Feature Article

Designing Loudspeakers At Revel -
An Interview with Kevin Voecks, Director of Technology, Harman Specialty Group (Revel, Mark Levinson, Lexicon)

April, 2004

Sumit Chawla

Following is an interview between Secrets Senior Editor, Sumit Chawla, and Kevin Voecks, Director of Technology at Harman Specialty Group.

Sumit: Could you please talk about how Revel started?

Kevin: It started because Harman had been purchasing companies that were the best in their area for quite a few years, and it's a way that Harman became best in category in so many areas. There are numerous pro-audio companies, for example, and you just can't find a studio without Harman brands (both our Pro Audio) and very well known consumer brands. At the same time Mark Levinson, one of the Harman companies, is known all over the world as the best in audio electronics. It's quite remarkable how uniform that is, and the company could point to Mark Levinson and say we have the best electronics, and they are known as such.

At the same time, while there are (Harman) brands that are extraordinarily successful in loudspeakers, both in consumer and pro, in most audiophile markets they were not known as �the best�. So the question is: how do you go out and acquire a company like Mark Levinson that combined in a unique way, real engineering, and the soul of high-end? You know, understand it; getting it about high-end! There just wasn't a speaker company that really had both elements - not honest to goodness engineering, rigorous real engineering, and really having the attitude of real music lovers - of sensitivity to the finesse that is required for really great sound quality. Fortunately such a company didn't exist, and so Dr. Harman had to create one within the organization. Revel is only the second company that Harman has ever created from scratch. Harman Kardon is the only other one. All the others were acquired.

Sumit: When and why did you join Revel?

Kevin: That situation was an opportunity to start up a new company, but rather than doing it in a traditional way, you know, on a shoestring, we were able to start up with the best engineering resources at our disposal in the world, both in terms of facilities, know how and engineering experts throughout the field, experts that often have specialized knowledge, certainly transducer design and even specialized within transducer design and materials. I mean, you name it! (For example), On the mechanical side there are experts in the organization, and they are all available to Revel, at our disposal. So it was a unique opportunity to create a high-end speaker company with the goal of simply creating the best sounding loudspeakers. In fact, that's the only thing Dr. Harman said at the inception of the company - �Make the world's best loudspeakers.� No restrictions on the kind of technology. For example, if we had decided that electrostatics were best, even though that's a completely different kind of technology than you've seen in the Harman loudspeaker brands. There were absolutely no restrictions. We determined with double blind listening tests and engineering research that dynamic loudspeakers although refined, in numerous ways, make for the most accurate loudspeakers.

Sumit: Were you designing loudspeakers from the start of your career?

Kevin: Yes. That started with a little company I had called Symdex. I had a high-end HiFi store while I was in college called �Natural Sound,� who are still one of our dealers in Massachusetts, and I was going to school with a specialty of designing loudspeakers. I started a little speaker company because it was very obvious, having the store, where the problems were in the systems. Loudspeakers were the big issue, and so I did some research and came up with a speaker. (Later) we started up Mirage loudspeakers in Canada, and then Peter Snell had an untimely death. He had been a good friend since my retail days when we sold his loudspeakers, and his work was far ahead of his time. Many of the beliefs that he had clear back to the 70s were against the main beliefs in the audiophile world, but most of them turned out to be right. And now that we have a lot of years of research behind us, that's proven to be the case. Things like high-order crossovers, and things like being concerned about phase in crossover regions in transitions, but not overall absolute phase kind of issues, the importance of both on-axis response and off-axis response, and power response were pretty unheard of. Still are really! They are all things that we've refined for years and been able to make much better loudspeakers because of that. I became the chief engineer for Snell for a number of years before Revel.

Sumit: How has your experience at Revel been compared to the previous companies you were at?

Kevin: I've been used to working on a shoestring, certainly relative to at Revel. Revel is almost an embarrassment of resources. 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. It's very exciting!

Sumit: Could you elaborate on the research facilities that are accessible to Revel?

Kevin: Absolutely. Before I was at Revel I was using the Canadian National Research Council because it had the best measurement facilities and Dr. Floyd Toole's work there was absolutely best in the field. He went to Harman and continued that great research along with Sean Olive who is now in charge of subjective testing or double blind listening tests at Harman. That's just a priceless resource because the bottom line is how something sounds. Normally listening tests are done in a way where it's anecdotal and when you do double blind listening tests, you eliminate the variables, and it's certainly about how the speaker sounds.

