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Does a guitar cab baffle really have to be birch ply,?

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Does a guitar cab baffle really have to be birch ply,?

Postby SecretSam » Sun Feb 04, 2018 10:10 pm

Like it says in the title.

All the guides say use 15mm void-free birch ply. But is marine ply materially as good if you seal it?

Birch ply is a bit hard to find on the arse end of Africa.

Moisture is unlikely to be a problem in the current once-in-a-thousand-years-but-whoops-global-warming drought.
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Re: Does a guitar cab baffle really have to be birch ply,?

Postby Sam Spoons » Sun Feb 04, 2018 11:01 pm

I've built cabs (guitar and PA) out of various different types of ply and doubt there will be any perceivable difference in the sound if you use reasonably decent quality plywood. Durability is, IMHO, the main reason why birch ply was specified. When, many years ago, I worked in a timber merchants birch ply was simply the best quality building plywood.

For a guitar speaker marine ply should be at least as good as birch ply but anything reasonably hard/void free (look at the cut edges) will be fine.
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Re: Does a guitar cab baffle really have to be birch ply,?

Postby ef37a » Mon Feb 05, 2018 8:19 am

Good quality ply (from any tree IMHO!) is the best material for guitar cabs mainly because it combines high impact strength with reasonably low weight. The other two common materials, chipboard and MDF are neither as strong nor as low in density. The front baffle of a 4x12" is very largely speaker!

Now, some widdlers claim ply sounds better and they put this down to the fact that ply of any given thickness is not as rigid as chipboard or MDF and the flexing imparts a pleasing 'colour'. Yer pays yer money! Of course, a flexible enclosure is the last thing we want in a hi fi/monitor speaker!

I had the perhaps rare experience of being in a not very large room when a 200W valve amp was being tested at top shout into 2, 4x12s. I was wearing defs and they cut a lot of the mids and HF signals but I could clearly hear the cabinets creaking and groaning under the pressure!

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Re: Does a guitar cab baffle really have to be birch ply,?

Postby Hugh Robjohns » Mon Feb 05, 2018 9:55 am

ef37a wrote:Of course, a flexible enclosure is the last thing we want in a hi fi/monitor speaker!

It ain't necessarily so!

The BBC developed the use of thin-walled (flexible) enclosures for very good engineering reasons, as outlined here by Alan Shaw of Harbeth:

Alan Shaw, Harbeth wrote:What underpins the BBC's thin-wall cabinet philosophy (and I was surprised to read that exact word in one of Harwood's papers recently) is the observation that a perfectly cast bell will ring on for many seconds. Conversely, a bell with a hairline crack will sound leaden and hardly ring at all. It's the same with cabinets: if the panels are all rigidly glued together then at some critical frequency or other a note or notes in the music will trigger the cabinet's natural structural resonance. In such a rigid structure, there is nothing that can be done to suppress the ringing - and each time that note reappears, it tops up the ringing which then becomes a permanent drone underneath the music.

Conversely, in a thin-wall cabinet, the lossy joints (i.e. removable baffle/back and the generally 9-12mm thin panels used throughout the box) each act as an acoustic hairline crack. They inhibit the build-up of resonance. Simple as that really!

Now, let's not kid ourself that it is possible to kill cabinet resonance stone dead. It isn't. Not with any approach to cabinet design because the sound pressure inside the cabinet is huge. What the thin-wall approach does is to move unwanted resonances downwards in amplitude and frequency so that they are adequately buried below the music and then pushed down in pitch. Note that I said adequately. Providing that the resonance, be it from the cone, cabinet or even recording - whatever the source - is x dBs below the fundamental, the BBC proved that it was completely inaudible. Once inaudible to trained listeners on all types of music/speech, that is the end of the matter. Inaudible to the trained listener is as good as the solution needs to be. It is neither necessary nor cost effective (nor good engineering) to continue pushing for a degree of theoretical excellence that nobody can appreciate but everyone must pay for. That pragmatism keeps our speaker affordable - and sounding natural.

What we seem to be lacking in the industry today is the good old fashioned common sense that was abundant when serious researchers with zero commercial interest (i.e. the BBC) had their hands on the tiller. Thank goodness that they thoroughly documented their efforts for posterity since physics, acoustics and our hearing are the same now as fifty years ago. Now it seems we are all conditioned by marketeers to chase theoretical perfection which is far, far beyond what our ears can reliably resolve.

Shaw discusses the ideas and practicalities a lot more in his own Harbeth Forum here:

http://www.harbeth.co.uk/usergroup/forum/the-science-of-audio/speaker-design/215-bbc-style-thin-wall-cabinets-why-so-special

..and it makes for interesting reading. The original BBC R&D department papers on this are all publically available.

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Re: Does a guitar cab baffle really have to be birch ply,?

Postby ef37a » Mon Feb 05, 2018 10:14 am

Thank you Hugh. I was aware of the Harbeth Thin Wall design concept.

The fact remains however that most high quality speakers are built on the 'BSH' principle*. Indeed many SoS reviewers report that the oh so scientific 'knuckle' test results in "dead as Lead" sound.

*The celebrated G.A.Briggs of Wharfedale fame actually mentions brick as perhaps THE best speaker cab material. The thin wall design does however move! This takes some of the energy from the motor and reduces sensitivity. Not usually a problem in high end hi fi speakers and monitors I will admit.

Skinning cats here.

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Re: Does a guitar cab baffle really have to be birch ply,?

Postby The Korff » Mon Feb 05, 2018 10:22 am

That is a very pleasingly no-nonsense quote. Thanks!
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Re: Does a guitar cab baffle really have to be birch ply,?

Postby SecretSam » Thu Feb 08, 2018 8:32 am

Thanks for the replies, Gents. That is good news.

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Re: Does a guitar cab baffle really have to be birch ply,?

Postby SecretSam » Sat Feb 24, 2018 11:27 am

Found a company that has fancy CNC machines for cutting clever shapes. They stock birch ply and knocked me out a perfectly-cut baffle for a tenner, including the wood. If anyone has a use for an accurately cut circle of off-cut plywood that is two saw-blades less than 283mm diameter, let know know :-)

To anyone building speaker cabs in Cape Town: CAM Craft are your new best friends.
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