Measurement setup and basics
In general, so-called couplers with clearly defined volumes and built-in, properly calibrated measurement microphones are used to measure the transmission behavior of headphones. The rest is then also based on the object to be measured. The setup can be used just as well for plug-in headphones (in-ears) and small headphones (e.g. from hearing aids) as for simpler headphones and headsets as so-called on-ears (headphones with supraaural cushions). For such headphones (on-ear), the “artificial ear” according to IEC 318 can be used, which I have now followed with the implementation.
The thick over-ears, i.e. circumaural or ear-enclosing headphones, are not easy to handle when it comes to measurement and, above all, reproducibility. This is because there are not yet any truly standardized couplers. The reasons for this lie in the difficulties of measurement technology and the many influencing factors that make reliable reproducibility almost impossible. Therefore, such circumaural headphones are mainly measured with appropriately modified couplers for supraaural headphones by using an additional flat plate as a support for the circumaural cushion (see picture).
Important reference point for headphones: the Harman curve
The so-called Harman curve is an (optimal) sound signature that most people prefer in their headphones. It is therefore an accurate representation of how, for example, high-quality speakers sound in an ideal room and it shows the target frequency response of perfect sounding headphones. It also explains which levels should be boosted and which should be attenuated when using this curve as a basis. This also explains the term “bathtub tuning”, which is often quoted, but in which the Harman curve is completely misused and exaggerated.
For this reason, the Harman curve (also known as the “Harman target”) is one of the best frequency response standards for enjoying music with headphones, because compared to the flat frequency response (neutral curve), the bass and treble are slightly boosted in the Harman curve. This “curve” was created and published in 2012 by a team of scientists led by sound engineer Sean Olive. The research at the time included extensive blind tests with different people testing different headphones. Based on what they liked (or didn’t like), the researchers found and defined the most popular sound signature.
Tuning headphones can be really problematic due to the human anatomy. Everyone has a slightly different pinna and ear canal, which affects how individuals perceive certain frequencies. In extreme cases, there is a difference of a few dB from person to person, which explains the small differences in some measurements with artificial ears. Furthermore, if the sound is not absorbed, it is also reflected by other surfaces. Theoretically, a torso could also be included in the test setup, but that would be far too time-consuming.
Frequency response
The Harman curve just mentioned is shown as a dark line in the diagram. Let us first evaluate the unsmoothed measurement. If we superimpose the green curve and the purple curve (Harman), interesting findings emerge with a dip at approx. 6 and 7 KHz respectively.
If you smooth the curves, the whole thing definitely doesn’t look like an extreme bathtub. The mids are naturally lowered somewhat and there is also sufficient low bass. The level stability isn’t exactly epic, but it’s enough. Here, too, we see the slight high-frequency dip, but this can be eliminated quite easily with a proper parametric equalizer (see tutorial).
Parametric equalizer
As I wrote in detail in the tutorial on the APU and PEACE equalizers, you can also adjust the curves and thus the sound to perfection at low level. SteelSeries also describes its own equalizer in the GG software as a “parametric” equalizer, but this is just a simple graphic equalizer and pretty useless because you can NOT adjust the parameters of a selected frequency range. Except for the volume. So the whole thing is a kind of overstretched bell filter, nothing more.
Cumulative spectra
The cumulative spectrum refers to various types of diagrams that show the time-frequency characteristics of the signal. They are generated by successively applying the Fourier transform and suitable windows to overlapping signal blocks. These analyses are based on the frequency response diagram shown above, but also contain the time element and now show very clearly as a 3D graphic (“waterfall”) how the frequency response develops over time after the input signal has been stopped. Colloquially, this is also known as “decay” or “decay”. Normally, the driver should also stop as quickly as possible after the input signal has ceased. However, some frequencies (or even entire frequency ranges) will always decay slowly(er) and then continue to appear in this diagram as longer-lasting frequencies on the time axis. This makes it easy to recognize where the driver has glaring weaknesses, perhaps even particularly “rattling” or where, in the worst case, resonances could occur and disturb the overall picture.
Burst decay
The “burst decay” refers to the gradual fading or decay of a signal or wave after a sudden impulse or “burst”. Such a burst can be caused by a sudden injection of energy, for example, and the subsequent decay describes the way in which the system returns to its initial state. The burst decay is used in the context of analyzing signals in audio processing. It is important to understand burst decay as it can affect the performance and quality of a system. In audio engineering, for example, burst decay can cause distortion or unwanted noise if it is not properly controlled.
Here, it is the engineer’s job to design systems that minimize such interference and handle burst decay effectively. With CSD, the plot is generated in the time domain (ms), whereas the burst decay plot used here is plotted in periods (cycles). And while both methods have their advantages and disadvantages (or limitations), it is fair to say that plotting in periods can be more useful for determining the decay of a driver with a wide bandwidth. The small 40 mm drivers do an almost perfect job, you can leave it that way. There is not much resonance, only in the lowest bass.
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