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MBL’s Juergen Reis explains the 101 X-Treme Mk II and much more

Today we have a video, along with an extended transcript, of a presentation on how loudspeakers actually work. This is not a beginner’s guide, though those new to audio may enjoy it if they’re technically oriented. Loudspeakers are complex, especially in real rooms with real listeners, and that complexity has led to simplifications and assumptions that either aren’t quite right or are outright wrong among many in the audio community. We hope to address some of this in a helpful way.

I had the opportunity to record and interview Juergen Reis, chief engineer of MBL, who was giving a talk to an audience at the 2024 Munich high-end Expo. Subsequently we discussed some of the research which leads to his key points. In his talk, Juergen explains how the MBL 101 X-treme mk II speakers are designed.  I think his presentation is interesting as much for those interested in other brands of speakers as for those curious about MBL loudspeakers. That’s because good loudspeakers are designed around electrical and mechanical and acoustical and neurological principles that apply to all audio systems. Juergen mostly talks about these principles, and I’ve edited the transcript to focus there and to expand on some of what we heard in Munich with the conversations I had with him afterward.

Why Juergen? Well, I believe, and Jon Valin believes, that the 101 X-treme is one of the great loudspeakers currently on the market.  I’ve heard several other speaker designers privately volunteer that the 101 X-treme is one of the “greats”. The relevance of the 101 X-treme for this presentation is that logically it should be interesting to understand some of the principles behind making a great speaker. As I say, these principles are pretty much universally applicable ideas, and the designer of one of the great speakers is likely to focus on the critical ideas that distinguish “great” from “good”.

TBM

 

Phase Coherence Is Important

Juergen starts with the logic for some of the physical parameters of the 101 X-treme.  

“The midrange and the tweeter (of the 101 X-treme) are on the same axis helping them to be time coherent to your ears and even though the bass radialstrahler is larger diameter and so the radialstrahler “melon” is closer to your ear than the tweeter, the rear of this bass radialstrahler has a counter piece which is farther away from your ear so as an end result the actual source location, which can be measured and also listened to, is along the center line. So, this main driver array is totally time coherent. But what about the subwoofer? You can see that our sub tower is not in the center axis, not even close. But since this subwoofer system is not something you buy afterwards, but rather belongs to the 101 X-treme system, it is designed to be phase aligned with the main radiastrahler tower. The sub is active and I have designed the crossover slope of the crossover so that it is faster than its position would imply and I also have an all-pass filter inside that can be adjusted so you can then move the sub electrically forward 3 meters and back 3 meters without moving the sub mechanically.

“You may ask ‘why is it necessary to move this so far in front or in back of its physical position?’ This is because of different flexibility in the walls of each listening room. So, the thing is that say you physically measure the sub position as one meter behind the main tower. If you then measure the impulse response you will notice that, depending on the room, sometimes the sub is acoustically one and a half meters back and sometimes 2 meters and sometimes a half meter depending on how the walls react. So we allow adjustment of the acoustical positioning by plus or minus 3 meters in position. This also allows the user to put the sub in the position he or she likes for appearance reasons. Then they can align the whole system acoustically. This adjustment can be done via measurement or by ear if you or your dealer knows what locked-in bass sounds like. Also, some customers have this large sub and they want to really hear the sub so this is a matter or different taste, but we allow for that. But the correct timing is only when you don’t hear the sub as separate at all.

Vibration Control

“The next piece we need to address is that when a driver starts to move some energy goes into the cabinet, and then the energy decays a little bit later so that when the drivers stand still the cabinets still vibrate. Sure, there are some sophisticated manufacturers that can build very stable cabinets, but often if you apply an accelerometer and measure the vibrations even a very heavy loudspeaker system can vibrate. So, this weight or mass is not the indicator per se of how stable the cabinet is. There’s a manufacturer in South Africa who originally came from the UK and they have fairly light cabinets made with resin and epoxy layers which are very rigid, while there are some heavy cabinets that still vibrate. Mostly, we eliminate cabinets entirely on the 101 X-treme to solve this problem. Then, especially for the 101 tweeter and midrange we have no secondary part that is not responsible for sound, meaning we have no spider that is just there for holding the voice coil in place. Everything that is moved besides the voice coil itself creates sound pressure and this also means that you can listen with the system at very, very low levels because no energy is wasted. Mechanically, the voice coils of the upper and lower tweeters and midranges and bass radiastrahlers move up and down at the same time. These opposing forces cancel each other out because the upper and lower driver assemblies are tied together with these very massive steel rods. So, you will notice that if you will lay down on the floor and put your ear on the floor you will hear nothing from the speaker through floor vibrations. Then you may also argue ‘what’s with the sub using a wood cabinet?’ But the drivers are in a push-push configuration for force and between the driver baskets we have aluminum rods that keep both baskets tied against each. Again the vibration forces cancel out. Also, between the magnets we have some absorbing material so in case the basket is try to vibrate we prevent them from vibrating. So, I can play very heavy bass notes and you can put your ear on this cabinet you will hear nothing from the cabinet.

