure.

The bigger reason for being skeptical of Dynaco's claims is that they simply misrepresented the true technical nature of the so-called aperiodic enclosure. They made it seem like something that was technically sophisticated, when all it really is, is a quasi-sealed speaker, i.e., a speaker that does not use the Helmholtz radiator principle, with a large hole in the enclosure in order that the enclosure does not have to be as ginormous as what the driver needs. When something this simple has been given a marketing spin that makes it seem like more than it truly is, I tend to be distrustful of that company's claims, generally.

Something that you provided, a response from Dynaco to the Stereophile review of the A-25, is a further example of the same kind of thing:

...I looked up the contemporary Stereophile review of the A-25 and found the following in the manufacturer comments (italics mine) :

The aperiodic design is not a bass-reflex approach, since there is no acoustic output through the port. The characteristics of the "plug" in the port are quite critical, necessitating individual adjustment of each system. [...] This added acoustical impedance damps the woofer, improving its response to transient signals. Examination of the woofer cone motion shows that, with this aperiodic design (on which patents are pending), the cone follows the input signal all the way down to DC with far greater precision than is the case with either bass-reflex or acoustic-suspension designs.—Dynaco

There are obvious technical errors in this. Certainly it is true that the aperiodic design is not "bass-reflex", i.e., it does not employ the Helmholtz radiator principal. But it is ludicrous for them to say that the reason, that it is not bass-reflex, is that there is no acoustic output through the port. There is definitely acoustic output through the opening. At low frequency, this output is strongly out of phase with the direct output from the woofer, such that destructive interference is very strong and becomes greater as frequency moves lower. As for the statement that the "added acoustic impedance damps the woofer", there is probably a sliver of truth to this, but it is not a correct understanding of the reason the big opening has a suppressing effect on the woofer's primary resonance and the related impedance peak. The extent to which the spring effect of the enclosure (enclosure compliance) exacerbates the woofer's intrinsic resonance is diminished by virtue of the large opening in the enclosure. The claim that this "damping effect" improves the "response to transient signals" is further meaningless. Other than DC signals (which should never be allowed to reach any loudspeaker), all audio signals are transient signals. And no explanation is provided by why or how transient signals in particular would benefit from the supposed improvement in the "damping effect". Whoever wrote this response to Stereophile was wearing their marketing hat.

 

[Aperiodic]

To me, it does not seem entirely unreasonable that this sort of fine tuning on a unit-by-unit basis was actually done, by Dynaco. I am a little skeptical, but I think it is not entirely unreasonable. Regardless, I think it is pertinent to have a correct understanding of what the aperiodic enclosure actually was (and is). The comment from the late Jeff Bagby, quoted for us in this thread by Dennis Murphy, sums it up pretty nicely:



To be clear, the so-called "aperiodic enclosure" (I dislike this silly name) is equivalent to an acoustic suspension (sealed) speaker with a large opening added for the express purpose of leakage, in order to obtain an acceptable Qtc using an enclosure that is much smaller than what the woofer otherwise requires. Beyond this, there is only the fact that the driver is more efficient than a driver that would work well in that smaller enclosure would otherwise be. The advantage in efficiency, however, does not equate to improved sensitivity at bass frequencies, because of the leakage through the large opening.

If the amount of acoustic impedance placed at the big opening was in fact fine-tuned at the factory, what this would actually amount to is the fine-tuning of the effective enclosure volume, i.e., adjusting the effective enclosure volume to make it a better match to the Qts and Vas of the driver. Whether this would or wouldn't be especially beneficial would depend on the statistical unit-to-unit variance in the Qts and Vas of the driver. Vas is actually an expression of the stiffness of the driver suspension, i.e., the spring effect within the suspension. Qts is an expression of the severity of the primary resonance of the driver, which depends partly on the spring effect within the suspension, and also on the damping, which derives from the mechanical loss of the suspension and also the electrical loss due to Ohmic heating of the voice coil. These driver parameters are known to vary significantly from unit to unit, for any given model of driver, even with modern drivers.

