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AR 2ax and flat energy response


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I just picked up a pair of AR 2ax's in beautiful condition. The cabs are almost spotless.

All the drivers have been replaced at some time with original AR drivers.

The woofers in 1988 and the tweeters and mids in 91. I got these from the original owner and he gave me the service bills from the official AR dealer at the time. Not sure why the drivers were all replaced.... the owner said he never played them loud but he brought them back to the dealer at regular intervals and they recommended the replacements!

Anyway, the woofers and mids look like the ones in original AR brochures from that era but the tweeters have the little metal tabs on the front and are wired from the rear. The original terminals for the front wiring are still there. They are AR but newer looking than the tweets in my LST's..... like the ones in my AR12's .

Questions.... how do these newer tweets compare to the originals?

What is meant by 'flat energy response' in the AR literature as opposed to frequency response?

Were the published freq resp curves measured at the 'normal' or the full up positions of the pots?

Where can I get new 15 or 16 ohm pots with the same power rating as the ones used in the vintage AR speakers?

I'll send photos later in the week.

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What a wonderful can of worms this topic opens up. IMO, it goes to the heart of what high fidelity and accuracy are about. It's an issue Vilcher and Kloss struggled with nearly sixty years ago while most other manufacturers don't have the insight and haven't considered it to this day. And the design of their products no matter how expensive or complex in their execution reflects it.

We've discussed the fact that in the early days of stereophonic sound in the late 1950s and early 1960s, many recording studios used Altec A7 Voice of the Theater speakers as studio monitors. I met one salesman at a trade show who bragged he had just sold 58 to Columbia studios. These speakers had a harsh peaky treble and so when recordings were equalized to sound balanced on them, they sounded dull on more accurate speakers like ARs.

Look at any tweeter response and you will see that as frequency increases, the off axis response falls faster than the on axis response. This means that the total radiated power cannot be flat unless the response is electronically compensated for. But if it is, the on axis response will have a peak so you can't have both flat on axis response and flat total power response from a single tweeter simultaneously. Then compare a tweeter with a woofer and a midrange and you will see that they are almost omnidirectional relatively speaking. Why is this important? Because much of what we hear from loudspeakers in real rooms is reflected off of other surfaces and if the total power response reaching our ears isn't flat, the tonal balance of instruments won't sound accurate. But even if the radiated response were flat, the characteristic of many materials to absorb high frequencies selectively would still make the total power response reaching our ears less than flat. (I've been working on this problem for 16 years.)

In a concert hall, instruments sound very different than in a small room. Not only is the overwhelming majority of what we hear reflected sound but as the reflections die out, the high frequencies die considerably faster than the middle and low frequencies. This is evident from the lower RT at hf than and midf alnd lf. This changes the subjective tonal balance of the instruments making them overall more mellow and is one reason we like hearing live music in preference to recordings. Unless this reverberant component is also reproduced, that tonal quality cannot be duplicated, they are inseparable. And sadly, that ability is currently well beyond the state of the art. (I've also been working on this one for 31 years.)

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  • 1 month later...

Ok, then are the published Freq res curves published by AR composites of several microphones around the rooom?

Does that infer that the freq res curves for other brands are single point front mic'd only?

I do find that my advents are brighter but not necessarily in the highest range but in the mid/upper mids. Does anyone have a freq response plot for the original large Advents so I can compare with the published curves I have for the LST's.

My newer AR's are also brighter than my older ones IE my 12's,48's 58's are brighter than my LST's,AR2ax's and my AR4x's. Did AR succumb to the brighter/louder marketing strategy?

My 'relatively newer' speakers are several different models of Paradigms and Angstroms...Canadian brands which use 8" bass drivers. These have been well reviewed. All have subjectively more bass than the AR's and the high end is much like my newer AR's relative to the mid and bass. These speakers were designed using our government testing facilities and have very flat response curves. I suppose some of this (bass)has to do with baffle step compensation...yes? (I have yet to build Pete B's BSC network for my Advents)

Do speaker manufacturers routinely design in Baffle Step Compensation and if so approximately what year would that have started.

Although my newer AR's are brighter than the classics, the bass is essentially the same. To get somewhat the same (level) bass as my Paradigms I listen to the AR's against a wall. They sound too thin out in the room, where the newer Paradigms sound better away from walls.

Sorry if I seem to be rambling, but I'm having trouble finding a single affordable 'perfect' set of speakers ....... LOL :-)

ps: Those of you who have done your own freq res testing, what equipment do you use and how/where do you do this. I'm assuming few of you have anechoic chambers.... LOL Any time I have done any measurements, the curves were wild and did not look anything like published curves and had huge peaks and valleys.

