Amp Open Back Question S S S

Philip Mitrakos · From: The Beach South East Florida

Â I would like to know why so many amps have full open backs instead of closing the back of the amp to give the speaker something to bounce off of .

Philip Mitrakos · From: The Beach South East Florida

This is a question that is commonly asked, but no one can usually explain why thereâ€™s a difference in tone. Well, weâ€™d better put that right!

All speakers have what is known as a Free Air Resonance (Fs) and is measured in Hz. It is the frequency that the speakerâ€™s cone will naturally resonate at with greatest acoustic output and with the least input power, when the speaker is not mounted in a speaker cabinet... just sitting on a bench connected to a signal source. The frequency is different for each speaker type in a makerâ€™s range, but is often around the 75 to 110Hz range for a typical 12â€ guitar speaker.

When that same speaker is mounted in an open back cabinet, it will still exhibit high acoustic output at around those same frequencies. But, letâ€™s suppose that a speaker naturally resonates at 80Hz... which is almost the same frequency as open E on the 6th string of a guitar (82Hz). So, playing that note sets off the speaker and gives a very high output at that frequency. Thatâ€™s why many amps can sound quite boomy around that note and is more noticable with tube amplifiers and also modern transistor amplifiers that employ constant current drive... like Marshallâ€™s FDD - Frequency Dependent Damping - used in many of their tranny amps.

So, now letâ€™s fit a closed back on that same speaker and cabinet. What happens now, is that the sealed cabinet puts an increased load on the speaker. By this, I mean that the air is trapped inside the cabinet, so the speaker has to compress and stretch it... causing more opposite tension on the speakerâ€™s cone. So the cone becomes stiffer and raises the speakerâ€™s resonant frequency, usually by two and a half notes up to open A on the 5th string (110Hz). This resonance rise is pretty much the same amount for any size cabinet, but air temperature seems to make it vary a small amount. Many speaker cabinets have half backs with an extra half that you can screw onto the back to make it totally closed, providing the option of both types.

Tonally, both have their place. Closed backs seem best suited to rock music, where open backs seem nicer for all other genres. The trouble I find with closed backs, is that they sound boxy and the higher resonance seems to get in the way for me. But then Iâ€™m more of a swing jazz/blues/country player, so not surprising really.

Some find that even the lower resonance of open back cabinets can be too much, so they fit speakers with a low resonant frequency, well below open E (82Hz) at around 55Hz. This has the affect of making the tone rather flat or cold in tonality, but for some styles it works well. Larry Carlton had a Mesa Boogie fitted with an EVM12L (Fs: 55Hz) when he was with The Crusaders and sounds great with his Gibson ES335.Â

The Celestion G12H30 Heritage model was brought back into production for this reason a few years back. Itâ€™s plain paper edge has a resonance of 55Hz too and was designed originally as a bass guitar speaker. It has a regular guitar cone, but without the plasticised surround to stiffen up itâ€™s suspension. Hence the low Fs. A more supple edge lowers the FS... thatâ€™s why most hifi bass speakers have very supple rubberised edges to the cone, and can reproduce very low frequencies... but this is not required for 6 string guitar!

This was something I found when looking for sugestions ..it was a copy and paste

Jim Pitman · From: Waterbury Ctr. VT 05677 USA

Recall the "Acoustic Suspension" stereo speaker days?
To me the closed back sealed cabinet was the best sounding rig for music reproduction in general. In a addition to raising the resonance, a speaker designed for a closed back sealed cabinet will exhibit less bass distortion with a flatter bass response.
Correct me if I'm wrong, Philip, but a sealed speaker will be less efficient. One needs more amp power for acoustic suspension.
I'm no fan of the more popular modern ported stereo speakers. I think they are boomy, albeit more efficient. I have an old 50W Marantz driving a pair of Scott acoustic suspension speakers that I really like for music listening.
When it comes to guitar and PSG performance, I too like the sound of an open back cabinet better though.

Ian Rae · From: Redditch, England

I like the new style of thread where you ask a question then answer it yourself. This avoids all the misunderstanding of OPs that sometimes arises.

