Here's a simple question: Why does low impedance not mean higher efficiency?

I have two headphones:
 
1. Hifiman HE-560 with measured DC impedance of 42 ohms. 
2. Sennheiser HD800 with measured DC impedance of 350 ohms.


Using my Oppo HA-1 SS amp with balanced and unbalanced outputs, the HD800 always requires a lower volume setting versus the HE-560 to obtain the same perceived sound level.   

Other owners of both headphones have noted the same thing. 

Why would would a HP with lower impedance require more power to drive it to the same level?

 

Read this and play with the calculator at the bottom: http://www.apexhifi.com/specs.html

The simple answer is that impedance simply doesn't directly relate to efficiency. The Ohm rating of a headphone is simply the electrical resistance of the driver circuit. Push a given amount of voltage into a driver circuit and you'll get a given amperage flow, determined by the resistance. Everything about the driver design, can design, etc. affects how efficiently the headphone turns power into sound. In other words, how many milliwatts it takes to create a given dB SPL.

At a given voltage a low impedance headphone will draw more amperage, so that may be some people's definition of "easy to drive", but it doesn't tell you much about how much volume you will get out of it at that point.

The HD800 will put out over 100dB at 1mW, the HE560 just 90dB. So, it will take more voltage to push 1mW through the HD800, but it will take far more total power (far more mW) for the HE560 to match the volume of the HD800.

So, in a way, low impedance headphones aren't really any fundimentally easier to drive than high impedance ones. Each headphone will take a given mW to produce a given dB. I think the main issue with high impedance headphones is that many portable sources are very limited in voltage output, so they simply can't push very many millliwatts of power into headphones with high impedance. But all will be ultimately limited in millliwatts as well, so there is no guarantee that a low impedance source will be more easily driven than high impedance ones from a given source. If your amp (whether it be in a phone or whatever) maxes out at 1v, you won't be able to push very many mW through a high impedance headphone. But if it is limited to 5mW, you won't be able to push any more power than that through any headphone, regardless of impedance.

It's late, I hope I'm making sense.

The efficiency or sensitivity of the headphone could depend on the strength of the magnets, the physical and acoustic properties of the driver, the acoustic properties of the chamber, the proximity to your ears, and many other factors. Given these few examples it is easy to see that a headphone of lower electrical impedance could have lower efficiency.

Why do you think impedance has anything to be with efficiency? In loudspeakers high efficiency often sound horrible then others sound great. For example paging speakers can be very efficient because they have large peaks in the speech range, but they sound horrible.  

dprimary said:

  Why do you think impedance has anything to be with efficiency? In loudspeakers high efficiency often sound horrible then others sound great. For example paging speakers can be very efficient because they have large peaks in the speech range, but they sound horrible.  

Click to expand...

??? Efficiency as others have pointed already out, is a measure how much power (watt) is needed by the driver to produce a certain spl or how much spl the driver can produce when powered by 1 watt. Just an electrical characteristic of the driver.
 
The efficiency doesn't have anything to do with the sound quality of the driver.
It just tells you which kind of amp will be technically a good match for the driver to reach a certain spl.

gr8desire said:

  Here's a simple question: Why does low impedance not mean higher efficiency?

I have two headphones:
 
1. Hifiman HE-560 with measured DC impedance of 42 ohms. 
2. Sennheiser HD800 with measured DC impedance of 350 ohms.


Using my Oppo HA-1 SS amp with balanced and unbalanced outputs, the HD800 always requires a lower volume setting versus the HE-560 to obtain the same perceived sound level.   

Other owners of both headphones have noted the same thing. 

Why would would a HP with lower impedance require more power to drive it to the same level?

 

Click to expand...

 what Music Alchemist linked gives an excel that let you try a few values and experience how much sensitivity can dominate over impedance on the electrical side of things.
the reason simply being that impedance is a decimal system where decibel is a log one. so to go from 1 to 60ohm you multiply by 60 and you'll recover the result on voltage and power in a pretty linear way. but to go from 1 to 60db in sensitivity you multiply by 1000. and that's why impedance while playing a part, shouldn't be thought as a good way to estimate efficiency.
 
 
on the very practical side of things, you take you hd800 with it's nice and soft driver, and the coil made of a lot of turns to get more magnetic pull/push right next to the magnet, and you can guess that it won't need too much effort to move the membrane by a given amplitude(that will decide upon the loudness).
now think about your HE-560 with the membrane that is strongly damped from the start, and how the magnetic field is created by play of surfaces instead of the very effective coil.  even without too much electrical knowledge, you can guess that moving that well damped membrane by the same amplitude as the hd800 will require a lot more effort.

There are a lot of variables that determine sensitivity. Impedance by itself won't usually tell you much. Take for instance, the akg q701 (62 ohm), beyerdynamic dt 880 pro (250 ohm), and sennheiser hd 650 (300 ohm). In terms of loudness, the AKGs are actually the quietest and the sennheisers are the loudest at equal volume settings at 1khz - the exact opposite of what you would expect if you thought lower impedance meant louder.
 
Impedance by itself is most useful when you are comparing otherwise identical versions of the same headphone, for example, dt 990 600 ohm and dt 990 250 ohm. There you know the 600 ohm version is quieter.
 
At http://www.innerfidelity.com/headphone-data-sheet-downloads you can see the voltages needed to get different headphones to 90db at 1khz. This allows you to compare the relative loudness of all headphones there.