GX Barefaced Guitar Cabs - The History

 

The reinvention* of the guitar cab

(*this isn’t hyperbole - read on!)

The story starts when rehearsing and gigging with my band in the mid 2000s. Our guitarist was using an open-backed Sound City 2x12” valve combo from the ‘70s, which was a pretty epic sounding piece of kit. Early on I persuaded him to elevate it on a chair and turn it on its side (so the speakers were vertically stacked) and that really helped with audibility. At one gig he had to use an Orange 4x12” and we’ve never had such a messed up mix as that - the cab was so directional and so loud out in the venue (if you were in the line of fire) that he was too loud for a select few people and too quiet for everyone else. And this was a venue where we were all going through the PA system too!


Around that time I started working on what because Barefaced and as that became an actual business I did more research, designed and manufactured new products, gathered more feedback and learnt as much I could about the complexities of sound. A notable place where I spent a lot of time reading, learning and thinking was Siegfried Linkwitz’s excellent website. If you have a look there you’ll see that he’s designed the weirdest looking hi-fi / studio monitor speakers, which rather than having speakers in boxes (monopole) use a mix of dipole, cardioid and omnidirectional designs, which are implemented using open baffles, transmission lines and H and W frames. So why are these speakers so weird compared to mainstream loudspeakers?


Most hi-fi speakers are designed to sound best in a room with good acoustics and the inevitable sonic problems that are caused in almost all rooms are simply accepted. Go to a demo room that’s set up for really good hi-fi speakers and that room will have loads of acoustic treatment. Mr Linkwitz has instead approached things from the opposite direction - rooms will always affect your sound so how do you design a speaker to be minimally affected by the room?


The answer is actually quite simple and is a result of how our brains have evolved to process what we’re hearing. Most sound sources in nature (and most acoustic instruments) are broadly omnidirectional - small things make high frequencies, large things make low frequencies, which is simply a result of the physics of sound. Loudspeakers are a bit like that - a tweeter is smaller than a woofer - but not enough. Look at the vast difference between a buzzing mosquito and a singing whale! The size ratio 1” (25mm) tweeter vs an 18” (450mm) subwoofer doesn’t really compare to a 3mm insect vs a 100,000mm cetacean!

Before explaining why this matters, we need to look at what really happens with sound in the real world. Every time we hear a sound it reaches us two ways - directly and reflected. This even happens outside most of the time, unless you’re in an absolutely vast open space with the sound source right against the ground. You’ll be familiar with reflected sound for two reasons - the most obvious are echoes, which is reflected sound with such a long delay that you hear it as a completely separate sound, like putting a guitar through a delay pedal. The second variety you’ll have heard is reverberation, which is where you don’t hear a separate sound but the original sound has a certain amount of sonic “space” added to it, as you’ll hear if you sing in a church or turn up the reverb knob on a guitar amp.


But there is one more variety of reflected sound you’re unlikely to have noticed - if echo is the sound you get when the reflected sound has the longest delay and reverb is the one with less delay, there’s a third where the reflected sound isn’t delayed enough for you to perceive it as an obvious sonic effect, it simply becomes part of the original sound once our brain has processed what our ears are taking in (we don’t “hear” what our ears hear, our brain is doing a lot of post-processing of the data coming from our inner ear).


If our sound source is omnidirectional (that means it puts the same sound out equally in all directions) then the reflected sound will be near identical (subject to the absorption of the reflector and through the air) to the direct sound we hear, just delayed according to the increase path length between source and ear. Because of the way our hearing system has evolved, if the direct sound and reflected sound are very similar then our brain simply makes the reflection part of the original sound. Never forget that our hearing system is first and foremost a survival system and secondly a communication system - if our brain knows it’s hearing the same sound from the same place then it doesn’t have to worry about the reflected sound. Make that reflected sound different and our brain questions if that second sound is actually something coming to eat us.


So the brilliance between Mr Linkwitz’s designs is that they aim to fill the room with the same sound, thus the reflected sounds are near identical to the direct sound. This results in a huge improvement in clarity because the reflections are integrated with the direct sound by our brains. Bear in mind that these loudspeakers are still designed to be listened to from the ideal listening position, just as with other fancy hi-fi equipment, it isn’t about having this superior dispersion so that everyone can hear them well - it’s all about making the reflected sound as similar as possible to the direct sound. There’s lots of other clever stuff going on with this designs (related to stored energy, active crossovers, excitation of room modes at low frequencies and more) but that’s not so relevant to our particular challenge. But if we move the listener away from the ideal listening position we find that even way off-axis they can hear the speaker well because of how it’s filling the whole room with sound.


Going back to our guitar cab conundrum there are a few facts we know for sure:

1. A cab with a single driver or two vertically aligned drivers has much better horizontal dispersion than a cab with two side by side drivers or a square 4x cab.
2. An open-backed cab is much easier to hear all around the room than a closed-back cab.
3. An open-backed cab has much less bottom end thump and fatness than a closed-back cab.


Anyone who has heard enough guitar rigs (whether they’re a guitarist or not) will have heard this, whether or not they’ve made the connections. If you haven’t realised this before, see if you notice it now! So can we solve these problems? Make a cab that’s easy to hear like an open-backed cab but have the bottom end warmth and roundness of a closed-back cab?


