Under-powering cabs

[Mark Reccord]

Fortunately, the internal resistance of the 9V battery greatly limits the current (and thus power) it can deliver.

Of course, hadn't thought of that.

 

[-Asdfgh-]

It takes twice the power to raise the sound by 3db.
Just so I don't have to use a bunch of X's to represent original Sound Pressure Level (SPL), let's presume that your 50-watt amp produces 100db with 50-watts of power (all hypothetical of course). Then let's step it up by doubling the power of the amplifier to find out how much it would take to be twice as loud to the human ear:

50-watts = 100db
100-watts = 103db
200-watts = 106db
400-watts = 109db
600-watts = 112db
That's all presuming everything is working perfectly with speakers that can handle the power.
​

Although 3db increments are measured as "twice as loud", the human ear hears a 10db increase as twice as loud. There's a 9db increase using 400-watts and a 12db increase at 600 watts. So it's reasonable to say a 500-watt amp is twice as loud because it probably does produce 10db more than the 50-watt version and you would indeed hear that as twice as loud.

This is why amplifier power is only half of the formula. The other half of the formula is the speakers you use or plan to use with it. The better the sensitivity of the cab, the less power it takes to bring the SPL of your gear up to gig levels. It's the combination of the amp and the speakers that give you the final SPL your combination of amp and cab(s) can produce.

If I am looking for a new cab the things I look at are frequency response (is it really a bass cab), sensitivity and weight as a heavy cabinet I won't get to a gig is of no use and then I hope to find a max SPL figure rather than just power handling. I know what SPL figure, based on existing gear, covers all bases (no pun intended) for me, so if the max SPL is around that, all is good and I'd prefer that to RMS Watts as I am unlikely to need to turn it up to be louder than what I currently have.

 

[agedhorse]

In general, if you know the maximum power handling in RMS units and the AVERAGE sensitivity, you can calculate maximum AVERAGE SPL.

The problem with these numbers is that if you don't know how the sensitivity or SPL numbers are derived (are they average, are they due to a peak in the response curve, are they based on RMS or peak power), you still don't know what you are really receiving.

 

[MrKpawz]

but you don't red-line your car every time you get in it.

Theres still a 2004 corolla somewhere, maybe, that'll disagree with you

 

[-Asdfgh-]

In general, if you know the maximum power handling in RMS units and the AVERAGE sensitivity, you can calculate maximum AVERAGE SPL.

The problem with these numbers is that if you don't know how the sensitivity or SPL numbers are derived (are they average, are they due to a peak in the response curve, are they based on RMS or peak power), you still don't know what you are really receiving.

Yes, I've always been a bit wary of such figures but have had no way to compare things with any more accuracy. Often the maximum SPL, sensitivity and power handling figures don't quite match up, throwing more doubt on the basis for the calculations. But I am not sure what else to base things as I expect I'd get short shrift if I turned up to a shop with my SPL meter and some test input recordings and suggested I wished to connect my existing amp and then crank it up as far as it will go... Earplugs in first, of course.

 

[agedhorse]

There is also real world max average SPL in calculated and measured versions. All manufacturers that I know of use the calculated method as it’s difficult to measure because the numbers are not static.

The difference between the two methods is called power compression, commonly associated with a rise in VC temperature but in fact contains a large amount of electro-mechanical linearity factors. These E-M factors is what the IASYS power limit threshold measurement set algorithm is based upon because it’s non-destructive.

 

[-Asdfgh-]

There is also real world max average SPL in calculated and measured versions. All manufacturers that I know of use the calculated method as it’s difficult to measure because the numbers are not static.

The difference between the two methods is called power compression, commonly associated with a rise in VC temperature but in fact contains a large amount of electro-mechanical linearity factors. These E-M factors is what the IASYS power limit threshold measurement set algorithm is based upon because it’s non-destructive.

I wish I understood all of that. Still much to learn!