VCL DI-4212 P

January 2025 – 4-way full range dipole speaker – Audio Technology woofers, midwoofer and midrange – Mundorf AMT – Bliesma 2 dome tweeters back-to-back


Project scope


VCLLabs has launched a new loudspeaker project: the VCL DI-4212-P, a “state-of-the-art” 4-way dipole loudspeaker. This project builds on the knowledge and experience gained from previous studies and developments. Our VCL DI-215-W design, a 3-way dipole loudspeaker, was already a great success, delivering exceptional performance and valuable insights through extensive measurements. With the development of the new 4-way dipole, we aim to further refine and enhance our achievements.

First, we want to apply Audio Technology transducers in a dipole application. The Audio Technology Flex Units are particularly interesting because they can be customized specifically for dipole configurations.

All the Audio Technology transducers used in this loudspeaker feature underhung voice coils. We are interested in evaluating the benefits of this design.

Another goal is to assess the performance of a 4-way design compared to a 3-way version. By adding a midwoofer to the 3-way dipole system, we can evaluate how the additional component impacts overall performance. This loudspeaker can also be configured as a 3-way system, allowing direct comparison between the two configurations.

In addition to the Mundorf AMT dipole tweeter, which we have used in other projects, we want to test a “back-to-back” tweeter dipole concept using two Bliesma T25B-6 dome tweeters.

As with all our speaker projects, we will experiment with various crossover filter types to gain deeper insights into their performance differences in a dipole loudspeaker setup.

The construction of these speakers is currently in progress. Once completed, additional measurements will be done, and updates will be published on this page.


Contents




Chapter 1 Specification Headlines

  • System: 4-way full range dipole with a active digital X-over using miniDSP
  • Woofer: 2 x Audio Technology 12 D 77 customized
  • Midwoofer: Audio Technology 10 A 77 customized
  • Midrange: Audio Technology C-Quenze 18 H 52 06 13 SDKA LR
  • Tweeter: Mundorf AMT17D2.2
  • Low frequency response: F3 = 35 Hz
  • Sensitivity: 81 dB at 1m, 2.83 Vrms, full space
  • SPL at maximum excursion of the woofers at 35 Hz @ 35 – 20000 Hz: 98 dB, 1m, full space
  • Crossover: Different filter types at 120, 550 and 2500 Hz
  • Dimensions: width x heigth x depth = 46 cm x 152 cm x 34 cm



Chapter 2 Cabinet


Cabinet version using one AMT tweeter


Cabinet version using two dome tweeters back-to-back



The speaker has two large baffles, each 4 cm thick. One measures 80 x 40 cm for the two woofers, and the other measures 36 x 40 cm for the midwoofer. Between the two baffles are two smaller baffles for the midrange and tweeter. The midrange baffle is mounted on the woofer baffle, and the tweeter baffle is placed between the midrange and midwoofer baffles.

The vertical pole has a U-shape. A cover can be screwed onto the back of the pole. On the cover are four pairs of terminal plugs for connecting amplifiers. Inside the pole, the wiring can be routed.

To assemble the speaker as a single unit, five trapezoidal frames and a vertical pole are used. Three trapezoidal frames are screwed to the back of the woofer baffle, and two trapezoidal frames are attached to the midwoofer baffle. The five trapezoidal frames are screwed to the vertical pole at the back. On the sides, two side panels are screwed to the woofer and midwoofer baffles to connect the two baffles. This creates a standing structure with the woofer baffle at the front and the vertical pole at the back.


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Chapter 3 Transducers – Acoustical measurements on IEC baffle


For this design, we chose to use Audio Technology transducers for the woofers, midwoofer, and midrange. We have had excellent experiences with these drivers in past projects. They perform exceptionally well and are very solidly and reliably built. Additionally, their Flex units allow the driver parameters to be customized for a dipole application, a feature offered by very few speaker manufacturers.

For the tweeter, we opted for a Mundorf AMT tweeter on one hand and two Bliesma dome tweeters back-to-back on the other. We are familiar with the Mundorf from our previous dipole project, the VCL DI-215-W. The Bliesma dome tweeters are well-known and compact enough to create a back-to-back dipole tweeter.

For all new speaker drivers used in a new project, we measure them on our IEC 225 Hz baffle. The SPL on-axis at a distance of 1 meter and 2.83 Vrms is measured, as well as the impedance at 2.83 Vrms. Using these two measurements, it is possible to create a fairly accurate infinite baffle response of the driver, which is used in the further design process.
With the IEC 225 Hz baffle, the SPL can be accurately measured above 650 Hz without diffraction. The SPL below 650 Hz is calculated from the impedance measurement and the other TSP (Thiele-Small parameters). Both SPL responses are then spliced together to achieve the infinite baffle response.



