If you’ve been following my posts you’ll know that I’ve spent a lot of time recently researching guitar loudspeakers and their behaviour in different cabinets. What’s most fascinating is that, in contrast to the world of hi-fi loudspeakers, no-one seems at all interested in the acoustic properties of guitar loudspeakers. As David B. Weems remarks in the 3rd edition of Designing, Building and Testing Your Own Speaker System: “The same laws of acoustics prevail for musical instrument speakers as for stereo speakers, but you wouldn’t know it by inspecting many commercial musical-instrument speakers”. He goes on to talk about the apparent preference for open-backed cabinets but he obviously isn’t entirely sure why this should be. It’s a good question: Why do guitarists use open-backed cabinets?
There are problems with open-backed cabinets that the hi-fi designers of today would not tolerate but it was not always thus. Referring to The Why and How of Good Reproduction by G. A. Briggs, it is clear that, back in the 1940s and 1950s, the speaker manufacturers knew very little about the physical properties of a loudspeaker that would influence the cabinet proportions; they knew only the resonant frequency, the frequency response and the impedance. The Thiele/Small parameters, that are taken for granted today in hi-fi circles, would not appear until the 1960s/70s so back in the 1940s/50s creating a cabinet that would allow a speaker to do its job properly was trial and error and the cabinet (probably ported) would have been large. This was the world in which Leo Fender built his first amps.
In the mid 1940s, Leo Fender selected a Jensen speaker for his new amp because he liked the tone it produced. It is important to realise that, at that point in history, all speakers were general-purpose speakers. Jensen speakers from the 1940s were made for hi-fi but ended up in guitar amps because – by today’s standards – they were heavy-duty speakers. Jensen stopped making speakers in the 1960s but the relaunched, Jensen-branded speakers are, by all reports, faithful to the originals so we have a pretty good idea what Leo Fender was working with. Leo’s motivation was to make product that was mass-producible and affordable (i.e. cheap) just like a radio set. If a radio set had a back it would be lightweight and vented to allow the valves/tubes inside to stay cool. Removing the back of a radio would reduce resonances and rattles which would make it sound better (we have Gilbert Briggs word for that). Under these circumstances why would Leo Fender want to put a back on his guitar amp? It would make the amp, larger, heavier, harder to cool and would increase his R&D costs. It was a no-brainer to leave the back off.
So open-back guitar amps/cabinets were produced not because they sounded better than the alternatives but because they sounded good enough and were cheaper than the alternatives. I think that was a good design decision at that time but what puzzles me (and David B. Weems) is why, 60 years later, we are still taking the same approach. That’s a subject for another day.