Hello, i am Rohan an audio engineer and i have just joined head fi so a HI! to all,
What is impedance?
Simply the AC resistance of the "voice coil" + magnetic field coupling of the magnet inside the headphone.
What is its impact?
From simple electronics,
we know Voltage = Current X Resistance
and Power= Voltage X Current (or P=VI)
now lets take an example,
For a headphone to deliver 2 watts of power (a headphone never outputs such high power just an EXAMPLE)
it can be
at 1 Volt with 2 Amps of current i.e P=2X1=2watts
also at 1 Volts with 2 Amp's of current the headphone has an impedance* of Resistance = Voltage/Current thus, R=1/2 Ohm (just an example not actual values)
the same output with 2 Volts and 1 Amp. and R=2/1=2Ohms
*life is int that easy impedance is actually quite complex and different driver designs have different impedance's at different frequencies and amplitude (voltage, instantaneous or loudness)
Why are high impedance headphones manufactured?
1.) The displacement (amount of movement) of the diaphragm (the thing that actually vibrates and produces sound) is better controlled via a more accurate flux (magnetic field to pull and push the diaphragm). This gives better accuracy and more performance at lower frequencies.
2.) The impedance is increased by using a different wire for the voice coil and most importantly more turns. More turns or loops creates a larger field (area of magnetic influence), in lay mans terms bigger magnetic area for the coil to move more efficiently.
3.) It also allows SS amps to work more efficiently with lesser distortion have a look at data sheets and a graph of distortion vs output impedance for most audio op-amps and you'll get the idea.
Problems with high impedance
1.) Difficult to drive for low powered devices which are configured with lower gains.
2.) Most need an amp with a different gain (most headphone amps are more geared towards current rather than voltage i.e low impedance than hi impedance)
Do high impedance headphones sound better than low impedance ones?
Actually, in headphones "sound stage" or "airiness" or simply the experience depends upon the following,
1.) Frequency response, the curvy graph shows that volume decreases or increases with change in frequency this is actually the complex magnetic field pulling and pushing with different amount of force at different frequencies, impedance affects this in complex ways but to make life easy, it makes it more somewhat "straight" or linear.
2.) Distortion, represents the amount of "change" from the actual signal to the real signal (input signal to output signal) the reason why this is measured by a square wave is because square waives are difficult to drive as they have a "part" DC in them which in theory does not generate flux. Impedance does not a large effect on this.
3.) Build and design, (to long i cant type so much :P )
So to conclude,
High Impedance is only a ddifferent approach in energy delivery in basic terms its relative to rather faster flowing electrons (hi impedance) or MORE but slower electrons (low impedance) how they effect sound is dependent on the magnetic filed generated and diaphragm construction (physical response of diaphragm).
Hope this clears the air.