3.1 – Woofer Audio Technology Flex 12 D 77


For the woofer selection, we started with an existing Audio Technology Flexunit, the 12 D 77 25 10 KAP. We adjusted several parameters to increase the Qts and achieve maximum sensitivity with the existing D magnet for a dipole application. Additionally, we opted for an underhung voice coil for this woofer. The voice coil resistance was slightly increased to allow two woofers to be connected in parallel for this project.


Audio Technology Flex 12 D 77 mounted in IEC baffle



Audio Technology Flex 12 D 77 (custom) – Measured Infinite Baffle response at 1m on-axis , 2.83 Vrms


Audio Technology Flex 12 D 77 (custom) – Measured Impedance at 2.83 Vrms on IEC baffle


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3.2 – Midwoofer Audio Technology Flex 10 A 77


For the midwoofer selection, we started with an existing Audio Technology Flexunit, the 10 A 77 25 10 KAP. We adjusted several parameters to increase the Qts and achieve maximum sensitivity with the existing A magnet for a dipole application. Additionally, we opted for an underhung voice coil for this midwoofer. The voice coil resistance was slightly decreased to improve the sensitivity.


Audio Technology Flex 12 D 77 mounted in IEC baffle


Audio Technology Flex 10 A 77 (custom) – Measured Infinite Baffle response at 1m on-axis , 2.83 Vrms


Audio Technology Flex 10 A 77 (custom) – Measured Impedance at 2.83 Vrms on IEC baffle


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3.3 – Midrange Audio Technology C-Quenze 18 H 52

For years, we have been using the Audio Technology Flex 5 H 52 17 06 SD midwoofer in a 3-way speaker with excellent results.
The Audio Technology C-Quenze 18 H 52 06 13 SDKA LR, which we are choosing for this project, features a Kapton Aluminum underhung voice coil and the LR-magnet system. It is highly recommended by Audio Technology for dipole applications, and Troels Gravesen calls it “My favorite midrange.”


Audio Technology C-Quenze 18 H 52 06 13 SDKA LR mounted in IEC baffle


Audio Technology C-Quenze 18 H 52 06 13 SDKA LR – Measured Infinite Baffle response at 1m on-axis , 2.83 Vrms



Audio Technology C-Quenze 18 H 52 06 13 SDKA L – Measured Impedance at 2.83 Vrms on IEC baffle


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3.4 – Tweeter Mundorf AMT17D2.2

The first tweeter that we have chosen for this project is the Mundorf AMT17D2.2. This dipole tweeter is performing wery well in the VCL DI-215-W and is one of the smallest dipole AMT tweeters that can be found.


Mundorf AMT17D2.2 mounted in IEC baffle


Mundorf AMT17D2.2 – Measured Infinite Baffle response at 1m on-axis , 2.83 Vrms


Mundorf AMT17D2.2 – Measured Impedance at 2.83 Vrms on IEC baffle


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3.5 – Tweeter Bliesma T25B-6

In addition to the Mundorf AMT dipole tweeter, we will test a “back-to-back” tweeter dipole concept using two Bliesma T25B-6 dome tweeters. This Bliesma tweeter is very well known and it is a high performing driver. Its outer dimensions are very suitable to realize a compact back-to-back tweeter concept.

Bliesma T25B-6 – Datasheet Infinite Baffle SPL and impedance responses at 1m on-axis , 2.83 Vrms


This page will be updated as soon as these tweeters are at our home and the acoustical measurements on IEC baffle are performed.


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Chapter 4 Dipole radiation simulation of the transducers on the baffle


4.1 – Woofer dipole radiation


Simulation in Leap

2 x Audio Technology 12D77 on a baffle with dimensions W x H = 540 x 880 mm. The baffle width for the Leap simulation is calculated as the actual baffle width of 460 mm plus two times the baffle depth of 40 mm.


SPL on axis (black) and SPL on infinite baffle (pink) at 3m, 2.83 Vrms in full space


Dipole transfer (blue) compared with +6 dB slope curve (yellow)

The dipole transfer is the ratio of the SPL on axis over the SPL on infinite baffle. In normal conditions this dipole transfer is a +6dB slope below the dipole peak.
The deviation of the dipole transfer below 80 Hz is caused by a Leap artifact at low frequencies. Some correction can be calculated on the dipole transfer below 80 Hz. The corrected dipole transfer is shown in the plot below.


Corrected dipole transfer up to 80 Hz (violet) compared with +6 dB slope curve (yellow) and the uncorrected simulated dipole transfer (blue)


Using this corrected dipole transfer together with the woofer infinite baffe response, the SPL on axis response at 1m, 2.83 Vrms can be calculated. It isexpected to be more accurate and closer to the practical SPL, that will be measured.


Corrected SPL on axis (black) using corrected dipole transfer and SPL on infinite baffle (pink) at 1m, 2.83 Vrms in full space

This SPL curve can be used to make the loudspeaker sensitivity analysis.
Choosing F3 = 35 Hz, it is expected that the sensitivity at 1m,2.83 Vrms in full space will be close to 81 dB.


