a Power vs Volume Question

Greg, you are confused, do some research and you will see you are wrong on all counts except that 6dB is 4x power.

Elvick's chart clearly shows a doubling of power for every 3dB. Robert Harley's book on high end audio states “each increase in SPL of 3 dB requires a doubling of amplifier power output.”

Yes, 6dB is 4 times the power but that is because it is 3dB plus 3dB which is double times double (2x2=4) however another 3db for a total of 9dB would be 8 times the power (2x2x2=8)

Your formula is for voltage, not power.

dB power is 10 log p1/p2

dB voltage is 20 log v1/v2

Why is one 10 and the other 20? Since power is proportional to the square of the voltage this keeps the dBs in step. An increase in voltage of 6dB will result in a 6dB increase of power. A few examples:

If I double the voltage I get 4 times the power (2 squared = 4)

Voltage dB = 20 log 2 = 6 dB
Power dB = 10 log 4 = 6 dB

If I produce 5 times the voltage I get 25 times the power (5 squared = 25)

Voltage dB = 20 log 5 = 14 dB
Power dB = 10 log 25 = 14 dB

and it isn't delta. Delta means the "amount of change" so the delta from 200 to 300W is 100W. The formulas use the ratio so you would take the log of 300 divided by 200.

tsk,tsk:)

cheers

Mbacinello, yes it holds true, a watt is a watt.

The confusion is caused by manufactures who state sensitivity in terms of voltage instead of power. It takes 2.83V to produce 1W into 8 ohms. This same voltage produces 2W into a 4 ohm speaker.

This means a 90dB/1W/1m 4 ohm speaker will be a 93dB/2.83V/1m 4 ohm speaker since the voltage spec is actually 93dB/2W/1m. This can can be misleading but the manufacturer will say that since most amplifiers are voltage amps and can produce twice as much power into 4 ohms as they can into 8 ohms they are justified in inflating their sensitivity by 3dB.

To add to the discussion;

1) Each additional speakers gives you 3 db SPL

2) Each time you double the distance you lose 6db SPL ( 2 meter is -6 db, 4 meters is -12db )

3) Line arrays drop at 3 db per meter when are very close to the speaker....as you get into farfield they behave just like other speakers

4) The room will help reduce the loss of the sound level with distance through reverberation.

5) Speaker close to a rear wall add up to 3 db SPL in the bass.

6) Speaker in a corner add up to 6 db SPL to the bass.

If you are serious about audio reproduction then consider this: a drum set can produce 115 db SPL and a symphony orchestra can produce 110 db SPL, marching bands and rock concerts can achieve more.

A drummer sits at the back of the stage. A conductor stands several feet back from the orchestra.

A system that can achieve 105 db spl continuous cleanly at the listening position (with 10 db of head room) is roughly what you need to cover all but the most extreme situations.

He Shadorne; I like that!!

I also read somewhere that you lose 3 dbs output going from a 8 ohm load to a 4 ohm load with the same wattage.