It is thus reasonable that a speaker manufacturer would seek to achieve better quality control, by adjusting the effective size of the enclosure to accommodate the variation in the driver's Qts and Vas. However, if this was in fact done, I would not regard this as any vindication of the aperiodic design, given that equivalent unit-by-unit optimization may be done with any true acoustic suspension speaker by adjusting the amount of internal damping. It would probably be more difficult to do with a sealed speaker, i.e., it would likely require removing the woofer, but it is nevertheless possible to do this to the same end. (And other than this potential, uncertain advantage of the aperiodic design, there is no genuine benefit in using a driver that inherently requires an enclosure very much larger than the enclosure that is intended to be used.)

I am generally skeptical of the marketing claims that Dynaco made, which probably aren't any worse than what was common then and common now, but nevertheless lacking in believability. For example, I recall seeing a marketing brochure where two graphs of system impedance were shown, to illustrate the effect that the opening has for the severity of the impedance peak. In that brochure, what they showed was the difference for the completed speaker with the big opening, compared to what the impedance peak would be using that same driver in the same size enclosure without the opening. The impedance peak was not made less severe than it is for a typical acoustic suspension speaker without a big opening but using a driver that is better suited to the size of the enclosure.

The bigger reason for being skeptical of Dynaco's claims is that they simply misrepresented the true technical nature of the so-called aperiodic enclosure. They made it seem like something that was technically sophisticated, when all it really is, is a quasi-sealed speaker, i.e., a speaker that does not use the Helmholtz radiator principle, with a large hole in the enclosure in order that the enclosure does not have to be as ginormous as what the driver needs. When something this simple has been given a marketing spin that makes it seem like more than it truly is, I tend to be distrustful of that company's claims, generally.

Something that you provided, a response from Dynaco to the Stereophile review of the A-25, is a further example of the same kind of thing:



There are obvious technical errors in this. Certainly it is true that the aperiodic design is not "bass-reflex", i.e., it does not employ the Helmholtz radiator principal. But it is ludicrous for them to say that the reason, that it is not bass-reflex, is that there is no acoustic output through the port. There is definitely acoustic output through the opening. At low frequency, this output is strongly out of phase with the direct output from the woofer, such that destructive interference is very strong and becomes greater as frequency moves lower. As for the statement that the "added acoustic impedance damps the woofer", there is probably a sliver of truth to this, but it is not a correct understanding of the reason the big opening has a suppressing effect on the woofer's primary resonance and the related impedance peak. The extent to which the spring effect of the enclosure (enclosure compliance) exacerbates the woofer's intrinsic resonance is diminished by virtue of the large opening in the enclosure. The claim that this "damping effect" improves the "response to transient signals" is further meaningless. Other than DC signals (which should never be allowed to reach any loudspeaker), all audio signals are transient signals. And no explanation is provided by why or how transient signals in particular would benefit from the supposed improvement in the "damping effect". Whoever wrote this response to Stereophile was wearing their marketing hat.

I don't have an issue with anyone debating the merits of the approach. It was, after all, 1969 when this speaker reached the market, and technology has advanced a lot. What I take issue with is accusing (directly or otherwise) people who aren't around to defend themselves of not doing what they explicitly said they did. There's a word for what they were being accused of: Fraud. Dynaco was one of the most respected names in audio and a lot of the people who have five figure systems today- and a lot of the members of any audio board- probably started with them.

As far as acoustic output through the port, I suspect it was, at the very least, attenuated significantly by passing through tightly stuffed fiberglass, compared to your garden-variety bass reflex tube. The larger two-chambered models (A35, A40XL, A50) probably had deep bass behavior more like the acoustic-suspension competition, since that is essentially what they were.

Remember, word of mouth was crucial back then. This was before you could con people into paying thousands of dollars for a power cord by giving a manipulated demo at an audio show, or literally stick a $599 CD player inside an outer enclosure and charge $3K for it. Not to say there weren't flim-flammers back then, but they usually didn't last long once there was a critical mass of screwees. Today, the business is dominated by BS. So maybe that's where the inclination to regard any claim as BS comes from. I don't know.

Personally, I think Dynaco walked the walk. Almost all the speakers in the small-bookshelf category I heard in that era had, to varying degrees, loose and/or one-note bass. The A-25 didn't. Maybe all the reviewers who commented on the tight bass were wrong though, I don't know. Maybe Dyna was onto something. They certainly weren't scam artists.

