VCL EX32

June 2025 – 2-way monitor with SB Acoustics transducers
Open source design – Low budget speaker


The VCL EX32 is a concept study for a budget-friendly monitor speaker. It incorporates carefully selected SB Acoustics drivers: the 6 inch SB17MFC35-8 midwoofer and the 1 inch silk dome SB26STAC-C000-4 tweeter – both offering excellent performance relative to their cost.

To enhance low-frequency extension, a vented-box (bass reflex) enclosure is used. The cabinet dimensions (W × H × D) are 25 × 49 × 30 cm, excluding the speaker stand.

The estimated bill of materials for a pair of speakers, including a good quality crossover, is approximately €700.


Contents



Chapter 1 Specification Headlines

  • System: 2-way, vented box system, passive X-over
  • Midwoofer: SB Acoustics SB17MFC35-8
  • Tweeter: SB26STAC-C000-4
  • Low frequency response: F3 = 50 Hz
  • Sensitivity: 83.5 dB at 1m, 2.83 Vrms, full space
  • SPL at maximum excursion, 50 – 20000 Hz: 94 dB, 1m, full space
  • Crossover: LR4 at 2500 Hz, optional Elliptical 3rd order at 2500 Hz
  • Impedance: value between 5.6 and 33 Ohm
  • Cabinet dimensions, exclusive stand: W x H x D = 25 cm x 49 cm x 30 cm


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Chapter 2 Estimation Bill Of Material

  • 2 x Midwoofer SB Acoustics SB17MFC35-8: €146
  • 2 x Tweeter SB Acoustics SB26CDC-C000-4: €95
  • Components good quality crossover: €250
  • Cabinet material: €100
  • Cabinet Accessoires: €100
  • Total with good quality crossover €691 euro


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Chapter 3 Speaker System


3.1 Midwoofer SB17MFC35-8

The SB Acoustics SB17MFC35-8 is a 6 inch midwoofer with a polypropylene cone. It is placed in a vented box system.
This a very well performing driver. The 4 Ohm version has been measured by HifiCompass, see in following link.


Midwoofer Vented-box System – Small Signal Analysis

The calculations for the vented-box system response are based on the article “Vented-Box Loudspeaker Systems” by Richard H. Small, which serves as the primary reference. A summary of the key parameter values and resulting calculations is provided. For a more in-depth explanation of the methodology and theory, please refer to the original article.

SB Acoustics Midwoofer Parameters:

  • Resonance frequency (fs): 33 Hz
  • Equivalent compliance volume (Vas): 39 L
  • Total Q factor (Qts): 0.37
  • Mechanical Q factor (Qms): 4.9
  • Voice coil resistance (Re): 5.7 Ω
  • Ratio Res = (Bl)2 / Rms = 69.6

System Configuration:

  • Enclosure volume (Vb): 20 L
  • Tuning frequency (fB): 40 Hz
  • Cabinet walls lined with damping material

System Response Parameters:

  • Transducer time constant (Ts): 4.8 ms
  • Vented-box system time constant (Tb): 4.0 ms
  • Compliance ratio (α): 1.95
  • Total system Q factor (QT): 0.37
  • Leakage losses QL: 7

Using these values, the frequency response of the system on an infinite baffle can be computed according to the formulas presented in the reference article.

The green curve represents the vented-box response, while the red curve shows a Butterworth B4 alignment at the same system tuning frequency for comparison. The chosen system design is more heavily damped than the B4 alignment.

With the current parameter values, the -3 dB cutoff frequency (F3) is approximately 50 Hz.

An initial estimate for the port configuration suggests using two tubes with a diameter of 35 mm and a length of 135 mm. The Monacor MBR-35 bass reflex tube is a suitable choice for this application.

These response curves include cabinet losses, with a quality factor of QL = 3. Damping material is applied to the internal walls of the enclosure.

To verify the amount of internal filling and fine-tune the port resonance (targeted at 40 Hz), the midwoofer’s impedance curve should be measured. The impedance dip between the two peaks should occur at 40 Hz. The upper impedance peak, near 65 Hz, is expected to reach approximately 35 ohms.


Impedance midwoofer SB17MFC35-8 in the cabinet


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3.2 Tweeter SB26STAC-C000-4

The SB Acoustics SB26STAC-C000-4 has a textile dome. This tweeter is similar to the SB Acoustics SB26STCN-C000-4, which has a smaller front plate. Last one has been measured by HifiCompass and it is performing very well, see following link.


