How and why the AVTX closed-back design works

It would seem perfectly logical that the first step one should take when trying to impart a combo with a sound similar to a stack would be to close up its back. An opened backed cabinet will not sound anything like a closed-backed 4x12”, regardless of how big it is and how many speakers it houses. 

Why? 

Logic tells us that in an open backed situation, sound is allowed to “escape” through the back while, in a closed-back situation, the sound has only one way to go – forward. That fact goes a long way in explaining the unique tightness, low end “thump” (focus) and projection Marshall 4x12” cabs are world famous for.  

To give a combo the same sort of desirable sonic properties as a 412 is to close its back – right? WRONG! The problem is, when you close the back of a relatively small cabinet (and all of our AVTX combo cabinets are relatively small compared to a 4x12”), you inadvertently “mess with” the inherent resonant frequency of the speaker you load into it. And, the smaller the box, the worse said “messing with” becomes. 

What is a speaker’s resonant frequency? All guitar speakers are designed with a resonant frequency which is at the low-end of the guitar’s sonic spectrum, somewhere between 75 Hz and 100 Hz.  The resonant frequency of a speaker is the frequency at which it is most efficient – thus resulting a peak in the bass response where its output is raised above the normal level. Once the frequency of the signal hitting the speaker goes lower than this resonant frequency, there’s a sharp roll off in speaker efficiency (loudness) and notes are projected by the speaker in a very puny (quiet) fashion. 

What happens to a speaker’s resonant frequency when you put it inside a small, closed-back cabinet?

As soon as you “trap” a speaker inside a relatively small, closed-back cabinet, the resulting close confinement raises the speaker's resonant frequency. The smaller the closed-back box, the worse this unavoidable “messing with” becomes and the amount by which the speaker’s resonant frequency is raised, gets bigger.

Closing the combo’s back increases projection, tightness and focus, the only problem is this – these desirable sonic additions we’ve achieved are occurring in the low mids as opposed to the lows. By closing the back of the cabinet, we’ve forcibly raised the resonant frequency of the speaker. To make matters worse, because the speaker’s all-important resonant frequency now resides in the low mids, the lows we wanted to enhance are so puny, they’re effectively inaudible. By closing the back of our small combo, although we’ve got a great low mid “thump” happening, we’ve effectively lost a low octave.

What can be done to restore those missing lows and move the desired thump and projection? You choose.

Choice # 1   

Increase the size of the closed-back cabinet to a point where the resonant frequency

of the speaker being used is hardly “messed with” at all and remains in the lows, as

opposed to being forced up into the low mids by the smallness of the cab. True, this is

a surefire solution to the problem we are faced with…but it has a huge negative

attached to it, namely size. One of the main reasons people buy combos is the fact

that they are relatively small and portable.  

Choice # 2

2.  Find a speaker that has a resonant frequency that is so low that when it is forcibly raised by being put inside a small, closed-back cabinet, it still remains in the lows as

opposed to being pushed up into the low mids. This is where the specially designed

Celestion Extended Bass Response speakers used exclusively in AVTX come into play. They have a lower than normal resonant frequency to counteract the “messing about” a small closed-back cabinet is responsible for.