open vs. closed back cabinets!

[nero1985]

whats the big difference?

my amp has a closed back, so i just removed the back cover (made of wood) and im going to try it in a little while, im just wondering before i even test it, what should i expect from this?

how about is i drill holes on the back?

[nero1985]

WOW!

i just did it!


its great!!!!!!!!!!!!!!!!!!!!!!!

more volume and more BALLS!

this is great!!!! why do they make them closed back when they sound better as opened back?!?!?!?!?!?


again..

WOW!!!!!!!!!!!

[nelson]

Closed back cabinets have a better bass response.

[petemoore]

Country players like open back for the drummers guitar monitor, it lets the sound out the back too.
 Definitely a difference in the bass response/definition...
 Most of the sound that 'would' go out the back is directed foreward, this is why closed cabs project. Also the 'eddy's of air traveling from high pressure [front] to low pressure [back] [and opposite depending which way the speaker is pushing it] is eliminated.

[R.G.]

Closed back cabinets raise the lowest frequency that the speaker/box can put out because the air inside acts like a spring, raising the speaker's natural resonant frequency. The air spring and box resonance may or may not help the sound of a given woofer unless careful design is done to make them work together.

Open back cabs have little effect on the speaker's resonance, and so the speaker is largely uncontrolled at its mechanical resonance. This makes for more prominent (if muddier) bass and the increased possibility of exceeding the speaker's available excursion, damaging the speaker.

[petemoore]

drop a rock in a pond
 Drop a rock in a pond, right next to a dam, notice because the wave can't go > way, it goes even stronger <way
 This is a metaphor of what walls and floors 'do' to soundwaves.
 closed back cabs back is a wall, but it's sealed 'inside'
 'projects bass'is an oft used term with 4x12'' cab sound descriptions...which when compared to a 2x12''' open back...be that as it may 'pretty good term to throw around.
 There's a bunch of complex stuff going on with either of these designs, Leo, being known as the 'frugal one' seemed to favor the open back design, simple,Good, popular...like any design that has a 'comparator', it will likely be said to have it's strong points and maybe weak points.
 Enclosures get the natural compression caused by the increasing air pressures inside the cabinet when the speaker cones move farther from 'idle' position.
 Definitely a different sound.
 Put an open back cab next to a wall with a cushion closing off as much of the back port as possible [watch melting it on the tubes !! and build up of temperature, get the speakers going a bit, notice the change in the sound.

[nero1985]

so the open back can damage my speaker??

i have a crate 65 watts amp and its open back, and many artists use open back cabinets (specially back in the day)

i think it sounds way better as an open back and louder as well but i dont wanna damage the speaker

[petemoore]

I would suppose it's designed for that, use as a guitar amp.
 Speakers...you've tried pushing in a speaker cone [carefully distributing the gentle force so as not to distort the shape of the cone unduly], it is much harder to get 1/4'' travel than it is 1/64th.
 The coil alignment structures of the speaker are also motion dampers,  [the cone and the 'flat accordian' looking 'dummy cone' under the cone both restrict movement of the cone/coil from going past the coil 'boundaries' [fif it's a factory amp/speaker, theyr'e designed to work as sold. {hhopefully and probably].
 You should not worry about blowing your speaker any more than you did before reading these posts.
 I think open back speakers might not last as long as enclosured ones...lol...hard to say for certain where your amp fits into my 'contradiction' but probably in the heavy duty speaker aspect of it.

[RandomRedLetters]

open back wont damage your speakers, Like said before the differense is really in frequency respose. Try it in a band situation, I do believe closed backs work better in rock situations, while open backs are nice for clean tones (jazz, blues, country etc)

[brett]

Hi.
I've had clean (MusicMan) and dirty amps (Marshall) and favour open backs for both.  Why?  
1.  Efficiency.  They are louder because they're not wasting electricity compressing the air in the cabinet.  If you want to compress air with electricity, buy a compressor, I reckon.
2. There's lots of sound that's going out the back (nearly half).  You're probably thinking that's wasted.  Nothing could be further from the truth.  You want to control the use of that sound.  Parallel to walls, at 45 degrees, near, far, etc - the options are great.
3.  There are probably other reasons, but the two above are fairly convincing for me.

Of course, for stadiums these things don't apply, so a stack of 4 x 12s is probably attractive for that.

Anyway, it's great that your Crate is cranking now that it's opened up.  Have fun.

