Juergen Reis, head of R&D talks about the MBL 101 E MKII

by Laurent Thorin | Oct 2022 | INTERVIEWS, MBL

We had the opportunity to have a long conversation with Juergen Reis during our visit in Berlin in the offices of MBL. The technical director of the German manufacturer devoted a lot of time and availability to explain us in detail the genesis and the functioning of his favorite loudspeaker. Here is the exhaustive report of our exchanges.

When was the first version of the MBL 101 born?
The first version of the MBL 101 loudspeaker was introduced in 1986. MBL was founded in 1979 with the idea of having an omnidirectional loudspeaker system as “the” perfect loudspeaker. It was a sort of 3 way-system driven by 2 voice coils. It was a challenge those days, to bring this idea into a product. But at the end, the MBL 100 was at least the “proof” that this idea can be working. Even it was working in principal, this early version of the speaker needed to be sold together with a parametric equalizer to compensate the sound into a more homogeneous and balanced character. After the MBL 100, it was still a long way to go before the MBL 101 was born.

I joined the company in 1982 and after 2 years, in 1984, I stepped up as the Chief Engineer. I started with analyzing the early MBL 100 speaker and began to change / exchange most parts, except some parts from the big melon, to bring this idea / product onto a higher level, to a High End loudspeaker, as a 3-way system with 3 voice coils and a 3-way crossover and for most, no longer need for an external equalizer. After working 2 years on the development, the MBL 101 saw the light in 1986 as a high resolution and homogeneous / balanced sounding speaker, with a natural sound radiation.

How did you come up with the idea of manufacturing this type of transducer going from low mid to high?
When I first heard these early MBL 100 at the “Internationale Funkausstellung” in 1981 (the largest Hifi / TV show in Germany those days) there was already something in the sound that has drawn me into it, that even with this very early version, the natural presentation of how sound radiates and how musical instruments have been presented, made my decision that after I finished my study of electro acoustical engineering in 1982, to ask for an interview and for the job, and got it, and was very happy about that.

How many different versions have you designed?
If you ask for MBL 101 versions, I developed 8 different versions; 4 versions as 3-way systems (without internal subwoofer), and 4 versions as 4-way system with internal subwoofer. But you may want also to add the MBL 101 X-Treme to it, a 4-way system with external active subwoofers and 2-times 3-way 101 Melon system, stacked on top of each other, in a D’Appolito alignment. And if you then want to add also the “smaller versions” of the omni speaker systems, like MBL 111, MBL 116, MBL 120, MBL 126, MBL 121 and the other MBL loudspeakers, I have developed in total more than 30 different MBL loudspeakers.

One might think that the vibrational behavior of the 101 is similar to that of pulsating spheres. Was this the goal?
Yes, the individual drivers act like pulsating spheres, this is right. This is one of the best ways to create an all omnidirectional sound wave. Having these individual drivers in the center line direct on top of each other, the woofer, the midrange driver and the tweeter ensures, that the sound of all three drivers do reach the listener at the same time, as the sound of each driver virtually origins from the center (as they all radiate like spheres). Another advantage of doing this is, that the acting membrane is basically also the “wall” of the internal volume, so there is no passive cabinet. All what you see and hear is the active surface. There is no energy that is otherwise be lost in a cabinet, when the driver / membrane starts to move, and there is no energy that is stored and radiated afterwards through the cabinet, when the membrane stops. So no delay when build up the sound and no decay / time smear when the sound stops, leading into a high transparent sound.

What is the membrane of each of the three spheres made of?
Ok, lets begin with the tweeter. The membrane of the tweeter is out of a 1-layered unidirectional carbon fiber. This material is ideal for bending lamella of higher frequencies. With choosing the right portion of carbon fiber and resin, you can adjust the best balance between sound propagation speed to mass, also called Young’s Modulus, and the internal damping. Additionally, this material, does not age, so it behaves still the same after 10 years, as it has in the first week. The membrane of the midrange driver is out of 2-layered cross woven bi-directional carbon fiber. So it is softer as the one for the tweeter, as it has not to go as high in frequencies, it does have a bit higher weights, in order to go lower in frequencies and does have a higher internal damping. The membrane of the bass driver is out of a mix of aircraft aluminum and magnesium. Aircraft aluminum is cooled down very slowly (and not in oil) in order to prevent any stress in the material and the magnesium acts like a lubricant for the aluminum atoms, to stay flexible all the time.

