The Radialstrahler mbl 101D Loudspeaker

mbl-Akustikgerate GmbH
Einemstr 20a
D-10785 Berlin

Note: Since we originally ran the surround sound review of the unique mbl Radialstrahler speakers in 21st, we have received a number of reader inquiries about their design and construction principals. We therefore asked mbl to provide a white paper, below, that discusses the engineering effort that went into making this extraordinary loudspeaker. Since that review appeared, we have received a new pair of 101D speakers. They are actually superior in many ways (especially in the bass) to the already excellent 101D review pair we had. These improvements clearly reflect an ongoing commitment to musical perfection and engineering excellence by mbl -- Ed.
You seek the ultimate reproduction of music. You want sound in your home like that heard in a concert hall, music that naturally envelops you. A sonic experience like this demands a balanced combination of direct and indirect acoustic waves. Getting just the right ratio of volume and time offset between direct primary sound and diffuse secondary sound will produce the highest level of quality and musical enjoyment.

For reproduction in your listening room, the sound sources must generate a sound field that is similar to that of instruments in a concert hall. However, even the best quality hi-fi speakers that use conventional cone and dome drivers cannot produce such a natural sound field. They radiate sound in a completely different manner than musical instruments. This quest for natural sounding music created the impulse for us at mbl to go well beyond the bounds of convention and to develop our own unique speaker concept.

The experience of 3-D sound

What we wanted would have been impossible to achieve with any existing systems as they mark the end of today├.s conventional speaker physics. The realization of our idea needed a isotropic speaker with high impulse accuracy. That is why we developed a new loudspeaker principle and made it fit for series production: the Radialstrahler. A loudspeaker no longer must be the weakest component in your stereo system. Just compare the mbl 101D specifications with conventional high-end speakers. Even more important, listen to the mbl 101D because it is your ears that set the standard for the ultimate listening experience.

The new speaker design principle: The mbl 101

The Radialstrahler took more than twenty years of research to develop. It is unique. It has nothing in common with traditional transducers. With its novel technology, the Radialstrahler's sound qualities are so outstanding that even listener positioning turns out to be of minor importance.

We have applied a construction principle that could only be facilitated by the use of new materials. In the Radialstrahler, lamelloid transmitter parts, grouped in a column, are forced to vibrate. The result is an uncolored, neutral, and homogeneous sound field that has a three-dimensional character and naturalness.

Figure 1. The 101D and its three Radialstrahler drivers

Lamellae, computers, and Astronautics

The Radialstrahler lamellae are made of flexible transducer strips and are the result of an exemplary cooperation. After our engineers calculated the Radialstrahler's basic shape, we then optimized the transducer systems in cooperation with Professors Dr. J. Sternberg and Dr. H. Fritz at the Aerospace Research Department of the Technical University of Berlin.

During an extensive long-term series of tests, the team discovered that the lamellae for the woofer produce the least coloration when made of an aluminum/magnesium alloy. But that├.s not all. To have the woofer sound nothing but neutral, we had to: 1) coat the inside of the lamellae with special materials to get a sandwich construction, 2) take acoustic measures of each single segment, and 3) interconnect the strips with special V-shaped, die-pressed beads of Subronyl.

Figure 2. mbl 101D, its three Radialstrahler drivers and their unique Lamellae strips
Photo Detail: Tweeter, upper left; Midrange, lower left; Woofer, right

Precision and cool coils

We take the liberty of calling the lamellae in the tweeter and mid drivers another engineering landmark. Each single lamella is produced labor-intensively by hand from carbon fiber compound. It is then die-formed under high pressure at a specific temperature. Assembly of the parts, which have been meticulously checked, again demands very fine mechanical precision. For instance, to reduce weight, the voice coil of the tweeter moves free and without a spider in the air of the magnet. This requires a manufacturing tolerance of less than 1/100 millimeter.

In order for the speaker to handle peak power of 2200 W, we used a highly effective cooling system in the voice coils. The voice coils of the tweeter and mid driver float in ferrofluid cooling material. The net effect is a reduction of thermal distortion. As a consequence, we achieve neutral and distortion-free reproduction of short peaks (no compression) which many other speakers simply can not do.


The Radialstrahler tweeter is built of 24 segments using unidirectional carbon fiber (a pulsator group). The shape of the segments is precisely calculated to get minimum distortion regardless of power and frequency. We coated their surface with a thin anti-resonance film. To close the power loop between magnet, voice coil and pulsator group, we use a solid one-piece carbon fiber block. In combination with its pure 4N copper wiring, the performance of the Radialstrahler tweeter is outstandingly good.


This is the most critical part of any speaker. After lengthy research, we discovered that a group of crossweaved carbon fiber segments was the perfect design. These segments were carefully damped by a special rubber material applied on the inside, and also with an anti-resonance coating used on the outside. To close the power loop, we connected the solid AlMg bearing with 4 super-refined steel rods with a magnet. The bearing is single side coated with a heavy tar plate to remove resonance. The voice coil is glued with a special high temperature glue (the same glue as used in the brakes of cars) and burned in at 230┬░C (500┬░F). To get maximum "speed", we don├.t use a spider for centering the voice coil. Its wiring is also 4N copper.


Integration of Deep Bass with the Low Mid Driver

The MBL 101D has absolutely integrated low bass response. The result is a seamless four-way system.

Schroeder frequency alignment.

