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So all you guys said makes sense. Thanks a lot. I just checked my ear bud impedance. It was 16 while my headphone impedance is 32.

Now I'm a little confused. So high impedance headphones (> 100) need to be driven by amps with higher VOLTAGES not IMPEDANCES and lower impedance headphones need less volts and less impedance?

So does loudness have to do with both SPL and impedance?

Ok, some maths here, so bear with me.

Voltage = Current * Impedance.

What the source or the amp provides is Voltage (AC).

The impedance decides the Current.

So, for 1V RMS, 16 Ohms impedance, Current (RMS) = Voltage/Impedance = 0.0625 Amps.

And, for 32 Ohms impedance, Current (RMS) = 0.03125 Amps.

Average Power is defined as Pavg = Vrms x Irms

For 16 Ohms its = 1 x 0.0625 = 0.0625 Watts.

For 32 Ohms its = 1 x 0.03125 = 0.03125 Watts.

So, a higher impedance headphone consumes less power for the same voltage.

SPL defines the max decibel sound level created for 1 milli-watt of power transferred to the headphone.

As you can see, a higher impedance headphone will need more voltage to deliver 1mW than lower impedance one. Once you supply this 1mW, a higher SPL headphone will be louder than lower SPL one.

Generally it tends to be the case that lower impedance headphones are also sensitive, meaning high SPL. So you don't need to pump the volume up.

Your HD439 is a combination of higher impedance and lower SPL when compared to your IEMs.