Jim sums it up well - closed back better for hifi, but a flat frequency response generally not want you want for a real instrument.

Real difference though, you can't cram any junk into a closed cab.
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Geoff Noble · From: Scotland

I'm building a couple of 12" cabs at the moment and I've been looking into whether I should make them closed, opened back or ported.

I've read the theory that you have posted Philip and read reviews of people's opinions on the resulting sound. Many said the closed back had a tighter, more even response but required more power and had a bit of a flat,(dead), sound. Others said the open back didn't have such a tight bass response but more ambience coming around from the back, or reflected from a wall etc, and a bit more life in the sound.

The dimensions I'm using are based on the Cube 60 but increased by around 5-10%, so a bigger Cab. If you look closely at the Cube it has 2 small front ports and a very small gap between the amp head and the cabinet back, so has an element of open back.

I figured they used the ports to increase the bass response of the cab as it is a small cab and probably needs more bass response.

I originally thought of copying this design but thought that my bigger cabs will give me the extra bass response so ports would just confuse matters. I'm going to try both open and closed back to see what works best for me, I'll also try different sizes of "gap" to see if this can be tuned sweet in any way.

The speakers I'm using will either be 2 x Eminence Legend 1275 or a couple of old McKenzie 12" that I recovered from an old amp, whatever sounds best.

Watch this space. (It could be a few months before I complete these!).
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Jim Pitman · From: Waterbury Ctr. VT 05677 USA

Geoff - I'm a fan of the Emminence Legend. That's what I currently have im my amp.

Erv Niehaus · From: Litchfield, MN, USA

It seems like most guitar amps are open back and most bass amps are closed back.

Ian Worley · From: Sacramento, CA

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Georg Sørtun · From: Mandal, Agder, Norway

I'm more curious about why so many have 15" speakers to "move more air and get more lows" in open-back cabinets that are so small - short free-air distance from front to back of membrane - that the produced volume naturally falls off from well above 100Hz downwards.

Be that as it may; the traditional open-back cabinets used by steelers, are - despite their low efficiency - good and "musical" enough for their purpose, and they are easy to build.

Ian Worley · From: Sacramento, CA

Precisely Georg. An efficient 10" speaker installed properly tuned and ported closed cabinet designed to that speaker's Theile-Small parameters will invariably produce a much more robust low frequency output than random 15" speaker "X" placed randomly in a generic open back box. As you note, the size of the box does matter for open back designs too. Small cabinets with big speakers don't produce/project low frequencies efficiently. The "move more air" thing is, as you allude, something of a myth when one bypasses both the art and science of efficient enclosure design.

Jerry Overstreet · From: Louisville Ky

I've built a couple dozen cabinets for my steel rigs. The latest ones are closed in kick back monitor style housing 12's w/ports. They are very loosely based on Thiele/Small parameters. This is more critical than with open back models, but not nearly as critical as when building cabinets for sound reproduction, home stereo or studio monitors.

I do follow some general guidelines to a point, but generally build them to suit my own design needs.

You can get away with almost anything in open back cabs. Generally, the larger the box, the better the bass response, but it's nice to have some general guidelines.

You can find several guides for building cabinets at many sites online by doing a search.

Many years of experimenting with cabinet designs I have found open back cabinets have a wider dispersion and somewhat mellower tone. This may let other band members hear you better on stage.

Closed backs are more directly focused and have more volume at the very front of them. I like them a lot in a stereo configuration where I'm using panning, rotary and other modulated effects.

After 20 yrs. of so of experimenting, I'm still not sure which design I like best. It varies by playing situation and from era to era.

These are my thoughts and findings to date. You may feel free to ignore or disregard them as you see fit.

I'm inclined to believe mfgs. who build open back combo units just use the dimensions of the amp chassis and the speaker in designing a cabinet. I'd be surprised to learn that there's much more thought given it.

Jim Pitman · From: Waterbury Ctr. VT 05677 USA

On the contrary Donny. For the last seven years or so there's a bunch of audiophiles who are into "Single Driver" designs using a very wide range, typically small, less than 10" speaker, in large ported cabinet. Speakers designed for this have very lightweight cones that are ultra efficient to get the most out of a single ended low watt triode tube amp.

