[Sound Science #61] A difference of just '1mm' changes the sound—The scientific reason why the same earphones sound different to different people
Hello, this is NUARL!
It is a pleasant time to take a walk while listening to music and breathing in the fresh air for a little change of pace in your daily life.
However, have you ever felt while walking, 'Huh? The bass sounds thinner than when I was listening at home,' or 'It feels like the sound has become a bit more distant.' Many people have likely had the experience of adjusting their earphones after feeling that way.
Actually, that is not just your imagination. This time, we will unravel the mystery of our 'surprising ear canal sensitivity' and 'individual differences in sound caused by a few millimeters of misalignment.'
This may be sudden, but have you ever had the experience of actually listening to earphones that are highly praised in online reviews and feeling, 'Huh? They don't sound that good to me...'?
Actually, this is not your ears' fault, nor is it the earphones' fault. 'Even when the same earphones are played at the same volume, the sound that actually reaches the eardrum is completely different for each person' is an acoustical fact.
This time, we will unravel the mystery of our 'surprising ear canal sensitivity' and 'individual differences in sound' while incorporating scientific research data.
'Ear canals' are small musical instruments
When we wear canal-type (earplug-type) earphones, the ear canal becomes a 'small, closed tunnel' with the eardrum as a dead end.
In acoustics, a tube closed at one end like this causes a phenomenon called 'quarter-wave resonance,' which amplifies sounds of specific frequencies. At this time, the resonant frequency f is determined by the following calculation formula based on the tube length L and the speed of sound c.
$$
f = \frac{c}{4L}
$$
In other words, if the length of the 'tube' that is the ear canal changes, the sound that is emphasized also changes. A study by Souza et al. at the University of Washington published in 2014 shows a surprising fact.
The sound changes just by the insertion position of the earphone tip (such as the earpiece) shifting by a 'few millimeters' inside the ear canal.
The sound pressure level in specific high-frequency ranges (3–4 kHz and above) fluctuates by as much as 20 dB (100 times in terms of sound energy).
The reason why certain sounds can feel piercing or, conversely, muffled just by the earphone being pushed 1mm deeper or pulled slightly outward is due to this minute change in physical space.
The 'shape of the ear canal' is unique to everyone
What is even more decisive is that the shape of our ear canals (thickness, length, degree of curvature, and volume) is, 'a shape unique in the world,' just like a fingerprint.
Even if the same earphones are inserted to the same depth, there is a large difference in the sound pressure generated inside (especially in the low-frequency range) between people with narrow ear canals and those with wide ones. Also, if the distance to the eardrum is different, the high-frequency bands emphasized by the aforementioned resonance will also shift for each person.
Earphones designed so that the sound reaches Person A's eardrum as a 'well-balanced, clear sound' are converted into 'harsh high-frequency sound' or 'overpowering bass' when passing through Person B's ear canal. This is the biggest reason why 'the way it sounds is completely different even under the same conditions.'
There is no such thing as perfect sound for everyone
Since everyone's ear shape is unique, it is physically impossible to create 'earphones that sound perfect to everyone in the world in exactly the same way.' That is precisely why we tackle this hurdle of 'individual differences' with various approaches.
Physical approach Our wired model, the 'NX1 Chapter2,' adopts a mechanism that allows you to replace the earphone tip (nozzle) itself. This allows you to physically fine-tune the volume inside the ear canal and the distance to the eardrum (the aforementioned tube length L), optimizing the sound source position and resonance balance for your own ears.
Digital technology approach The 'Audiodo Personal Sound' feature included in the 'Inovatör' measures individual hearing characteristics and corrects sound in frequency ranges that are difficult to hear, ensuring they are heard correctly. This allows even discrepancies in how sound is perceived due to differences in the shape of the left and right ears to be brought closer to a neutral state through digital correction.
Being conscious of 'just 1mm' to find the best sound
And this rule that 'the difference in wearing position changes the sound' is also a very important point for open-ear earphones that do not seal the ear canal.
For products like the 'νClip,' an ear-cuff type that you wear by clipping it onto the ear cartilage, the 'angle and distance of sound radiation' from the speaker part to the ear canal changes dramatically depending on whether the clipping position is slightly higher or lower, or further back or forward.
If you feel that the 'bass is thin' or the 'sound is distant,' please try fine-tuning the wearing position. There is bound to be a 'sweet spot' unique to you where the sound resonates with surprising richness.
How you hear music is unique to you. Please try to find the most comfortable position while communicating with your own ears.
☕️Summary
To summarize what we discussed this time, it is as follows.
A shift of just 1mm changes the sound: The ear canal is like a small musical instrument, and moving the earphone position by just a few millimeters can cause certain sounds to be emphasized or muffled.
Ear shape is the same as a 'fingerprint': Everyone's ear canal shape, including volume and length, is 'one of a kind in the world.' Therefore, there is no such thing as 'perfect earphones for everyone.'
Finding your own correct answer: It is important to find the 'sweet spot' that suits your ears through physical adjustments (such as nozzle replacement), digital correction, or fine-tuning the wearing position for open-ear types.
How you hear music is unique to you. Please try to find the most comfortable position while communicating with your own ears.
Thank you for reading to the end this time as well!
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📚[References]
Research on changes in sound pressure level due to insertion position shifts: Souza et al. (2014). "Compensating for ear-canal acoustics when measuring otoacoustic emissions". The Journal of the Acoustical Society of America.
The mechanism of 1/4 wavelength resonance in the ear canal: Chasin, M. "The etiology of the REUG: Did we get it completely right?". Hearing Review.