Category Archives: Guitar Loudspeakers

The Origins of the Open-backed Speaker Cabinet

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.

Guitar Speakers @ Home (Part 3)

underlay1After much experimentation and extensive listening/playing tests, it became obvious that 12mm plywood is a little too flexible for the larger panels of loudspeaker cabinets and bracing is necessary. You can see the diagonal braces that I added in the picture (one per side-panel); their addition was enough to tighten up the bottom-end.

The carpet underlay that you can also see in the photograph was purchased from the Carpet Underlay Shop.co.uk and has been very effective at cutting out internal reflections and further improving the “punch” of the speaker’s delivery. At a little over £2.00 (ex VAT) per square metre, it’s something of  a bargain.

The sound quality is pretty astonishing; it puts a smile on my face every time I play through it. Right now I can’t get enough of it – I really must get around to posting some new sound samples.

Guitar Speakers @ Home (Part 2)

cabinetThe picture shows a Jensen MOD 6-15 speaker in a 50 Litre enclosure. It’s a big box – around two feet tall – but, because the speaker is designed to work in an open-backed cabinet, it’s smaller than it needs to be for maximum bass response (as calculated using the Thiele/Small parameters provided by Jensen).  However, the bass roll-off is not dramatic and the bass response is restored by pushing the cabinet up against a wall (that’s the proximity effect at work).

So how does it sound? It’s killer – a joy to play through. There is no lumpiness or bass emphasis as there is with an open-backed cabinet (because of the inherent comb filter). In fact, on paper,  the frequency response of this speaker is very similar to the coveted P10R Alnico speaker – a fact that was confirmed by my tests – but it costs a fraction of the price. The only downside is that the MOD 6-15 speaker is 5dB less efficient than its Alnico sibling but that’s not really a drawback when you’re playing at home; 1W has power to spare.

This definitely the way to go for those who want a good sounding setup for home use at spouse- or parent-friendly volumes. I know this sounds unlikely – after all it’s using a cheap and cheesy 6 inch speaker like that in a Champion 600 – but the large cabinet allows the speaker to work to the best of its ability. The rule here is that it’s better to have a cheap speaker in the right cabinet than an expensive speaker in the wrong one.

Guitar Speakers @ Home

Not only is one watt too loud for the home but guitar speakers are not designed for home use. Guitar speakers are largely designed to be stuck into open-backed cabinets which means they are incredibly sensitive to placement. It’s fine if you’re up on stage with a few feet behind the cabinet but at home you’ll want to push your amp out of the way – up against a wall or in a corner – and that will cause the sound coming out of the back of the speaker to bounce around and interfere with the sound coming out of the front of the cabinet. The frequency response of the system – including the room – will be “lumpy” with some frequencies being affected more than others.

The way to fix this is (obviously) to put the speaker in a closed back cabinet but guitar speakers are designed for use in an open-backed cabinet so require a relatively large closed-back cabinet to provide a sensible frequency response. Jensen provide a good range of small speakers that ought to work well in a domestic environment and they publish the Thiele-Small parameters required to design the cabinet. Celestion don’t do either; they have one eight inch driver and that’s as small as they go. Celestion do not appear to publish Thiele-Small parameters.

What does this tell us? It tells us that products aimed at guitarists are remarkably conservative; in many ways they haven’t changed much over the decades. Guitar amplifiers first appeared in the 1930s/1940s which is around 30 years before Thiele and Small published the results of their study into the parameters that were important to the design of a loudspeaker enclosure. Designers back then were working blind so no wonder they left the back off the cabinets; all resonance problems were solved immediately (as long as you kept the cabinet in an unconfined space).

So how do you build a speaker cabinet for a guitar that will work well in a home environment? The answer is not clear to me yet but I want great tone at an acceptable volume and I want to be able to play along with acoustic instruments without dominating the sound so I’m going to have to sort it out. Perhaps the best idea is not to use a guitar loudspeaker at all but this is a bit radical and I do like the sound of Jensen speakers. While I’m pondering this, here are a couple of interesting articles on the subject of guitar loudspeakers:

Note that the Duncan’s Amp Pages article suggests a Qtc should be between 0.707 and 1.0. This is the hi-fi approach but cannot be achieved by any of the traditional Jensen speakers below 12 inches in diameter as the Qts is greater than 1.0 (and the Qtc cannot be less than the Qts) . The  modern Jet series creates some exceptions to this but the smallest Jet speaker is 10″ in diameter which, I would argue, is too large for home use.