And as a “short” excursion how I get into the for example all carbon fiber tweeter: When I started analyzing the early MBL 100, I realize, that beside the construction, that the optimum materials have not been chosen (by a long margin). So over a period of several months, I tested and measured every material that I could get a hold on, different metal materials like steel, copper, brass, aluminum, … different plastic material, different paper and different wood materials and created a large spreadsheet with Young’s Modulus (stiffness to weight ratio) and Logarithmic Decrement (damping to weight ratio) and found out, that a composite material like CFK (carbon fiber composite) or GFK (glass fiber composite), would be the right choice for the tweeter and the midrange. And happily the bass player of the 2 bands I played during that time, and later very good friend of mine, founded the first CFK company for guitar and bass guitar necks in Germany. So we hang out together a lot, I was listening to his CFK guitar necks and gave him feedback and he told me what his has changed in the CFK for composition ratio between carbon fiber and resin, and the different pressure while backing and the different temperature curve while backing. This was then the start for me to change the membrane of the tweeter from epoxy to CFK (for details see above), changed the material support from aluminum to a different CFK, changed the center rod from brass to another different CFK, change the voice coil former, … All this brought me a judge success concerning the linearity and the frequency smoothness of the tweeter, and it was logical, that I continued doing similar with the midrange driver. But beside changing also here nearly all materials, I changed totally the size and the shape of the midrange driver and for most, also the position of the midrange driver from below the bass driver (near the floor) to in between the bass driver and the tweeter, as this is logically the “one and only place”, where a midrange driver should be and with another very important point, to gave them a separate voice coil, that is no longer shared with the bass melon and so I could design for each driver their own individual crossovers and brought finally the speaker to a next step / level, that it was working now without any external equalizer and to a much higher neutrality and resolution.

Did you have any problems with the filtering?
I have chosen Linkwitz-Riley crossover topology with 4th order, meaning 24 dB / Octave. This gave me an all in phase behavior of all drivers, that does mean for example for the Tweeter and the Midrange driver, who crosses over at 3500 Hz, that at this frequency, or also at 3000 Hz or at 4000 Hz, the MT and HT does radiate always in phase / same time to each other. That means that the vertical radiation pattern is symmetrical to the horizontal plane, meaning if you move your head 10 cm up, you got the same / similar sound as if you move your head 10 cm down. I have choses the crossover frequencies in that way, to take care of the maximum power, the voice coil can handle, have taken care the maximum movement of the driver, so that the distortion stays low, haven taken care, that the impedance does not drop below the nominal 4 Ohm, and for most to take into consideration, what the best suitable frequency area of the specific driver is.

How is the restitution of the extreme grave register guaranteed?
From the MBL 101 Version D in the year 1997 on, the MBL 101 has an integrated subwoofer below the bass melon, to add an extra octave in the lowest register. This is a 4th order bandpass, so you actually don’t see the 12 Inch driver that is inside, and you do see only the 2 ports, that connect the front chamber of the bandpass to the outer world and radiate the sound.
There are several reasons, why I have chosen an bandpass sub. One is, that due to the sealed rear chamber of the bandpass, the 12 inch driver has a closed air chamber behind that does suppress the otherwise excessive movement below the port resonance in order to stay always in the linear range of the voice coil and magnet assembly and mechanical construction. The other advantage is, that with an bandpass, you don’t need the otherwise very large capacitors and inductors for the crossover, as the bandpass itself does form acoustically this 4th order high pass and 4th order lowpass acoustically. And as it forms this 4th order lowpass, it does also filter out any unwanted overtones in the bass and does only radiate the sound as it is.

