[SilverEars]

HE-6 is hard to drive and it's spec'd as efficiency of 83dB.  Sennheisers are spec'd as sensitivity.  Which of the two are more demanding on amp's capabilities?

 

[cjl]

Efficiency and sensitivity are the same thing. You do have to watch the units though - some headphones spec dB@1mw, while others spec dB@1VRMS.

 

[SilverEars]

  Efficiency and sensitivity are the same thing. You do have to watch the units though - some headphones spec dB@1mw, while others spec dB@1VRMS.

Yes, but the power draw and voltage draw of the load will be different.

 

[mikeaj]

  Yes, but the power draw and voltage draw of the load will be different.

 
What does that have to do with efficiency and sensitivity?
 
 
Or wait, are you just asking for a conversion between units for Sennheisers (which?) and the HiFiMAN HE-6? e.g. HD 800 @ 600 ohms, 102 dB SPL / 1 V -> 99.8 dB SPL / 1 mW? It's higher impedance and more sensitive so generally less demanding to play at a given volume.

 

[SilverEars]

Sensitivity is dB/V and Efficiency is dB/mW.  A headphone will output loudness depending on voltage or power which differentiates the two.  
 
I'm curious which(efficiency or sensitivity at loudness constant) is more demanding on the amp's power capabilities.  

 

[Roly1650]

Efficiency and sensitivity are the same thing. You do have to watch the units though - some headphones spec dB@1mw, while others spec dB@1VRMS.

Not strictly correct although the two terms are used interchangeably. Efficiency is acoustic power out divided by electrical power in multiplied by 100 to get a percentage. Speakers can very from less than 1% to about 3%, efficiency, on a good day, going downhill, with the wind behind them. Understandably speaker manufacturers would be nuts to let anyone know that, so efficiency and sensitivity have become interchangeable words.
Sensitivity is what you are quoting, not sure how that's measured in headphones, but with speakers it's SPL @ 1 meter @ 2.83 volts input. If it's being done honestly, the speaker impedance must be stated, a 4 ohm speaker is going to look like it's more sensitive than an 8 ohm speaker as 2.83 volts = 1 watt @ 8 ohms and 2 watts @ 4 ohms. I guess with headphones the same thing applies as the load impedance is a factor.

 

[ab initio]

  Sensitivity is dB/V and Efficiency is dB/mW.  A headphone will output loudness depending on voltage or power which differentiates the two.  
 
I'm curious which(efficiency or sensitivity at loudness constant) is more demanding on the amp's power capabilities.  

 
Unfortunately the answer to your question is "depends".
 
The headphone sensitivity and efficiency are related via the headphone impedance, R.
 
P = V^2 / R
<->
V = sqrt(P * R)
 
Recall that V = I * R, where I is the current. Then you can relate the power and current as
 
P = I^2 * R
 
In the end, whether it is more difficult for an amplifier to supply sufficient current or sufficient voltage to drive a headphone loudly depends on the amplifier's construction.
 
For example, a battery powered amplifier might only have a voltage swing of +/- 1 volt or so, but might have very low output impedance and be capable of driving high current through low impedance loads. A 300 ohm headphone like the HD 650 might require a voltage swing in excess of 1 V, hence this amplifier will clip before you can turn up the volume high enough to achieve your desired loudness.
 
Conversely, some amps might be capable of very high voltage swings, but not capable of driving a lot of current. This may be characteristic of some OTL tube amps. These amps would be very happy sending low current, high voltage signals to a 300 ohm headphone; however, they will peter out before they can send enough current to a headphone like the HE-6. This happens because more power will be dissipated within the amplifier due to the voltage division of the signal between the headphone impedance and the amplifier output impedance.
 
There are other situations in which a amplifier can't keep up with the demands of the headphone.
 