 

[MrPeabody]

I don't have an issue with anyone debating the merits of the approach. It was, after all, 1969 when this speaker reached the market, and technology has advanced a lot. What I take issue with is accusing (directly or otherwise) people who aren't around to defend themselves of not doing what they explicitly said they did. There's a word for what they were being accused of: Fraud. Dynaco was one of the most respected names in audio and a lot of the people who have five figure systems today- and a lot of the members of any audio board- probably started with them.

As far as acoustic output through the port, I suspect it was, at the very least, attenuated significantly by passing through tightly stuffed fiberglass, compared to your garden-variety bass reflex tube. The larger two-chambered models (A35, A40XL, A50) probably had deep bass behavior more like the acoustic-suspension competition, since that is essentially what they were.

Remember, word of mouth was crucial back then. This was before you could con people into paying thousands of dollars for a power cord by giving a manipulated demo at an audio show, or literally stick a $599 CD player inside an outer enclosure and charge $3K for it. Not to say there weren't flim-flammers back then, but they usually didn't last long once there was a critical mass of screwees. Today, the business is dominated by BS. So maybe that's where the inclination to regard any claim as BS comes from. I don't know.

Personally, I think Dynaco walked the walk. Almost all the speakers in the small-bookshelf category I heard in that era had, to varying degrees, loose and/or one-note bass. The A-25 didn't. Maybe all the reviewers who commented on the tight bass were wrong though, I don't know. Maybe Dyna was onto something. They certainly weren't scam artists.

All that anyone has said is that they have some difficulty believing that it was practical (or particularly beneficial) to fine-tune the acoustic impedance of that big hole for each individual speaker that passed through the assembly line. For my part, I've said that I am somewhat skeptical of this but allow that it is possible. I think that for you to say that it is tantamount to accusing someone of fraud, when I or anyone else expresses skepticism that this was actually done, is taking it over the top. The reasons to be skeptical are strong and manifest. Perhaps the truth is somewhere in the middle. Perhaps there was a period of time, early in the production of the speaker, when this was actually done for a while because the optimal amount evaded theoretical calculation. Perhaps, because this had actually been done for some short while at the beginning of production, this gave someone the sense that it was okay to allow the public to think that this was done for each and every speaker that came through production. Obviously I do not know what the truth is, but if I had to take a guess, my guess would be that there is an element of truth to it but that it is not particularly likely that this was done for every speaker that came down the assembly line.

I want to comment specifically on this:

"As far as acoustic output through the port, I suspect it was, at the very least, attenuated significantly by passing through tightly stuffed fiberglass, compared to your garden-variety bass reflex tube."

It seems obvious by intuition that there wouldn't be any point to the big opening if it did not allow acoustic energy to escape from the enclosure.

Certainly the acoustic output through the opening was diminished via the acoustic impedance (stuffing) placed over the opening. However the function of this opening in the enclosure was not even a tiny bit similar to the function of a port in a ported, "bass reflex" speaker. The similarity is superficial. In a true ported, "bass-reflex" speaker, the Helmholtz resonator principle is used in order to introduce a secondary resonance lower in frequency than the primary resonance. In a true ported speaker, the air inside the port tube is analogous to a mass suspended on a spring; the air inside the enclosure provides the spring. The net effect is a mechanical oscillator, tuned to a particular frequency. The rationale for doing this is that it increases the efficiency, and thereby the sensitivity, at frequencies very close to the frequency of oscillation. There are disadvantages, which I will not go into here, because I'm only wanting to point out that any comparison between the aperiodic speaker and a ported speaker is a meaningless comparison. They are completely different approaches that have very little in common.