In regular living areas there exists a frequency (about 60 to 80 Hz) under which bass is no longer radiated into the room. The bass response below this point, called the Schroeder frequency, is made up of an irregular arrangement of standing waves. A loudspeaker design with a linear alignment of the low frequency region will produce inaccurate bass response in a regular living room. This undesirable result is confirmed in the anechoic chamber. To avoid this problem, we gave the MBL 101D a gentle domination right above the Schroeder frequency. As a result, the lower mid unit creates the rhythmic pattern of the bass instruments, while the sub woofer adds low-end solidity and warmth but without disturbing the rhythmicality of the music. This is a great advantage of the 101D design and is very important for sound quality.


The Subwoofer system is manufactured using a bandpass construction. This construction leads to an acoustical second-order high pass, which results in a slope of 12 dB/octave. This smooth slope ensures very rapid and accurate bass reproduction. Further, within the sealed back enclosure of the woofer, there is an internal subsonic filter that eliminates excessive excursions of the speaker at low frequencies. Thus, the woofer always operates in a linear region of the magnet field, and there is no possibility of chassis overthrow.

Intrinsic to this system is also a second-order low pass filter, which helps prevent the dispersion of very small distortion by-products by the speaker.

With this unique system, it is possible to have a very high frequency response, great dynamic range, and high volume levels; yet also achieve low speaker excursions by the woofer, thereby minimizing distortion.

1. Bandwidth

Using a bandwidth of 2 octaves, mbl can create a very low 3-dB point with a relatively small woofer system. The 3 dB down point is 27 Hz, and the 8-dB point (usually given in data sheets) is 20 Hz.

To get a fourth order Linkwitz-Riley low pass filter, mbl added a second order electrical low pass filter to the intrinsic second order low pass of the bandpass. In order for this type of configuration to operate correctly, mbl compensated by modifying, electrically, the second impedance maximum of the bandpass by paralleling a serial RCL network. This creates very smooth system impedance behavior. Further, mbl reduced the resonance of the port with this serial RCL network parallel design.

2. Subwoofer-Driver

The woofer is a 12" driver with a 4" voice coil. The voice coil is composed of black anodized aluminum that has a very low thermal resistance, approximately 1 K/W. This gives the system large power handling capability. The voice coil wire is also constructed of high temperature material able to withstand temperatures of up to 360¡C. The wire is glued with the same glue used in the fabrication of motorcycle brakes, and is able to withstand a temperature of 260¡C. As a result, it is almost impossible to destroy the speaker. The airgap is 10 mm high and the moving coil has a winding width of 24 mm. Thus, the speaker possesses an extremely high linear throw, with a maximum of 14-mm peak to peak. The maximum mechanical throw is two times greater -- 28 mm maximum peak to peak.

3. Cabinet

The cabinet is manufactured out of a 30-mm thick MDF (medium density fiber) board, which has both excellent rigidity and a damping coefficient. Additionally, mbl used a rectangular cross configuration in the cabinet to stabilize the walls and to hold the magnet with a polyurethane polymer support. This construction is highly effective in protecting the magnet from any movement or resonance.

Sound optimization and adjustment of Radialstrahlers

Our engineers' extensive research also produced another special technical delight. In the mbl 101D, you can adjust the speaker's tweeter, mid-range driver and woofer to sonically match different electronic components, as well as rooms with complicated acoustics. A control panel with different setting possibilities is located at the back of the speaker, on the crossover housing. Three jumpers allow for activating various functions.

Note: The electrical characteristics of the crossover network are the same in all the different positions described below; only the molecular microstructure of its signal path is changed. This change is achieved through the use of different materials.

Tweeter Adjustments

SMOOTH: This setting is used to quiet electronic components that tend to hiss.

NATURAL: This setting is recommended by mbl for neutral sounding high-end electronics combined with neutral cables.

FAST: This setting is recommended when the electronic components are not very good in their treble response (e.g., too dull).

Midrange Adjustments

NATURAL: This setting is recommended by mbl for neutral sounding high-end electronics combined with neutral cables.

RICH: This setting emphasizes the "chest note" range so that the sound becomes more lively and colored. It is recommended for amplifiers and cables that tend to sound dead.

Lowrange Adjustments (bass reproduction)

SMOOTH: This setting is recommended for well-damped and acoustically dry rooms.

ATTACK: Recommended for rooms in which the bass tends to boom or seems too overwhelming.


System: 4-way.

Frequency range: 20 Hz to 40,000 Hz.

Impedance: 4 ohms.

Sound Pressure Level: 81 dB/W/m (2.83V/ 2p).
linear max. 106 dB.

Crossover frequencies: 110 Hz, 600 Hz, 3500 Hz
Linkwitz-Riley, 4th order.

Acoustic center: 109 cm.

Power, continuous: 320 W / 500 W
peak power 2200 W.

Woofer low: 300 mm, 12-inch cone driver (mbl).

Woofer high: Radialstrahler TT100 (mbl).

Mid-range: Radialstrahler MT 50, cross-directional CFK (mbl).

Tweeter: Radialstrahler HT 37, unidirectional CFK (mbl) Finish: High-polish black piano or satin black.

Dimensions: 400 x 400 x 1800 mm (BxTxH)
16 x 16 x 70 inches (W x D x H).

Weight: 80 kg, 176 lbs. (each).

Copyright 1999, mbl All Rights Reserved,
mbl-Akustikgerate GmbH
Einemstr 20a
D-10785 Berlin
Telefon 030 / 851 80 74 ┬Ě Telefax 030 / 851 80 62 ,

21st, The VXM Network,