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In this app note, we will walk through EQ-ing your headphone based on measurements taken with the miniDSP EARS measurement rig.

To implement the EQ, the app note will assume that you have a miniDSP HA-DSP headphone amplifier or another miniDSP device such as the 2x4 HD. However, the technique can be used with many other devices and also with software implementations, as long as they have a full parametric EQ capability.

For devices that don't have a full parametric EQ or that can't have an REW measurement signal run through them, see the related app note Headphone EQ for iPhone (or Android, or DAP).

• Why EQ?
• Measuring the headphones
• Applying EQ
• An example EQ
• Wrapping up

Why EQ?

There are a lot of factors that affect a headphone's sound, but the frequency response is the primary one. It determines not just the overall balance of bass, midrange and treble, but also the distribution of harmonics that are part of every musical note.

Headphones (and IEMs) are complicated things when it comes to their acoustics. Unlike a speaker, which can be measured for accuracy in an anechoic chamber, headphones can only be measured when they are on a "head" (either real or artificial). Sound bounces around in the tiny confined space between the headphones and your ears, and travels down the ear canal until it eventually reaches the eardrum.

Professional artificial measurement "heads" attempt to simulate this environment, and as result are very expensive. Despite their cost, they still produce very different results, as shown in the graphs in this article by Audeze. Furthermore, we all have different anatomy: studies show the acoustic signal at the eardrum varying by 5 dB in the bass, for example. Factors such as the positioning of the headphone, the seal of the pad to the head, and so on, will change the measurements. Finally, different people have different subjective preferences.

So, when you run headphone measurements for EQ, it's important to treat the measurements as a guide. You always need to listen to the result of any EQ based on the measurements, and adjust it so that it sounds right to you.

Some additional points:

• Because there are a lot of factors affecting the sound of a headphone, you probably won't be able to EQ a headphone you don't like into one that you do. It's best to start with something that basically sounds good to you.

• The headphone designer(s) will have traded off all of those factors when designing the headphone. So correcting only the frequency response may not produce the expected result. Again, you need to listen for yourself and decide how to apply the EQ.

• Remember Fletcher-Munson! Our ears' sensitivity to high and low frequencies depends on the SPL, so do your listening adjustments at the level you normally listen. (Don't be afraid to add some bass and treble if you prefer to listen at lower levels.)

• In this app note, we suggest using just a few bands of EQ, say no more than 5. This will make it easier to make manual adjustments when auditioning.

• Don't try and flatten every little bump and "wrinkle" in the measured response. Just work to make the response closer to flat.

• Regardless of what the measurements say, work within the natural capabilities of the headphone. A bass-light headphone may not respond well to a lot of boost in the bass, for example.

• Any modifications or "mods" to the headphone will change the response, so you may need to redo your EQ. Even different earpads change the response. (You can measure the change with EARS.)

The basics of parametric EQ

There are two main types of EQ filter. A peaking filter boosts or cuts frequencies around the center frequency. This example (from the HA-DSP) shows a 10 dB boost at 200 Hz with a wide bandwidth (Q=0.7) and a 10 dB cut at 2000 Hz with a fairly narrow bandwidth (Q=4):

A shelving filter boosts or cuts the signal above or below the set frequency. Shelving filters are typically used to "bring up the bass" or "take down the treble". This next example shows a shelving boost of 6 dB (Q=0.7) centered at 100 Hz, and a shelving cut at 2000 Hz (Q=0.5):

Measuring the headphones

The EARS page on the miniDSP website now has a compensation type named "HEQ", for "Headphone EQ". Go there now, enter your serial number and select HEQ to download the calibration files. These calibration files help simplify the process of EQing headphones.

Now run a measurement on your headphones, loading the file L_HEQ_xxxyyyy.txt into REW for the left and R_HEQ_xxxyyyy.txt for the right. (See the User Manual for details.)

Use the All SPL tab in REW to create an average of the left and right measurements. Optionally, you can take several measurements on each side with the headphone moved a bit each time, and average all of them.

The intent of the HEQ compensation is that, when it is used to measure a "neutral sounding" headphone, the resulting measurement will be approximately flat. It is similar in intent to the "preferred headphone target response" identified by Olive, Welti and McMullin (Ref. 1) but adapted for the EARS.

Let's look at a couple of examples. Headphone A is a good quality headphone that is generally considered to be "dark". Subjectively, we felt that it was withdrawn on vocals but also a little hot in the high treble. Bass is solid but it doesn't have a lot of sub-bass. Its response when measured with HEQ is shown in Figure 1 in red. You can see that it measures low (compared to the flat line) in the 2 to 7 kHz area, which explains the "darkness."

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