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Chapter 4 Cabinet


Cabinet Mechanical drawing

These are the cabinet dimensions as used for this concept study.

On the bottom there is an internal chamber to mount the crossover filter.
All panels are 18 mm thick, the front panel is 22 mm.
The transducers are mounted flush with the front panel


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Chapter 5 Responses of the transducers in the cabinet


5.1 SPL and impedance of the transducers in the cabinet


These are the SPL and impedance curves of the transducers placed in the cabinet in full space at 1 meter, 2.83 Vrms . The SPL responses on infinite baffle are also shown to see the impact of the cabinet on the response when placed in a full space domain. These simulated curves will be used for the first crossover filter design.


SPL midwoofer SB17MFC35-8 in the cabinet in full space (blue curve) and on infinite baffle (pink curve)


Impedance midwoofer SB17MFC35-8 in the cabinet


SPL tweeter SB26CDC-C000-4 in the cabinet in full space (red curve) and on infinite baffle (pink curve)


Impedance tweeter SB26CDC-C000-4 in the cabinet


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5.2 SPL off axis, Polar diagrams, Power and Directivity Index of the transducers in the cabinet


Horizontal SPL off axis response of midwoofer SB17MFC35-8 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Vertical SPL off axis response of midwoofer SB17MFC35-8 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Horizontal SPL off axis response of tweeter SB26STAC-C000-4 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Vertical SPL off axis response of tweeter SB26STAC-C000-4 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Horizontal polar diagram woofer SB17MFC35-8 in cabinet in free space
at 80 – 160 – 320 – 640 – 1280 – 2560 Hz


Vertical polar diagram woofer SB17MFC35-8 in cabinet in free space
at 80 – 160 – 320 – 640 – 1280 – 2560 Hz


Horizontal polar diagram tweeter SB26CDC-C000-4 in cabinet in free space at 1.2 – 2.5 – 3.8 – 5.1 – 6.4 – 12.8 kHz


Vertical polar diagram tweeter SB26CDC-C000-4 in cabinet in free space
at 1.2 – 2.5 – 3.8 – 5.1 – 6.4 – 12.8 kHz


SPL on axis (black) and Power (blue) of midwoofer SB17MFC35-8 in the cabinet in full space


SPL on axis (black) and Power (red) of Tweeter SB17MFC35-8 in the cabinet in full space


Directivity Index DI in full space of midwoofer (blue) and tweeter (red) in the cabinet

The Directivity Index plot shows that, to keep the DI of the sum response flat, the crossover frequency can be chosen around 2.5 kHz.


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Chapter 6 Crossover Filter


Filter Targets

LR4 at 2500 Hz

Sensitivity is 83.5 dB. The -3 dB cutoff frequency (F3) is 50 Hz.


Acoustic centers of the transducers

For this concept design, the acoustic center of the midwoofer is assumed to be 20 mm behind the acoustic center of the tweeter.


Schematic


Filter components

For a good quality crossover, air core wire coils, MKP capacitors and 10W MOX or carbon resistors, are recommended.


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Chapter 7 Responses of the total speaker with crossover filter


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

The filtered SPL of the midwoofer and the tweeter are not perfectly designed on target in their filter stop bands. The passive filter design has been made to compensate for the acoustic center offset between midwoofer and tweeter in the best way and make the filtered midwoofer and tweeter phases aligned.


Impedance



Phase of the filtered drivers, full space
Woofer in blue, tweeter in red, sum in black, phase target in grey


SPL of the sum on and off axis at 3m, 2.83 Vrms, full space
0 degrees in black, 30 degrees in green, 60 degrees in blue


Power response of the filtered drivers and the sum at 3m, 2.83 Vrms


Directivity Index in full space of the filtered drivers and the sum
woofer in blue, tweeter in red, sum in black


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Chapter 8 VituixCAD design VCL EX32


A crossover design has also been created in VituixCAD, using the on- and off-axis SPL data curves from the LEAP simulation. This VituixCAD project is particularly useful for making adjustments to the crossover schematic when fine-tuning the speaker during a practical build.

The VituixCAD project and associated data files are available free of charge in a ZIP archive upon request. To obtain them, please contact us at info@vcllabs.com.



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