Head+Cab vs (open back) Cab - how much is due to the 'back'

[Jay Menon]

I wonder when comparing the sound / tonality between:

- A Head + cab (often closed back) and
- A Combo (almost always open back)

...how much of the difference in sound is due to the open vs closed back nature of the cabinet? Does cab construction / design make a really big difference?

[Mixedup]

It certainly makes a difference in the room, and I used to like miking the rear of the speaker in a multi-mic setup too, which is obviously rather easier with an open-backed cab.

But how much of a difference it makes when miking speaker close up to the grille I'm less sure, as I've never tested it specifically — and there are two many other variables between cabs, speakers and instruments to say that the differences I've heard when miking can be attributed to that.

If you have an open or semi-open back cab, though, you can test this for yourself by covering up the gap with a bit of wood

[Wonks]

Guitar cabinet construction (and size) can make a reasonable difference to the sound. (Bass cabs are an entirely different kettle of fish)

Small cabs do sound 'boxier' than bigger cabs, even if open backed.

With closed cabs, the drivers moving inwards do compress the air within the cab slightly, so provides extra resistance to the cone movement in that direction and that extra pressure will initially help the cone move outwards. But after the cone reaches the centre point of travel, the cone is then trying to lower the pressure in the cab, so will impede the cone's travel outwards.

On some closed-back cabs you may get some acoustic treatment/wadding to reduce mid+ high frequency resonances.

With an open backed cab, there is no such increased/decreased air pressure for the cone to have to move against (except that provided by the inertia of the air in the locality). They certainly don't have any acoustic wadding applied, so standing wave resonance between the two sides of the cab and top/bottom of the cab are at their maximum influence (and one reason why smaller cabs sound 'boxier' than bigger ones).

So you are likely to get increased cone movement with open backed cabs, which is probably frequency dependent, so you get slightly more cone travel at low-frequencies compared to a closed back. This can lead to a slightly 'tighter' sounding bass end on a closed back cab.

Also, as open backed cabs radiate a lot of sound backwards, then you are also going to get the effect of that sound hitting the floor/walls/ceiling behind the cab and the reflections mixing with the projected sound from the cab. If close miking, then this effect will be negligible, but will make more of a difference to room mics and obviously the guitar player in the room.

Semi -open backed cabs will provide some extra resistance to air movement, providing the air gap is sufficiently small, and so provide a feel some way between the open and closed back cabs.

Construction materials also play a part. The sturdier the construction, the less cabinet vibrations will affect the sound you hear. The amount of panel bracing (from none to some) will also affect the level of panel vibrations and the vibration frequencies.

[Mixedup]

I always fancied trying one of these (on Jack Ruston's recommendation), which can do semi-open or closed, but I've not got around to it yet...

http://www.audiokitchen.co.uk/speakers

[Hugh Robjohns]

I wonder when comparing the sound / tonality between:

- A Head + cab (often closed back) and
- A Combo (almost always open back)

...how much of the difference in sound is due to the open vs closed back nature of the cabinet? Does cab construction / design make a really big difference?

Yes, the cabinet type and construction make a really big difference. But whether you hear that depends on whether you are talking about what you hear in the room, or what you hear over a close microphone inches from the speaker. Obviously, a close mic will tend not to pick up a lot of the room sound.

I think Wonks has given a lot of useful info already about some of the key differences, but this is a complex topic and there's a lot of different interacting effects and parameters here.

Clearly, the rear wave-fronts from a speaker will tend to escape into the room from an open-backed cab. Obviously, they are of opposite polarity to the wave-fronts from the front of the speaker, so the room sound will be heavily influenced by the way that rear sound gets reflected back into the room to interfere (acoustically) with the frontal sound.

At very low frequencies, with a near omni dispersion and diffraction around the (relatively small) cabinet, there will be a strong cancellation of LF output curtailing the LF response dramatically. This is unlikely to be an issue with a guitar, but would be rather unhelpful with a bass which is why there are no (or none that I've seen) open-backed bass combos!

