Volume db calculation help

[Jinto]

I was curious as to how loud 95db sounded like and realized that I have no easy way to tell. So I thought to calculate out the maximum volume of my Ipod Touch with a Shure SCL3 and estimate.

So what I know:
Shure SCL is 115db/mW with 26ohms impedance.
Ipod Touch outputs 30.6mW at 10ohms.

So Power = Voltage^2/Resistance. 30.6mV = V^2/10. V is constant so it would be 11.7mW.

11.7mW /1mW is 11.7 times the power. Doubling the power results in 3db. 3*log2(11.7)=10.6db. So the maximum db is about 125db.

So if I set my volume bar to about half, that should be around 62db? Or is there much more to this? Thanks.

 

[MisterSparkle]

Hmm, can depend on the nature of the iPod touch's volume control. I might set mine up tonight to a load and measure what it's output does.

Seems as if there's nothing tricky going on inside, just a plain logarithmic volume control.

 

[obobski]

most volume controls, at least if its a pot, don't move in a linear fashion (at least how you're thinking), they adjust voltage in a linear manner, thus adjusting output levels
when you adjust the pot, you're changing voltage (potential difference), not output power

basically at 50% you get half output voltage, which means roughly 20-40% output power

so in the case of your ipod, peak output is something like .5V into 10ohms, 50% out is around .25V, which will give you around 6.2 mW

now this is at 10 ohms, not 26 ohms, the rub is, Apple hasn't published the amplifier's characteristics at higher/different impedance, which is key, because it isn't guranteed to be a linear/constant relationship, and the reality is the #'s are probably ugly (so it might be like 69.39837% output power, or something obnoxious along those lines)