Does the impedance of my headphones and amplifier matter?

Headphone impedance and its effect on frequency response explained.

In this article:

  1. Does the impedance of headphones and amplifier matter?
  2. What is headphone impedance?
  3. Is my headphone input impedance flat?

 

Does the impedance of headphones and amplifier matter?

Yes, and they can affect the sound quality of the headphones depending on the device they are connected to. Problems are caused by high output impedance devices, for instance, built-in sound cards in some Windows laptops (having 50 Ohm impedance or higher). Depending on how high the impedance of your output device is, the frequency response distortion can vary between being completely inaudible (1 dB maximum) and quite noticeable (we've measured a maximum distortion of 7 dB in worst cases).

 

You should not be worried, though, if you're using an external audio interface or headphone amp as professional devices mostly have low output impedances. You should also not have any issues when using the built-in outputs of Apple devices (including the lightning to ⅛” audio jack adapter) or Android phones built by any of the major manufacturers (we've tested Samsung, OnePlus, and LG products).

 

To circumvent this problem, we recommend following the rule of thumb, known in the audio industry as the 1/8th rule - the output impedance of the audio source headphones are connected to must be 1/8th or less than the input impedance of the connected headphones. For example, if you have headphones with 32 Ohm input impedance (e.g. default in-ears coming with Samsung Galaxy 8, BOSE QC35 in passive, wired mode), the output impedance of the audio source should be 4 Ohms or less (1/8th of 32 is 4).

 

What is headphone impedance?

Headphone impedance describes the reactive resistance headphones apply to an incoming audio signal. In addition to being a good indicator of how easy the headphones are to drive (i.e. their maximum loudness), the characteristic impedance of the headphones can cause frequency response distortion and changes in the transient response. The impedance value of the headphones can change with frequency, meaning that some frequencies can be attenuated more than others when exciting the headphones (Figure 1).

 

Figure 1: Characteristic input impedance of Shure SE 535in-ears. Manufacturer stated input impedance value - 32 Ohms (Source)

Figure 2: Distortions in Shure SE535  frequency response depending on the output impedance of the connected audio source.

Green - 1 Ohm reference Yellow - 7 Ohm source Blue - 10 Ohm source Pink - 50 Ohm source

 

Unique to each headphone model, the effect of this type of distortion is shown in Figure 2. With a 50 Ohm source, the frequency response is significantly distorted in the 3kHz-9kHz range. While this is one of the worst-case scenarios we have found, make sure to keep impedance values in mind the next time you connect your headphones to a new audio source.

 

Is my headphone input impedance flat?

If unsure whether your headphones change frequency response when connected to various audio sources, check the impedance response of your headphones in one of the following resources:

  1. InnerFidelity.com - Headphone measurements section
  2. Reference-audio-analyzer.pro - Headphone report section

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