Because of having our pneumatic speaker mover, each of the speakers that we are listening to is precisely in the same physical location, so that endless problem that speaker designers have always had about how you compare two loudspeakers technically is completely eliminated. They are in exactly the same place, and they are switched so quickly that our physical aural memory is still intact, which is to the shortest of all our senses in that our aural memory only lasts a few seconds. It means that we are able to do more refined comparisons. You can discern smaller differences reliably when you do the really fast switch. Some people are concerned that rapid switching isn't their style--they like to listen for extended periods. The way we do listening, you can do it anyway you want. If you want to listen for half an hour before you switch, that's fine. There's no pressure to do it otherwise in the Revel listening test.

The ability to do accurate measurements is really critical. There's no replacement for a large real anechoic chamber. Basically the industry is using pseudo-anechoic measurements that are achieved electronically, and they don't work in that they throw away so much resolution depending on how large the area is in which you are measuring that you miss critical data. Not only do you get inaccurate low frequency measurements, but you also have problems with not being able to see resonances, or evidence of resonances.

Some of the research we've done includes the first really good research into the audibility of resonances. We need to know under what circumstances are they audible, and then we can do our design work so that we can make sure that they are below the audible threshold. This is one of countless areas where we've had to determine what's the threshold of audibility, and that's never a simple number. It's usually frequency and amplitude dependent, so we need to know all the circumstances. For instance, the audibility of resonances is dependent upon the kind of music. Resonances can actually be louder than the average music level for things like female pop vocals before we first hear them. Whereas for classical music recordings they can be at a much, much lower level, well below the average level of the music and you still hear them. The most sensitive listening test for resonances is pink noise, and so that's our standard. We want to make sure that the resonances in Revel loudspeakers are inaudible even with the most demanding possible test: pink noise.

Sumit: How well do the measurements correlate with the actual blind tests?

Kevin: That's a perfect question. Because we actually have come up with measurements based on listening tests, it's a full circle. We don't do measurements because we have cool equipment to do it, we do them only because we found an audible characteristic that will differentiate two speakers or one that's more refined.

So how do we measure that? How do we quantify that so that we can really engineer the speakers to perform better in that regard. So there's extremely good correlation between the measurements that we do, the set of measurements, and the sound quality. Looking at a set of measurements, if they're not good, we can say absolutely for sure that the speaker will not excel in listening tests. The converse is not completely true. Looking at a good set of measurements we could say this is a good speaker, but the difference between a really good and really great speaker is still beyond our ability to absolutely predictably measure. So it's that last level that we spend a lot of time on because it's not as direct a path.

Sumit: What is the selection process for the individuals who are to participate in the listening test? Do they undergo any training?

Kevin: Sean Olive has created a listener training program that makes listener's responses far more valuable to engineers. For example, instead of someone saying that it sounds �fuzzy in the bass,� they learn how to recognize frequency ranges so that they might instead say, �It sounds like there is a resonance around 150 Hz.� That is much more useful feedback. Years of tests have resulted in a massive store of data that Sean can �mine� for a variety of important information. For example, we know that everyone with reasonably �normal� physical hearing ability prefer the same speakers. It's not about �personal preference� after all. In addition, we know that everyone likes the same speakers for a variety of kinds of music. There is no need for �rock� or �classical� speakers. Knowing this, listeners are chosen for their ability to quickly and consistently come to conclusions. Their consistency is an important measure of the validity of their listening tests.

Sumit: Do the different loudspeaker teams within Harman, like JBL and Infinity, do any form of collaboration?

Kevin: There is at least informal collaboration. If there are discoveries made by a design team, then that basic improvement is shared. There is also Dr. Toole's whole research group which is all corporate, its all shared information and any company can choose how much of it to utilize. Then there is the Rapid Integration Lab, another research group that is doing some cutting edge research with practical applications that we are developing now and will be more so in the future.

Sumit: Can other groups utilize the newly developed transducers that are used in the new Performa series?

Kevin: The transducer technology is available to any one of our companies. It's a question of on each model whether there is the budget to utilize all of the refinements, sophistication that we are using in the new Performa models, for example. The transducer engineers like to show-off what they can do, and Revel is the place where they can best express their capabilities.

Sumit: Revel is part of the Harman Specialty Group which includes the Mark Levinson and Lexicon brands. Is there any collaboration with those groups?

Kevin: Well, I work equally for Mark Levinson, Lexicon and Revel. And there is absolutely synergy there in terms of things like low frequency research, how best to do bass management, how to do room equalization, which will be an option in Lexicon processors, for example. And it's very important that the loudspeakers' capabilities and how they integrate to the room is taken as a whole with room equalization processors that are to be used by our electronics clients. So there is a lot of connection there. The research group in Northridge definitely works together with Lexicon and Levinson engineering groups in Bedford.

