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effect of underpowering a cab?

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Devo-lution

Jun 24, 2009
535
4
#1
Due to recent changes in my gear I have acquired the following result to be my "rig"...

Markbass LMII 500 watt head

Markbass 4x10 head... 8 ohm, 800 watt

Having only learned recently how this ohm-thing works, my newfound conclusion is the following one: That would probably give the cab around 300 watt...

My question is related to the effects on the sound when giving a cab less than half of the watt-amount it can take. So, does it make for tonal differences? If yes, which ones?

I personally do like the sound I get, but a chance of improving my tone is always okay... I do wish to note that I go for a relaxed, clean sound usually in my rig.

thx
devo
 
#2
nope, no effect
it should be fine

800W is what the cab can handle, not what it needs.


If you do a search on underpowering you will find (first off: a lot of closed threads) somewhere a great explanation how the myth of underpowering originated.
 
#3
As far as tonal differences, it won't hit the "sweet spot" where it sounds full and open at the right amount of volume for what you need it for. I might not with 4 ohms either though, so don't worry about it and play.
 
#5
René_Julien;9024253 said:
If you do a search on underpowering you will find (first off: a lot of closed threads) somewhere a great explanation how the myth of underpowering originated.
Click to expand...

Or if you read the FAQ you'll find the thread that explains it, although it's probably linked in most of those closed threads.....
 
#6
Underpowering is a myth.

The wattage rating of a cab is simply the point at which the voice coil will melt, causing permanent failure.

Just because an amp is rated at 500 watts doesn't mean its putting anything near that to the speakers when driven at "normal" levels. You would have to have everything on "10" as well as a proper input level for the speakers to see the full 500 watts, and even then the speakers would most likely blow from overexcursion LONG before they would fail because the wattage rating was exceeded.
 
#7
coreyfyfe said:
Or if you read the FAQ you'll find the thread that explains it, although it's probably linked in most of those closed threads.....
Click to expand...

I think I saw Bill Fitzmaurice (not sure it was him) post an extensive easy-to-comprehend explanation about "underpowering". In a recent thread. (by recent I mean last 6 months)

I can't seem to find that post.


Shows how much of that explanation stuck with me. ;) :D



I'll try to wing it:
something about low-power (low-quality) poweramp-sections distorting and putting out square waves, which can hurt speakers.


I don't fully grasp it myself to try to explain it.
Cause a fuzz pedal also puts out a signal close to a square wave.
So I must admit I am a bit puzzled myself.


I understood about "underpowering" at the time it was explained to me.
But, I'm getting older. :)
 
#8
The difference in SPL between what you're rig will produce now and what it would produce if you had a 4 ohm version of your current cabinet is 3 dB. It is not a huge difference but is certainly discernable. You could get a second cabinet and then you would achieve a 3 dB increase in SPL. Bassically, each cab would receive approximately as much power as you are currently sending to the single cab. When you combine this the SPL increases by 3 dB.

The risk is not of underpowering the cabinet but of driving the head into clipping and then damaging the speaker.

Jim
 
#11
i think the square wave argument is based on things starting to look like a DC voltage.
i still don't really get it though. the driver is still moving back and forth, its not really dwelling at the high or low position.
 
#12
Although I admit I don't fully understand it, we at one time were unknowingly significantly underpowering our PA speakers, delivering 900W into speakers rated at 3kW. What happened was that the amp powering the speakers shut down and had to be repaired. The tech called it clipping, which to me is opposite of what I thought given the situation.
 
#14
jsbarber said:
The difference in SPL between what you're rig will produce now and what it would produce if you had a 4 ohm version of your current cabinet is 3 dB. It is not a huge difference but is certainly discernable. You could get a second cabinet and then you would achieve a 3 dB increase in SPL. Bassically, each cab would receive approximately as much power as you are currently sending to the single cab. When you combine this the SPL increases by 3 dB.
Click to expand...

I don't understand what you are trying to say here.
1 cab @8ohm
the same cab @4ohm is 3dB louder
2 cabs @8ohm is 3db louder

so according to your explanation:
two cabs togheter @8ohm each =as loud= one cab @4ohm

??? That doesn't seem right.
I think the two cab solution will be louder than one cab at half the impedance (but same impedance as the sum of the two cabs).

Or am I wrong about this?

Doubling power does not get you 3dB... I thought... ???
 
#15
René_Julien;9024602 said:
I don't fully grasp it myself to try to explain it.
Cause a fuzz pedal also puts out a signal close to a square wave.
So I must admit I am a bit puzzled myself.
Click to expand...

This one was by BassIan:
For the millionth (feels like it anyway) time, there is no such thing as underpowering a speaker or cabinet. What causes problems is being underequipped for the job. When this happens and a speaker failure results, it is the result of overpowering drivers or causing them to fail mechanically by overexcursion.

And this from Jerrold Tiers:
One thing that a very low powered amp MAY do that will hurt your speakers, is FAIL, and put out DC as a result of failing. DC (which is NOT produced by normal clipping) heats the voice coil, and can push it out of the magnet gap, so that the magnet structure is not available to soak up the heat by radiation from coil into the colder structure. That CAN fry the speaker, depending.....But DC is caused by a failure of the amp, not by a normal operation.

I think that covers what you were referring to. I couldn't find the BFM bit, but I was only looking through the FAQ ;).
 
#19
joeyjoejoe said:
but a square wave is not a dc signal.
Click to expand...

Exactly, otherwise distortion pedals (or overdriven amps for that matter) would destroy speakers pretty quickly. The issue is how far you push a small amp. If it's a smaller cab it will probably start farting out sooner, but a bigger cab takes the abuse (somehow) unnoticed until the amp starts to fail, which in turn causes mechanical excursion and harms the speakers. If I'm understanding it all correctly.
 

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