Under powered cabs...problem?

[Stumbo]

More speakers move more air. Two 2x12's with 250w would probably do better(sound louder with head room(clarity) than 400w one 2x12.

 

[CL400Peavey]

So I got a SWR sm-900, I'm thinking about buying a aggie GS212. The cab is 600w @ 4ohms. Should I hook it up in stereo mode and only push 400w or bridge it and send the full 900w and just watched the volume.

You gain nothing by bridging the amp.

 

[CnB77]

Under powering is a myth, read the sticky, or ask Bob from QSC the next time he is on here. It is a myth, an urban legend that won't die. Ask yourself this: if this underpowering idea was real, would getting a cabinet that supported fewer watts solve the problem? With the underpowering meme, it would. Simply ridiculous.

More importantly, if underpowering were real then using your volume knob would be dangerous

 

[guy n. cognito]

Do you think one 212 is loud enuff for bar gigs? And if I have PA help do you think it will do fine?

It depends on the bar, but I have used a variety of 212s at bar gigs, without PA support, and had plenty of volume on tap.

 

[smogg]

I was wondering if there is anything wrong with under powering cabs. So one had told me there is but I don't buy it. I'm think of buying a Aggie GS212 from a friend. It's 600w @ 4ohms, I'm only going to be able to push 400w to it. (only other 4ohm option is the full 900w to it).

On a side note Aggie GS212 vs Avatar B210?

I've been running a 500w head into a 600w 212 cab (Promethean into GB Neox 212T) for years and it's never been a problem, ever. Your set up should be plenty loud for a standard bar gig, especially with PA support. The aggie is a nice cab, built well and sounds good.
Enjoy & have fun.

 

[Bassmann1968]

Actually, UNDER powering is MORE likely to damage a speaker. The reason is that an amp slipping in to clip is VERY hard on a driver. You may not even notice that it's clipping by the sound once you figure the noise from the other insturments and the PA.

Over powering can damage a speaker, BUT it's unlikely for two reasons.

1) You'll hear the distortion sooner and turn down thus protecting the cab.

2) Cabs are fused. If you OVER power and hit the cab to hard and ignore the obvious distortion coming from the speakers, the fuse will pop. You replace a 10 cent fuse and you're up running again.

If you UNDER power the fuse will NOT pop because you never reached it blow rating. The drivers get to low power distorted signal and burn up.

I used to run an 8ohm 800 rated SWR Goliath III with a Crown K1 bridged to 1100 watts and NEVER had a problem but I was very aware of the cabs sound. It really started to sing at around 1 o'clock on the amps volume. I turned it up full blast one time just to see what it sounded like. It was LOUD.

SWR used to use a light bulb as a fuse. In the dark you could see it flashing. Pretty cool actually.

Da staunt der Laie, da lacht der Fachmann

 

[Downunderwonder]

So I got a SWR sm-900, I'm thinking about buying a aggie GS212. The cab is 600w @ 4ohms. Should I hook it up in stereo mode and only push 400w or bridge it and send the full 900w and just watched the volume.

It won't be hardly any louder bridged. Either way you could kill it from overpowering it.

If you push it as hard as possible with one channel it will be sounding arse and cooking. If you bridge it won't get a chance to cook because you'll shred the cones first.

The first 250W makes nearly all the noise. Either way you have to listen to the cab.

 

[Munjibunga]

Actually, UNDER powering is MORE likely to damage a speaker. The reason is that an amp slipping in to clip is VERY hard on a driver. You may not even notice that it's clipping by the sound once you figure the noise from the other insturments and the PA.

Over powering can damage a speaker, BUT it's unlikely for two reasons.

1) You'll hear the distortion sooner and turn down thus protecting the cab.

2) Cabs are fused. If you OVER power and hit the cab to hard and ignore the obvious distortion coming from the speakers, the fuse will pop. You replace a 10 cent fuse and you're up running again.

If you UNDER power the fuse will NOT pop because you never reached it blow rating. The drivers get to low power distorted signal and burn up.

I used to run an 8ohm 800 rated SWR Goliath III with a Crown K1 bridged to 1100 watts and NEVER had a problem but I was very aware of the cabs sound. It really started to sing at around 1 o'clock on the amps volume. I turned it up full blast one time just to see what it sounded like. It was LOUD.

SWR used to use a light bulb as a fuse. In the dark you could see it flashing. Pretty cool actually.

Nyuk, nyuk, nyuk. New around these parts?