Sumit: Revel started out by introducing the Ultima series and has since been introducing and improving the Performa line. Could you please comment on the two lines?

Kevin: Well, Ultima was meant to be much like its name implies: Ultimate kind of loudspeakers where we took all of the technology and applied it as best we could to get the best result. Performa the concept was simple: how can we reduce the cost while having the smallest possible impact on sound quality? How can we make the absolute best loudspeaker for the money? And I think that because of our engineering knowledge, because we know what things are important and what things are less important, because we understand that thoroughly, we don't have to put as much money in places with lower returns. Instead we can direct everything, focus it where it makes the most difference. Obviously the first place where you save money is in the enclosure which is an important part of the cost of a loudspeaker, and so by spending less money on visual aspects we can best optimize the value. So we think that the Performa models are just a phenomenal value. We've done blind tests with speakers that are very well known at five times their price and we easily beat them. So they're really a great deal.

Sumit: The Ultima series have not been updated in some time. Given that, have they stood the test in time?

Kevin: They have. They still outperform the Performa models, and maybe more importantly, we continue to have speakers come in from other companies that are talked about, and we have yet to have anything challenge the Salons for example. So they retain their place in the sonic category of blind listening tests.

Sumit: The Salon is the most expensive loudspeaker in the Ultima series. Correct?

Kevin: That's correct.

Sumit: What do they cost?

Kevin: Salons are $20,000/pair approximately.

Sumit: If someone had said that you could design a loudspeaker that cost twice as much, could you have designed a better loudspeaker at that time?

Kevin: No. We actually have been getting demands for more expensive loudspeakers for years and we have absolutely resisted just putting in a lot more drivers and making bigger speakers and collecting money. We'll only make more expensive loudspeakers when there's a reason: they have to be more expensive in order to sound better.

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. But we've looked at real world situations and found that the all-important side-wall first reflection is a function of the speaker's output at that kind of angle in the vast majority of listening rooms. So it means that you're going to be hearing that kind of sound. You will hear it with a slight delay, and in many rooms without very much attenuation. So optimizing the response at that kind of extreme angle is very, very important. 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: How does the price point limit you in so far as achieving these three goals?

Kevin: The three goals of direct sound, first reflection and power response I see as one basic goal. And price ranges such as that of the Performa F32 don't really limit that very significantly until you get perhaps to 12-15 kHz where the Ultima speakers have a rear tweeter that comes in just to optimize the reverberant field, not for the direct sound or the first-reflection. That refinement is an example of something that isn't the maximum bang for the buck (so isn't included) in the Performa range.

Sumit: You mentioned high-order crossovers a while back. Revel speakers have always used high-order crossovers. Several companies take the opposite approach i.e. they use low-order (first and second order) crossovers. What are the disadvantages of using low-order crossover designs? Are there any advantages to using low-order crossovers?

Kevin: Well, we were fortunate enough to have done research that has allowed us to know, to really understand, what are the characteristics that are important to sound quality and what are the characteristics that have some value but less, and those characteristics that don't have any sonic value and that allow us to make the optimum choices in the design. That all points very, very strongly to high-order crossovers because high-order crossovers are necessary in order to have low distortion which is way up there on the list of important sonic qualities.

High-order crossovers are important to have good dynamic capability without compression. It would really shock audiophiles to see how much the response of most high-end loudspeakers changes at different volume levels. They are like completely different loudspeakers when played even at moderate levels, and it is something that is very directly measurable. So we really focus on making sure that not only is the timbre really accurate, but that it changes as little as possible over a very wide dynamic range. Plus the distortion is below the audible threshold; resonances are below the audible threshold because our research has shown those are really important things.

If we used first-order crossovers, we would degrade the off-axis response, and therefore the timbre, we would completely degrade the distortion characteristics, we would loose our dynamic capability, our freedom from compression because tweeters and mid-ranges are then getting signals that are outside the frequency range that they are really designed to handle. So it's really mostly heat, and that heat makes the voice coil impedance go up, and as a result of that the filter network is mis-terminated because it's not seeing the termination impedance it expects to see, and then the response of the crossover is impacted.

So what that means is that when speakers heat up, voice coils heat up, the crossover networks don't work right anymore and you get peaks and dips in the response, several dB peaks and dips in the response. So with high-order crossovers and with all the things that we are doing in the transducer design to keep the voice coils cool which means we are generally using a very large voice coils which spreads out the heat over a large area, and we're using in some cases multiple woofers to further spread out the heat. We are using all of these techniques including the way we vent them which forces the air through the gap at the same time, the vents are designed in such a way so that they don't create noises. It's a very sophisticated approach to solving the problem with heat. But a big part of that is the high-order crossovers. It's an essential part of it.