The loading on the driver is also strongly affected by the open-backed cabinet, as Wonks has detailed. This has a significant affect on the low-frequency resonance of the driver itself and so the driver's parameters have to be selected specifically for use in an open-backed cabinet.

In a closed-backed cabinet, there is no rear radiated sound to worry about (or not much, anyway -- the cabinet panels can re-radiate sound in some designs, either deliberately or accidentally through poor internal bracing and damping). However, the rear wave-fronts can be reflected off the back of the cabinet and pass through the driver's cone to interact with the frontal sound, resulting in colouration from comb-filtering effects. Internal acoustic damping will obviously reduce this a lot.

The internal air pressure loading effects will also be significant, as Wonks detailed again, and this means that the driver parameters have to be specified with this kind of loading in mind.

It has been said that any science with more than seven variables becomes an art -- and speaker and speaker cabinet design definitely falls into that category!

H

[Wonks]

I always fancied trying one of these (on Jack Ruston's recommendation), which can do semi-open or closed, but I've not got around to it yet...

http://www.audiokitchen.co.uk/speakers

Personally I think that's pretty much a closed or open back selection (even Audio Kitchen say it's an open/closed back cab). Most 'open backed' cabs have a similar amount of rear venting to that cab in the 'open' position The opening would have to be a lot smaller to provide any noticeable and effective increase in air resistance. I'd be looking at between a 1"-2" gap at most. Also, in the 'open' position most of the driver is exposed, so rear cabinet reflections through the cone that Hugh mentioned in closed back cabs will be minimal.

It's certainly going to give you a choice of cab responses, but to make that a really versatile cab, you'd need to make that opening section in two parts, probably 1/3 and 2/3 of the height of the existing opening section, to give you closed, semi-open and open-backed characteristics.

P.S. I like the way they've put faux screw heads and washers on the opening part of the cab.

[Hugh Robjohns]

The opening would have to be a lot smaller to provide any noticeable and effective increase in air resistance. I'd be looking at between a 1"-2" gap at most.

At which point you're in the realms of a 'refex' or ported cabinet which is inherently a resonant box with all the related issues to deal with -- not least the altered resonant behaviour or the driver itself.

H

[Mixedup]

Sure. But as you said, Hugh, the variables here move us 'past' science and into art here. And if opening and closing makes a difference to the sound of the cabinet, that's one more artistic option to play with...

[Hugh Robjohns]

True!

[Wonks]

The early Fender amps had their drivers mounted on what seems to be referred to as a 'loose' baffle board, only held to the rest of the cab by a screw at each corner, which (allegedly) ,played a big part in the sound the amps created. The majority of amps these days(excluding old Fender copies) have baffle boards that are either glued into routed channels or screwed and glued to battens all round so the edges of the baffle board can't vibrate.

Another example of the cabinet construction having an influence on the sound.

[IvanSC]

...and this is one of the reasons I built my 53e or 5e3 or whatever *the* wide-panel tweed deluxe is.

Mine ( by sheer luck, I didn't go looking for it) is cased in 1950's pine and the box is built as close to original Fender spec as I could, including the floating baffle.

For whatever reason, the combination of that exact chassis and that exact cabinet is (to my ears) magic.
Admittedly I used a 20 watt celestion G12H intead of the speaker used by Fender, but it still sounds wonderful to me.

Just had a proper look and its a G12S from the Rola era.

[Jay Menon]

I have heard that old fender bassmans and Marshall JTM 45 (Or is it the blues breaker combo) sound amazingly similar... and that it is perhaps due to the open back cabinet construction.

[Sam Spoons]

I may be wrong but wasn't that because JM ripped off the Bassman circuit?

[Wonks]

With a few tweaks, but then again, none of Leo Fender's circuits were original (at least to start with), just different parts of circuits copied from GE valve circuit design guides.

[ore_terra]

well, I would say circuits are similar but adapted to the available components in each country. this makes them sound pretty different to my ears.