Room Modes

“Since our active sub is self-powered we also can adjust the dryness or the fullness of the bass with a Q factor control, and we have also two parametric equalizers to uh to reduce into account the two main room standing wave modes at specific frequences for the users room. We can compensate for this with adjustable EQ in terms of frequency and level and bandwidth.

Speakers Must Be Designed to Address Human Ear/Brain Processing

“Next we’re going to explain some factors in how the ear/brain combination perceives sound. It is particularly important to note that the ear and brain process information differently at different times from the initial sound through its harmonic decay to reflections in the room. The easiest to understand explanation starts with the so-called direct sound (sound that reaches your ear without any reflections). Direct sound in the first three milliseconds mainly is important for you to detect the direction from which the sound comes. This is important, but note that in first three milliseconds you have no idea what instrument is playing because the first 3 milliseconds do not contain the harmonic structure of the instrument which is its sonic signature.

“Now we come to the next time slot which consists of the first early reflections. When I was studying electrical engineering, I thought I would have to remove these first reflections in go in order to get only the sound that is coming from the speaker. This partially correct but also partially wrong.

“I start with the difference between a real source and phantom source. So when I talk to you I’m a real source which means that if I’m standing in the middle between the speakers you hear me in the middle, regardless of where you sit. When I record my voice and play back with loudspeakers my voice is then a phantom source (there is no speaker in the middle). Listers in the middle position hear my phantom voice also in the middle. But the listeners seated outside the center position will hear my phantom image collapse toward one side. This is especially so when I remove early reflections coming especially from the front wall (behind the speakers). This collapsing is something that I think will not benefit every listener particularly when you want to enjoy the music with a group of listeners. Certainly, at home when you are seated in the sweet spot or in studio mastering there’s some different needs. Most particularly we have a bit of a tradeoff between reflections that spread out phantom sources and widen the listening area vs. limited reflections from the first ~20ms, which may improve stage depth and instrument placement for a single central listener.

“Now imagine that I invited some friends up on the stage so three people are talking to you. Again, no matter where you are sitting the real positions of the real people will be evident. But you will also sense the size and reflectivity of the room (the soundspace – TBM). When we record this trio, you may also hear that the sound extends wider than the separation of the speakers. Now, if you absorb any early reflections from the sidewall, or if you have a loudspeaker which beams to you, then the soundstage will end where the speakers are.

“However, things aren’t quite so simple. The most important part here is that for the phantom images to work, the sound characteristic of the reflections should behave the same as the real sources would behave. That means if you have a loudspeaker that changes its frequency response with changes in radiation angle, then the reflections would have a totally different tonality.  The big thing is that your brain will detect that this does not really belong or act like the real sources, and your brain will deduce the location of the sound source. Your brain will collapse the image onto the speaker (deduced source).

“Note that it is not the case that good loudspeakers radiate like real instruments, because the instrumental radiation pattern is already captured in the recording. What matters so that your ear/brain is deceived into perceiving the phantom image is that the energy spreads into the room in a smooth and natural way. If the direct frequency response is smooth and linear, the reflected response should also be smooth and linear, although the tilt (slope) of the frequency response curve may decline the farther off axis you get. This is like an actual natural source would behave in the room, which is why your ear/brain looks for this characteristic to decide whether sounds are real or phantom. We want your ear/brain to think phantom sources are real, which is tricky. (This is a key to why omnidirectional speakers image well: they naturally have on-axis and off-axis sounds that have similar response shapes and thus are aided in creating phantom images – TBM)