Sumit: What crossover slopes are typically employed in Revel designs?

Kevin: Our networks are always tweaked to result in the smoothest possible transition between transducers. Because of that, their electrical characteristics don't meet the textbook definition of classical filters, which is why we don't specify them in our literature. However, the resulting response of the transducer/enclosure acoustic and the filter's electrical response is close to a 4th-order (24dB/octave) Linkwitz-Riley characteristic.

Sumit: Have you ever heard a good loudspeaker that uses a low-order crossover?

Kevin: I have heard good loudspeakers that use low-order crossovers. I haven't heard great loudspeakers that use low-order crossovers. They run into these problems. It's inevitable.

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. I actually credit Dr. Stanley Lipshitz at the University of Waterloo with bringing this to my attention, to put it mildly, in the 70's at Mirage. He and Dr. Vanderkooy came up with a box that would let you alter phase response without altering amplitude response, and by using it you could do listening tests to determine the audibility of phase errors, or time-coherence. It was quite evident then that if you are in an anechoic chamber or you are using earphones, you can detect the difference especially with special clicks that are made to hear it. You can't always say which is right or better, and as soon as you introduce the room it's 100% inaudible. The importance of phase is in the crossover region because that's an indicator of the blending of the transducers that are being crossed over. So in and of itself it's not a significant thing. And in order to try to optimize it you really sell your soul in terms of things we know are really, really important to sound quality. That is a key fact. It's not like it's cost free, audibly, to optimize that parameter.

Sumit: Have you ever thought of designing an active loudspeaker?

Kevin: Yes, we've looked at that and we have found that compared to active implementations of our existing loudspeakers, we're really able to achieve the kind of equalization that you would want to do in the mid-range and top-end with the passive filter networks. The response of Revels is so flat that you'd be hard pressed to see an improvement if you were to be actively equalizing just the loudspeakers response themselves. At low frequencies, obviously with subwoofers, that's an advantage in terms of size. And then we've taken advantage of the real opportunity there by providing equalization for the particular room situation beyond what the loudspeaker does.

Sumit: Would your ideal center channel speaker be the same as the left and right speaker?

Kevin: It's no question the ideal center channel would be the same as the left and right for the best possible timbre match. That's been a problem with home-theater because so far there are no perforated screens that don't ruin the picture even more than the sound. They degrade both, and that is unacceptable. Until and if someone comes up with a screen that is acoustically transparent and doesn't significantly degrade the visual image, we need to use center channels that will fit below a front projector or above or below a rear projector. So we match the timbre as well as possible, and we have adjustments that do a good job of compensating for these various positions. We actually do all of the anechoic measurements and the double blind listening test with the speakers on stands as one condition, sitting on a rear projector TV as another condition, and built in all of those tests to really optimize the response under all of those conditions. I can't imagine anyone else goes to that length.

Sumit: And you have a switch �.

Kevin: A switch, to switch the circuits in for the different locations.

Sumit: What is your opinion of the THX loudspeaker certification program?

Kevin: The THX program in general has been very, very important for the industry. I think if it hadn't existed, there'd be chaos right now, particularly in AV processors. Things like bass management would be a real mess. In loudspeakers, I certainly applaud their approach to looking for dynamic capability, looking for flatness of response, but in making speakers that go beyond very good to excellent we have to look for real flatness of response in a large number of off-axis measurements, not just direct sound, and as I said, optimizing the power response. In order to do that we don't want to limit vertical directivity in a manner that would degrade the off-axis response, which is how THX directivity is normally achieved. So we avoid doing that.

Sumit: What components and speakers do you use in your home-theater?

Kevin: Well, it's a great luxury, because we do some of our development work in my home-theater, one of the half-dozen rooms that we use as our primary listening rooms, and we do some of our training there, I have all of the Revel models in the system, so it changes constantly.

Sumit: What are some of the problems that you find in listening rooms?

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. Low frequencies are the hardest thing to deal with. They're kind of the most mechanically related thing. So the placement of the loudspeakers and the location of the listeners are extremely critical in a listening room with any loudspeaker, and room treatment at least at mid-range and high-frequencies is really critical. While our speakers have very accurate (response) even way far off-axis, it's still greatly preferable to properly treat the first reflection points in the room.