[Jay Menon]

From what I can see, it would appear that an open back cab allows that lovely shimmer for clean tones. For overdriven/saturated sounds however, a closed back cab really comes into its own.

To get the best of both worlds, I suppose putting a graphic or a parametric equaliser in the effects loop might possibly do the trick?

With a fender type amplifier with an open back cab, the clean tones are already there. For the overdriven sounds, what would one do - add some lower mid range in the loop...? Would it then become boomy?

Conversely, with a Marshall type amplifier and a closed back cab, the overdrive sounds are already there. So for the clean tones scoop the mids in the FX loop? (i.e. do the Mesa Boogie ‘V’ the thing on the equaliser)?

Intuitively, the latter seems more practicable and controllable...

[ef37a]

You might think that all valve combos are open backed for ventilation purposes and I am sure that was the thinking. However, when we sealed up a 2 x12 60W combo the actual temperature rise measured on the chassis rear panel was minimal!

I put the odd result down to the large area of cabinet that can radiate heat, even being wood. Then I think the thin paper cones of guitar speaker are probably practically transparent to heat rays?

It is, IMHO futile to compare combos and heads/cabs? Even if the amp is identical and (somehow!) you had a cab of exactly the same internal volume, they would I am sure sound quite different. One big reason is I am sure that valves were never intended to work effectively immersed in a very high pressure sound field. A 60W amp can easily produce 110dB SPL at a mtr so a pre amp valve must be enduring 130dB or so 100mm away, inside the box! In some combos, valves are mere mm away from the speakers.

Dave.

[John Egan]

I wonder when comparing the sound / tonality between:

- A Head + cab (often closed back) and
- A Combo (almost always open back)

...how much of the difference in sound is due to the open vs closed back nature of the cabinet? Does cab construction / design make a really big difference?

Hi Jay,
Back in the very early sixties, I chose an AC30 Super Twin (separate head and cab) rather than the more common combo. Although it didn't sound quite as nice, the greater projection I got from the closed cab was useful in big venues at a time when PA systems didn't exist.
Regards, John

[Jay Menon]

It is, IMHO futile to compare combos and heads/cabs? Even if the amp is identical and (somehow!) you had a cab of exactly the same internal volume, they would I am sure sound quite different. One big reason is I am sure that valves were never intended to work effectively immersed in a very high pressure sound field. A 60W amp can easily produce 110dB SPL at a mtr so a pre amp valve must be enduring 130dB or so 100mm away, inside the box! In some combos, valves are mere mm away from the speakers.

Dave.

So does that mean that one tends to get less interference / tube filament rattle etc with a Head & Cab set up rather than a Combo Dave?

[ef37a]

It is, IMHO futile to compare combos and heads/cabs? Even if the amp is identical and (somehow!) you had a cab of exactly the same internal volume, they would I am sure sound quite different. One big reason is I am sure that valves were never intended to work effectively immersed in a very high pressure sound field. A 60W amp can easily produce 110dB SPL at a mtr so a pre amp valve must be enduring 130dB or so 100mm away, inside the box! In some combos, valves are mere mm away from the speakers.

Dave.

So does that mean that one tends to get less interference / tube filament rattle etc with a Head & Cab set up rather than a Combo Dave?

Yes Jay, it is sometimes very hard to find a decent front end valve that has low enough microphonics. I even had POWER valves, EL34s, that 'sang'! Something I do not recall happening 40 years ago (N.B. When you buy new valves fit them immediately and blow the **** out of them. That way you have a chance to catch a rubbish one and return it..And you will!) .

"Back in the day" nobody in the audio industry would have dreamt of fitting a VALVE power stage inside a speaker* if they wanted high sound levels and some semblance of quality. The guitar amp grew from old radio set designs and I guess people just carried the idea forward?
My point was really that even if the combo remains stable (!) the high SPL hitting the valves must colour the sound? In a head the SPL is going to be much lower.

*The tweaky audiophool community bang on about even solid state amplifiers being microphonic? Largely bllx I think but capacitors in a high gain gitamp circuit certainly 'clunk'' well if you hit them with the taps open!