Reverb Times and Listening Position

“Now we get to a little bit of information about the reflections. If we continue on our journey farther from the time of the direct sound, the reflections get denser and then even denser. When we add these up, we get what is called sound energy. When I was studying for my engineering degree, I noticed two different models of speaker might measure the same in an anechoic chamber, but at home they sounded different. Why? Then I went to a hi-fi show in 1981 and I heard a very early model of the MBL 101 speakers and I recognized something sounded different and something sounded right. But why? Then I was at the CES show in Chicago and Floyd Toole (NRC and Harman speaker engineer — TBM) came in the room and asked me can you play pink noise (noise with each octave having the same energy – TBM). Floyd Toole was walking around and then we had a long discussion. Floyd had done a lot of measurements to understand the preferences of listeners for different loudspeakers. He realized, and I began to realize, that anechoic measurements are problematic because early direct sound is only in total four percent of what you hear, and direct sound is in total 12%. The sound energy (late reflections) is 44 percent of what you hear, and all early reflections are 44 percent. But our ear hears these different time regions differently.

“Now we come to a term called critical distance, or I would say optimum distance, where you have the perfect balance between room energy and early reflections. Now imagine I’m playing an instrument. Every room has a reverberation time and the reverberation time determines how far this critical or optimum distance is from the speakers. Optimum distance means the energy of the early reflections and the total room energy is the same. In this case, you have the perfect blend between the articulation part of an instrument, so you hear when I open my mouth to sing or when I pluck the guitar strings and the tonal balance of my voice or guitar. If you are get closer to me I’m still playing the same guitar, but you hear my finger plucking more distinctly yet you will be have more difficulty detecting if the guitar has a spruce or rosewood top. If you move farther away, you can more easily detect the resonance of the guitar but you lose the detail.

“I have had a recording studio for over 20 years and I’m a certified mastering engineer for over 15 years and so I have to have different rooms for different purposes. The Audio Engineering Society provides some clear specifications for the behavior of specific rooms. For example, the Mixing Room that also I have has a reverberation time of 0.25 seconds (T60). And the early reflections must be 24 db below the direct sound. With this room where I do mixing, I hear every error in the music. So, it’s good for mixing but I have no idea how the room sounds so it makes it hard to judge how this will later sound at home.

“Then we go to a Critical Listening Room (this is my most favorite room) which has 0.35 second reverb time with the early reflections down by 18 db compared to the direct sound. In this room you really get the magic of a perfect blend between the articulation of the what the musicians are doing with the tonality of the instruments they are playing and the room environment. So, this room, if every recording were great, would be the perfect room to listen in at home.

“But I have also a so-called Translation Room which the mastering engineer uses to check how this will sound in a typical customer’s environment. Sure, there exists no typical customer, but to help guide mastering the AES has even specified this in order to have a tool. This room has 0.45 second reverberation time and the first early Reflections have to be 12 db down. In this room I can enjoy and accept or ignore some timing or pitch mistakes and errors made the musicians. In this Typical Listening Room you can more be on the relaxed side compared to more on the analytical side with the Critical Listening Room.”

The point here is that the reflectivity of the room and the positioning of the listener closer or farther from the speakers can make a large impact on the sound we hear and our musical engagement. The more reflective the room, the closer you must sit to the source to regain a balance of articulation and tonality.

Summary Thoughts

I do want to emphasize what Juergen mentioned several times and that is that the factors he discusses are just some of the basics. The big takeaways I think are:

  1. we must understand how the ear and brain process information to understand speaker performance in the real world
  2. this is another way of saying that understanding the signal that initially comes from the speaker at say one meter is radically insufficient to understanding the result that you hear
  3. the room and listener positioning are also major factors in the resulting sound
  4. measuring to eliminate the room is a simplification that may greatly reduce predictive power, so what TAS tries to bring you are empirical tests of what speakers do all the way from the signal into the speaker to the brain
  5. it is possible to articulate a set of “rules” about setup to exploit these ideas, but setup cannot fully overcome a flawed loudspeaker design – we are working on codifying these rules

 Most of us can’t reason from theory or measurements to sound quality. It’s just too complex a subject, and we aren’t supplied with full measurement suites, and we don’t have enough interpretive experience of all those measurements, and some of what matters is only partially understood and measurements for these factors are not codified. Fortunately, we do have excellent instruments for perceiving whether stereo systems sound real: we have ears and brains and extensive experience with how sounds in general (and music in particular) sound in real environments.

 – TBM

Tags: FLOORSTANDING LOUDSPEAKERS STANDMOUNT VIDEO

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