What I have done with some passive crossover parts, as I am doing this with every MBL loudspeaker, I take care that the impedance does not have too high phase angles and so I added when needed a so called “load matching” circuit, to give the connected power amp always a little help with not too difficult to drive loads, and I have add also a small network to suppress port turbulences / resonances. The “rest” is done acoustically with the bandpass. The most difficult part of the band pass subwoofer was to adjust the group delay / speed of the woofer to the one, of the upper bass melon, as both have to work together hand in hand and should sound as one bass, and not as a bass and a subwoofer. For a bandpass, the only way you can do this is by changing the complete cabinet (and not just changing some crossover parts, as they are no crossover parts needed nor build in). So this was a bit tricky (and a lot of different cabinets) but finally succeeded and the subwoofer is totally integrated sonically with the bass melon. And to give the “final foundation”, we have supported the internal 12 Inch woofer via brass rods going from the inside, directly coupled to the woofer, down to the brass feeds of the MBL 101. So there is a direct coupled mechanical connection between the woofer and the floor, to hold the woofer firmly in place and to prevent mechanical vibration of the cabinet.

What do you think is the most striking innovation of its speakers?
This is by far the most important part / question, and this is also by far the longest answer, and this is maybe also by far the most “difficult” to understand, but really, these are the most important points of what this omnidirectional loudspeaker do and how it works from the acoustical standpoint. The goal of all loudspeakers in the world is, to transport the sound of a recording into someones home. This sounds “easy” but it takes a lot of understanding what does this mean and what has to taken into account / into consideration, when designing a loudspeaker, to reach this goal. So what does this mean? The frequency response should be neutral? Ha, this would be too easy. Lets put a speaker in an anechoic chamber and measure the frequency response. Doing so and developing that way, that the frequency response is flat does mean very little, to be exactly, this is not even 2 % of all what is important, when having the speaker at home. You see, that this point, that is measured and published the most (by manufactures and by magazines), have the least portion, of what is important. As this point is also covered in the next paragraph, we can ignore it here (as it has, measured alone, at the end, no real importance)

1: Listening Window: Measure the frequency response in a so called listening window. What does this mean? Measure the speaker within a range of + / – 30 degree horizontally and to + / – 10 degree vertically. We now have in total 9 points where we have measured basically all “direct sound” that reaches your ear without reflexions. Yes, this is important. This is the part of the sound that you hear in the first 10 ms. This is important to localize the direction of the sound source, and to judge the attack and build up of a tone, in order to distinguish similar instruments. And now to the MBL 101. As it radiates perfectly 360 degree, all measured data in all horizontal plane measured the same, so the sound in all directions is the same. And as all drivers are time coherent, and as I am using Linkwitz-Riley crossover topology, the vertical radiation patterns is symmetrical to the center plane. So the MBL 101 does succeed this first test with flying colors. This first 10 ms of sound, that is covered by the listening window measurements, do cover now about 20 % of what is important for the total so (PS: The number of 10 ms is mainly an orientation point, but it is widely used, as it matches also the widely used recommendation, to have no disturbing elements near the speaker, closer than ideal 1.6 Meter. We come to that point later, but this 10 ms is the time for the sound to travel 2 x 1.6 Meter). Should this measurement of the listening window, that covers the first 10 ms be flat? No. This part of a measurement should have a 1 dB tilt from bass to top. So what we see up to now is, that what is measured and published the most with loudspeakers (direct on axis frequency response), does count nearly nothing (yes, 2 % is not nothing, but nearly), and that part that reaches your ear in the listening window directly within 10 ms and should tilted 1 dB from bass to top to be right.

2: Early Reflexions: Let us go to the most important part, of how a loudspeaker does sound at home. This is the sound that reaches your ear between 10 and 50 ms. This is the “main part” of the music. It determines how you hear the room of the recording, the width, hight and depth, and how you hear where are all musicians are in the room and to each other, the main part of what they are playing, the main part of what instruments they play, etc. And this part is called “early reflexions” and as the word indicates, those parts you do not hear as a direct sound, those parts you do hear always and via reflexions. And to give you a number: This early reflexions part of the sound covers more than 40 % what is important for the total sound. So this is really the most important part.