[Doug_H]

If you have a spkr with a big meaty magnet, closing the back won't necessarily do anything for the bass response. I have a g12h30 in a 1x12 and I've taken the back off a few times. It disperses the sound around the room better and makes the highs more "airy" because of the reflections. In fact, I've even noticed more bass response with the back off. I'm going to open it up and put a 1/3 size back on it because I've been having trouble hearing myself when playing with the band.

Glad that helped you out, Nero.

Doug

[MetalGuy]

It depends what kind of music you're playing. To my years open back cabinets /combos/ are more blues and jazz thing. For heavy metal and stuff closed back 4X12" cabinets and stacks are the way to go.

[nero1985]

Hey guys thanks alot!!!!

the amp has a Celestion G12 AVT100 speaker wich is prettty mean sounding!!! i really enjoy the sound of the amp right now, the amp itself is dirty but now without the back i kinda get a clear sound as well so it gives me a different sound from everybody else and thats wat i was looking for, all im worried about is to not to damage the speaker thats all and since Mr. RG said it could im a little worried but ill keep playing it to see wat it sounds like,, thanks guys!

[Doug_H]

Don't exceed the power rating of the speaker and you should be fine. I don't think opening the back will damage your speaker as long as it is not specifically designed to be in a closed back cab. Many years ago there were hi-fi and car stereo speakers called "air-suspension" speakers that had to be in a closed (or was it open??? can't remember...) cabinets because of the way they worked. I'm already over my head talking about speakers, except to say that today with musical instrument speakers and specifically guitar speakers it is pretty common for people to mix and match them as desired into whatever kind of cabinet they prefer. Just make sure you don't exceed the power rating and have the correct impedance loads (not an issue here) and whether or not the cab is open or closed shouldn't be an issue. If you are concerned, contact Celestion and they can answer it for you specifically. They have a nice web site btw...

Doug

[petemoore]

I have a g12h30 in a 1x12 and I've taken the back off a few times
 That makes sense  :wink: the speaker doesn't care about compression enough to alter the volume control or whatever so's you'd notice...
 Now the cabinet itself can resonate, we had 'ol Jensen 13 ply cabinets, alot of it looked like 3/8'' plywood. They were LarGe and light, and got boards put over where the horns went. With EV PA type 15's driving, these cabs had very nice sounding resonance to them, Even the RCA 12's sounded better than...
 The other cabinets in this comparison were 'small brick' [3/4'' plywood] types, just big enough to fit horns and a 15'' in, about 'double deep' though. these didn't have resonance, also the volume was lower.
 The Jensen cabs went up in a housefire, I wish I had them.

[vanhansen]

Wanna get a real big sound, and I think petemoore was angling this direction, put the amp in a corner with the open back facing the corner.  Now that corner will project the sound coming out the back and it sounds like you have a bigger amp than you do.  I do this with my little 15 watt Crate and it sounds huge.  Can't do this at many clubs but it's fun at home.

[petemoore]

dAMM tABB lost it again...
 anyway that makes sense that a forcful speaker would care less about cabinet type.
 rock in the pond metaphor...this time simulates an open back cabinet in a typical situation...near..but about 15 feet from the wall.
 Throw the rock into the pond about 15 feet from the wall, and notice how the 'bounce wave' and it's artifacts interacts with the 'bigger clear wave' coming from directly where the rock started it...you've got a bunch of biger and smaller waves interacting.
 Now for a typical 4x12''...
 Build another wall 30'' wide, parallel to, and in front of the infinitely long wall by about 15 feet, drop the rock right in front of the 30'' wall.. notice how the 'bounce wave' [off the backwall' and it's artifacts is smaller, and influences the 'direct' wave less. The 30'' wall that simulates a closed back cabinet prevents the 'back' direction wave from 'bouncing' off the backwall.
 If you're looking for a 'direct punch' sound, or a more ambient [reverby] 'in the mix' sound, this metaphor may be of some help.
 The difference between oversized Jensen cabs with what looked like mostly 3/8'' plywood material, and the 'brick' cabs made from heavy duty 3/4'' plywood was quite remarkable, the cabinet resonance of the Jensens made a big difference in the overall sound, the ohter cabs were as small as a 15'' and horn would fit into...had a 'horn port'...the ports were blocked off then [which made not much difference ITC] the Jensen hornports were wooded over 'before'. I wish the Jensens didn't go up in Tim's housefire.
 Anyway if you have a large stage and are looking for a punchy sound this may help.
 Joe used a Fender Concert 4x10'', setting it right in front of his brothers drum set, Dave liked having the amp there.
 Cueing the drummer with the guitar can make a BIG difference in the tempo/length of a song, also provides feedback to the players for possible instant tightness.
 For 'clubbing' IMO, my 4 12 is much too directional to provide the drummer with enough information. Having the cab set right can bring a tightness and 'whole darn song'ness to a jam, without that it is much less fun...
 I'm seriously considering modding my Marshall 4x12'' for 'club use, and original by putting in pluggable ports. I love the punch and projection, but drummer cueing takes precedence.
 I hate having the drummer 'tell me' stuff that ain't quite right and not be able to 'talk back' to him. A drummer that can hear great guitar attack may add alot of otherwise missing content.

