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a decibel question

thehangingmist

thehangingmist

Dec 14, 2007
6,367
302
New Delhi, India
#1
a 100Watts combo amp with a 15" driver
approximately how many decibels would it be capable of hitting??
am only looking for rough estimations because the actualy output would also depend on the eq boosting and the strngth of the signal provided in. any help is appreciated

also, is there a way to calculate the output levels, in decibels, of combos or amp+cabs if one knows the total speaker sizes and amp's wattage?
 
#2
Ignoring power compression or excursion limitations, the max SPL of any speaker is the nominal sensitivity (somewhere between 90 and 105dB) plus 10 * log (RMS thermal power handling). A typical 100W 15" combo will have ~95dB sensitivity and 10*log100 is 20dB, therefore the total is 115dB.

Here's a response plot for a typical 100W 15" combo with tweeter:

Alpha15-3cuft.gif


Alex
 
#3
also think of this.
For every doubling of watts you only gain 3 db of extra volume.
But that doesn't mean pressure.
So in theory you only gain small volume increases for larger amps.
Why does an 8x10 feels so powerful.
Think of what your combining.

Also a tube head and a ss head at 100w both create the same volume but the tube sounds louder as it has an introduction of 3rd harmonic frequencies etc which trick your ears into thinking it's louder than it actually is.

This should help you make some good judgments down the line.
 
#6
varunkapahi said:
a

also, is there a way to calculate the output levels, in decibels, of combos or amp+cabs if one knows the total speaker sizes and amp's wattage?
Click to expand...
No. You may only calculate output levels if you have SPL and displacement limited power charts, which are not available from any manufacturer.
 
#7
Ok your all confused.
For example.
a 10 watt amp puts out 70 db.
a 20 watt amp puts out 73 db.

Or a 500 watt amp puts out 110 db
a 1000 watt amp puts out 113 db.

Thats what happens.
It's very simple.
(the db's are just for reference, I have no idea what db level what amp puts out and if certain brands with different technologies are louder at their wattages. But it's always a 3db difference if you double the power.

Just think though that air pressures different.
A large amount of speakers close together moves more air increasing air pressure also the introduction of complementary waves where similar or sane waves forms from the same sound combine and increase the total amplitude of the sound making it sound louder.
Ie: to scanners at a supermarket going off at the same time.
This is due to phase etc etc.

Very simple acoustics.
Which should benefit you with finding an amp, but usually not a light one.

Also you do know that watts is heat right???
 
#12
exactly.
Also your threshold of pain is 120db of sound, most clubs run theie systems at around 130db, so in other words you loose frequencies from your hearing extremely fast (even around a minute exposure to that volume and you've got permanent damage).


I think the people with gigantic stacks have a lot to learn from this thread.
I'm not saying I'd like a large stack (they look awesome).
I think from this the best possible choices would be:

A 100 w tube head powering an 8x10 for concerts
A 100 w tube head powering a 4x10 for medium and small gigs.
Simple.
Pressure and a sustained loudness level plus tube quality.
Haha.
 
#13
spector29 said:
Ok your all confused.
For example.
a 10 watt amp puts out 70 db.
a 20 watt amp puts out 73 db.

Or a 500 watt amp puts out 110 db
a 1000 watt amp puts out 113 db.

Thats what happens.
It's very simple.
(the db's are just for reference, I have no idea what db level what amp puts out and if certain brands with different technologies are louder at their wattages. But it's always a 3db difference if you double the power.

Just think though that air pressures different.
A large amount of speakers close together moves more air increasing air pressure also the introduction of complementary waves where similar or sane waves forms from the same sound combine and increase the total amplitude of the sound making it sound louder.
Ie: to scanners at a supermarket going off at the same time.
This is due to phase etc etc.

Very simple acoustics.
Which should benefit you with finding an amp, but usually not a light one.

Also you do know that watts is heat right???
Click to expand...



The OP is asking for a simple answer to a very complex question and several people have given right answers and also added that there are other factors that must be considered for the answer to make any sense in the real world. Acoustic physics combined with the electrical and mechanical physics in a music amp is a very complex subject and, while doubling amp power theoretically results in a 3 dB increase in SPL, that principle usually is overwhelmed by other real world factors that are more important to what the musician actually hears.




And a watt is a unit of power, not heat.



:)
 
#14
spector29 said:
there is really no reason to pay $300 extra for an amp putting out 500 w compared to one putting out 250w. Power wise.
Click to expand...
Not exactly.

When you run an amp at its maximum output, it will clip and produce square waves that can damage speakers. The 500w amp will have much more headroom, so it can achieve the same volume without being pushed so hard.
 
#15
watt IS a measure of power but is equal to one joule per second which can measure the amont of work done which i guess can be used to express heat in a twisted sorta way but im with hbarcat
 
#17
Yes but a watt is actually heat measured as a unit of power.

I understand all the acoustic principles and factors thats why I've explained so many.
You decide what is really worth what.
I understand the headroom principle as well.
But I am also a live sound engineer working on many big gigs and events.
Lets just say, it may be ok for an 8x10 ampeg rig cranking away at the Big Day Out but if you've got anything like that for a club gig then your already too loud, theres no argument for headroom when the venue doesn't allow for such volumes to start with.
Oh and think of yet another thing, especially from my point as a sound guy and a bass player, I really don't care what your if you can't hear yourself on-stage and constantly turn your amp(s) up as you don't know what EQ to set or your compression settings blah blah, all I care about is the the singer and drummer hearing the singer voice. I'll give you a little, but not a lot (often the band wants more and most engineers in indoor enviroments compete for FOH with the monitors.
I'll let you hear everything at whatever volume at a large outdoor concert but nearly everything else it all comes down to FOH sound so the audience likes it.
Simple and no arguments.

My only problems with choosing an amp and cab is weight, I'd like tube as the harmonics make it sound better when louder and theres not a great problem with headroom....But they weigh sooooo much.
I'd love a Trace stack but in reality it'll be awhile if I ever get to play in an outdoor medium to large concert.
 
#20
q1014 said:
spencer when u say square waves would it be charted on a graph as the line hitting the pinacle of its power and flatlineing and then coming back down?
Click to expand...

The graph would literally look like a bunch of squares. Flat at the peaks, and straight up and down. Square waves produce many more harmonics than sine waves, which means they can blow tweeters easily.


Here's some info: http://en.wikipedia.org/wiki/Square_wave
 

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