Design Kit VCL SDK00
2-way monitor with SB Acoustics transducers


The VCL SDK00 is a demonstration speaker design kit.



Dimensions W x H x D = 24 x 42 x 38 cm.
Midwoofer: SB Acoustics SB17MFC35-8 in a vented box system.
Tweeter: SB Acoustics SB26CDC-C000-4.
Bill of material estimation for 2 speakers is about 900 euro with a high quality crossover and 650 euro with a medium quality crossover.

Contents



Specification

  • System: 2-way, vented box system, analog X-over
  • Midwoofer: SB Acoustics SB17MFC35-8
  • Tweeter: SB26CDC-C000-4
  • Low frequency response: F3 = 47 Hz
  • Sensitivity: 82 dB at 1m, 2.83 Vrms, full space
  • SPL at maximum excursion, 47 - 20000 Hz: 93 dB, 1m, full space
  • Crossover: LR4 at 2500 Hz, optional Elliptical 3rd order at 2500 Hz
  • Impedance: value between 6 and 16 Ohm
  • Cabinet dimensions: width x heigth x depth = 24 cm x 42 cm x 38 cm

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Estimation Bill Of Material

  • 2 x Midwoofer SB Acoustics SB17MFC35-8: 120 euro
  • 2 x Tweeter SB Acoustics SB26CDC-C000-4: 90 euro
  • Components high quality crossover: 500 euro
  • Components medium quality crossover: 240 euro
  • Cabinet material: 100 euro
  • Cabinet Accessoires: 100 euro

  • Total with high quality crossover 910 euro
  • Total with medium quality crossover 650 euro


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Speaker System



Midwoofer



The SB Acoustics SB17MFC35-8 is a 6 inch midwoofer with a polypropylene cone. It is placed in a vented box system. Its price is around 60 euro.


Midwoofer Vented-box System - Small Signal Analysis

To calculate the responses of the vented-box system, the article "Vented-box Loudspeaker Systems" by Richard H.Small is used as the reference. In this post only the parameter values and the calculation results are summarized. For a more detailed understanding, the reference article can be used.

For the SB Acoustics midwoofer: fs = 33 Hz; Vas = 39 L; Qts = 0.37; Qms = 4.9; Res = 69.6
With a cabinet volume = 20 L, fB = 35 Hz, QL = 3 (cabinet filled with dampening material) and the serial source resistance = 0.5 Ohm,
Ts = 4.8 ms, Tb = 4.5 ms, alfa = 1.8 and QT = 0.40

With the above parameter values, the frequency response on infinite baffle can be calculated, using the formula as mentioned in the reference article.

The green curve shows the vented-box response, the red curve a Butterwoth B4 response at the same system frequency (to compare with). The system chosen for this speaker is more damped than the B4 alignment.
F3 = 47 Hz with these parameter values.
A first estimation for the port are 2 tubes with a diameter of 35 mm and a length of 150 mm.
These responses are a result of a calculation with cabinet losses (QL = 3).



Tweeter



The SB Acoustics SB26CDC-C000-4 has a ceramic cone. Its price is around 45 euro.


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Cabinet



Cabinet Mechanical drawing

These are the cabinet dimensions as they are used for the first simulations.


- On the back side there is an internal chamber to mount the crossover filter. The back panel of the cabinet has to be removable.
- The basreflex tubes have a diameter of 35 mm and are 150 mm long. The impedance curve should be measured to check if the impedance dip is around 35 Hz.

Cabinet Materials and construction

All panels are 18 mm thick, except the front panel is 22 mm.
The transducers are mounted flush with the front panel



Cabinet filling

Sheep wool is used as dampening material. The impedance curve of the midwoofer has to be measured to check the amount of filling. The impedance peak at 60 Hz should be around 35 Ohms.

Note: the cabinet chapter will be updated when a practical realization of this speaker is available

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Graphs of the transducers in the cabinet



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 shape on the response. These simulated curves will be used for the 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



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

Horizontal SPL off axis response of midwoofer SB17MFC35-8 in the cabinet at 0, 30 and 60 degrees

Horizontal SPL off axis response of tweeter SB26CDC-C000-4 in the cabinet at 0, 30 and 60 degrees

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

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

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

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

Directivity Index DI in full space of midwoofer and tweeter in the cabinet

DI of midwoofer in blue and DI of tweeter in red.

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

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Crossover Filter



Filter Targets

LR4 at 2500 Hz



Acoustic centers of the transducers

Acoustic center of midwoofer: 20 mm behind acoustic center tweeter.



Schematic configuration



Filter components

The coil and capacitor component types are described in the schematic in terms of Jantzen Audio Components for a high quality crossover.

High quality crossover components:
Coils: Cross Coil and Iron Core Wire Coil
Capacitors: Sup Z Cap, Cross Cap and BP Elco
Resistors: MOX 10W

Medium quality crossover components:
Coils: Air Core Wire Coil and Iron Core Wire Coil
Capacitors: Cross Cap and BP Elco
Resistors: MOX 10W



Filter construction

Note: this chapter will be completed during a practical realization of this speaker.

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Graphs of the total speaker with crossover filter



Impedance with crossover filter

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

The filtered SPL of the midwoofer is not perfect on target above the X-over frequency. It is designed to compensate for the acoustic center offset between midwoofer and tweeter and bring the midwoofer phase better aligned with the tweeter phase.

Phase of the filtered drivers

SPL off axis of the sum at 1m, 2.83 Vrms, full space

Power response of the filtered drivers and the sum at 1m, 2.83 Vrms

Directivity Index in full space of the filtered drivers and the sum

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