Impedance graph significance?

[Huuvola]

Um, what do the impedance graphs really mean? I noticed that the Denons have hardly any deviation in impedance throughout the frequency range, but all other cans (unless I missed some) have a significant hump somewhere along the range.
What does this actually mean, technically, sonically? Does this graph mean anything relating to sound or quality? I read the description on your website, but it was kind of broad: it did not help me to understand the real world differences say between the D7000 (super low impedance and deviation) vs. the HD800 (high impedance and very significant hump around the 100hz area).
I know that the HD800 has higher impedance which is fine, but why the hump? ...also present at the very high end of the frequency range.

[murrays]

The effect will be dependent on how well the amplifier copes with varying load impedance. An amplifier that has a high output impedance (e.g. series resistance) may cause a variation from an ideal frequency response. For instance, you may get a slight lift in the areas where the impedance is highest. This will show up differences between amplifiers. It should not be a problem for an amplifier with good drive ability and low output impedance.

[Uncle Erik]

murrays is right. Also, you have to keep in mind that impedance is not always constant in a pair of headphones. It will change depending on what it's being fed.

Comparing the headphones' impedance to the output impedance of an amp tells you how efficiently power is being transferred to the headphones. So, when the headphones' impedance varies, the power reaching the headphones will vary, too.

[Tyll Hertsens]

Originally Posted by Huuvola

What does this actually mean, technically, sonically? Does this graph mean anything relating to sound or quality?

Good question!

The big hump is the free air resonance of the driver. It's probably best to steer you here for an explanation---it's about speakers, but it's the same principle.

In terms of effecting the sound, I would suspect that the more severe the bump, the harder it is for the amp to drive the headphones near the resonance peak, which may have some effect on the sound.

We have noticed that the impedance curve does change when the headphones are put on the head---makes sense since we're talking about the electro-mechanical impedance characteristics, and the acoustics of the enclosed earpiece or having the driver within 1/4 wavelenth of the ear or head would directly effect the mecanical impedance of the driver.

The other thing we've noticed is that we think we can see if a headphone has spurrios mechanical vibrations in the curve as some minor wiggling of the impedance curve at certain frequencies.

[Huuvola]

Thanks for great info from all of you.

Originally Posted by Tyll Hertsens

Good question!

The big hump is the free air resonance of the driver.

So,that "acoustic optimizer" in the AH-D7000 that Denon claims is supposed to equalize sound pressure levels on both sides of the driver...perhaps that is responsible for eliminating the impedance hump? I wonder at the mechanism.

[majkel]

The higher the output impedance of an amplifier, the more emphasis appears for the frequencies affected by the bump. It's just a plain effect of a voltage divider. For "zero" output impedance amp - you don't experience the effect of the driver resonance.