The aperiodic speaker does not use the Helmholtz radiator principle. As such, it is more like a sealed speaker than a conventional ported speaker. In fact, the difference between the single-chamber aperiodic speaker and an acoustic suspension speaker like the old Advent is a very slim difference. As elucidated by Jeff Bagby in the comment that Dennis quoted, you can model an aperiodic design using a sealed speaker model/simulator that allows you to specify the enclosure leakage. If you do this and make the leakage much greater than it would be with any real acoustic speaker speaker, you will then be modeling an aperiodic speaker. Said differently, the single-chamber aperiodic speaker is equivalent to an acoustic suspension speaker with a big leak. The net effect is that, owing to the diminished spring effect of the air in the enclosure, the enclosure does not exacerbate the driver resonance to the extent that it would if the enclosure were tightly sealed. In other words, something partway between an acoustic suspension speaker and a raw driver suspended freely in the open air. This really is all that the aperiodic speaker really is. Imagine a spectrum with a true acoustic suspension speaker at one end of the spectrum while at the other end, a raw driver suspended freely in open air. The acoustic impedance that covers the big opening has the final say over the particular point on this spectrum where the aperiodic speaker falls.

In the design of an acoustic suspension speaker, the enclosure volume is chosen to match with the driver's Qts and Vas, such that the completed speaker's Qtc will be close to the value deemed optimal, which, depending on who you ask, should be somewhere between .5 and .9. (In practice, the enclosure volume is usually targeted and the driver is chosen accordingly.) The Qtc of the speaker is an expression of the severity of the primary resonance of the speaker. The primary resonance is directly consequential to, and closely associated with, the primary resonance of the driver itself, shifted higher in frequency and made more severe by the air spring effect of the enclosure. Thus, if Dynaco fine-tuned the amount of acoustic impedance covering the opening, for each speaker coming off the assembly line, what they were doing is making certain that the Qtc of every speaker was very close to the Qtc value that someone at Dynaco had decided was optimal.

The big opening effectively increases the enclosure compliance, thereby increasing the effective size of the enclosure. This effect is not substantially different from the effect of using a larger enclosure, or the effect of using a different driver with lower Qts and/or higher Vas. There is however one very important difference. The important difference, between an aperiodic speaker and a speaker made using the same driver but using a larger enclosure, is that with the aperiodic approach, the bass that escapes through the big opening (without which the aperiodic design has no rationale) is out of phase with the direct acoustic wave coming off the driver. The same as with a raw driver suspended in open air, the destructive interference increases in strength as the wavelength increases (frequency decreases). It is entirely apparent that the aperiodic speaker is sonically inferior, in an absolute way, to a larger speaker using the same driver, where the large sealed speaker and the small aperiodic speaker share the same Qtc. The small aperiodic speaker takes up less space in the room, but the bass you get from it is diminished in comparison to the bass you get from the larger, sealed speaker that has the same Qtc. The aperiodic design is merely a way to make the enclosure smaller at the expense of bass. Or, looked at the other way around, it is a way to use a driver that is more efficient than a driver that would work in the given enclosure would otherwise be. But the advantage in efficiency is not real, because of the destructive interference which becomes stronger as the frequency moves lower.

 

[Keith Conroy]

I don't have an issue with anyone debating the merits of the approach. It was, after all, 1969 when this speaker reached the market, and technology has advanced a lot. What I take issue with is accusing (directly or otherwise) people who aren't around to defend themselves of not doing what they explicitly said they did. There's a word for what they were being accused of: Fraud. Dynaco was one of the most respected names in audio and a lot of the people who have five figure systems today- and a lot of the members of any audio board- probably started with them.

As far as acoustic output through the port, I suspect it was, at the very least, attenuated significantly by passing through tightly stuffed fiberglass, compared to your garden-variety bass reflex tube. The larger two-chambered models (A35, A40XL, A50) probably had deep bass behavior more like the acoustic-suspension competition, since that is essentially what they were.

Remember, word of mouth was crucial back then. This was before you could con people into paying thousands of dollars for a power cord by giving a manipulated demo at an audio show, or literally stick a $599 CD player inside an outer enclosure and charge $3K for it. Not to say there weren't flim-flammers back then, but they usually didn't last long once there was a critical mass of screwees. Today, the business is dominated by BS. So maybe that's where the inclination to regard any claim as BS comes from. I don't know.

Personally, I think Dynaco walked the walk. Almost all the speakers in the small-bookshelf category I heard in that era had, to varying degrees, loose and/or one-note bass. The A-25 didn't. Maybe all the reviewers who commented on the tight bass were wrong though, I don't know. Maybe Dyna was onto something. They certainly weren't scam artists.

