# EQ with personal measurements (***)

Up to this point, EQ was used with measurements provided in different databases, and while some of them are available with the 5128∆ project that allows for cross database comparisons, unit variation is still a very big factor that should be addressed, especially if an IEM and/or brand are known for having mediocre quality control. To remedy this problem as well as a few others, personal measurements should be used. There are three ways in which measuring your own set can help when using EQ.

For the rest of this section, it is assumed that the measurement rig is an IEC-60318-4 compliant occluded ear simulator, aka a 711 coupler, and that all measurements of an IEM are done with the same ear tips unless stated otherwise.

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# Unit variation

Personal measurements are useful for determining whether or not a particular unit of an IEM is faulty or not. Not only that, it also serves as a useful indicator that some part of the IEM might require cleaning or fixing. For example, clogged filters, or debris in vents can drastically change the sound of an IEM, even if it measured fine out of the box.

Unit variation is usually not a major issue, but when it is, it can be confusing when trying to EQ. If a unit has significant variation that is not easily fixable through EQ (eg having a substantial drop/boost in the bass region), then it is better to simply return it rather than salvage it. These differences will be perceived by the listener, but even with seemingly large deviations in measurements, human hearing is not as sensitive and can be easily fooled by different biases.

The following image shows three different samples of the same IEM. Unit variation can manifest itself as either poor QC, revisions, or pre-production units. The differences shown are relatively significant, and are located in a region that seems to be polarizing considering how the correct final version has the least energy there.

The next one shows an example of an IEM that has somehow passed the manufacturers quality control stage. Notice how low in amplitude the whole range below 1 kHz is on the defective model; this large discrepancy would require filters with very high gain, and subsequently high preamp negative gain. Not ideal, and not worth using EQ to fix it.

For your own measurements, check to see if there are significant deviations, and if there are, simply EQ the unit to the desired FR following the guide. Re-measure your unit with EQ applied to see if the deviations have actually been removed and listen for any possible (albeit very rare) signs of distortion or other artefacts.

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# Channel Matching

Depending on quality control, some IEMs and individual units can have significant channel imbalances. Depending on the frequency range being affected and the volume difference, the imbalance can be audible: for most manufacturers, a deviation of 1dB is generally considered as acceptable. Fixing these imbalances relies on the same steps and ideas as previous sections, but requires separate and independent channel EQ.

If both sides have been measured to have the same resonance peak location and the same loudness, then comparisons and conclusions can be made between the two sides. The same rules as before apply here.

The image shows two IEM units, for which both their channels have been measured. For the Hola, this particular unit displays a consistent difference of around 1dB between channels; whether this is audible or not has to be determined through listening tests. If there's a need for correction, a simple preamp (or peak/shelf filter for focusing on a frequency range) can be used. The G10 also has slight differences, but because they are so narrow, they will most likely not be noticeable.

The following images shows an IEM for which there is a channel imbalance in the ear gain region. At first glance, the small peaks are not only different in level, but also not aligned between the two sides. Fixing this with the left image isn't so straightforward; the second image, with side 1 being compensated to side 2 offers a better look at what the differences are, and how to fix them.

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# Insertion depth and ear tips

Insertion depth changes not only the frequency of the canal resonance peak, but also affects the surrounding frequency ranges, and oftentimes significantly. While previously established rules state that measurements shouldn't be scrutinized above 10kHz, there is still some use in looking at insertion depth profiles of an IEM in order to not only see how it reacts to different coupling scenarios, but also to find a match for your own personal experience.

The following image is an extreme example of how insertion depth can affect FR. As the insertion depth changes, so do the surrounding frequencies' levels. Depending on the IEM's shape, intended insertion depth, and the ear tips being used, insertion depth location can shift drastically; not only that, there can also be damping, from the ear canal and/or IEM in that region, which can affect both the amplitude and Q of the resonance peak. It is therefore useful to have insertion depth profile done through your own measurement rig.

The following images shows possible FR changes attributable to ear tips. Ear tips usually influence the bass region because of seal, and treble because of insertion depth as well as other reasons outside the scope of this guide. Assuming proper seal, the bass factor can be ignored, leaving only the treble as being an affected factor. Depending on the shape and size of the ear tip, the resonance peak frequency can shift considerably. The image however does show changes despite them having the same resonance peak frequency; while it is usually recommended not to scrutinize measurements past 10kHz, it is an interesting thing to note that ear tips can change upper treble response, usually in the form of damping.