Sound On Sound x HEDD Panel on the Future of Monitors

The way that we listen to music has changed in many ways since the advent of speaker technology, yet the underlying mechanics of speakers themselves have barely changed in more than a century. Moving air in a way that pleasingly reproduces voices and sounds has been an evolving art and science, with new principles, techniques, and materials continually pushing the capabilities of speaker systems.

In November 2024, we came together with our friends at Sound On Sound magazine for a special event at London’s Tileyard Studios that featured a panel on what the future of professional audio and monitoring might hold: what advances might be had, what will monitors be made of, how will they be connected, how different (or not) will things be for audio professionals?

Below you will find an edited transcript of the panel which saw the audience in conversation with the four panelists: HEDD Audio co-founder and leading acoustic physicist, Dr. Klaus Heinz; Sam Inglis, Chief Editor of Sound on Sound; Chris Allen, founder of Room Sonics, who provide acoustic design support; and Chris Korff, monitor reviewer for Sound on Sound.

Look out for an edited video of the panel coming to our YouTube channel soon!

Audience Question: Where did speaker design start for Klaus? How did you get involved?

Klaus Heinz: This is maybe best described by the scene of a Dutch entertainer. There was a show, it was dark, and there were two big horn speakers at both sides of the stage, and nothing happened besides a Schubert quartet was playing. And then I came on to the stage and went to the speaker and was so astonished at the kind of music that came out, this was the original moment of being astounded by a large speaker representing music.

And that's really a kind of interest that hasn't disappeared until now. And, the early beginnings were ridiculous enough from today's point of view. I spent so many decades with speakers, went to scientific conventions and read the literature. Besides that I also made music by myself, more the classical stuff, and so it came together with the music and this inexplicable interest. I may remind you of the following phrase from a famous physician who listened to one of those lectures by [German physicist] Heisenberg. He came out and told a friend, “I didn't understand anything, but I want it.” That's the way it started.

Audience Question: What do you think is the most exciting technological development in speaker design right now or in the future?

Klaus Heinz: I’ll talk about the Lineariser because it is an ambiguous approach. There are books about loudspeakers that state phase linearity is not audible. And there's a good mathematical explanation for it. And this held true for 20, 30 years. And then, a German guy made a PhD work that started what became impulse response-oriented phase correction and, in the beginning, it was even worse than without phase correction, because the digital signal handling was bad enough to overthrow the linearization of the phase response. So, whether it was audible or not had to be explored. I tested it with some old, classical recordings from Decca that were made with orchestras. And there the difference was most audible. The phase response linearity really takes you a step forward. With modern productions where you have your mixing console with all these tricks, the original localization and phase response disappears, in importance at least, and so it is better. And it was a missing link in speaker quality because it wasn't possible to really apply it before.

Chris Korff: I think one of the, almost a low tech innovation, that's on the way would be material science, which is just developing lower weight, lower mass, higher rigidity materials that can do the same job that speakers have always done. The idea of gluing a magnet to the back of a piece of paper and making it wobble around in response to an alternating current is over a century old. Amazingly, paper is still one of the best materials you can use for that. But people are starting to work with things like graphene for high-frequency drivers. Focal used Beryllium, actually what Focal did was make Beryllium usable in a way that doesn't immediately kill you. The materials that are actually used, the interface between the electrical and the acoustic is where I think some of the biggest leaps are still to be made because loudspeakers are fundamentally steam age technology.

Klaus Heinz: I agree. In my former years, chasing diaphragm materials was the most important task for me because all the rest, the coil and the magnet, that's like an air pump, that's easily handled.

Chris Allen: What excites me in terms of most recent developments? It's probably class D technology and the integration of digital technology into speakers simply because of convenience. With new technology, whether it's a stand-alone room correction, or built into the speaker, we've got so much more accessible technology now to really get around some of these inherent historical problems, those horrible rumors which caused big bumps and things like that. They can be addressed pretty damn easily now. In the field that I'm in and the business of making things sound better, I absolutely embrace class D and DSP technology. It just makes things so much easier.

Audience Question: I was wondering if materials in monitors fatigue and deteriorate and maybe every ten years I should replace the woofer or the tweeter. Or can you expect the performance to stay the same until the end of the product’s life?

Klaus Heinz: The performance changes and the materials used 20 or 40 years ago are being retired. You cannot expect to have the same performance. I wouldn't claim that for our speakers either. That is the real truth. It is not awful, you know, but it is audible. And ten years, I think, is a fair amount of years before changing the speakers.

Host: And are there things we can do to look after our speakers better and ensure that they have a longer life?

Klaus Heinz: No [laughs]. We try. For example, if you use rubber surrounds, it's safer. Foam surrounds that were around for some time, they are disappearing.