Bob Hoffnar · From: Austin, Tx

Larry Allen · From: Kapaa, Kauai,Hawaii

Here in Hawaii more than half our gigs are outside. I made a Back cover from tolex and thin plywood that velcros on for these gigs to help project the sound...Larry Smile

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Dan Robinson · From: Colorado, USA

A bass player in my college-years band had an Ampeg SVT into a large (full-stack size) Marshall cabinet, ported, with one down-facing 18" speaker. We called it "the refrigerator."

He could play a low note, at mid-volume (beyond the SVT's "2") that masked every other sound in the room. My LP thru Twin Reverb with EV/SROs could not cut through. Still it was impossible to hear WHAT NOTE he was playing. Careful EQ was essential. Talk about low-resonance!

Tele-master Roy Buchanan played open-back Fender amps, turned up loud, but pointed away from the audience for his distinctive sound and sustain.

Philip Mitrakos · From: The Beach South East Florida

This is why I posted this thread ...to read other peoples ideas ..!

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Georg Sørtun · From: Mandal, Agder, Norway

George Redmon · From: Muskegon & Detroit Michigan.

For awhile back in the mid 90's I was running both together. I had my moded Fender Deluxe Black Face, and ran a custom wired Peavey closed bottom with 2 12" BW's. Plenty of power, we played outdoors as you can see. Still not sure if I really liked it or not. It was ok I s'pose. It did give me more bottom end. Then I went to just the little modded Fender Black Face Deluxe alone.

You said a white shirt? really?.....oh sorry, what's in that white cup?

Ian Worley · From: Sacramento, CA

Georg, I'm curious about the transmission line style vented enclosure. Does it require a larger box than a bass reflex enclosure, or are the box dimensions still calculated from the speaker's Theile-Small parameters? Is the length of the vent channel based on the speaker's resonant frequency, or can it be something lower or higher? With a single speaker does this additional low frequency resonance inside the box have any discernible effect on the speaker's reproduction of higher frequencies?

For the past couple of weeks I've been practicing through my Mesa 2x10 bass cabinet and I really like the tone vs. my open back 1x12 Celestion. It is a typical bass reflex with a short vent. The box is not that big overall but it's really heavy. I assume the density of the internal material used to enclose a transmission line channel is important too in minimizing unwanted resonances, does surface reflectivity make difference also?

I just ordered a new Eminence Double T 12 and want to try some different combinations. I know there is no holy grail, but it would be nice to maximize the speaker's overall efficiency while not letting the box get too huge/heavy or having to give something else up tone-wise.

Georg Sørtun · From: Mandal, Agder, Norway

Ian, I wasn't sure how to respond to your questions, as it can be rather difficult to explain "the inner workings of a non-traditional speaker-cab setup" using common terms - I have tried for about 50 years Smile

There's also a language barrier in place, as English isn't my first language. Anyway, I decided to give it a try...

A properly designed "cut transmission line" cabinet will tend to become a little heavier than a "bass reflex" cab - if built in the same materials, but not necessarily larger. It all depends on how well one makes use of angles inside the cab to avoid parallel surfaces and standing waves, to incorporate the transmission line(s) (TL) along the inner edges of the cabinet.

By letting no space go to waste, and tuning the entire construction for audible performance, home-builders can shave off size and weight while utilizing the TL principles for increased low-range efficiency and "musical preferences". Cabs on the market are unlikely to have been optimised in every internal detail, as such "invisible" optimisation takes too much time and cost too much money in production lines.

Most transmission line constructions are designed somewhat like this...

...which means they are tall but not very wide - nice at home but a disaster waiting to happen on a stage Very Happy

The ones I built back in the -70s - -80s were laid on their side and about 25 inches wide, about 18 inces deep and about 15 inchs tall (IIRC), with the speaker on one side and the port on the other - like a deep dual speaker cab. A low and steady cabinet well suited for stacking.
As I also incorporated a form for horn-loading of the speaker (and the loaded port) - for controlled spread more than anything else, the speaker was mounted about one third way into the cab from the front.

