Impedance

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Influence of impedance on tonal balance

These types of plots are not found on this website. Even though the impedance can have an influence to the actual sound it is often difficult to asses what the actual implications are to the tonal balance. For this the output impedance of the amplifier is of major influence.
On this website I have chosen a different way of showing what kind of influence output impedance has on the tonal balance.
When applicable some plots are used showing the actual effect of what a higher output impedance has on the actual sound. Usually a 120Ω resistance is used simply because it is used in certain norms.
Below the influence of a 120Ω output resistance on a HD650 compared to a low output resistance of 0.1Ω 

https://diyaudioheaven.files.wordpress.com/2015/01/hd650-new-pads-0-ohm-vs-120-ohm.png?w=614

Below the effect a varying impedance has on the tonal balance of a K612 driven from a Ω amplifier and a 120 Ω output R amplifier. This plot shows the impedance of the K612 is higher between 1oHz and 300Hz (opposite 1kHz) and also starts to rise again above 5kHz.
The tonal balance differences shown are caused by VOLTAGE DIVISION and NOT by less damping.

k612 0 ohm vs 120 ohm

It should be noted that a higher output resistance of an amplifier also changes the sound LEVEL. The 2 plots above do NOT show the level difference as that difference has been removed by changing the offset. The plots only show the tonal balance changes.

Despite popular belief the impedance of a headphone does NOT have to match that of the used amplifier.

Impedance plots:

Another type of plot is the ‘impedance plot‘ which also tells something about resonances and shows how MUCH a headphone will react to differences in the output resistances of amplifiers. This dependency is a tricky thing as not all amplifier manufacturers mention their output resistance, yet it can have (depending on the impedance behavior) quite some influence on the sound. The majority of the anecdotal reports of amplifiers having a ‘sound’ are related to this ‘phenomenon’.

A great resource for impedance plots is found on inner|fidelity

The impedance plot shows what the actual impedance is of the headphone at different frequencies. The impedance should always be measured when mounted ON a measurement rig as the impedance plot will be different when it is created with the headphone lying on the desktop. It stands to reason most independent websites that measure headphones are aware of this fact but may not be. Most (but certainly not all) dynamic headphones a certain frequency dependent impedance change is quite normal, regardless of the nominal impedance. The nominal impedance is always equal or higher than the Ohmic resistance you can measure with a multimeter. Only multi armature IEM’s (with crossovers in it) can have impedances below the nominal value. When you do measure the impedance with a multimeter in the Ω-setting it will at least give a good indication of the nominal value. The measuring current used in the majority of multimeters is so low you can NOT damage headphones doing this. Some dynamic headphones are so well damped internally they do not show peaks in the impedance plots. Orthodynamic headphones, as a rule, do not exhibit varying impedances over the entire frequency range. The significance of varying impedances and what effect it can have on the sonic signature is explained in the article resistance, impedance and other issues. Below a typical impedance plot of an orthodynamic headphone with a nominal impedance of 50Ω (HE-6 for instance)

graph impedance ortho

The plots themselves are pretty straight forward. Just like the frequency plot the frequency is on shown on the horizontal scale and the impedance, in Ohms (Ω) is on the vertical scale. When it shows a relatively large (but very wide) peak sometimes even 2 or 3 times the value of the nominal impedance (mostly in the 50Hz – 200 Hz region) the headphone will sound different on headphones with a higher output resistance (there will be more lows) compared to amplifiers (and portable players) with a low output resistance. Headphones with ‘flat’ impedance plot, or varying merely a few %, will sound very similar when driven from either a low or high output resistance amplifier. Note that most headphones also show a rising impedance in the highest frequencies. Because of this these headphones might sound warmer or a tiny bit more velvety or less aggressive on amplifiers with a higher output resistance.

The horizontal frequency scale is always logarithmic but the vertical scale is linear (in most cases). Sometimes the vertical scale is also logarithmic. In this case the raise in impedance is harder to see and thus seems more flattering. Something to take into account when looking at these plots.

Some headphones that already have a rather high amount of lows may start to sound ‘muddy/uncontrolled or bloated’ in the bass when driven from a higher output resistance amplifier. Examples: Sennheiser HD650/HD595.  Headphones that show relatively less bass on low output resistance amplifiers may become ‘fuller’ sounding on higher output resistance amplifiers. Examples: Sennheiser HD600/HD800, Beyerdynamic DT880, AKG K501/K601/K701/K702.

The impedance plot can also show problematic areas in the headphones. When a membrane/driver resonates the impedance increases  as it needs less energy to move/vibrate because the driver generates it’s own ‘counterforce‘ which lowers the current. When the applied voltage remains the same and less current is drawn (needed) this means the resistance will need to be higher. So small peaks somewhere in the plot between 200Hz and 10kHz will show areas where the headphone has some resonances and thus a certain degree of degradation in sound. How much degradation and the audibility of it depends on the frequency and severity of ‘ringing’. 2 of these peaks are shown in the plot below where 4kHz and 6kHz show some resonances.

graph impedance HPThe wide peak in impedance on the left of the plot above is quite common for most dynamic headphones as this is the natural resonance of the driver(membrane) and sonic ‘chamber’. The plot above is not of an actual headphone but will look much like it. The peak in the low frequency area (in this case at 75Hz) in headphones can vary between 40Hz and 150Hz and the peak shown above is almost a factor 2 (when referenced to 1kHz) and quite realistic. At 1 kHz most headphones (multi driver IEM’s can differ) are close to their nominal impedance which is usually mentioned in its specification sheet.The peak can also be lower in amplitude with a peak of just a few Ohms higher or be smaller or bigger in width.

More often than not there is also a slight variance in actual resistance value, even between left and right driver, so headphone A rated at 32Ω could very well be between 30Ω and 35Ω and may even differ slightly between left and right driver. This is nothing to worry about as long as the drivers sound equally loud.

The upper part of the frequency spectrum often shows an increase in impedance which is normal and is caused by the inductance of the voice coil.

In short: Impedance plots say something about resonances. The higher the peaks the more the sound of the headphone will vary when connected to an amplifier with a high (between 30 and 300 Ohm) output resistance. The higher the peaks are the more pronounced the frequencies in the same area are present.

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