Understanding Frequency Response and Equalization

Frequency response graphs are a useful tool for understanding what a pair of headphones sounds like, but they can often be challenging to understand, and don’t tell the complete story. Likewise, EQ is a powerful tool for sculpting your headphones sound, but without some knowledge behind it, you’ll end up breaking the nose off the Sphinx rather than chiseling out the David. Let’s start by trying to get a better understanding of Frequency Response graphs and how they work as a visual representation of a headphone’s sound.

Frequency Response

The first question you might have regarding frequency response is “What does that graph actually mean?” The graph represents the amplitude with which the headphones reproduce each frequency. So when you feed the headphones a signal that is the same across all frequencies, some of those frequencies will come out louder than the source signal, while others will be quieter. But what are these “frequencies” that I speak of?

Focal Celestee Frequency Response

Sound is made up of vibrations. The speed of those vibrations determine the tone of the sound. Frequency is how we describe that speed. Faster vibrations – or higher frequencies – create higher pitched sounds, while slower ones create lower pitched sounds. So bass is created by low frequencies, treble by high frequencies, and midrange in the middle. Frequencies are typically described in hertz (Hz) and kilohertz (kHz), with the range of human hearing being approximately 20Hz to 20kHz.

Charts and Graphs

So when we look at this graph below, we can see that the lower frequencies are elevated, which means that the bass is going to be louder in the output. The midrange is lower than the bass and treble, and the upper midrange and treble is almost elevated to the same level as the bass.

Empire Ears Valkyrie Frequency Response

In this one, you see everything looks much more even. There are some small bumps, but not a lot that’s elevated or recessed. This is much more “neutral” and is a more honest reproduction of the original recording.

HIFIMAN Arya Frequency Response

Another helpful tool is knowing where different types of sounds live in the mix according to audiophile and sound guy colloquialisms:

  • Rumble: 30Hz-40Hz (Subbass)
      • Punch: 100Hz-180Hz (Midbass)
      • Muddiness: 200Hz-500Hz (Bass into Lower Midrange)
      • Fullness: 300Hz-500Hz (Lower Midrange)
  • Snap: 800Hz-1.5kHz
      • Tinniness: 1kHz-2kHz (Upper Midrange)
  • Sibilance: 3kHz-7kHz (Treble)
      • Definition: 6Khz-10kHz (Treble)
  • Air: 15kHz-20kHz

  • This also demonstrates the challenge of tuning headphones or EQing them. You want to have Punch and Fullness, but not Muddiness. You want to have Snap and Definition, but not Sibilance or Tinniness. The best headphones will balance these things well, but also knowing if you’re particularly sensitive to Sibilance, for example, might tell you that you want to avoid headphones with elevated treble intended to increase the feeling of Definition.

    Need… More… Power!

    There are some things that a frequency response graph doesn’t tell you. One of those has to do with the Impedance of the headphones and the voltage of the amp. Impedance plays a part in how frequency response graphs translate into what you actually hear. Impedance describes a transducers resistance to being driven. A big part of that resistance is that it’s harder to generate low bass notes than treble. Higher impedance headphones often sound overly bright, and “bass light” when not given enough power. The lower registers are not receiving enough energy to properly engage, leaving only the easier to drive midrange and treble. To fix this, you need an amp which can provide a higher level of voltage to drive the bass as hard as the rest of the frequencies. 

    The Shape of You

    Another factor that affects how you hear headphones – and even more so IEMs – is the physical shape of your head, ears, and inner ear. Over-ear “circum-aural” headphones need to create a seal around the outside of your ear to produce the optimum sound. In some cases the shape of an individual’s head or ears might make this difficult. The shape of your ears themselves also affect the acoustics of the space that the headphones are creating, and can impact the final sound.

    With IEMs, there are even more factors, as the shape and length of your ear canals can impact your ability to properly insert the IEMs. Even if you achieve a good seal, there’s no guarantee that the IEM is pointed at the optimal angle to deliver music to your ear drums, and not just to the wall of your ear canal. So different aspects of the sound can be potentially lost or changed by the shape and construction of your ears.

    Equalization 

    Understanding how to read a frequency response graph can help you pick out headphones or identify characteristics that you don’t and don’t like. To take it a step further, having a good understanding of how frequencies can help you tweak the sound of your headphones with equalization (EQ). Equalization is the process of adjusting the frequency response of your audio source to change how the final output sounds.

    Various forms of equalization are happening at many different levels through the process of creating music. For an electric guitar, as an example, the guitar manufacturer selects pickups and materials that will generate certain types of tones from the guitar itself. The guitarist will then typically use a tone knob on the guitar, and the “Bass/Mid/Treble” knobs on his guitar amp to alter the sound from there. An engineer will record the song, and apply EQ to the guitar to create the final mix (and then EQ the final mix as a whole). When you play it, your player, DAC, and amp have some level of tuning, as do your headphones. And then finally, you might add your own layer of EQ to tune it to your preferences.

    Types of EQ

    There are two main types of equalizers that you can use to tweak your sound: graphic EQ and parametric EQ. Graphic EQ is what most people imagine when they think of EQ. If your media player has a built-in EQ, odds are it was a graphic EQ. Graphic EQ gives you a selection of “bands” where you can raise or lower a specific frequency to your liking. The benefit of the clasgraphicsic EQ is that it’s very easy to use, but its accuracy can be limited by the number of bands available. A 24-band EQ can provide a lot of options for subtle adjustments, but a 10-band EQ might be too large of a brush to effectively paint with.

     Graphic EQ

    Parametric EQs got their start in the studio, and have become more popular as easier to use digital versions have become available. They give you more precise, but also more complicated controls. Rather than just turn a specific frequency up or down, you select a frequency, a filter type, a Q Factor, and a gain level. This provides a ton of precision, and also the ability to shape and sculpt whole sections of the frequency response very quickly. However, to properly explain a parametric EQ, you probably need an advanced degree in mathematics.

    Parametric EQ

    So what should I EQ?

    The best use of EQ with headphones is to subtly tweak small aspects of the headphone that are good, but (in your humble opinion), but not quite good enough. Or to remove some bothersome characteristic that bothers you. As an example, the HIFIMAN Ananda has some elevations in the 3kHz to 4kHz range which you might hear as sibilance. You might also want to make the subbass response a little stronger to get more rumble out of it.

    HIFIMAN Ananda Frequency Response

    In this case you could do a very small cut the 3kHz-4kHz range, and then either use a classic EQ to increase the subbass levels, or use a parametric low shelf filter starting at around 65Hz with a parametric EQ to more uniformly elevate the whole of the subbass.

    So how do you actually apply the EQ? There are a number of options, ranging from digital audio players from brands like iBasso or Astell&Kern with EQ apps built in, or you can use Roon, Audirvana, or any other number of apps to provide an EQ for PC and Mac.

    The Bottom Line

    In a world where you can’t always try out every set of headphones that you want to buy, understanding frequency response graphs and what each frequency means in terms of the tuning and sound of the headphones will enable you to make more educated decisions with your headphone purchases, and being able to use EQ can help maximize your enjoyment of headphones that seem almost perfect. Altogether though, there’s more to the sound of a headphone and music in general than numbers and charts. Sooner or later, you’ll need to actually use your ears. And you might find that what you see and what you hear don’t always line up.

    Headphone frequency response graphs provided courtesy of In-Ear Fidelity.