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I think AR's early published FR curves were for the most part on axis anechoic chamber measurements although later, AR used highly reverberant test chambers to measure total radiated power but I don't recall that these were every published. Perhaps someone who worked at AR could comment. Other testers such as those in the hobbyist magazines of the day used an average or composite of room response down to low frequency where they spliced it to a near field measurement. I think Hirsch-Hauck averaged 8 such measurements. What results you get are highly dependent on HOW the measurements are made and the correlation between perceived tonal balance and measured frequency response is IMO something the industry has never satisfactorily addressed, at least to my way of thinking. My own experiments indicate that as total power transfer gets flatter or total power radiated gets flatter for that matter, there is a qualitative difference which cannot be duplicated by altering the on axis frequency response of a speaker which radiates its energy more irregularly with frequency, especially one which radiates more narrowly with increasing frequency. Wider dispersion translates IMO into sound which is less shrill and more musical even when the spectral balance is on the brighter side. This is especially important with compact discs which many complain sound too shrill or harsh. The tendency towards brighter speakers in the market may be due to the observation that they are more ear catching in rapid fire AB comparisons by inexperienced listeners in typical demo rooms and therefore tend to sell better. Once the purchaser inevitably become fatigued by them, they rarely simply recognize their mistake and trade for something more accurate, they search enelessly for ways to mitigate the problem by buying exotic wires, vacuum tube amplifiers, and other components with a hi frequency rolloff. This has lead to the rise of cottage industries which have done very well meeting this demand. And of course, the last thing in the world these people would think of is using an equalizer to fix the problem. They are convinced that this will cause unacceptable distortion even though they are pervasive at nearly every level of recording and playback in their favorite medium, vinyl phonograph records.

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>I think AR's early published FR curves were for the most part

>on axis anechoic chamber measurements although later, AR used

>highly reverberant test chambers to measure total radiated

>power but I don't recall that these were every published.

From the very earliest days, AR published both on- and off-axis frequency-response curves. It is not just the broad dispersion that was important, but the *smoothness* of that off-axis dispersion. On-axis performance, while very important to many of today's loudspeaker testers, and with the advent of gaited measurement techniques, was not as important to AR as the integrated-power response. Integrated-power response, above about 200 Hz, was measured in the reverberant-test chamber. Some power-response measurements were made in anechoic chambers with (averaged) different microphone positions, but the reverberant chamber was best for this purpose. AR published the integraged-power curves as well.

The example below is of the AR-3a 3/4-inch tweeter, on- and off-axis. These were anechoic-chamber curves, for sure, but still demonstrated the excellent performance of this tweeter. I am not aware of any other tweeter, other than the Allison tweeter, that can match the off-axis performance of this tweeter.


Fig.1 AR-3a on- and off-axis anechoic response curves

>own experiments indicate that as total power transfer gets

>flatter or total power radiated gets flatter for that matter,

>there is a qualitative difference which cannot be duplicated

>by altering the on axis frequency response of a speaker which

>radiates its energy more irregularly with frequency,

>especially one which radiates more narrowly with increasing

>frequency. Wider dispersion translates IMO into sound which

>is less shrill and more musical even when the spectral balance

>is on the brighter side.

I really have to agree with this observation as well. Speakers with extended or relatively flat integrated-power response always sound more natural, less-shrill, than those with powerful and relatively flat on-axis response but diminished off-axis response. You are required to listen in a "sweet spot" to be able to enjoy the more directional speakers, and they tend to bring on the old dreaded "listener fatigue" over time.

--Tom Tyson

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The ability to produce sound only down 5db from 15 khz to 18 khz 60 degrees off axis is remarkable, practically unique even by today's standards. Even the most expensive tweeters such as the Scanspeak D2905/9900 Revelator ($800 per pair) find it impossible to match. Nevertheless, in comparison to radiation at lower frequencies, even this is not ideal. LST recognized this limitation not only providing greater hf power handling but better dispersion as well. Were this not considered important, the drivers could have all been mounted on a flat baffle instead of angled side panels. In this regard, LST was IMO AR's greatest loudspeaker because it recognized the problem and made a serious effort to deal with it. Alison and others plagerized the solution to wide acclaim.

One of the most neglected aspects of high fidelity sound reproduction in the home is the study of how speakers launch their sound geometrically into space and how interaction with the room determines the perception of sound. Speaker manufacturers speak in generalities but specifically, their designs don't reflect profound knowledge of it and performance does not usually accurately mimic the sound of real musical intruments even as they would be heard in the same room. Within the limited paradyme of 2 channel stereophonic sound reproduction, a format which surprisingly has lasted this long, there are still real improvements to be made but most manufacturers seem to be playing it safe with a endless variations of me too designs.

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>The ability to produce sound only down 5db from 15 khz to 18

>khz 60 degrees off axis is remarkable, practically unique even

>by today's standards. Even the most expensive tweeters such

>as the Scanspeak D2905/9900 Revelator ($800 per pair) find it

>impossible to match.

Compare the response of a hand-made Hiquphon OW1 3/4-inch dome tweeter (one held in the highest regard, incidentally) to that of the mundane AR-3a 3/4-inch tweeter. The vertical division resolution of the Hiquphon tweeter graph is 10-db-per-major division, which favors a "smooth" appearing response, whereas the AR resolution is 5-db-per-major division, showing more squiggly motion. Both measurements are anechoic-chamber measurements with the tweeters radiating from flat baffles into half space. The important thing to observe is the off-axis response of the AR-3a tweeter compared to the OW1. It is whole magnitudes better off-axis than the otherwise superb OW1 tweeter -- the reason: the OW1 uses a "semi-horn" to extend the upper response efficiency and on-axis response, at the expense of off-axis response. It is fairly difficult to have one with the other.


