Why fix the frequency response of headphones and do you need to do it?
Studio headphones are an important tool for sound engineers. Previously, we have already discussed the main problems of monitoring in headphones and today we want to focus in more detail on the topic of amplitude-frequency response.
1️⃣ What is the amplitude-frequency response?
The amplitude-frequency response displays the relative gain level of a particular frequency in a sound device. It is usually presented in the form of a graph, whose horizontal axis is the frequency range, and the vertical axis is the gain level.
2️⃣ Differences between different headphone models
Unfortunately, most headphone models do not have a flat frequency response and they all sound different. There is no standard for "right" sound in the music industry, as different manufacturers do their own research and get different results. For example, the frequency response of over $1500 studio headphones designed specifically for mixing:
And here is the frequency response of another popular open studio headphones costing about $500:
Both models are professional mixing instruments, but they have a fundamentally different sound. If you rely only on your feelings, then mixing in these headphones will lead you to completely different results. In the first case, you run the risk of getting a mix that is too bright, since when mixing you will try to compensate for the lack of high frequencies. In the second case, you will try to compensate for excessive brightness and get a dull and unimpressive mix.
3️⃣ Why do you need to correct the frequency response of headphones?
There are several benefits of correcting the frequency response of headphones, and we will look at each of them in more detail.

Tone fidelity
The most obvious benefit of correcting the frequency response of headphones is to improve the fidelity of instrument sounds over the entire frequency range. If the frequency response of your monitoring is non-linear, then at some frequencies the monitoring will increase, and at others it will attenuate relative to the normal level. You will have a constant desire to compensate for this by equalizing in the opposite direction. When listening to your tracks in the right acoustic conditions, you may find yourself over-boosting at frequencies where your headphones have dips and cutting too much where your headphones give rise.

Balance of instruments in the mix
An unbalanced headphone frequency response can lead to a distorted representation of the mix, resulting in the end result not sounding as intended. For example, if your headphones are swamped with mids, then you will have a tendency to push vocals and solos into the mix too high. To avoid errors in musical balance, it is important to use headphones with a flat frequency response, which will allow you to get an objective view of the mix and help avoid unpleasant surprises when playing on other systems.

Low frequencies and bass
One of the biggest problems with headphones is that they often have low-frequency inaccuracies. Some headphones will have too much bass, others will have too little. A flat frequency response can help improve the transmission of this frequency range.

Frequency masking and detail
Dips/boosts in the frequency response can cause some elements of the mix to be perceived as fuzzy, blurry, or completely out of audible against the backdrop of headphone frequency resonances. This phenomenon is called “frequency masking”. Because of the masking, hearing fatigue increases and there is a constant desire to increase the monitoring volume, which creates an additional load on the ears. It also leads to poor mixing decisions, as important details may be missed. In headphones with a flat frequency response, the auditory masking caused by spectral peaks and dips is eliminated, resulting in increased detail, reduced hearing fatigue and errors.

The creative component of mixing
Among sound engineers, distrust of the sound of headphones is common. Producers very often like to use references when working and try to copy their sound using analyzers, instead of just listening to music and creating “with the flow”. Each track is individual and to keep this uniqueness, creative solutions are needed, and they must be based on ear and inner feelings. To make such decisions productively, you need to trust your monitoring. If the monitoring “lies”, then the depth of creativity tends to zero. Undoubtedly, you can copy someone else's idea, but unfortunately, your own sound solutions in your track will not be enough to distinguish it from the background of a huge mass of other compositions.

Realphones corrects the frequency response of headphones, which, combined with binaural emulation of studio acoustics, makes them a reliable monitoring tool, allowing you to get rid of common headphone mixing errors and develop creative approach in sound engineering work.
4️⃣ How is the frequency response measured and calibrated in dSONIQ?
In order to control the frequency response of monitors in the studio, it is enough to use a high-quality measuring microphone. However, when it comes to headphones, things get much more complicated.

The frequency response of the headphones is formed only after they were put on the ears and took up the working position. This is due to the tightness of the ear pads to the listener's head, the interaction of the speakers with the acoustic resonator of the headphone cups, as well as with the listener's auricle. Many years of trial and error have led us to the conclusion that to obtain the most accurate results it is necessary to directly measure the auditory sensations, because even the most advanced microphones are not able to fully reproduce the real acoustic conditions of the ear and reconstruct the acoustic signal that hits the eardrum.