 

[Bob Lee (QSC)]

You can blow a speaker by "underpowering" it, which is to say you drive a saturated level that exceeds 71% of the speaker's RMS level. With bass amps often there is a limiter on the output so with a 400W amp you can put out a saturated 400W signal that is virtually square wave. Square wave signals pass 41% more continuous power than an RMS signal (400W RMS only peaks at 400W). For example, if you slam the limiter of a 300W amp that is capable of fully saturated output (tube, older class AB solid state, all class D) you can blow a 400W RMS speaker.

But slamming a limiter like that is either a deliberate tonal effect or careless abuse. If your amp use doesn't fall into those two categories, underpowering is totally safe. I drive my 700W cab with a 500W amp on purpose, because it has lights that comes on at half power and at full power (limiter/saturation), so I'm aware when I'm exceeding 1/3 and 2/3 of the cab's power handling. Having 40% more cab RMS power than amp power is best, because then even a total moron would have trouble blowing it, but it's still not impossible.

I think you're mixing up a few concepts.

I'd like to point out that it's quite rare to get anything that really approaches a square wave out of an amp.

 

[Bob Lee (QSC)]

Actually, UNDER powering is MORE likely to damage a speaker. The reason is that an amp slipping in to clip is VERY hard on a driver.

No.

If you could put a clean signal output of an amplifier into a loudspeaker, and then clamp the amp output voltage down so that the same signal flat-tops at a certain point, you'd find that the clamped signal, because it reduces the power dissipated in the driver, is easier on the driver. It would probably sound like crap, but it would not be harder on the driver.

 

[Tuned]

Underpowering a 700 Watt RMS power handling cab by -40% translates to an amp power of:
420 Watt
Doubling the Wattage due to clipping calculates into:
840 Watt

Overpowering a 700 Watt RMS power handling cab by +40% translates to an amp power of:
980 Watt


Conclusion:
840 Watt will fry the voice coils but 980 Watt results in good cooling voice coils.

Having 40% more cab power handling means underpowering at 71% or -29%, not 60% or -40%. 71% x 141% = 100% Maybe I should have specified that with the number 71% in my previous post. Oh wait I did... I guess I shouldn't have hid it so well.

Clipping does not double power output, it only adds 41% to the continuous output compared to sustained RMS sinewave output. Fact is that without clipping, normal bass play does not sustain that precise a level over time, so going from a signal that doesn't clip at all to full saturation will be more than double, but that's not my point.

My point is that 41% more cab than amp will require either a concerted effort or abject abuse to blow the cab.

 

[Bob Lee (QSC)]

Having 40% more cab power handling means underpowering at 71% or -29%, not 60% or -40%. 71% x 141% = 100% Maybe I should have specified that with the number 71% in my previous post. Oh wait I did... I guess I shouldn't have hid it so well.

Clipping does not double power output, it only adds 41% to the continuous output compared to sustained RMS sinewave output. Fact is that without clipping, normal bass play does not sustain that precise a level over time, so going from a signal that doesn't clip at all to full saturation will be more than double, but that's not my point.

My point is that 41% more cab than amp will require either a concerted effort or abject abuse to blow the cab.

If you want to get into the math of signals, especially into two or three significant digits, you need to be more cognizant of terminology and actual mathematical relationships.

First, there's actually no such thing as "RMS power." RMS is voltage or current. The peak voltage of a sine wave is 1.414 times the RMS voltage. Maybe that's what you were trying to say? If you compare a sine wave output whose peak is X volts with a square wave whose value is X volts (peak = RMS), the square wave will indeed develop twice the average power in the load that the sine wave does.

Square waves are often misunderstood, too. How many times have you heard someone claim that a square wave is all or almost all harmonics? In a perfect square wave, approximately 90% of its power is in the fundamental frequency; about 10% in the third harmonic, about 4% in the fifth, and so on.

Fortunately, the output of your amp will rarely, if ever, be an actual square wave.

 

[CL400Peavey]

Fortunately, the output of your amp will rarely, if ever, be an actual square wave.

What about all those guys using tone generators live?

 

[B-string]

What about all those guys using tone generators live?

I don't think you missed the point that high power audio amps are designed to reproduce sine waves ? The purity of a square wave has nothing to do with what will come from the output of an audio amp.

 

[Tuned]

If you want to get into the math of signals, especially into two or three significant digits, you need to be more cognizant of terminology and actual mathematical relationships.

First, there's actually no such thing as "RMS power." RMS is voltage or current. The peak voltage of a sine wave is 1.414 times the RMS voltage. Maybe that's what you were trying to say? If you compare a sine wave output whose peak is X volts with a square wave whose value is X volts (peak = RMS), the square wave will indeed develop twice the average power in the load that the sine wave does.

