[husafreak]

I hear references to how efficient or how hard to drive headphones are. And I read about how well headphones mate with certain amps, or not. At present I have three pairs of Headphones:
Fostex TH-X00           25 ohm       94 db
Sennheiser HD600     300 ohm     97 db
NAD VISO HP50        32 ohm       100 db
From what I have picked up, and I may be wrong here, the most sensitive cans are the NAD's at 100 db and the least sensitive are the Fostex at 94 db. So I assume that the NAD's need the least twist of the volume knob for the same sound pressure level. They are the most efficient.
But I seem to have picked up that 25 ohm cans are easier to drive than 300 ohm cans. Does that mean the Fostex are actually the easiest load for an amplifier? Even though the Sennheisers, for instance, have a higher sensitivity?
So I figured I would ask here about the interaction (or not) between sensitivity and load. I really don't get the whole "load" thing.
Please realize I believe sound quality has nothing to do with all this!

 

[HUM24H]

Impedance:
 
This is effectively the resistance the headphone has against the power flow coming out from the device, so the higher the resistance the less current flow makes its way to the drivers inside the headphone. The lower the impedance, then the more current that will flow to the drivers. But you also have to take into consideration output impedance of the device you are playback music on, so you almost want to match the output impedance with the impedance of the headphones but this is almost never the case, but simply put the higher the output impedance of the device and the lower the headphone impedance you end up getting a larger loss in power. But the same goes with lower output impedance and higher impedance headphones. Generally though output impedance of most devices is relatively low and the power output is high enough to drive a large variety of headphones on the market.
 
Sensitivity:
 
So with sensitivity, the dB is literally the sound level being outputted. So a headphone that says 90dB/mW is basically that a 1mW of power the headphones will release 90dB or SPL (Sound Pressure Level). The higher the sensitivity, the more efficiently the headphones are able to transform the electrical current into sound pressure.
 
So really the higher the impedance of the headphone, the more output power you will need from the device playing the music. But a headphone with a higher sensitivity will be much more efficient a delivering sound pressure from its drivers and therefore should not need as much power to be driven loud.
 
People are more than welcome to correct any points I have missed or said incorrectly but this is my basic understanding of it

 

[MindsMirror]

There are several electrical properties which are all related to each other by Ohm's Law, which are Voltage, Current, Impedance or Resistance, and Power.
 
The sensitivity of a headphone is specified in terms of power. To confuse you, some manufacturers specify the sensitivity based on voltage instead.
 
Amplifiers have a maximum gain which determines the ratio of the input voltage to the output voltage.
 
The volume knob on the amp controls the voltage.
 
Amplifiers also have a maximum voltage (not necessarily determined by the gain or volume knob), and a maximum current. Depending on the impedance of the headphone, either one of those could limit the maximum power it can deliver.
 
Amplifiers will also have an output impedance, which will reduce the output voltage depending on the headphone's impedance.
 
 
As you can see, there are quite a few variables (I've probably missed some), and because of Ohm's law, if you change one thing you will change another, so it can get a little complicated. It's really hard to make a general statement like "higher impedance headphones are harder to drive," or "the headphone with higher sensitivity needs less gain," because that always depends on multiple other things. I think it's best to just look at each combination of headphone + amp that you are interested in, do the calculations, and see what the numbers say.

 

[mindbomb]

You listed sensitivity in db/mw. This has to be taken into consideration with the impedance. If you have the sensitivity in db/v, this will give you a number that is directly comparable between headphones. You can convert mw and impedance to v through some formulas. It comes out to 110 db/v for the fostex, 102db/v for the sennheiser, and 115 db/v for the NAD. So the NAD is the loudest, followed by the fostex, then the sennheiser.
 
This is assuming output impedance of the amp is negligible.

 

[fjrabon]

Sensitivity tells you how loud a given mW measurement will make a headphone.  so, for example, a HE6 has something like 83.5 dB/mW sensitivity.  As a rough idea this will mean that 1000 mW will drive it to a peak SPL of 110 dB, give or take.  
 
headphone resistance simply tells you just that, the headphone's resistance, and strictly speaking the headphone's impedance tells you nothing about how loud an amp will make it.
 
However, amps will vary their mW output depending on the headphone's resistance.  So, you will need to know exactly what your amp's output is at the given headphone's resistance, to then be able to tell if the amp will drive your headphone loudly enough.  
 
Generally speaking, as a headphone resistance rises, amps put out less power.  For example, the Schiit Magni puts out the following measurements:
130 mW at 600 Ω
260 mW at 300 Ω
1,000 mW at 50 Ω
1,200 mW at 32 Ω
1,800 mW at 16 Ω
 
So, it's not so much that higher resistance headphones are harder to drive, as much as it is for most amps they put out less power at higher resistances, hence the idea that higher impedance headphones are harder to drive.  However, this isn't always the case.  Consider the Schiit Valhalla for example:
450 mW at 600 Ω
800 mW at 300 Ω
180 mW at 50 Ω
 
It actually hits its peak output at 300 Ohms, instead of the normal 32 ohm peak for a lot of headphone amps, and it outputs more power at 600 Ohms than it does at 32 ohms.  
 
Further, with headphones, the lower their impedance, the lower you want the output impedance of the amp, to allow for electronic dampening.  This is complicated, but essentially all you need to know is that you want the amp's output impedance to be 1/8 or lower of the headphone's impedance.  So, for example on a 32 ohm grado, you would want the amp's output impedance to be 4 ohms or lower.  
 
So, in figuring out if an amp matches up well with your headphone you will need to know: your headphone's impedance, the amp's power at that impedance, and the amp's output impedance.  

 

[Music Alchemist]

Here are some links to help you determine the power requirements of a headphone:
 
http://www.apexhifi.com/specs.html
http://blog.jdslabs.com/?p=1285

 

[husafreak]

Wow, thanks for all your responses! Taken together it's a mini graduate seminar and it helped me understand what is going on better. I should probably point out that I am just getting into headphone listening seriously and having a lot of fun with it. The HD600's are at least 20 years old and I have been using a solid state Creek headphone amp all that time as well. I recently got the HP50's and then the TH-X00's because I needed some closed back phones. But what really got me thinking was a new Garage 1217 Ember II tube amp I got for Christmas. That sucker allows changing input resistance and output resistance. There is a table in the manual that shows how the output power into different loads changes with different output resistance settings. Until tonight that kind of went over my head. Now I understand what is going on And why I prefer the low output resistance setting for my low impedance phones and a higher setting for the HD600's. That kind of goes along with the rule of having the output impedance lower than the phones impedance. Thanks again.