The impedance of the earphones is only part of what determines the audio output level you can expect from your setup.
Impedance is a broader term for
resistance that includes dynamic factors, including inductance and capacitance, which can vary at different frequencies, while a strict definition of
resistance is refers only to a linear DC
impedance values. However, for general understanding, you can think of them as being about the same.
The resistance of the phones determines the maximum electrical power the amplifier can produce. It is determined by the forumla:
P = E^2 / R, where
P = Power (watts)
E = Signal (volts)
R = Resistance/Impedance (ohms)
As you can see, the maximum power falls as the resistance rises.
Acoustic power (loudness) is not the same as electrical power. The actual loudness (in acoustical watts) is also determined by the
efficiency of the transducer (earphone or speaker). A highly efficienct earphone or speaker having a higher impedance can produce more audio output than one having a lower impedance, but also being less efficient.
Another factor is the design spec for the amplifier. If your system is designed to drive any load from (for example) 4 to 50 ohms, you can use any headphones that fall within that range. Depending on the quality of the design, some amplifiers may produce more distortion when driving a lower impedance, which is why you want to know the impedance range spec'd for the system by the manufacturer.
Once you know the Shure phones you're considering are within the spec'd range for your setup, the best way to know if you will like them is to test them with your MP3 player. It's simply a question of whether they play loud enough, and they sound good to suit you. Try several different phones in a side by side comparison, and take your choice.
WARNING!!! Listen carefully, and
DO NOT play your system at ear splitting levels. You can and
WILL permanently damage your hearing. :Q
Originally posted by: harrkev
The fact is that you get maximum power transfer if your input impedance matches the output impedance of the driver. But this assumes that your output impedance is fixed, and you can adjust the imput impedance of the headphones.
That is not correct. The concept of impedance matching is more significant in older transformer coupled circuits and in other tuned circuits, but if the load impedance matches the source impedance, half the power will be lost in the source.
The output source impedance of most direct coupled solid state amps will always be far lower than the load presented by the headphones (small fractional ohms), and they are designed to work into a higher impedance. Essentially, the source impedance appears in series with the load and determines the maximum current or power the circuit can deliver to it.
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