Square waves are often misunderstood, too. How many times have you heard someone claim that a square wave is all or almost all harmonics? In a perfect square wave, approximately 90% of its power is in the fundamental frequency; about 10% in the third harmonic, about 4% in the fifth, and so on.

Fortunately, the output of your amp will rarely, if ever, be an actual square wave.

Yes, granted, I screwed up the terminology. I know how to use gear within an inch of it's life without damage, explaining it isn't my forte. Similarly your explanation of square wave harmonic content adds to 104%. We all **** up explaining what we do day in day out without having to explain.

Correction: RMS power peaks at 1.41 times its rating. However, modern amps are rated at peak power, so a 500W amp can only do ~355W RMS without clipping/limiting. If it can handle full saturation it can do 500W square wave no problem, but 500W square wave will fry a 700W RMS speaker if you either try to do it or let some bonehead crank your gear.

The difference with square wave vs RMS is that the voice coil has to endure 41% more current without any increase in cone excursion, so there's more heat without more air moving around to cool the coil. Using an amp's limiter to fool a speaker into cheating its excursion limit just lowers the thermal tolerance of the voice coil.

 

[CL400Peavey]

If it can handle full saturation it can do 500W square wave no problem, but 500W square wave will fry a 700W RMS speaker if you either try to do it or let some bonehead crank your gear.

Maybe, but more likely that 700w RMS is a thermal rating and you are reaching the mechanical limits at a much lower level than 500 watts. For the vast majority of bass cabs thermal ratings dont mean spit because their mechanical limits are so much lower.

 

[ThisBass]

Having 40% more cab power handling means underpowering at 71% or -29%, not 60% or -40%. 71% x 141% = 100% Maybe I should have specified that with the number 71% in my previous post. Oh wait I did... I guess I shouldn't have hid it so well.

Clipping does not double power output, it only adds 41% to the continuous output compared to sustained RMS sinewave output. Fact is that without clipping, normal bass play does not sustain that precise a level over time, so going from a signal that doesn't clip at all to full saturation will be more than double, but that's not my point.

My point is that 41% more cab than amp will require either a concerted effort or abject abuse to blow the cab.

ok, no problem,
so we do the math again:

Underpowering
71% of 700 Watt:
497 Watt
doubling due to square wave
996 Watt

Overpowering:
141% of 700 Watt
987 Watt


If 497 Watt is too weak all the way and therefore you have to crank the amp to (so called) squares (which is nearly impossible IMO) so you have to crank (to clip) the 987 Watt as well.
Not as much like the 497 Watt amp but still a very lot.

But if you are at the edge with 497 Watt means that an increase to 987 Watt will (probably) provide a little headroom.

 

[sratas]

this board has tons of threads regarding underpowering and related truths or myths.

I SUGGEST EVERYONE TO TAKE A CLOSE LOOK AT THE TECHY SECTION OF THE BAREFACED BASS WEBSITE, one of the most useful and coming from a top notch cab builder.

These are referenced statements in my opinion, more than simply a word of mouth coming from a buddy or similar.
here is the link, use it!

http://barefacedbass.com/technical-information/mythbusters1.htm

 

[JHAz]

Yes, granted, I screwed up the terminology. I know how to use gear within an inch of it's life without damage, explaining it isn't my forte. Similarly your explanation of square wave harmonic content adds to 104%. We all **** up explaining what we do day in day out without having to explain.

Correction: RMS power peaks at 1.41 times its rating. However, modern amps are rated at peak power, so a 500W amp can only do ~355W RMS without clipping/limiting. If it can handle full saturation it can do 500W square wave no problem, but 500W square wave will fry a 700W RMS speaker if you either try to do it or let some bonehead crank your gear.

The difference with square wave vs RMS is that the voice coil has to endure 41% more current without any increase in cone excursion, so there's more heat without more air moving around to cool the coil. Using an amp's limiter to fool a speaker into cheating its excursion limit just lowers the thermal tolerance of the voice coil.

FWIW the difference between 100 watts and 141 watts at eight ohms is roughly 3.5 amps versus 4.1 amps. Most amp specs I've looked at lately give an RMS rating.

You might consider the possibility that a guy that designs amps for qsc and has been an officer in the AES might actually have a better than vague idea what he's talking about.

And the technical stuff on the Barefaced site is quite good. For a dabbler like me, entertaining and educational.

 

[jastacey]

I have a GS212 that's 8 ohm cabinet ..... and I use it on occasion with the Tone Hammer 500 or my ThunderFunk 550-B .... it's plenty loud for a bar gig ..... if you get the 8 ohm cabinet, you'll have the ability to get another 8 ohm cabinet, so you'll load most recent manufactured amps correctly at the 4 ohm rating