SPL on axis and horizontal off axis 15 30 45 60 75 90 degrees at 3m, 2.83 Vrms in full space


Horizontal polar diagram 80 – 160 – 320 – 640 – 1280 Hz at 3m in full space


SPL on axis and vertical off axis 15 30 45 60 75 90 degrees at 3m, 2.83 Vrms in full space


Vertical polar diagram 80 – 160 – 320 – 640 – 1280 Hz at 3m in full space


SPL on axis and the power H+V (blue), H (orange) and V (violet) at 3m, 2.83 Vrms in full space


Directivity Index H+V (blue), H (orange) and V (violet) at 3m in full space


Dipole beaming

If we examine the DI (directivity index) curves of this dipole system, we see that the DI increases above a certain frequency, or in other words, the directivity of the driver in the dipole baffle increases at higher frequencies. This directivity is caused, on the one hand, by the dipole effect and, on the other hand, by the driver’s directivity at higher frequencies. The dipole directivity starts at low frequencies and decreases toward higher frequencies, depending on the baffle width, which determines the path length from the rear to the front side of the dipole system.

It would be interesting to observe only the effect of dipole directivity as a function of frequency, excluding the driver’s directivity. The dipole directivity can be calculated by dividing the total DI by the DI caused by the driver’s directivity. In the plot below, the total DI is shown along with the DI resulting from the driver’s directivity and the DI resulting from the dipole directivity.


Directivity Index H+V (blue), driver directivity on infinite baffle (grey) and dipole driectivity (pink)


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4.2 – Midwoofer dipole radiation


Simulation in Leap

Audio Technology 10A77 on a baffle with dimensions W x H = 540 x 436 mm. The baffle width for the Leap simulation is calculated as the actual baffle width of 460 mm plus two times the baffle depth of 40 mm. The baffle heigth is calculated as the actual baffle height of 356 mm plus two times the baffle depth of 40 mm.


SPL on axis (black) and SPL on infinite baffle (pink) at 3m, 2.83 Vrms in full sphere


Dipole transfer (brown) compared with +6 dB slope curve (orange)


SPL on axis and horizontal off axis 15 30 45 60 75 90 degrees at 3m, 2.83 Vrms in full sphere


Horizontal polar diagram 80 – 160 – 320 – 640 – 1280 Hz at 3m in full sphere


SPL on axis and vertical off axis 15 30 45 60 75 90 degrees at 3m, 2.83 Vrms in full sphere


Vertical polar diagram 80 – 160 – 320 – 640 – 1280 Hz at 3m in full sphere


SPL on axis and the power H+V (brown), H (orange) and V (violet) at 3m, 2.83 Vrms in full sphere


Directivity Index H+V (brown), H (orange) and V (violet) at 3m in full sphere


Directivity Index H+V (brown), driver directivity on infinite baffle (grey) and dipole directivity (pink)


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4.3 – Midrange dipole radiation


Simulation in Leap

Audio Technology 18 H 52 06 13 SDKA-LR on a circular baffle with diameter of 246 mm and 18 mm depth. The baffle diameter for the Leap simulation is calculated as the actual baffle diameter of 210 mm plus two times the baffle depth of 18 mm, which equals 246 mm.


SPL on axis (black) and SPL on infinite baffle (pink) at 3m, 2.83 Vrms in full sphere


Dipole transfer (green) compared with +6 dB slope curve (orange)


SPL on axis and off axis 15 30 45 60 75 90 degrees at 3m, 2.83 Vrms in full sphere


Polar diagram 320 – 640 – 1280 – 2560 – 3840 Hz at 3m in full sphere


SPL on axis (black) and the power (green) at 3m, 2.83 Vrms in full sphere


Directivity Index (green) at 3m in full sphere


Directivity Index H+V (green), driver directivity on infinite baffle (grey) and dipole directivity (pink)


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4.4 – Tweeter AMT dipole radiation


Simulation in Leap

Mundorf AMT17D2.2 on a baffle with dimensions W x H x D = 70 x 80 x 12 mm. For the Leap simulation a box with same dimensions is choosen with two drivers in opposite phase, one on the front and one on the back. A box with some depth is preferred as baffle model to obtain the best diffraction simulation at a few kHz.