The dSONIQ team has taken many frequency response measurements of various headphone models and developed a sophisticated approach that includes not only complex microphone measurements but also specific hearing tests. Through testing with different frequency signals and a clear methodology, we have obtained results that most closely match the human auditory experience. Due to the sensitivity of human hearing and careful tuning of the reference speaker system, the dSONIQ measurement error is estimated to be within ±1 dB. The level step at which an engineer hears a difference is only about 0.3 dB. In addition, dSONIQ technology takes into account the individual characteristics of the ears of our engineers, subtracting them from measurements and minimizing their impact on the final result.

Based on this data, two generic correction profiles were created for closed and open headphone models, as well as standard correction profiles for more than 200 headphone models.

Generic profiles allow you to use Realphones emulation for closed and open headphones of any model, but to take advantage of headphone frequency response equalization, you must use the standard correction profiles for supported headphone models.

Standard correction profiles are based on the average frequency response of several pairs of the same model. Although headphones of the same model are manufactured using the same technology and quality control, manufacturing tolerances can always affect their frequency response. This can lead to variations from pair to pair, as well as a difference between the left and right speaker. The spread between instances can reach 6 dB (±3 dB) or more.
The graph shows the frequency response curves measured for several pairs of headphones of the same model
5️⃣ But what about the Harman curve?
We are constantly asked by advanced users about how to apply the Harman curve in Realphones.

In simple terms, the Harman curve is the extra bass boost and treble roll-off that is applied to a flat, corrected headphone frequency response in order to make the hearing panel feel closer to the sound of a flat speaker in a well-sound room. Emulation of binaural phenomena and acoustics of the location will be replaced by this curve.

In Realphones, frequency response correction is designed to be used in conjunction with room emulation, and a similar effect occurs when emulation is enabled. Therefore, additional Target-curves from third-party studies do not need to be applied. If you're interested in the details, we've written a separate article on the Harman Curve.
6️⃣ Conclusions
Correcting the frequency response in conjunction with the emulation of the studio acoustic environment and reflections is an extremely important step to improve the quality of monitoring in headphones. Many experienced sound engineers argue that it is necessary to use headphones without software enhancements, as they were intended by the manufacturers. After all, it’s not in vain that they conduct research and develop new models for years? It is not that simple.

Headphone frequency response is formed due to many factors that engineers cannot influence sufficiently to guarantee a flat frequency response. The manufacturer may change the design of the speakers, the headphone housing and the acoustic resonators of the cups. Each of these elements has many characteristics that also affect each other. But just a linear frequency response by itself will not give the desired result when mixing and will not be able to provide you with the necessary translation. This is where the design features of the headphones come into play, due to which the acoustic environment in them is fundamentally different from the reference listening conditions for acoustic systems.

In the case of studio monitors, linear frequency response and neutral sounding is a much more predictable task than with headphones. Unfortunately, however, studio monitor manufacturers cannot influence the acoustics of your room. In this case, it directly affects the linearity of the frequency response. No matter how reliable the monitors are “from the factory”, unfavourable room acoustics can ruin everything.

All of the above does not negate the fact that in headphones you can mix a mix with high quality without correcting the amplitude-frequency characteristic and emulating space. But, unfortunately, a high-quality and desired result requires a lot of experience, but not everyone is able and willing to adapt to this. The labour and time costs of creating a mix increase significantly, because the sound engineer will have to constantly make adjustments for unwanted sound colouration, unreliable transmission of timbres and space during monitoring, which greatly interferes with the creative process and productivity. And if you're just starting out in mixing and don't have an understanding of reliable monitoring, then the chances of getting the desired result are obviously reduced.

Studio headphones have been around for decades and were originally conceived as a way to complement monitors, not replace them. Based on our experience, we have found that the use of headphones for mixing can be greatly expanded with software. We assume that the manufacturers did not think about the fact that the emulation of the space of the studio and other rooms can drastically affect the capabilities of the headphones.

We are successfully cooperating with some manufacturers and we want to note that they have already begun to pay attention to this. For example, companies such as ESI and FLUID Audio already use Realphones technology with their headphones and encourage their users to use our software security.

Time does not stand still and now you have a great opportunity to use your headphones as a full-fledged alternative to studio monitors. We hope that this article has helped you gain a deeper understanding of the topic of frequency response and the effectiveness of its correction.
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