How do I determine how many dB of output my guitar cabinet produces at a given volume setting?

Question

I'm playing a Peavey Triple x*x 412 slant rated at 400W with a Marshall Mode Four head rated at 350W. I'm wondering how I could find out the decibel rating output of this setup at any given volume through a mathematical means or anything that doesn't require buying (or borrowing) a dB meter.

Answer 1

Nice setup.  I need something like that for my bass (LOL).

The sensitivity of the speakers is listed as 97.8 dB / W / m

http://peavey.com/products/browse.cfm/ac...

Which is pretty loud compared with alot of other speakers.

So... that 350 watts is going to have to be split 4 ways.  Each speaker will be driven by 87.5 Watts.

97.8 db per Watt, times 87.5 Watts = 8526 decibels?!? Ummm, something is wrong, here.  That's not how it works.  deciBels go up on a logarithmic scale.

10 * log(87.5 Watts / 1 Watt) = 19.42 decibels louder

total: 97.8 + 19.4 = 117.2 dB SPL at 1 meter

BUT... it gets better!  117 dB SPL is for each speaker.  There are 4 of them which means 4 times the noise, or +6 dB more.

Grand total: 123 dB maximum SPL at 1 meter

note: that is 1 meter away from the cabinet.  As you get further away, the SPL will decrease.  If you are in a small room with no curtains or sound dampening materials, you'll get close to 120 dB at max. volume on the Marshall.

Now, for every increment on the volume control down from maximum... everytime you decrease the power level of the amp by 1/2, the SPL will go down by 3 dB.  But, is is hard to equate a volume control position with an absolute power level.  I would have to know a lot more about the internal workings of the Marshall to try and figure that one out.  1/2 the power may very well be going from a 10 to a 9, or it may be from a 10 to an 8.5 -- I can't know for sure.

Also... keep in mind that that 350 Watt rating is for a pure sine-wave and not for real music.  In reality you will be lucky to maintain notes on your guitar to produce even 1/2 of that power (-3 dB SPL from the max.)  In true reality, even if the Marshall is cranked up all the way, and you play some notes, you might peak at 1/2 the power and sustain 1/4 of the power (-6 dB SPL from the max.).  But, even 117 dB SPL is above the "pain" threshold of hearing, so the place will be 'rockin'.

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