 

[Keith Conroy]

Its hard to keep up with all the information being traded on the A-25 Speaker system. I started to reply to Aperiodic's last post, but then just gave him a like. I completely agree with the comment that this was a different time in Audio History. From a science & technology standpoint things were very different. I do also agree using the words, "Your accusing someone of Fraud is pretty strong language"??

Without splitting hairs on how and why Dynaco got there? It seems clear the A-25 was a very successful speaker of that era. It also generally got good reviews. I heard them and thought the bass detail sounded very good. It doesn't really matter what I think. However they sold a crap load of speakers, I must not have been the only one enjoying them?

Design is always about trade-offs. There is no way we will ever know the full story behind why Dynaco chose Aperiodic loading. Some of the historic quotes were provided. However no one commenting worked for the company during this period. No one commenting was in the engineering project meetings during the A-25 system design. We also will never know completely how they did their QC on the A-25 over a number of years production?

In my personal view, its pretty harsh to take the whole company Dynaco to task over a couple paragraphs. This information is what 40 years old plus? Usually there is always a back story on such things. From all I read about David Hafler he was a talented engineer and a pretty straight up guy that really liked audio............I would just say "THX Dynaco for producing a very good speaker during this time in audio history!!

 

[MrPeabody]

I prefer the smooth, natural bass rolloff of the old acoustic suspension speakers. Ported speakers generally, with the notable exception of true subwoofers, exhibit a more abrupt bass rolloff, which to my ear is not substantively different from a resonance located at the "knee" of the rolloff. Rarely have I ever heard a ported speaker where this effect was not conspicuous to my hearing and a strong deterrent to my enjoyment of the speaker.


One of the reasons that I had been looking forward to the measurements of this speaker is that I was curious as to whether the harmonic distortion in bass would be any better than it typically is with a typical modern speaker that relies more strongly on the driver suspension to supply the centering force for the diaphragm. The theory that has long been expounded, in support of acoustic suspension speakers, is that the spring effect of the enclosure, being a good deal more linear than the spring effect of the driver suspension, leads to reduced bass harmonic distortion. This idea has long appealed to me, but I had never seen any objective proof that it materializes in practice. We do not see any evidence of this in Amir's measurements of the distortion of this speaker. I won't draw any rigid conclusions from this one example, but it does suggest that the suspension's contribution to distortion is secondary to the contributions of other sources, such as the motor, and possibly the lack of rigidity of the cone. With respect to the motor, lack of constancy and linearity in the effective coil inductance is a well-known, significant source of harmonic distortion. Which of course is why the better modern drivers generally use shorting rings on the interior pole in the vicinity of the coil. I am thus inclined to speculate, based on what I see here, that the improvement in the linearity of the restoring force, to the extent that it is improved with an acoustic suspension speaker, is not adequately sufficient to compensate for the other, more dominant sources of non-linear distortion. At least this is what the results we see here suggest.


Buy the hay, for anyone looking for a modern off-the-shelf driver suitable for use in an acoustic suspension design, there are a handful that are fully useable. A year or so ago I was able to identify about ten drivers that are very good choices for acoustic suspension use, where the theoretical F3 will be between 35 Hz and 40 Hz (with enclosure volume chosen for Qtc in the ballpark of .7). These drivers range in size from 10" to 15", and the required enclosure volume covers a broad range. It turns out that the true difference between these drivers and the drivers of yore is efficiency. As long as you don't object to a speaker with somewhat lower efficiency compared to the early acoustic suspension speakers, these newer drivers are actually better in most all other respects, including the theoretical F3. Note that in order to insure that the enclosure will not be larger than you may desire, it is necessary to consider Vas alongside Fs and Qts. The apparent need for Vas to be small (i.e., for the suspension to be stiff), in order that the enclosure will be small, is contrary to intuition and is a misleading consequence of the way the Thiele-Small parameters are defined and the consequential formula you use to calculate the volume. The formula for calculating the enclosure volume involves the driver's Qts, and Qts depends partly on the stiffness of the driver suspension, thus allowing Vas to enter the calculation surreptitiously. In general for the drivers in the list I came up with, both the theoretical F3 and the theoretical enclosure volume are small compared to the old Advent (for Qtc slightly smaller than it is with the old Advent). You end up with a lower F3 and with a smaller enclosure, and even with less of the Qtc-related mid-bass response hump. But there is no free lunch; the price you pay is lower efficiency and lower sensitivity. The list I came up with, that will give these desirable results, is a list of six drivers: Dayton RSS265HF-8, SB Acoustics SW26DBAC76-4, Scan-Speak 30W/4558T00, Peerless XXLS-300F50AL01-04, Dayton RSS315HFA-8, and Scan-Speak 32W/4878T05.

