[EssKay]

I'm looking for some help from someone who was paying attention in high school.
 
I've just got some D2000s to complement or possibly replace my ATH-M50s and as I'm switching between them I'm having to turn the volume up on the amplifier (Fiio E17) two 'notches' with the Denons to get the same subjective volume. This in itself is fine but from the specs I was expecting it to be the other way around if anything as the Denons are a bit more efficient:
 
D2000 - eff. = 106 dB/mW; impedance = 25 Ohms
 
ATH-M50 - eff. = 99 dB/mW; impedance = 38 Ohms
 
Is this something to do with the impedance or that my evaluation of relative volume is out? (The latter would explain why I now have a headache although it's more likely that I've just been listening too long at too high a volume).
 
System details: some sort of Mac>Audirvana> Fiio E17> Cans
 
(Fiio E17 headphone impedance range is 16 - 300 Ohms).
 
I'd be very grateful if someone could set me straight.

 

[stv014]

In reality, the ATH-M50 is more efficient, based on the measurements available at innerfidelity.com. I will calculate the actual maximum sound pressure levels assuming that the amplifier section of the E17 is identical to that of the E11 (which has been measured by NwAvGuy). According to this graph, the E11 would output a maximum of about 1.3 Vrms into 24 Ohms with low distortion, and slightly less than 2 Vrms into 38 Ohms (these are only approximate values).
The Denon was measured to require 0.073 Vrms for 90 dB SPL, while it is 0.041 Vrms for the M50. The E11 has only 0.5 Ohm output impedance, so I ignore it to keep the calculations simple. Note that the M50 is more efficient by ~4.5 dB, while the D2000 is less efficient by ~9.5 dB than what the manufacturer claims. At the same voltage (i.e. same setting of the volume control with no clipping), the M50 is louder by about 5 dB. The maximum SPL at the maximum voltages with no clipping distortion:
- M50: ~123.5 dB
- AH-D2000: ~115 dB
The difference further increases, because the amplifier can output higher voltage with an easier high impedance load. Of course, headphone measurements are not very accurate, so take these results with a grain of salt.
 

 

[EssKay]

Stv014  - many thanks for your detailed answer, very helpful and much appreciated.

 

[MalVeauX]

Heya,
 
You'll find a lot of low impedance headphones are more difficult to drive. Look into dampening and output impedance and how it effects current.
 
Very best,

 

[TMRaven]

Actually the lower the impedance the more current it needs to be driven, so it can definitely be harder to drive.  Although I'm not too sure what the number is where it starts becoming like that. 

 

[crimsonOkami]

From what i understand, driver and speaker resistance have an indirect cause with sensitivity.   driver sensitivity is a totally different thing than the resistance a driver has,  High resistance drivers can be highly sensitive and low resistance drivers can have very low sensitivity, and vice versa.  When looking at sensitivity it's usually labeled as SPL, and it is measured based on decibels and watts. 
 
i could try to describe it more but this guy basically says it better:
http://hometoys.com/emagazine.php?url=/htinews/feb04/articles/polk/impedence.htm

 

[EssKay]

Thanks all for your quick and very helpful replies.