[labrower17]

So I am fairly new to the high end audio world. I have a pair of V-Moda Crossfade M80's that I use with my desktop. I have been wondering what Impedance is for a while and I was looking at a pair of headphones recently (Beyerdynamic DT 990) and they have three options for Impedance when purchasing (32ohms, 250ohms, 600ohms). Can someone explain impedance for me as well as terms such as ohms?
 
Also I am going to build a new desktop this summer and I am wondering if I should pick up a soundcard such as the Creative Sound Blaster X-Fi Titanium HD or the Xonar Essence STX. What benefits would a sound card like this offer me?

 

[Strangelove424]

Impedance is a measurement of how much resistance a circuit/device (which includes headphones and amps) has to an electrical current - how much it "impedes" electricity. Ohm is a unit of measurement (named after a scientist) that describes the amount of resistance a given device has. In a headphone with high resistance/high ohms/high impedance (all the same thing) the headphone needs more voltage to overcome its high impedance compared to a low impedance headphone, and thus requires more power to reach the same listening levels. So why would anyone want a high-resistance headphone that takes more power to reach the same listening levels (such as the Beyer models that feature 600ohm editions)? Usually this has to do with amp/headphone matching. If you have an amp that's designed to output alot of power @ high ohms (like a tube amp), low impedance (low resistance headphones) will be too sensitive to voltage, and the driver will not be well controlled by the amp. With more resistance comes better dampening, meaning that the driver's movements are more controlled, and there is less ringing/resonance of the driver following the initial electrical impulse of the actual waveform. The driver comes to a stop more quickly, and thus the sound becomes cleaner and better defined. This is the reason that the 600ohm DT880/990 sounds smoother than the 32ohm version - the driver's movements are being dampened more, causing less ringing at certain frequencies.
 
As Tyll states at Inner Fidelity, skip the 32ohm version. Its a home listening audiophile headphone compromised by having to be a mobile headphone at the same time. If you go for the 250ohm/600ohm version though you will definitly need an amp. A soundcard that contains both a DAC and AMP (with good measurements) will set you up nicely to have a good clean signal, and amplify it properly for high resistance headphones. I have the STX and it can drive my DT880/600s perfectly well. Some people go with the 250ohm DTs for better impedance matching, but it sounds fine to me with 600ohm cans, and can drive them into eardrum bursting levels. With the 600ohms I'm usually at medium (not high) gain and 50% on the volume knob. My experience with Creative X-Fi drivers has been that they are buggy and that the sound quality is so-so. (edit: they came out with this new card recently though http://www.newegg.com/Product/Product.aspx?Item=N82E16829102050, you might want to look at it. It uses virtually the same exact parts as the essence STX - i'd go so far as to say "clone" - and specs look almost exactly the same... personally I trust Asus more, their card is powered by the PSU not the motherboard, and they send out each STX with its own test report graph, but I'm very curious about this new sound blaster card) There are external DAC and amp options as well.

 

[jaddie]

I've always hated the use of the term "resistance" when describing impedance, yet it's hard not to use it.  Perhaps to define the difference...
 
Resistance is the resistance to electron current flow, and is considered constant for DC and AC signals of any frequency.  Resistance is measured by applying a small DC voltage and measuring current flow.
 
Impedance also resists current flow, it's a little more complicated.  Impedance is a composite of three electrical properties: resistance, inductive reactance, and capacitive reactance. Inductive reactance comes from a coil of wire, or something acting like a coil, and resists current flow more as frequency goes up.  Capacitive reactance comes from a capacitor, or something acting like one, and resists current flow less as frequency goes up.  Roll all three together and you have impedance.  Impedance may not be constant with changing frequency.
 
Headphones and speakers have all three properties, but we only talk about the composite, impedance, because it's easier.  Technically, impedance should be specified with a graph of impedance vs frequency for a complete understanding of what it means to us.
 
Here's an example of an impedance curve that changes a lot with frequency:
http://graphs.headphone.com/graphCompare.php?graphType=0&graphID[]=4068
 
Here's an example of an impedance curve that doesn't change much with frequency:
http://graphs.headphone.com/graphCompare.php?graphType=0&graphID[]=360
 
It's not that one curve is better or worse, but it shows that a single figure impedance spec is kind of pointless.

 

[Strangelove424]

I glossed over some details, but since capacitance isn't a part of the the normal testing procedure/spec sheet, its not a factor when shopping for equipment. Yes, resistance is a bad word to use because "resistance" has it own scientific meaning separate from impedance, but its hard not to use that word when describing impedance. (thesaurus only has so many options... restrain/retard/deter doesn't really capture it) Sensitivity can be high, impedance can be high, but you'd still have a relatively easy-to-power headphone. Sensitivity can be low and impedance can be relatively low (like the K701) and then the headphone would be hard to power, despite having a low ohm figure. So driving a headphone has more factors to take into account. And as you mentioned, impedance can vary by frequency, and change the sound signature of the headphone. That can contribute to people having different experiences with the same headphone on different amps (i.e. HD600/800, where the upper frequencies respond like a 300hm headphone and lower ones like a 500ohm headphone). For someone just getting into the hobby though, concerned about basic amp matching, while the single impedance number may not describe the exact response of the headphone over the entire frequency spectrum, it will get them in the ballpark for power requirements and amp/headphone matching.