Why are the frequency response of headphones so curves

How to read the frequency response of headphones correctly? What affects it, what frequency response is perceived as flat, what is the Harman curve? Finally, how to correct their sound in a minute and without an equalizer? More on this later.

Headphone frequency response problem
With the frequency response of the speakers, everything is simple – the smoother it is, the more neutral the sound will be in the end. However, in order to obtain maximum neutrality, you will need to drown out the room with a few cubic meters of mineral wool. Not an option for everyone.

And here headphones come to the rescue, which take the influence of the room out of the brackets. However, if you look at their frequency response, then almost always on the graph there will be a roller coaster with giant humps and dips. For example, below is the frequency response of the Beyerdynamic DT 990.

Wired headphones Beyerdynamic DT 990 (250 Ohm) gray

Why, despite the frequency response curve, are they used by many sound engineers to mix music? How to read their frequency response correctly and correctly predict which model is best suited to your tastes? And most importantly, how to correct the sound?

If you start to understand, then with headphones everything turns out to be much more complicated than with speakers. Their timbre balance is influenced by a bunch of things – from the design itself and the ear pads to the structure of the auricle, the volume level of listening to music, listening conditions and even the age of the listener.

Influence of the head and ears
If you sit a person in front of a sound source, then the audio waves will bend around the head and shoulders with distortion. Researchers have found that there is a whole set of body-related distortions called the Head-related transfer function (HRTF):

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1. The influence of the head itself. It drowns out the high frequencies, but does not create problems for the mids and lows.

2. Shoulders and neck. They also create acoustic shade.

3-4. Auricle. It is not just that it has a complex shape – it is easier for a person to localize sound this way. However, the shape of the ears distorts the frequency response of the sound wave – it raises the upper middle and smoothes the highest frequencies.

5. The auditory canal. Together with the earpiece, it forms a closed system in which resonances appear.

 

HRTF will be slightly different for the left and right ear. The values ​​will change depending on the direction the sound is coming from (waves blowing directly into the face behave differently than waves blowing sideways). For covering headphones, you need to simulate the influence of the head and shoulders, for in-ear headphones, you will have to add an imitation of the influence of the auricle.

All these studies resulted in compensating frequency response curves taking into account HRTF. They are sensitive to the position of the source. The speakers are usually placed in front of the listener and form a triangle with him. Headphone drivers shoot directly into the ear and form a straight line with the listener. Therefore, there are several compensating curves, depending on the type of measurements:

 

If you put a person in front of a column that blows directly into him, you get the Free Field, FF curve .
If you put a person in a room with several speakers pointing in different directions, but not at the listener, you get a Diffuse Field, DF curve .
In total, a sine with a flat frequency response on the approach to the eardrum turns into one of the graphs above. The frequency response of the headphones is measured on a mannequin with ears with built-in microphones. Therefore, to get the correct measurements, you need to subtract the effect of the artificial head from the raw graph using one of the curves above. The smoother the final graph, the smoother the headphones will be.

 

For example, one of the most sound-neutral Sennheiser HD 600 headphones, used in many studios as a reference. On the left is a raw chart without compensation, on the right – with compensation.

Firstly, all this introduces obvious confusion in understanding the frequency response graphs – it is not always indicated how measurements were taken, and whether compensation was applied. Secondly, even on the compensated graph, one can see a 1-2 kHz dip and irregularity at the top. Failure is an imitation of the influence of the shoulders and torso, and irregularity is the resonances that occur in the closed space formed by the ear and earpiece. A slight irregularity is the norm, it is not felt by ear.

Sennheiser HD 600 Wired Headphones gray

[2.0, female, 12Hz – 40500Hz, 300Ω, wired, cable – 3m, gray / blue]
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The influence of preferences and the Harman curve
Experimenting with HRTF misses an important thing – taste preferences. In the end, sound neutrality and balance are subjective things. Therefore, at the beginning of the 2000s, engineers Todd Welty and Sean Oliver Harman, since the tenths, have been conducting research on groups of listeners to find out which frequency response will be perceived as the most neutral and balanced, but taking into account the preferences of the majority.

 

For the experiments, the method of double-blind listening with different headphones was used. The listeners did not know their model or what they looked like. They even wanted to freeze their ears and whiskey to exclude tactile sensations, but the lawyers did not allow it. Based on the preferences of the listeners, an averaged curve was derived, which now bears the name of Harman. It is slightly different for in-ear and in-ear headphones.

 

64% of listeners preferred headphones with a curve close to the obtained one – it seemed to them the most even, neutral and balanced.
15% of listeners, mostly young and middle-aged men, preferred a 3-6 dB boost in bass starting at 300 Hz relative to this curve, and a 1 dB boost above 1 kHz.
21% of listeners, most of whom are women and people over 50, would prefer, on the contrary, to turn down the bass by 2-3 dB, but they would also add 1 dB to everything above 1 kHz.
After the publication of the research, many manufacturers have released models that are tuned to this curve. Of the in-ear headphones, these are JBL Live 200, 500, and 650, Samsung Galaxy Buds, JBL Reflect Flow, the most famous of the covering headphones are AKG N700 NC, K361, and K371.

 

Frequency response of Galaxy Buds + tuned along the Harman curve

TWS Headphones Samsung Galaxy Buds + Black

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Disputes about whether to focus on this curve when creating headphones do not subside, because the frequency response of some models known and loved by listeners strongly disagree with it. Some people think that the bass is too high in the curve, and a drop of more than 10 dB in the middle frequencies strongly colors the sound.

Influence of psychoacoustics
Our hearing is not linear and is most sensitive to the 1-5 kHz region, and the least sensitive to the lowest and highest frequencies. For example, a sound at 3 kHz with a volume of 20 dB will feel the same as a low frequency hum at 60 kHz and a loudness of 50 dB. In other words, at low volumes, bass and treble are less picked up. However, this sensitivity changes with the volume of the sound. At a volume of 100 dB, the perception becomes almost linear. It should be noted here that, according to the WHO , headphones are usually listened to at a volume of 75–105 dB.