We know from our research that the first reflection points are audibly very critical, and especially the side wall reflection points. So I like to see absorption, at those first reflection points, at least unless the room is quite large, and otherwise diffusion in the room overall. For imaging characteristics it's optimum to be sitting on the center line, but that introduces challenges at low frequencies in most rooms which can be accommodated by diaphragmatic absorbers to the side or the use of multiple subwoofers, and our work is on-going. We've got some great results using multiple subwoofers in order to solve problems of dips in response that you can't solve with any single subwoofer location, and also getting consistency of response throughout the listening area.

Sumit: This leads into my next question on the use of multiple subwoofers. Do you recommend that?

Kevin: Absolutely. We've done really quite definitive research on that, and we found the optimum number of subwoofers is four, and you get a huge improvement in most rooms going from one to two subwoofers.

Sumit: I'm assuming that the work you are referring to is the white paper on the Harman website by Todd Welti.

Kevin: Yes. And there is a much more complete version of that that was an AES paper. We were fortunate enough to be involved in that research, and I can say that it's not just theoretical. That's the way it works in the real world.

Sumit: The suggested configurations in that white paper are not in the Revel owners manuals. Any particular reason for that?

Kevin: The complication of explaining to people how to choose the positions. But when we introduce this new measurement software (for the Ultima Sub 30 and Perform B15), which is free, it will allow people to do very high-resolution in-room measurements. That's really a breakthrough. By doing those high-resolution measurements they can determine experimentally, for sure, where the right places are. We're working on a wizard driven process to simplify the process for users. It's really challenging because most rooms have physical limitations for one reason or another: we can only put the subwoofers along these walls or in these possible locations. It's very hard to solve all the endless assortment of specific installation issues.

Sumit: Will any special measurement equipment be required for this or would a radio shack meter work?

Kevin: The system will compensate for the response errors of an average Radio Shack Sound Level Meter, but a calibrated microphone is strongly recommended. They can be purchased for less than $100, and the improvement that can be achieved by being able to make accurate measurements is tremendous.

Sumit: What are your thoughts on DVD-Audio and SACD?

Kevin: I wish they were being marketed better; especially DVD-Audio; it's been marketed horrendously. I doubt that there is any sonic difference between the two systems, but unfortunately the examples that are out there for DVD-Audio are lacking overall, and there are some really nice examples of SACD recordings. There's no question that a two-channel recording cannot capture sound realistically because it's missing that important component of a sense of space. For years everyone has tried to simulate it in various manners, and some of them are really quite effective, like Logic 7. But we can record it, and we can reproduce it, so I am hoping that one of them succeeds.

Sumit: Do you ever listen to your CD collection using Dolby Pro Logic II or Logic 7?

Kevin: Absolutely. For years I have listened to most two-channel recordings with either the Mark Levinson stereo surround processing or Logic 7.

Sumit: You are working on a line of in-wall speakers. What were some of the challenges that you encountered during the design process?

Kevin: Well, the first question with the in-walls is: is it possible to make in-walls that sound good enough that we want to call them Revels? And fortunately everyone agreed that we just wouldn't make them if they didn't sound like Revels. We were able to determine that yes, it is indeed possible. You can really get everything except depth of image with the in-walls, so that's a win! We had to do a lot of research because no one has really taken a rigorous approach towards in-walls. We had to learn how we could deal with the limitations and in some cases advantages that exist with in-walls. You know where the boundary is which is one huge advantage with in-walls, because they're in this plane. We've learned that we can outperform most free-standing loudspeakers in a double blind listening test. We've literally put big sections of wall on our pneumatic speaker mover with our in-walls installed versus free-standing loudspeakers. The real reason is that our design goal is to match the timbre of Revel free-standing loudspeakers, and that's what we've achieved very closely. It's only the secondary reason of comparing them against other people's free-standing speakers.

Sumit: Will we ever see a Revel speaker in a car?

Kevin: Perhaps.

Sumit: Who makes the speakers for the current Mark Levinson car audio system?

Kevin: They are made by Harman and in Harman facilities, so all the design work is done by Harman. We are in a number of automotive brands, and it's very exciting to see the process where the cars are designed from the ground up with the audio system. They go hand in hand which provides it a huge advantage over any after-market possibility.

Sumit: What are some of the new products in the development pipeline that you can talk about?

Kevin: We actually have just come out with the M22, F32, C32 and the SUB30, the I20 two-way and the I30 three-way in-walls are coming in the spring. There isn't anything else that I can talk about right now.

Sumit: It was a pleasure talking to you, Kevin. Thank you.

- Sumit Chawla -

All images courtesy Revel


� Copyright 2004 Secrets of Home Theater & High Fidelity
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