[MindsMirror]

Today I was doing some experiments with my headphones and a multimeter and discovered something that I thought I would share. To those of you who already fully understand how electrical damping works, this won't be anything new to you, but it might prove interesting or helpful if you don't fully understand all the aspects of how or why it works.
 
Here's what I did. I plugged my headphones into the first output of a headphone splitter. I connected the second output of the splitter to my multimeter, so it is essentially measuring the voltage across the headphones and/or the voltage at the output of the amp. Then I connected the splitter to my various amps and played some sine waves into them.
 
I was initially just trying to measure the output impedance of the amplifiers by measuring the voltage under load, then unplugging the headphones from the splitter to measure the open voltage of the amp. Then I discovered while I was testing my Logitech speaker's amp (which has a high output impedance, damping factor of ~1), that if you cover the back of the headphones (they are Grados, open back), the voltage across the headphones decreases. When I did this with my Magni amp which has low output impedance and a very high damping factor, the voltage remains constant.
 
Covering the back of the headphones essentially creates more physical resistance to the driver's movement. When the physical resistance of the driver is increased, the amp with the low damping factor provides reduced voltage across the driver, so it has less electrical control over it. With the high damping factor amp, physically resisting the driver has very little impact on the voltage across it, so the amp has more electrical control over the driver.
 
This experiment won't work for all headphones. I tried this with some sealed headphones. Just sealing the ear cup didn't work, it doesn't provide significant enough resistance to the driver. The back of my Grado is a relatively small enclosure, so sealing it created enough air pressure to physically resist the drivers movement.

 

[stv014]

Covering the headphones increases mechanical damping, and "flattens" the primary resonance (usually in the bass range for full size headphones). If you measured at a low frequency like 60 Hz, then the driver impedance is of course lower with less resonance. The same effect can be seen when the impedance vs. frequency of the headphone is measured both when it is worn and when it is not.

 

[MindsMirror]

  Covering the headphones increases mechanical damping, and "flattens" the primary resonance (usually in the bass range for full size headphones). If you measured at a low frequency like 60 Hz, then the driver impedance is of course lower with less resonance. The same effect can be seen when the impedance vs. frequency of the headphone is measured both when it is worn and when it is not.

Actually with the Grados, nothing really happened when I tried to seal the ear side. The earpads are foam, so you can't really achieve a seal on that side. People here generally say that Grados don't need an amp as much as some other headphones. Might that be part of the reason? Without a seal on either side of the driver, there is less mechanical damping, so they don't need as much electrical damping either.

 

[SilverEars]

  Covering the headphones increases mechanical damping, and "flattens" the primary resonance (usually in the bass range for full size headphones). If you measured at a low frequency like 60 Hz, then the driver impedance is of course lower with less resonance. The same effect can be seen when the impedance vs. frequency of the headphone is measured both when it is worn and when it is not.

I looked up a speaker design and with the vent open it flattens the resonance frequency if the vent is open.
 
http://diyaudioprojects.com/Technical/Aperiodic/
 

 

 

[Roly1650]

I looked up a speaker design and with the vent open it flattens the resonance frequency if the vent is open.

http://diyaudioprojects.com/Technical/Aperiodic/

 

I'm not sure I trust that graph, that's unlike any impedance curve I've ever seen for a vented box, usually they look much more like twin peaks, like this:




I have a feeling your graph is showing acoustic impedance, not electrical impedance, even though it's labelled as electrical impedance, but I could be wrong. In any case, I think the volume of the average headphone cup is too small to be directly comparable to a speaker cabinet and certainly not a vented/bass reflex box. On an open headphone the port diameter is pretty much the same as the driver diameter, unrestricted it should act much more like an open baffle loudspeaker imo.

 

[MindsMirror]

  I looked up a speaker design and with the vent open it flattens the resonance frequency if the vent is open.
 
http://diyaudioprojects.com/Technical/Aperiodic/ 

The cabinet of the speaker is much larger than a headphone enclosure. Perhaps when it is sealed the cabinet resonates at that peak frequency, and opening the port allows some of the pressure out to reduce that resonance. The enclosure of a headphone is much smaller. While the driver might resonate at a low frequency, the enclosure will not, so sealing it would damp the mechanical resonance of the driver and flatten that resonant peak.

 

[SilverEars]

The graph I posted looks to be more of an approximation to illustrate the affects of the electrical bass resonance from adding vent on the enclosure.  The graph Roly posted is an electrical impedance of a real speaker measurement where the vent is tuned to reduce resonance at 39Hz which resulted in the twin peak resonance around it.  Although it's for a speaker, it shows the affects of changes in electrical resonance based on an opening of the space behind the driver, although that space is very large. I'm not entirely sure what enclosing openback headphones actually does without looking at impedance measurements when closed and open.  Perhaps since the space from the driver to the grill is slight, it has different affect.

 

[stv014]

Here is a quick test with closed headphones that have vents (DT770 Pro):

 
Blue: not worn (but still mostly closed because of the clamp), the vents are open; this has the largest resonance peak
Red: worn normally with the vents open: the resonance peak is now flattened somewhat
Green: not worn, but the vent is closed on the channel tested; there are two samples, since not being able to perfectly close the vents made the results not too well repeatable. But it can be seen that the low frequency resonance is reduced, while the impedance is now actually higher around 200 Hz

 

[SilverEars]

Cool, this answers my question.  Thanks for the graph.  It's interesting how the electrical resonance peak changes with mechanical damping although not a significant affect.  Would be interesting to see the output values with vent closed using different output impedance values.

 

[castleofargh]

  Cool, this answers my question.  Thanks for the graph.  It's interesting how the electrical resonance peak changes with mechanical damping although not a significant affect.  Would be interesting to see the output values with vent closed using different output impedance values.

the IE80 would make for a nice guinea pig, flat impedance over freqs and adjustable back vent.  or simply ask Rin if he comes back from the secret bunker where he spent the summer with Elvis and nwavguy, to measure the different impedance responses when he plays around with obstructing or creating pinhole bores on IEMs. 
 
the more we talk about damping of drivers and resonance, the more I think stirling motors for some reason... probable a lack of pringles energy for my brain.