I used a KEF B200-G element. That's a "short coil in long gap" type of speaker, well suited to be driven by the 80W RMS amps I used. This rather small element weighs a bit for its size, but since I only needed one speaker pr cabinet it was nothing compared to having a couple of 12" or 15" speakers in a more traditional cab.
I chose this particular element because I already knew the KEF 104aB reference speakers inside out - still have them, and the more power-tolorant KEF B200-G was the closest thing I could get on the market back then. I wouldn't know for sure what element to choose today if I was to build such cabinets again.

Two different-length TLs behind the speaker ended up in one "inverted TL" about half way through, resulting in much less prominant resonances and higher efficiency.
As there is no such thing as a "true transmission line" - a true transmission line is by definition "endless", the principles I followed are best describe as being somewhere between a Quarter Wave Enclosure and a Tapered quarter-wave pipe. In short: the TL's "breathe" real lows in sync with the actual element, somewhat like a bass reflex construction but tuned much lower - a very practical compromise.

My construction was further "complicated" by the use of a KEF "passive radiator" at the port to get optimal performance with a minimal-sized cabinet. This passive element made it possible to get away with shorter TLs by "increasing load and phase-delay on the TLs", but makes it a lot more difficult to describe my overall design. If you're interested I should be able to draw up my basic design, and publish it here or send it to you privately.

The "cut and loaded transmission line" was about 3 feet long at one side of the speaker and a little longer at the other - curling around inside the edges of the cabinet and ending up behind the passive element. I tuned them by ear and measurements - using music and generated sound to get them "just right" and well balanced, and the passive element at the end of the TL just barely started contributing in-phase lows from around 70 Hz - acting somewhat like a "bass reflex" (see: "Tapered quarter-wave pipe" linked to above). Pretty even and clear overall reproduction down to about 25 Hz - with no dominant resonances, and projecting good "tonal" lows to well below 15 Hz. Overall efficiency was measured to about 35% around 40 Hz back then (most traditional open-back cabs have more like a 5% efficiency that low, and a good bass-reflex cab maybe as much as 25% but often with a more "boomy" character).

As I used thick MDF (18mm and 22mm) for the entire construction, those cabs of mine became heavy - wasn't a problem back then.
Such cabinets can be built a lot lighter by using thin plywood sandwiched with other materials and stiffened with ribs in critical areas. Dampening cab-resonances becomes slightly more complex then, but creating "dead panels" isn't all that difficult.

In addition to having no parallel surfaces or 90 degree angles anywhere inside the cabs, I "roughened" all internal surfaces and "curved" all angles by applying a layer of fine and course woodshavings with glue that was left to set. This, and a thick layer of foam - similar to what you put under wall-to-wall carpets but with several layers glued together in some areas, efficiently dampened off internal reflexes.

Needless to say that each of my cabinets took a while to build and tune, but they were easy to replicate and mirror for PA use (mirroring pairs was not really necessary but I did it anyway), and, apart from the one cabinet that was dedicated for playing steel and therefore was built a bit smaller, I didn't have to bother with size and weight.

Len Amaral · From: Rehoboth,MA 02769

I don't have a scientific approach to my liking for open or closed back speakers. However, I have used and experimented with both. Other than a Polytone amp with a closed back, I prefer open back cabs. Even when I acquired closed back cabs I would eventually open the back. I had used one closed and one open in a stereo setup once, but in mono I always preferred open back.

The Polytone is a different animal and a one trick pony. But a heck of a trick.

Bill A. Moore · From: Silver City, New Mexico, USA

I have built a lot of speaker cabinets, but I think the best guitar cabinets have an open back. Playing through a tuned port, closed back seems to muffle the sound, (to my ears). I had noticed while playing on larger stages that using one of the rack covers behind the open back cab increased the bass response. I have built a few with an oval opening behind the speaker with good success. I got the idea from seeing commercially availlable cabinets, and don't know the science behind it. I make the opening approx. 1/3 the diameter of the speaker, with the length approx. the full diameter.
This little practice amp has a lot of bottom end, for it's size!

Geoff Noble · From: Scotland

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