And I think it is easy to understand, that the MBL 101, as a loudspeaker that radiates 360 degree, that this is exact the reason, why the MBL 101 does sound “so natural”, concerning the tonality of each instruments, the spatial placement of each music instruments and the sound stage. And yes, this is where MBL is world wide unique and yes, this is what has drawn my attention when I first heard the early version of the MBL 100 and yes, this is, why I joined MBL.

And what else does happen, due to the (we can say) perfect Early Reflexion behavior of the 101?
Natural Sweet Spot: As I explain at every HiFi show, with a conventional direct radiation loudspeaker you have to sit exact in the middle between both speaker in order to have a center image. I am sorry, but I don’t understand why people at home do accept this behavior, as this is not, how it is in the real world and how it should be then at home. Compare this to the MBL 101. You do not need to sit exact in the middle, you can even be outside of the middle seat at all and you will still have the “correct” sound stage, as it is with real musicians. Yes, when you are far left you hear the left channel louder, as you hear with real musicians then the left musician louder, but still, you have a sound stage also between the speakers, and the sound does not collapse to one channel.

Sound Stage Merging: I can also explain this point, if you like, but it is a bit more difficult to explain, and so maybe a bit too long. Please let me know. But I think this would be too much.

Measurement: I also think, that explaining this measurements would be too much and too long. I can say, that you do measure in total 30 different angles and add this together with a weighting. But what I wanted to say about those measurements: Those look “perfect” when they have a tilt of 5 dB from Bass to Treble, so not a flat horizontal line. I noticed this in the very early stage when I was developing the MBL 101, without being possible to read anything about this. But luckily nowadays this is well proven and accepted, and matches perfectly what I have found out 35 years ago and since then, how I develop and tune each and every MBL loudspeaker model and version. During production: when we check our loudspeakers in the factory during production, we combine the first 10 ms measurement (Listening Window) and the 10 – 50 ms measurement (Early Reflexions) together with an adaptive weighing function and this gave us a tilt of 4.5 dB from Bass to Treble. So without all knowledge, and without listening, you would think that these type of measurement look wrong, but they are totally right. And to repeat the beginning of this paragraph: The one position measurement in an anechoic chamber as a flat line, does not give you any information, of how the loudspeaker will sound in a room. Thats not a so easy fact isn’t it?

3: Sound Power: Ok, when we have 30 measurement points to define the “Early Reflexions” area (from the radiation angle) with the time from 10 ms to 50 ms, we will come to 72 measurement points (area) and to all what comes after 50 ms to define the total “Sound Power”. This is all the sound, that flows into the room and determine, how the speaker / room interaction does sound. Here you define the more global characterization of how the speaker does sound in your room. This is a warm sounding system, this is a full bodies sounding system, this is a bright sounding system, etc. It is all about the character of the speaker in your room, when the music decays. You may ask, is this also important? Yes it is, and also with about 40 % of the “Total Sound”. Even this part takes not much when it comes to details, but it takes much about the “Feel”. This is the curve that you get, when you measure with steady state signals (so without time window) your speakers in your room at your listening position. This is the point, whether you can enjoy for long term and relax while hearing music, or if you feel stressed after a period of time. This all goes also with the Sound Pressure to Sound Energy coefficient. This should be homogeneous as well.

As similar as I described in the Early Reflexions point, the MBL 101, as is radiates perfectly 360 degree in the horizontal plane and has a symmetrical behavior in the vertical plane, we got a very smooth response in the “Sound Power” discipline. And guess what, how should a “perfect” Sound Power measurement look like? It will have a 10 dB tilt from Bass to Treble. Sure they are some variations, depending what literature you take as reference. From the very early Bruel & Kjaer to the so called Harmon curve. And there are also some “house” curve, but all have the above mentioned similar character and do differ mainly on the extremes (low bass, high treble).

Do you have recommendations for placement in the listening room in order to benefit from the best omni-directional radiation?