Now, if you’ve taken any notice of our Barefaced bass cabs you’ll know that they’ve been designed to have far superior dispersion than conventional bass cabs, thanks to vertical alignment, direct coupled midrange domes, constant directivity waveguides, our Barefaced Line Array crossover, etc. Why don’t we just apply the same design principles to a guitar cab, borrowing from high-end hi-fi, studio monitor and PA speaker design. Well, the big problem is that most of the tone of a guitar cab comes from the speaker in it. Replace that, for instance, 12” speaker with a 2-way design like our Big Baby 2, or a 3-way design like our earlier Big Baby T, and it’ll lose the colouration that sounded so good with electric guitar. Strike one.


So why don’t we keep the speaker the same and build some kind of horn or waveguide or diffraction lens in front of that 12” speaker? Unfortunately any of those mechanisms will also change the sound of the speaker, by affecting how it couples with the air in the room. Yes, we can improve the dispersion by firing the speaker through a narrow throat and amplifying that diffracted sound through a horn but the horn changes the acoustic impedance load on the speaker, changing its vibration pattern and thus its sound, and then the horn apparatus will filter the sound according to standard horn theory, again changing the sound. Strike two.


But what about the sound that’s coming out of the back of the speaker? Can we use that?


What happens with the sound coming out of the back of the speakers in a closed-back cab like a Marshall 4x12”? It bounces around the inside of the cab and gradually dies away. Some of it that internally reflected sound escapes through the cones (they’re only thin pressed cardboard) and adds/subtracts to the sound coming out of the front of the cones (depending on the relative phase). So basically most of it is wasted whilst the rest slightly messes up the sound from the front of the cone.


What happens with the sound coming out of the back of the speakers in an open-backed cab like a Fender Twin? Much of it goes straight out of the hole in the back of the cab, most of the rest bounces round the inside of the cab before escaping through the open back and a little bit gets reflected back through the cones and out to the front. The sound coming out of the back of the cab is opposite polarity to the sound coming out of the front of the cab, or 180 degrees out of phase. In the midrange and treble frequencies this doesn’t matter at all - the wavelengths are so short that by the time they reflect around the room and meet the sound from the front of the cone they just contribute to more output (non-coherent mixing). However, in the lows the longer wavelengths mean that the rear output wraps around the whole cab, as does the front output and because they’re 180 degrees out of phase they cancel each other out, massively reducing the output in the lows. In other words, when the front of the cone pushes to pressurise the room in the lows, the back of the room pulls and de-pressurises it. This is why open-backed cabs produce much less bass than closed cabs.


So here’s the question: Can we make a cab where we release the broadband output from the rear of the cone into the room without its low frequency output cancelling out the frontal output’s lows?

About five years ago this led to my idea of putting two different speakers side by side in a guitar cab, putting a low pass filter on one of them, and then putting a slot/horn apparatus behind the full-range speaker in an open-backed cab and the other speaker in a separate ported sub-enclosure. And four years ago that got made and it worked rather well. It used a guitar driver as the full-range speaker in the modified open-backed cab and our 10CR bass guitar driver as the low-passed one in the ported cab. The concept behind this was as follows:


1. Guitar driver’s forward output is as normal.
2. Guitar driver’s rearward output is diffracted through the slot and extra gain and dispersion is provided by the horn plates - taking what’s good about an open-backed cab and making it better still.
3. Bass driver’s forward output only fills in the from the mids and down so minimally affects the tone and keeps the dispersion as wide as with a single driver.
4. Bass driver’s rearward output is enhanced by a tuned port, providing similar efficiency in the lows to a closed back cab with more speakers.


So we basically have two cabs, one which is basically an improved open-backed cab which acts as a dipole in the mids and highs with figure 8 output, and one which is a ported cab which acts as a quasi-cardioid in the mids and as an omnidirectional monopole in the lows. Much easier to hear thanks to the figure 8 mids and highs output and big chunky lows available from the high excursion ported bass driver.


The downside of this design was it lost some of the characteristic guitar cab colouration, it required a minimum of two drivers and had quite complex electronics. And there was also the matter that although the bass driver could handle tons of power, the guitar driver couldn’t handle as much, and most guitar amps don’t have very much power, so people might not get the best out of it and those that did might run the risk of blowing the guitar driver. And some funny stuff about how valve amps would handle the impedance changing as you switched through the multiple settings (4-way rotary switch plus 2-way toggle). But I felt there was potential, it would just require more real world testing, lab testing and probably many more prototypes.


Whilst first writing a patent for this in 2015, I had the revelation that it might be possible to make a simplified version, where the slot/horn apparatus was mounted in a sealed cab (so the only opening was the slot) and that it might be possible to adjust the dimensions of the cab, the slot and the horn to tune this as a Helmholtz resonator as in a ported cab, whilst also keeping the improved diffraction and relatively uncoloured transmission of midrange and treble of the slot/horn apparatus. And eventually, once various bass cab projects were completed, we set to work on this.


A few years and many prototypes later we had the final pre-production version of the Barefaced GX complete, which from the front looks like an entirely conventional guitar cab - but its unique Augmented Vent Diffractor apparatus (AVD) is visible from behind, and that’s where the magic happens!


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