I'm fairly certain that we have a very comprehensive thread in Sound Science about calculating voltage and current requirements from headphones given senstivities in dB @ 1mW vs dB @ 1V
 
Cheers

 

[ab initio]

http://www.head-fi.org/t/668238/headphones-sensitivity-impedance-required-v-i-p-amplifier-gain
 
Here is the thread @xnor was keeping track of voltage and current requirements of different headphones. Unfortunately, xnor won't be coming back, so maybe somebody else wants to inherit this list and maintain it?
 
This was an absolutely fabulous resource. It lets you see the current and voltage requirements of a pair of headphones, which then allows you to compare these requirements with your amplifier specifications to see if there are any incompatibilities.
 
Cheers

 

[SilverEars]

Damn, that's very helpful work he has done.  He states"The following table is based on measurements, not manufacturer specs."  

 

[mikeaj]

  Sensitivity is dB/V and Efficiency is dB/mW.  A headphone will output loudness depending on voltage or power which differentiates the two.  
 
I'm curious which(efficiency or sensitivity at loudness constant) is more demanding on the amp's power capabilities.  

 
The figure in dB/V is larger than the figure in dB/mW for pretty much all headphones, if that's what you're asking.
 
If you have 1 V input, if the load is 1000 ohms, then the power input will be 1 mW. If the load is less than 1000 ohms, like practically all headphones, the power will be greater than 1 mW. (if it is greater than 1000 ohms, the power for 1 V input would be less than 1 mW, but let's ignore this case)
 
So the figure quoted in dB/mW will be less than the one quoted in dB/V: they will be closer the higher the impedance is and further apart the lower the impedance.

 

[cjl]

Not strictly correct although the two terms are used interchangeably. Efficiency is acoustic power out divided by electrical power in multiplied by 100 to get a percentage. Speakers can very from less than 1% to about 3%, efficiency, on a good day, going downhill, with the wind behind them. Understandably speaker manufacturers would be nuts to let anyone know that, so efficiency and sensitivity have become interchangeable words.
Sensitivity is what you are quoting, not sure how that's measured in headphones, but with speakers it's SPL @ 1 meter @ 2.83 volts input. If it's being done honestly, the speaker impedance must be stated, a 4 ohm speaker is going to look like it's more sensitive than an 8 ohm speaker as 2.83 volts = 1 watt @ 8 ohms and 2 watts @ 4 ohms. I guess with headphones the same thing applies as the load impedance is a factor.

True enough. I was referring to how the terms are commonly used in audio, but you are correct that the efficiency is abysmal in all cases. As for sensitivity in headphones, there's obviously no real meaning to the "@ 1 meter" portion of the measurement, so it's just dB SPL at the listener's ears at either 1mW electrical power level (more common) or 1VRMS input (less common, but used by Sennheiser). It's also important to note that it's usually at 1kHz - unless the headphone has a perfectly flat impedance curve and frequency response, power level at other frequencies for the same volume will be different. The rating at a specified power (1mW) rather than a specified voltage (1V) does help somewhat correct for the widely ranging impedance of headphones, but it introduces other confusions, such as the possibility of a high-sensitivity, high-impedance headphone that requires a lot of voltage (but almost no power) to get to a reasonable volume level (for example the HD800), or conversely, a low-impedance, low-sensitivity headphone that doesn't necessarily need a huge amount of voltage, but needs a lot of current and power (for example most planar magnetic designs). I'm not convinced the voltage specification is a lot better though, since as you point out, it biases the numbers towards low impedance designs.

 

[castleofargh]

I use xnor's post, also http://www.mediafire.com/view/dzwe1g73q6pbhnx/headphone%20power%20table.pdf  as both seemed to break free from the manufacturer specs(doesn't need to be different but it's sure is nice to have it from measurements).
and when I'm lost because of what units are used I use the excel offered here (just clic on the excel picture to download) http://www.apexhifi.com/specs.html

 

[esldude]

  Damn, that's very helpful work he has done.  He states"The following table is based on measurements, not manufacturer specs."  

Yes, but I suppose xnor shows that the truth will set you free.  He has been freed from this forum.