Fig.1 The excellent (on-axis) Hiquphon OW1 3/4-inch hand-made, fabric-dome tweeter.


Fig.2 The mundane but also excellent (on- and off-axis) response of the hard-dome AR-3a 3/4-inch tweeter.

In my opinion, the OW1 (as superb as it is) is not comparable to the AR-3a dome at wide off-axis angles. It is hampered by the recessed location of the dome below the top plate and the appearance of a "semi-horn" area around the mouth of the dome. The design goals were very different, and the resultant sound reflects the difference in design approach.

--Tom Tyson

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The OW1 is down about 4 to 5 db 30 degrees off axis in the 15 khz range, about what the AR tweeter is down 60 degrees off axis. Since falloff is usually very rapid beyond a certain angle you'd expect the OW1 to be down at least 10 to 15 db by 60 degrees very significantly poorer than the AR tweeter. (It is interesting that most audiophile speaker systems today use 1" tweeters not 3/4" tweeters which you'd expect to be even worse.) While the AR tweeter is excellent on a relative basis, it is less than ideal on an absolute basis. The total radiated energy is still far from flat and selectively reflective properties of many surfaces would make the total energy transfer between the tweeter input and the listener even further from flat. The additional tweeters in LST can help flatten direct radiation off the main speaker axis but unfortuantely they all have the same energy response and therfore cannot compensate for reflected total power response errors. The most popular solution today is to pull the speakers away from the walls and/or apply absorbtive material to surfaces behind the speaker on the front walls. This absorbs indirectly radiated middle and low tones removing them to some degree from the reverberant field and flattening overall power response. A better result can be obtained by providing additional indirect tweeters having a rising power output with frequency flattening the total energy transfer response. This preserves the acoustics of the room and is much closer to the way many musical instruments radiate energy and sound in real rooms.

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>(It is interesting that most audiophile speaker systems today use 1" tweeters not 3/4" tweeters which you'd expect to be even worse.)<

That was a comment I made to Ken sometime back when he said that drivers had gotten better over the years. My reading of AR's literature said that the physics involved dictated the vanishingly small tweeters.

I know you can fool mother nature sometimes, but overcoming this size vs wavelength issue didn't sound like something you could do with a better voicecoil, dome, whatever.

I thought you and Tom might get a kick out of reading this:


Seems there's nothing new under the sun, even when it's new. As I was reading it I thought, "Roy Allison must be very proud."


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Thanks for the reference Bret, a most interesting article and one that gives tremendous insight into what some of the best thinkers in speaker design are doing. Revel is probably one of the few places where meaningful research is actually happening in the advancement of high fidelity today. Here's some excerpts of what Kevin Voecks, Director of Technology at Harman Specialty Group had to say. I recommend reading the entire interview.

"When I talk to other engineers in the speaker field, it almost feels like, gee it’s not fair to have such phenomenal resources at our disposal! Literally, no one else does."

Most other speaker designers from what I gather from their products are tinkerers trying anything and everything, at best tweaking ideas that have been around a long time.

This high powered audio think tank reveals a lot of things I have to agree with in this article.

"the importance of both on-axis response and off-axis response, and power response were pretty unheard of. Still are really!"

"Sumit: I find that quite commendable. When designing a loudspeaker what are the three most important aspects of a loudspeaker according to you that one must get right?

Kevin: Timbre is the overwhelming aspect. Based on our blind listening tests timbre is the thing that differentiates between good and bad loudspeakers, but also between good and great loudspeakers. So timbre is kind of a broad term. It incorporates balance, frequency balance, or it can be thought of very roughly as frequency response. That’s a little dangerous because, not to infer that one on-axis measurement tells you what the frequency response is in a loudspeaker. It doesn’t. Other areas like off-axis response are very critical and we’ve learned that very far off-axis response like 60-75 degrees is very critical. Almost no one even measures it, let alone designs loudspeakers that are optimized at that sort of angle."

"And then the power response, the reverberant field that we hear a little later in time is also important. So we literally design for all of those areas: the direct sound, the first reflection sound, and the reverberant field, because we know that all those three things are huge contributors to the timbre, to our perception of the speaker’s timbre."

"Sumit: Do you feel that time-coherence is an important property of a loudspeaker?

Kevin: No. We’ve done a lot of research in the area. In fact when I made my first loudspeakers at Symdex in 1976, I thought that it was important because that was the fad at the time. With some people, it’s still a fad. But if you look at the way the human hearing functions, you’ll see that time-coherence isn’t important."

"Kevin: Listening rooms are our biggest problem, clearly, and it’s the combination of the way that the loudspeakers interact with the room and the room characteristics overall."

Lot of interesting ideas here and food for thought. Putting the products AR produced as the result of their research and what is being cranked out today, it's easy to see why these old products have stood the test of time for so long.

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