noobie: what is the power needed?

[truzz]

Hi,

Sorry for the stupid question,
I have a headphone which has a sensitivity of 97dB SPL per mW,
what is the value of power needed to get 85dB?

Thank you in advance.

 

[JohnRoberts]

Not much,  8 dB above one mWatt is 6-7mW or so.

JR

 

[strangeandbouncy]

Hi,

  very sorry JR, but I think you've gone the wrong way . . . He wants 8dB down from 1mW, not up . . .


          Kindest regards,



            ANdyP

 

[truzz]

That's right!

I have 1 mW giving me 97dBspl,

what mW do I need to get 85 dBSPL ?

 

[Val_r]

Let's see if I can help you.

Let us assume that you have a Sennheiser HD600, which has 97dB SPL/mW sensitivity and 300 ohm impedance.
You know that the voltage at the load is sqrt(P x R), or sqrt(.001x300) = 0.548V.

Then you need a 12dB decrease in sound pressure level, which means a factor of 3.98 fold.

So you would need a voltage of .548/3.98 = 0.138 Vrms, or a current V/R=0.00046A
then find the power: P=V x I, or .138x.00046 = 0.063mW

Hope it makes sense.

 

[PRR]

> what mW do I need to get 85 dBSPL ?

Very very few.

12dB down is 1/16th the power, 0.063mW.

BUT: you probably want 85dB SPL AVERAGE. That is a GOOD listening level. Speech/music is dynamic, transient. My town is at average 50 foot elevation, but it runs down to the sea and up to my 140 foot ridge. An 85dB SPL average has instants down to zero/nothing and peaks much higher than 85dB SPL. Depending on our tolerance for distortion, we allow 10dB to 20dB "headroom" above the average. At 10dB headroom you will have clips in every loud beat. At 20dB headroom you won't have a clip in a month of music. Where headroom is cheap, 16dB-18dB is a popular target. Where headroom is costly, as in AM radio or battery headphone amps, 10dB-12dB may be a tolerable headroom.

So the quick answer is to give yourself a 1mW power, and don't use it as a fuzz-box.

How can you know how many mW your amplifier puts into this specific impedance? Since there is no real standard impedance, amplifier makers can't quote an exact answer unless your phones happen to be at their test impedance.

 

[truzz]

Hey, thank you val_r and PRR.

Yes, I need an average level. I thought that the manufacturer quote of sensitivity was an RMS value, am I missing something?

I need to build a passive volume control, a stepped attenuator, and I want it to be at mid (12 o'clock) when I'm listening to music at 80/85 dB SPL.
Does it make sense?
So should I stick to 1mW at mid position?

Thanks a lot.