SPL on axis (black) and SPL on infinite baffle (pink) at 3m, 2.83 Vrms in full sphere


Dipole transfer (red) compared with +6 dB slope curve (yellow)


SPL on axis and horizontal off axis 15 30 45 60 75 90 degrees at 3m, 2.83 Vrms in full sphere


Horizontal polar diagram 1.20 – 2.56 – 3.84 – 5.12 – 6.40 – 12.80 kHz at 3m in full sphere


SPL on axis and vertical off axis 15 30 45 60 75 90 degrees at 3m, 2.83 Vrms in full sphere


Vertical polar diagram 1.20 – 2.56 – 3.84 – 5.12 – 6.40 – 12.80 kHz at 3m in full sphere


SPL on axis and the power H+V (red), H (orange) and V (violet) at 3m, 2.83 Vrms in full sphere


Directivity Index H+V (red), H (orange) and V (violet) at 3m in full sphere


Directivity Index H+V (red), driver directivity on infinite baffle (grey) and dipole directivity (pink)


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3.5 – Tweeter back-to-back dipole radiation


Simulation in Leap

Two Bliesma T25B-6 Be dome tweeters are placed back to back in opposite phase in a box with dimensions W x H x D = 72 x 500 x 30 mm. The rear tweeter is placed 60 mm higher than the front tweeter.

Front side


Rear side


SPL on axis (black) and SPL on infinite baffle (pink) at 3m, 2.83 Vrms in full sphere


Dipole transfer (red) compared with +6 dB slope curve (orange)


SPL on axis and horizontal off axis 15 30 45 60 75 90 degrees at 3m, 2.83 Vrms in full sphere


Horizontal polar diagram 1.20 – 2.56 – 3.84 – 5.12 – 6.40 – 12.80 kHz at 3m in full sphere
Not normalzed to the SPL on-axis


SPL on axis and vertical off axis 15 30 45 60 75 90 degrees at 3m, 2.83 Vrms in full sphere


Vertical polar diagram 1.20 – 2.56 – 3.84 – 5.12 – 6.40 – 12.80 kHz at 3m in full sphere
Not normalzed to the SPL on-axis


SPL on axis and the power H+V (red), H (orange) and V (violet) at 3m, 2.83 Vrms in full sphere


Directivity Index H+V (red), H (orange) and V (violet) at 3m in full sphere


Directivity Index H+V (red), driver directivity on infinite baffle (grey) and dipole directivity (pink)


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Chapter 5 Acoustical measurements of the transducers in the dipole baffle

This page will be updated as soon as these acoustical measurements are performed.


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Chapter 6 Digital Crossover


6.1 – Digital Crossover design using simulation results


A first digital crossover filter has been designed to conduct an initial evaluation of the behavior of the sum of the filtered drivers of this dipole speaker – both the on-axis and off-axis SPL response – the power response and the directivity index. Additionally, the crossover frequencies can already be selected based on the simulation results. An LR4-type filter was chosen because it provides an interesting reference point to start with and to compare with other filter types at a later stage. The use of simulations also offers the advantage of easily examining the horizontal and vertical polar diagrams, which is more complex to achieve with measurements, especially in the vertical direction.


Simulated SPL responses of all drivers in the dipole baffles at 1m, 2.83 Vrms in full space
woofer = blue, midwoofer = brown, midrange = green and tweeter = red

For the simulation of these SPL responses in the dipole baffles, the driver datasheets and the IEC baffle measurements were used to create the most accurate transducer models for these simulations.


Simulated DI responses of all drivers in the dipole baffles at 3m in full space
woofer = blue, midwoofer = brown, midrange = green and tweeter = red

The crossover frequencies are chosen at 120 Hz, 550 Hz and 2500 Hz. This ensures that the individual driver DI curves have sufficient overlap to achieve the flattest overall DI value for the summed response of this dipole speaker.


Crossover filter targets LR4 120 550 2500 Hz


SPL on axis of the filtered drivers and the sum at 1m, 2.83 Vrms in full space


SPL horizontal off axis of the sum at 0 , 30 and 60 degrees at 3m, 2.83 Vrms


Power response of the filtered drivers and the sum in free space at 3m, 2.83 Vrms in full space


Directivity Index in free space of the filtered drivers and the sum
woofer = blue, midwoofer = brown, midrange = green and tweeter = red


As expected, the power and DI responses show that the midrange has less power above 1kHz. This is caused by the driver itself beaming at those frequencies. A remedy could be to choose a smaller midrange, but experience so far has shown that midranges smaller than 6.5 inches perform less well in these dipole applications.
The midrange behavior above 1 kHz that we observe here has not yet been measured. Off-axis measurements are needed to accurately map the power behavior. We will carry out these measurements as soon as the speaker is ready. Previous dipole projects have taught us that there is certainly a power dip above 1 kHz, though the exact shape of the curve will likely differ in the measurement.
We have decided to start with the 6.5-inch Audio Technology midrange, check it all with measurements and listen to it. Potentially we do additional tests later with other midranges, including smaller versions.


6.2 – Digital Crossover design using measurement results


Note: this page will be updated as soon as the acoustical measurements of the transducers in the dipole baffles are performed.


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Chapter 7 Listening tests


Note: this page will be updated as soon as the listening tests are performed.


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