1. Recommendations for placing the MBL 101 in a room are basically the same as for all type of loudspeakers. There is really no difference here. You should have a good balanced room, where you are happy to be in, not too hollow and not too dry, so if you feel comfortable when you speak to someone and feel comfortable when you listen to someone in that room, then this is a good start. If you want some numbers here, an average RT60 (reverberation time) of about 0.45 seconds is a “good” room for listening music. 0.3 is a bit on the dry side (so more like a mixing / mastering room) and 0.6 is a bit on the “live” side, if you don’t have much furniture in your room. It is also always a “cultural” thing and in what kind of rooms you are grown up and used to live in.

2. Try to stay away from too close placed furniture so minimum 80 cm, or better 1.6 meters between the speakers and any furniture would be good. But, as I said above, this is the same with every type of loudspeakers and is not a question of having omni speakers or not.

3. Set them up irregular concerning the distance from the side walls to the back wall, so for example 1.2 Meters from the back wall and 2 meters from the side wall or vice versus and have nothing between the speakers, that would disturb the image of the sound stage.This is, in no way, different to “conventional” direct radiation loudspeaker. The “Omnis” do not need any special need or care. But they will benefit from a good room and a good placement, as every other conventional loudspeakers will do too.

Is this enclosure difficult to power? Could we use them with an amplifier from the Corona series?
The short answer: No the MBL 101 is not difficult to drive, but I strongly recommend to connect the MBL 101 to components, that “match the performance”. And yes, in principle, you could drive them with an amp from the Corona line, but I would never recommend this. Here are the details: To begin this explanation, all MBL omni loudspeakers have the same sensitivity, meaning they produce the same SPL (Sound Pressure Level) per Input Wattage, so no matter if you have the smallest MBL 126 or the MBL 101, as there sensitivity is the same, they are same “loud”. To continue with, sure they do sound different, have different “finesse”, different resolution and different ability to listen “through” the music, but here in this case, they have different bass extension so you feel a different loudness level, when driven with the same amp. But the important part here is to know, that the different MBL loudspeakers have different maximum SPL. So even the 126 does produce the same SPL with 1 Watt input as the MBL 101, the MBL 101 can play much louder and can withstand higher input wattage, and so in the end, you can also fill larger rooms with the MBL 101, as you can fill with the MBL 126.

And to your second part of the question: Yes, the MBL 101 is very transparent to what you feed it, so you clearly and easily hear every improvements what you are doing at the source side, but also detect very easily every weak part of the connected system. It is not the case, that the MBL 101 is “difficult” to drive, in now way, but it is more that it is magnifier glass to what you connect to it, as it is also a magnifier glass to the music. With the MBL 101 you do hear things in the music / recording, that you haven’t heard before with other speakers, and this is great, but you do also hear the weak components in your system, and this has nothing to do with “difficult” to drive. I think / hope you do understand what I mean. All MBL loudspeakers do have specific load matching circuits in the crossover to make sure that the phase angle of the impedance does not reach 60 degree, so that the power amplifier is not stressed too much. And all MBL loudspeakers do have nominal 4 Ohm and all do not go lower than 3.4 Ohm at any frequency. So we took great care, not to stress an amplifier too much.

But the different MBL loudspeakers have different level of sound quality, transparency and performance, so even they are all similar friendly to a power amp, and even they all produce the same SPL with the same wattage, it is worth to match the different sound quality levels of the speakers to that of the connected power amp and front end. So yes, the corona integrated C51 amp that can drive the MBL 126 loudspeaker very well, and it can also possibly drive the MBL 101 loudspeaker to the same SPL, but in no way and under no circumstances I would ever recommend these “matching”. The corona line is a good product line, but the noble line is a better one and the reference line an even more better one. So to answer your question: For the MBL 101 loudspeaker, I would recommend using MBL 9008 power amps in Mono. And “alternatively” the MBL N15 will also work and sound good, but will behave and sound differently. So in the end, it is a question of taste and space what you choose, and how your room is arranged. You have to listen to in your room with your setup to judge.