[brett]

RE: putting the amp in the corner of the room and using the "natural megaphone effect".

The passive megaphone seems a super-efficient device for the human voice.  Just ask any of us old hippies who rallied in the 60s and 70s!  But there's always a catch.  What are the limitations to using this effect?  Loss of lows or highs or what?  Voices seem to become high and nasal, I think.
cheers

[R.G.]

Acoustic horns are transformers - they transform high pressure/small area vibrations at their throat to low pressure/wide area vibrations at their mouth. They do this quite efficiently if a whole range of stuff is correctly done.

The acoustic impedance of a speaker is mismatched to the acoustic impedance of air. Air is thin, low grip stuff. Speaker cones are dense, tough (relative to air) high mass things. So the typical direct radiator speaker spends all its energy controlling where the cone is and only a tiny fraction of the electrical energy put in goes into moving air.

An acoustic horn transforms the air impedance to a much higher load at the speaker. This makes the air relatively harder to move compared to the speaker cone, so relatively more of the electricity you put into the cone comes out as air movement, relatively less has to be spent on moving the cone because small cone motions at the throat become big (but low impedance) motions of air at the mouth. A perfect acoustic transformer is a wonderful thing, just like a perfect electrical transformer.

Alas, they suffer the same problems too. There **isn't** any perfect acoustic horn.  An acoustic horn resembles a pipe with a steadily increasing diameter. If the rate of flare is too little, then the horn is enormously long, and you get resonances from just the pipe's shape. If the rate of flare is constant, you get a straight sided horn which is short, but has ugly resonances and a poor bandpass response - the highs and lows don't "see" the same impedance transformation.  The "right" flare rate was the subject of intense research in the 1920's through 1960's. It turns out that the right flare rate is something like an exponential or tractrix. These shapes give enough pipe-y mouth to keep highs and lows together, and fast enough expansion to get a big mouth.

Oh, yeah. Big mouths. The lowest frequency the horn does well, even if it's otherwise perfect, is the frequency where the diameter of the mouth is 1/4 of the acoustic wavelength. With 1000ft/sec sound, a 20Hz horn mouth needs to be about 12-16 feet across. Guitar's 82 Hz lows make this a more-reasonable 3-4 feet across, but it's still big.

And distortion. Air is a non-linear material. It squeezes more linearly than it evacuates, so with high enough levels of pressure/depressure from a speaker cone at the throat, the de-pressure waves approach atmospheric pressure, and the air in the throat distorts. So there's a limit to how hard you can drive the throat of a horn.

The corner-megaphone or corner horn expansion trick is real, but it's a really imperfect horn throat.

[WGTP]

Boogie and others use 1/2, 3/4 and full closed back cabinets for altering the bass response, dispersion, etc. of their cabinets.  A slick design is to open the top 2 speakers in a 4-12 cabinet and leave the bottom 2 closed.  This way you get the a combination of both types.  Also, using different speakers in a cabinet to blend the characterestics of both is becoming popular.   :shock:

Putting a hinged back on the cabinet might be beneficial for making adjustments and aiming the sound coming off the back.  :roll:

Using a cabinet with a speaker on the back and front might provide both the bass response, power handling and dispersion etc. desired.  Also, if 2 different speaker were used, 2 different primary sounds could be available.  For example a clean type and a vintage type.  Or 2 facing forward and 1 facing back.  Or a 12" in front and a 10" in back.  Many possiblities.    :twisted:

Speakers have been neglected too long as a major tone influence.