Lightweight cab musings ...

[SamIAm]

Very grateful to have been one of the lucky BCers to receive a Bayma SM212 Driver from the generous @JohnDaBass (Thank you!!)

 

Now the housing.  I'm not knowledegable enough to design one so I'm looking at the fab BC112 designs on here.

 

With a bad back, I'm looking to create a 'light as a feather' cab (<10Kg) and am investigating what options might be viable, but again ... I don't know what qualities a bass cab should have to sound good.

 

Are the sides suppossed to be totally rigid, with no give/ressonance at all or do they need to have some flex?

Does thickness matter (apart from strength) ... if one had a paper thin material that was totally rigid, would it likely work?

What are the issues (if any) of using thin (8mm or 12mm) ply?

Does anyone have experience of building a cab using a sheet of foam/balsa layered wrapped in a composite such as fiberglass?

Any thoughts on a 3D printed honeycomb casing (In reality this would entail a number of pieces glued together as my 3D printer bed is not big enough to do a cab side in one print)

 

S'manth x

[Bill Fitzmaurice]

12mm plywood is all you need, if it's properly braced. Thinner than 12mm can be done, with curved panels, which increases their resistance to vibration. Knowing how to brace cabs requires a fair amount of building experience. Sandwich constructions can work, but require even more experience to be successful. My personal cabs are primarily made of 3mm and 6mm plywood, but I had 30 years of cabinet building experience when I designed them.

[nekomatic]

Ha, I drafted something prefaced with ‘I’m not an expert but’ and meanwhile an actual expert has shown up (and luckily, not contradicted anything I was going to say too badly). 
 

You want the cabinet to be as rigid as possible to avoid resonances which will colour the sound. From memory of trawling through various cabinet building threads, I’m fairly sure it was Bill who stated the most effective bracing to start with is a batten tying the middle of one panel to the same place on the opposite side. Running a signal generator app on your phone into your amp and feeling each panel for vibrations as you ramp the frequency is adequate to identify whether you need to add any more. 
 

I think commercial PA speaker cabinets often use moulded plastics, and there are companies like GR who make bass cabinets from carbon fibre, but I’m not sure how feasible either of those is for the home builder. I think you will be very pleased with an SM212 in a Basschat 112 enclosure though. 

[nekomatic]

One practical point if you want to minimise weight: a tweeter is surprisingly heavy, and the coils for the crossover are a fair amount of copper as well, so if you aren’t sure whether you need a two-way design you might want to try it with the Beyma on its own first and only add in the HF side if you feel it’s necessary. This is the sort of thing you can take your time faffing about with when you build it yourself. 

[Downunderwonder]

Trying not to strain our collective grey matter too much here....

 

Stiffness of a panel is related to how well its ends are secured, the material, and its thickness. How much it flexes then depends on how hard you push on it and the span.

 

If you put a brace in a panel you effectively halve its span. That's why braces are much more efficient than using thicker material.

[Bill Fitzmaurice]

Span is the key. My benchmark is to not have more than an 8 inch/20cm un-braced span.

[JPJ]

Companies like GR using sandwich construction will most likely be using a pre-preg (pre impregnated) carbon fibre mat cured in a vacuum oven. There are some air curing pre-pregs available but they are eye wateringly expensive and you still have to think about how you are going to finish it to seal it against moisture ingress. Alternatively, you can buy carbon fibre sheets either as pure carbon or foam cored from company’s such as these (no affiliation) but then you have to think about how you are going to construct the cabinet, oh and carbon sheet isn’t cheap either. 

[JPJ]

Thanks Bill, I’ve found the same by experimentation ( in other words lots and lots of failed attempts) 😂

[Balcro]

When you say "sounding good", the question must be "what sounds good to you". Given that you have the Beyma SM212, I find that there have been subtle changes to it over time. My own files from 2014 show an Fs of 40.5Hz, but according to the listing under "Discontinued Speakers" it has a Fs of 43Hz. Such subtle alterations by manufacturers over a production run are normal, but it might be sensible for any plans to be based around the higher Fs.

An extract from Phil Starr's inital briefing on the  general design spec,

https://www.basschat.co.uk/topic/227904-1x12-cab-design-diary

Capable of keeping up with an unamplified drum kit (120dB across most of the frequency range)
Compact (60l or less)
Neutral /clean sounding
Easy to handle
Good low frequency power handling
Readily available components, and easy to build
Value for money.

I stand to be corrected, but I believe the SM212 was the driver that was finally chosen for the BC Mk.1.

If you're basically happy with a little "warmth" and an overall even presentation of sound, then building to a 60L internal volume will give you that. Please see the "recommended internal volume" here.

https://www.beyma.com/en/products/c/low-mid-frequency/112SM2128/altavoz-sm-212-8-oh/

 

The DATA Sheet is here:-
https://www.beyma.com/speakers/Fichas_Tecnicas/beyma-speakers-data-sheet-low-mid-frequency-SM212.pdf

 

There will have to be compromises in the design - building a 60L box with 12mm panels and some bracing may turn out to be heavier than 10Kg. The Basschat "massive" will undoubtedly advise you further on developing your loudspeaker.

 

The best of luck.