Audience: I would like to see manufacturers talking about this and offering a service where if you bought the speakers from us ten years ago, now you can buy replacement parts to keep them fresh, instead of simply trying to sell the new model, because I don't have enough incentive to buy the whole thing again.

Klaus Heinz: You want to open Pandora's box for manufacturers [laughs]. That's what you want to do.

Chris Korff: The car industry does all right by offering, I think in the EU and by extension the UK for the time being, ten years after the discontinuation of a car, the manufacturer still has to provide parts for it. Other industries survive by having these impositions placed upon them, you know, being user serviceable.

Klaus Heinz: I would claim that a good rubber surround is not affected after ten years really. Our tweeters use Kapton folds that are used in airplanes to prevent fire and so on, that is probably the most stable fold you can have. So, because of the materials used for our tweeters we try to have really good rubber surrounds on them because I'm familiar with the problem. And, yeah, I don't want to have trouble after ten years. Not now at my age [laughs].

“Progress in monitor design really comes from materials and application of these material advances.” - Klaus Heinz

Sam Inglis: Well, there certainly are manufacturers who will still service 20 and 30 year old speakers. I'm not actually sure what HEDD Audio's policy on that is, but yeah, it's not unheard of.

Audience Question: Are there still improvements to be made beyond the current generation of monitors, considering we've already had a big leap over the sort of speakers from even ten years ago. How much better can speakers get and how can they be made so?

Klaus Heinz: Speakers are really different and modern materials are really much better. A paper cone has a natural ten-year problem. But now there are materials that really withstand the pains of time. So, the question now is quite different from the same question ten years ago. I mean, so many materials are involved, so many glues are involved. The overall engineering quality of our speakers, as well as all of our competitors, has improved an awful lot. And many details need to be mentioned, from materials to glues to test methods. And, it is really different from what I see in these big old speakers that I sometimes see in the studios. It has become much better.

Audience: Do you think there are more improvements that still could be made? Will there be further improvements in the future?

Klaus Heinz: I'm not a prophet, you know [laughs]. But what I can say is that speaker technology is so old and hasn't changed really, at least with respect to the voice coil systems, that I am confident that materials scientists or engineers, they work all the time, and progress is mainly made in materials. Big new ideas after Oscar Heil’s Air Motion Transformer I cannot see or find. I would like to have one and so I expect that the current materials limitations is what makes such new innovations out of reach for now.

Chris Korff: If I can also add. This is from speaking to Phil Ward, who was supposed to be here today, one of the things that he's mentioned from a commercial point of view is that when Klaus designs these speakers, he's not just doing it for fun. He's going, “oh, there is a need for these high-end speakers. I will build them and people will buy them.” And that commercial need can only be met if it's financially viable for people to build them. And one of the things that I've heard can be very beneficial in that regard is computer modeling.

So you would take your composite materials that you're making the cabinet out of and subject them to a lot of virtual tests and see, “oh, there's a weak spot there. There's a resonance there.” Which means that you can do a huge amount of sort of pre-stress testing without having to make a prototype or different marks, which might be why we're seeing more high tech monitors from a higher number of manufacturers, rather than just a few manufacturers that have the R&D budget to build different marks before they achieve the sort of the pinnacle of a range.

Klaus Heinz: And it depends on the big industries to come up with new materials, like the Kapton folds. It really changed, at least for me, the situation. Same with new magnet technology. When I found the original AMT it was a big block, using big magnets, not very practical. But for audiophiles it was a revelation, myself included. I hadn't heard such good high frequency reproduction before.

Fortunately, neodymium magnets came up at the same time and that enabled me to shrink this big block of a tweeter towards what we have now. This was material progress and not an original idea. So if materials get better and so many big companies are working on different folds and different kinds of plastics, the progress really starts from there. And we are in an old fashioned business. Speakers work the way they have since they were invented 120, 150 years ago. Small progress in concept only really, all the rest is material and serious application of these materials. But the speaker itself is old stuff.

Sam Inglis: One very interesting product that I saw demoed quite a long time ago now, was a speaker invented by Dragoslav Colich, who's the main guy behind Audeze headphones, and he'd managed to create a planar magnetic full-range speaker, which was sort of desktop-sized, and I don't know what happened to them or whether they ever became a commercial product. Is that something you could do, for example with the AMT, in principle? Could you make a full range speaker with it?

Klaus Heinz: Yeah, but then I have to leave the dimensions I have, you know. It would be huge and heavy and it would be nice for audiophiles but it's not a broad application anymore. So the same old restrictions remain. We are in times of climate problems and the worst machines that you all use are loudspeakers because the efficiency is like 2 to 3%. That would not fulfill any requirement of today's electrical machines. This limitation of 2 to 3% means they are slow. It would be nice if a speaker came up that was like 50 or 60% efficient. But that's what we have to live with.