If I still owned the pair of Advent 5012 that I sold in 2009, I'm not so sure that I would re-foam the woofer. Re-foaming the woofer would be the least costly option, but somewhere between doing that and buying a brand new pair of costly speakers, I would consider replacing the woofer with the RSS265HF-8 and replacing the tweeter with a modern tweeter, perhaps the Seas DXT. The crossover would obviously have to be completely redone. This would be significantly more costly than a simple re-foaming of the woofers and a mild refurb of the crossover, but I expect that if this were done properly by someone with the right skills, the result would be a significantly better speaker. But less efficient.

 

[MrPeabody]

Its hard to keep up with all the information being traded on the A-25 Speaker system. I started to reply to Aperiodic's last post, but then just gave him a like. I completely agree with the comment that this was a different time in Audio History. From a science & technology standpoint things were very different. I do also agree using the words, "Your accusing someone of Fraud is pretty strong language"??

Without splitting hairs on how and why Dynaco got there? It seems clear the A-25 was a very successful speaker of that era. It also generally got good reviews. I heard them and thought the bass detail sounded very good. It doesn't really matter what I think. However they sold a crap load of speakers, I must not have been the only one enjoying them?

Design is always about trade-offs. There is no way we will ever know the full story behind why Dynaco chose Aperiodic loading. Some of the historic quotes were provided. However no one commenting worked for the company during this period. No one commenting was in the engineering project meetings during the A-25 system design. We also will never know completely how they did their QC on the A-25 over a number of years production?

In my personal view, its pretty harsh to take the whole company Dynaco to task over a couple paragraphs. This information is what 40 years old plus? Usually there is always a back story on such things. From all I read about David Hafler he was a talented engineer and a pretty straight up guy that really liked audio............I would just say "THX Dynaco for producing a very good speaker during this time in audio history!!

It is good to point out that these speakers were all designed 50+ years ago. For those of us who were around back then, looking at these speakers today is analogous to what it would have been like in 1970 to comment on speakers that were designed back around 1920.

That said, when Dennis used the descriptive phrase "nothing burger" to characterize the aperiodic thing, this was on the mark. When Aperiodic took umbrage at the characterization, I was motivated to second what Dennis said and explain the reasons. The aperiodic thing has bugged me since the first review of the A-25 I read in Stereophile so many years ago. It was apparent to me even then that it was hokey. Even then, I immediately concluded that all this could actually accomplish is to allow the enclosure to be smaller than a tightly sealed enclosure would need to be with the given driver, thus raising the question of why a different driver weren't used. When there is a question this obvious and it isn't ever answered or even mentioned, something smells fishy.

Something that has long been common with entrepreneurs in the audio field (and similarly in other areas of applied technology), is the practice of exaggerating the significance and uniqueness of any attribute that will potentially differentiate the product that someone wants to promote, from the competing products. This practice is pervasive, and we take it for granted. But I've never discerned a reason why other people should be expected to play dumb and refrain from pointing out when a claim that someone has made doesn't hold water. Are we supposed to continue playing dumb fifty years later? Should we continue indefinitely to bite the lip, or does a time eventually come when it is appropriate to say plainly what the thing truly is?

Other than the convenience of the small size of the enclosure, what advantage does the aperiodic approach have over a larger speaker using the same driver and with the same Qtc? There is no advantage other than the convenience of small size, while there is the significant disadvantage of destructive interference between the wavefront directly from the driver and the wavefront exiting the big opening.