BTW, This ^ is one reason Bstar went to solid state front ends in all but 3 of their amps.

The Series One amps for instance have pre with about 12dB of gain, zero hum, zero microphonics and much lower noise than a triode. Overload capacity is about 3V rms, better in fact than the usual ECC83 stage. Much cheaper too! Win-win?

Dave.

[ef37a]

Have just read about a Peavey 30W combo that is very hard on EL84s, it shakes them to failure apparently long before the rather high cathode biased dissipation has taken its toll.

I would therefore suggest that if a combo can DESTROY valves acoustically it must have a large influence on the sound quality?

Dave.

[Folderol]

Wow!
That's not just bad design - it's pathetic. I wonder if the {cough}designers{cough} actually know what a valve is?

[ef37a]

Wow!
That's not just bad design - it's pathetic. I wonder if the {cough}designers{cough} actually know what a valve is?

Well Will, I do have a BIT of sympathy for Peavey? The fact is that modern valves are largely crap and do not cope with abuse nearly so well as old Mullies and Brimars.

I remember VALVED car radios where the PSU/PA was in a case in the ENGINE compartment and you had to get a pretty thick, pretty stiff umbilical through the bulkhead.
Those valves were exposed to high heat and appalling vibration but seemed to give several years of service?

Unless the whole guitar amp/valve mnfctring industry get their fingers out and collaborate I can see in ten years valve amps being only for the very rich and famous.

If anyone wants to know I am reliably informed that 'JJ's are currently about the best valves around.

Dave.

[John Egan]

If anyone wants to know I am reliably informed that 'JJ's are currently about the best valves around.

Dave.

Dave,
I prefer the sound of JJs and they do seem to give less problems, although a friend of mine is not keen on using them inverted, because he says they get shaken out. I don't use JJs 6V6s though, since I read that the design and sound is quite different from the originals. I wonder if you would agree.
I find EHX valves to be good as well.
Regards, John

[Wonks]

My guess would be that any mechanical vibration would form a small feedback loop to very slightly stretch out a sound - though I have doubts whether the amplitude of any such sound produced could be seen or heard above the noise floor of the amp.

Any vibrations are just the sound that the amp's already produced, and there will be two types of vibration. The first type will be direct coupled vibrations from the combo's cabinet being shaken directly by the speaker movement, the second will be any vibrations caused by air pressure changes as the speaker moves.

I would guess that the direct mechanical vibrations reach the valves first with a very short propagation delay, whilst the airborne vibrations will arrive slightly later with somewhere between a 0.5 ms and 1 ms delay; dependent upon the distance of the valves from the speaker (and it will be slightly different for each valve).

With a time delay, there will be some interference between the two vibrations, though probably not until you get up above 1500Hz (or maybe higher) are you likely to see any full constructive or destructive interference at certain frequencies. But I suppose any constructive interference could fall in line with a resonant frequency of the amp chassis or a valve sitting in its base (especially if already a bit loose), or internal components of the valve itself. But at these higher frequencies, the amplitudes should be a lot less You'd need to fit accelerometers to the chassis and the valves themselves to detect this.

To see if vibrations affected the sound, you could probably set up a simple test rig using two amps of the same make and model. Combo No.1's and No.2's controls are set to the same positions. Combo no.1's amp output is connected to the speaker(s) in combo No.2. Combo No.2's output is connected to a speaker emulator/load box. The output from this is recorded by a DAW. Combo no.1 could be fed either from a real instrument or from the DAW via a re-amp box playing back some pre-recorded guitar signal. I suggest the latter is preferable as it's easier to compare periods of silence and periods with signal. Also peaks in the guitar track may be easier to cross-reference .

It would then be simple to see if any detectable sound was imprinted on the background noise of combo No.2.

Best to do in a studio set-up, as you'd want to get the amp running near maximum for the biggest vibrations. And you'd want to do it a well-treated room to avoid any room mode resonances affecting the results.