Chris Allen: Are there innovations within the cabinet itself? The electronics, the crossover? The amp? Unfortunately, I’m probably not the right person to ask. I don't design speakers. But again, leaning into the fact that we are embracing more and more digital technology, historically class A and B amps are the way we drive a lot of the speakers and produce a sound, and still are for a large part. But again, class D digital technology isn't to be laughed at anymore. You know, it's not as naff as it was back in the 80s and 90s. It's got a bit of a stigma, at least it used to, but I think every speaker brand has at least tried or fully embraced class D. And so that's a whole different approach, right?

In terms of crossover technology and amplifier technology, for one it makes it really light. I would say from my, you know, novice opinion that it would be class D technology where improvements still lie. And the way that we're driving the amps now has become much more efficient from class D technology. And then within that, the features that we can include, we can have fully parametric EQ built into it at no real extra cost, apart from an extra processor, you know, versus class A and B was quite big and clunky and heavy and has its sonic characteristics and its attractions but there's no denying how good class D technology is making things for us.

Klaus Heinz: Perhaps I may add that, when I started, crossover filters were an issue of big discussion. They're not anymore, you know, if you have reasonable drive units, this type of filtering is okay. If you have reasonable crossover frequencies and reasonable drive units, then the directivity of the speakers is rather optimized. In earlier times you had like this big Christmas tree, right. Broad characteristics in the low end getting smaller to the crossover point, being broad again into the next crossover point, so for a three-way system you had a nice tree there. And, today's good designs can avoid that. The best so far are the Klein & Hummel speakers, which have a very linear directivity versus frequency response, but everybody can do it. There are no more secrets and, again, we need another model.

Chris Korff: Just to add, we've talked about how costs across the amplification and DSP are already incredibly efficient and incredibly advanced. And we've already talked about how inefficient the copper coils glued to the back of a piece of paper are. But there was a technology I was reading about which is current sensing, which isn't to do with efficiency but with accuracy. And it's not even new. It's from 1999 or something.

In a standard loudspeaker voltage goes in, you pipe it through the voice coil in the back of the speaker and the voice coil moves in response. But the voice coil also heats up as it moves, and it has this current going through it, and the current sensor, which is woefully underused, it's a brilliant technology but it's not used very often, essentially measures the current going through the voice coil and compares it to the input current. And it means that no matter how hot the voice coil is, the current going through the voice coil will be enough to move the speaker the right amount in relation to the input signal, which essentially eliminates thermal compression or negates thermal compression, which is one of the big issues when you've got a speaker running for a very long time. It gets hotter and hotter and hotter, and the excursion becomes less for a given input signal. So yeah, Google it, current drive I think it's called. And it's in a paper from 1999.

“In the field of making things sound better, I absolutely embrace class D and DSP technology. It just makes things so much easier.” - Chris Allen

Audience Question: [inaudible question about Klipsch horns and placements in large spaces, like a nightclub]

Chris Korff: So I think for large scale stuff, when you really want to fully fill a room with lovely, lovely bass then fidelity isn't always the best thing. If I was in a club I'd want to listen to a big stack of bandpass woofers or W bins or some Funktion One. I think what humans find enjoyable to listen to isn't necessarily the same as what's accurate and highly faithful to the input signal. I think they’re just two different listening experiences that I don't necessarily think have to learn from each other. If there was such a thing as a perfect speaker, there’d only be one kind of speaker.

Klaus Heinz: If I may add a personal remark to that. When I started, I visited all good speaker designers in the world, like five. One of them was Paul Klipsch in Hope, Arkansas, where I tried to discuss with him the disadvantages of his loudspeakers. The advantages were plain to hear, and especially with regards to our previous discussion about efficiency. If you have a Billy Cobham drum solo, you know what I'm talking about, right? But the disadvantages of early reflections with the folded horns are apparent with classical music. It is not right. Whatever the discussion, it is not right. And, so for a universal speaker approach that serves all communities, it is not viable. And I may add that when the visit ended and we were far away from the nearest airport, Paul Klipsch took me to a private airplane, it was like 1972 or so, but he’d just started learning to fly and so I was nervous! But he was very active and had a very impressive personality. And, there are so many people that are still excited after all these decades, half a century and so on, about what he did that it was evidently very impressive.

Audience Question: [inaudible question about issues in room design and space usage.]

Chris Allen: Yeah there's always plenty of obstructions and limitations when you come to treating a space. And the clouds or the absorption above the mix position is often one of them. Whether it's in a rented accommodation and you can't drill holes in the ceiling or if you've just got exposed pipework or something like that, it's just not realistic. And there's not really much you can do unless you can build a gantry or something where it's going to support the clouds. As I said before, you've got to make compromises somewhere, you know. We were talking about how clouds overhead are a key place for early reflections, but there's plenty of early reflections on the left and right as well. So you can still go a long way and make huge gains by just treating your sidewalls. There's plenty of scenarios where we don't have the real estate to put bass traps in the corner either. But, you know, bass traps don't necessarily have to go in the corner, if you go thick enough on the rear wall it can go a long way to help. You're still going to get a lot of benefits by covering as much as you can.

