This cabinet is designed to weight less than a ton. To do so, thin (10mm) particle-wood (MDF or Medium Density Fiber) is assembled like an aircraft wing: the total strengthness results from the whole structure rather than on the mass. The driver weights 4kg (8 pounds) and the total mass should not exceed 8 kg. It is efficient (42Hz @ -4dB) and provides a deep sound to mimic an upright bass with a 4-string fretless bass on more or less small stage.
It is intended to provide a rich, deep and transparent sound for a jazzy-style bass player with at most 3 other musicians and 50 people. Use it with a tube amplifier, or a 100W Mos-Fet amplifier. Good results can also be achieved with a home amplifier like the NAD3020i (setup for 4 ohms, soft clipping, and only one channel used of course)!
Since the speaker is designed for professional public-adress, it reaches its best efficiency at high power. This cabinet, however, allows the speaker to reproduce low frequencies at low power. But this choice led to reduce the power handling capacity. Thus, despite the 350W RMS of the Eminence beta 15, this cabinet is unable to manage high power without reducing the low frequencies. To play loud, use an equalizer to reduce low frequencies.
(before paint, filter and tweeters)
Musical tests Tests are performed with a DIY (of course) TDA7294-based, 70W RMS Mos Fet amplifier, which has a very wide bandwidth, and an off-the-shelf HIFI amplifier NAD 3020i (classic bipolar 2N3055/2N2955 transistors). In both cases, the "bass" must be set to the minimum as the volume is increased.
The instrument is a Cort B4 fretless bass (four strings, active). The bass response is conspicuously but smoothly dimmed from E to G#, according to the software simulation. Facing a big Fender Rumble 100W and a small Trace Elliot Boxer 30, this cabinet handles far deeper bass. A pair of tweeters is added (not figured on the blueprint).
Vibrations in the prototype The back panel is screwed in this prototype, and vibrations appear at high power. This is partially fixed with battens. This cannot be totally fixed because a large box assembled with 10-mm MDF is too weak to manage high, alternative air pressure at low frequencies. Note that only the rear panel vibrates, and this rear panel is not assembled with inner reinforcers, nor it's glued. That is, the whole structure is quite efficient, but the rear panel should be assembled like other panels, and include a hatch to access the speaker.
Build this bass cab to hear the natural sound of a jazz bass, without missing an octave in lower notes. Not for hard trash metal.
Highly recommended option:
- either med/high Monacor MHD-152 compression tweeter + 2-way passive filter, 3,5kHz, 12dB/octave. Add a L-pad (constant impedance attenuator) or a power resistor because of its high (110dB) sensitivity.
- or use two good old, very low-cost, ultra-reliable, smooth, linear, low distorsion piezo-electric tweeters Monacor MPT-177 (formerly KSN-1177) with a simple and ubiquitous filter.
5-string bass concerns A 4-string bass guitar is able to produce 42 Hz (low E). For a 5-string bass guitar, which produces 31 hz, you should choose an appropriate boomer (e.g. lower Fo) then recalculate the box. In a cabinet design software, see the SPL (Sound Pressure Level) plot: when you edit the parameters while designing the cabinet, the tuning frequency of the vent may be very different from the resulting lowest frequency of the whole cabinet. Choose a vent tuning frequency higher than the resonance frequency of the speaker (that is "Fs" in Thiele and Small parameters).
- 51 x 79 x 40 cm (outer dimensions), 146 liters
- tuning: 37 Hz
- 42Hz @ -4 dB = 93,6 dB SPL
- up to 3500 Hz with Beta 15 only, full-range with tweeter
- one 120 x 80 cm, 10mm-thick MDF panel (rear + front)
- three 40 x 120 cm, 10mm MDF panel (sides + top + bottom + inner reinforcers + vent)
- four 15x30 mm battens, length: 2 meters
- wire, connector (Jack/Speakon/...), vinylic glue
- Eminence Beta 15, option: tweeter + filter
How to cut the parts: all 40 cm sides are already cut in 120x40 MDF!
- Sides: 79 x 40 cm
- Bottom and top: 40 x 49 cm
- Front and back: 77 x 49 cm
- Rectangular vent (Mach=0,06): front 26x9cm (INNER dimensions), length 20cm
Use small parts of MDF for reinforcement parts. Two triangles beside the driver: about 5 x 15 cm. Other parts: square-shaped (may be curved like showed on photos), sides about 17 cm, extrusion of square angles for 15 x 30 mm battens. Central reinforcement part: 5 cm wide.
Great thanks to Juha Hartikainen's box-design software WinISD ! SPL response curve and group delay:
- The MDF (Medium Density Fiber, a kind of wood made out of thin particle) must be cut with accuracy. Draw the line then cut either at the left OR at the right side of the line, but NEVER neglict the thickness of the blade.
- NEVER use glue before you are sure that the parts may be correctly assembled.
- Use a slow vinylic glue (a white glue especially intended for wood). Strongly press the parts during 20 minutes.
- Strongly maintain the part before cutting it
Assembling the box: left, bottom, right.
The front (viewed from back, top is on the floor), vent hole and speaker hole.
The inner reinforcement parts and the vent.
Bottom, sides, front, vent, rear battens.
Central reinforcement part preparation...
... and assembly.
One more view...
... one more, to see how the reinforcement part are assembled...
... and one more.
Eminence beta 15 driver, 10 cm MDF battens on back, absorption moss (half the depth for sides), connector, wires. The rear panel IS NOT glued in this prototype.