Klaus Heinz: Maybe it’s a good moment to mention a limitation of all monitors. There have been measurements about violin reproduction and the directivity of violins from tone to tone and they are so different. You wouldn't believe it if you would see the diagrams. That means, the dispersion into the room is one of the sensations in the violin. It’s why it sounds so special. It’s not something you can capture in a file, you know. So then to limit the reflections of the room makes sense because we don't want to hear the room, we want to hear the recording but still spatial information, in many ways, is totally cut off from the original musical events.

Audience Question: Do cardioid loudspeakers offer any advantage in most real world rooms?

Klaus Heinz: Because the spatial impression you get, of course, is from the room you are in and not the recording room I think they are questionable for real monitoring tasks and they give different and additional difficulties, but you may enjoy it, you decide. For monitors they cannot be a good solution from my personal opinion.

Chris Korff: I would differ, I'd say the less extraneous low frequency energy your monitors are throwing away from you and then back into what you hear the better, everything that comes behind the monitor and then gets thrown back into the mix with everything that's coming out in front is going to muddy waters. Anything a monitor manufacturer can do to negate energy leaving the back of the speaker and then joining the soup out the front is going to make the front firing sound better I think, intuitively.

Klaus Heinz: That's the ideal moment to make a remark about our subwoofers, because we deal with this problem. So, the HEDD subwoofers have an analog output for the satellites with a controlled delay of 10, 20, or 30ms. And so, the phase response between subwoofers and satellites, which normally is lost and is the main cause for the bad sound of most of the satellite systems, is compensated for. It's not forced back, but at least in respect to the subwoofers it does solve the problem. In future loudspeakers I certainly would like to introduce that.

Chris Allen: Just to share how I would calibrate subs, the first thing I would do is find the best position of the sub in the setup’s front plane. You do need some testing equipment to be able to measure the phase of the sub, and also relative to the speakers, because that's how phase works. And get that in the very best position you can within the limitations. So you might not be able to put your sub in certain places, but within the space that you have, do put the sub in different positions. Compare the phase correlation to your left or right or center speaker, for example, and get it as close as you can, and then just adjust the phase just to kind of marry them up.

Klaus Heinz: And that does not mean that the reproduction of the thing is moved backwards or forwards in time. That is a different aspect. And to be clear, our solution is not good for live monitoring, it’s not pleasing at all.

“Anything a monitor manufacturer can do to negate energy leaving the back of the speaker and then joining the soup out the front is going to make the front firing sound better I think.” - Chris Korff

Audience Question: I know the HEDD monitors have got both AES and analog inputs, and I'm nervous about moving to use a speaker with AES ins because I've always used Prism D2As with nice analog monitors. Now, because I do also need to change the volume level that I'm sending to my speakers, is it better to use the Prism D2As analog going into the speakers, or use an AES input which I assume is limited to the maximum of 24 bits with a digital volume control. Should I go digital?

Sam Inglis: I would think that would depend on whether your speakers are internally digitizing the audio. If the speakers are all analog, I think you should probably stick to going into them analog.

Chris Korff: I think the point is whether digital attenuation is better than analog, because obviously you don't want to send a full range, zero dbFS signal through your studio monitors and then attenuate digitally by 50, 60dB only to then be coming closer to the noise floor of your digital audio system.

Chris Allen: So the situation is that you've got a really nice premium Prism sound converter. And that is taking care of the digital to analog and analog to digital conversion. Is there a benefit or not to bypassing the D2A of my Prism, going out digital and going into the AES input? Or should we maintain analog output from the Prism and go to the analog input of the HEDD, which we know is going to do some A2D as well?

Klaus Heinz: As a principle, it's wrong to do it that way. We do, but we don't have another choice. But if you talk digital to analog conversion question, that's religion. I mean, we all have very different opinions. I do not have the practice or experience with different data converters and so on. But I wouldn't expect it to be important with modern DA converters. They are so good, and even the cheaper ones are so good that I cannot find it a limitation of any kind, in my opinion.

Chris Korff: If you've got an analog monitor controller lying around, you might as well use it. But I wouldn't be happy just piping an AES3 output from my computer straight into an AES3 input onto a pair of monitors. Because if your computer shits itself, you're going to get zero dbFS of hash destroying however many thousands of pounds worth of drivers. You should use whichever interface, analog or digital, is most convenient. You’d shave almost a full millisecond off your latency if you use the digital input. But other than that, whichever is most convenient, whichever is least likely to blow the speakers up, frankly, especially if you've got a Prism converter anyway, you’re not going to hear any difference.