The spatial sensation when listening to stereo music with earphones is due to the auditory system’s ability to localize sound sources based on the time difference between the sound arriving at the two ears, sound shading effects, head and auricle reflections, and interference.
When you listen to stereo music through earphones, the sound reaches your ears at slightly different frequencies, giving you a sense of space, like a concert hall spread out in front of you. How does this effect work? This sense of space is the result of artificially reproducing the directionality and distance of sound that we naturally experience in our daily lives through earphones.
The human ear recognizes the type of sound source by detecting its frequency distribution. This goes beyond simply recognizing the loudness and timbre of a sound, and gives us the ability to determine where it’s coming from and how far away it is through a variety of acoustic cues. However, it doesn’t detect any direct information about the location of the sound source. To compensate, the human auditory system uses multiple cues from the interaction between the two ears and between each ear and the side of the head to determine the location of the sound source. This process is very complex, and our brain uses it to create a three-dimensional, lifelike acoustic environment.
The location of a sound source is perceived using the horizontal and vertical direction of the sound and the distance to the sound source, but the accuracy depends on the location and type of sound source, and there are large individual differences. For example, some people are very sensitive to sounds coming from a certain direction, while others may not be able to distinguish sounds from the same direction. The distance to a sound source is also estimated using the correlation between the loudness and distance of familiar sounds, such as voices, which we unconsciously use in our daily lives. This is based on our ability to unconsciously associate loudness and distance in everyday life.
If the sound source is directly in front of the listener, the distance from the sound source to both ears is the same, so there is no difference in the time it takes for the sound to arrive at both ears. This makes us feel like the sound is coming from directly in front of us. On the other hand, if the sound source is skewed to the right of the listener, the sound will arrive at the right ear first, creating a time difference between the two ears. The greater this skew, the greater the time difference. The order of arrival and the time difference are important clues to the horizontal direction of the sound source. This principle plays a very important role in how we localize the various sounds we hear around us.
If a sound source is at the level of the listener’s right ear, it will sound smaller to the left ear because of the head. This phenomenon is called “sound shading,” and it happens mostly in the high-frequency band. This is because for high frequencies, the sound travels through the head and is blocked by the head and does not reach the left ear, whereas for low frequencies, the sound travels through the head and reaches the left ear. These characteristics lead us to understand the frequency-dependent nature of sound when we perceive the location of a sound source. The sound shadowing effect is most pronounced for high frequencies (above 1,000 Hz), but rarely for low frequencies (below that). This phenomenon is a particularly important clue for determining the horizontal orientation of high-frequency sound sources.
Meanwhile, before reaching the ear canal, sound is reflected in many directions by the interaction of the sides of the head and the flexion of the auricle, and the reflected sounds interfere with each other. This process plays an important role in determining not only the direction of sound, but also the distance, height of the sound source, etc. Even for the same sound, the effect of the interaction varies depending on the direction in which the sound reaches the ear: differences in vertical as well as horizontal direction have an effect. This interaction results in a change in the frequency distribution, as the interference makes some frequencies of sound smaller and others louder. This is also an important clue to the direction of the sound source. Furthermore, all of these complex processes come together to allow our brain to reconstruct a three-dimensional picture of the acoustic environment around us, which plays an important role not only in listening to music but also in our everyday experience of sound.
As you can see, the human auditory system is a highly sophisticated and complex system that relies on a variety of cues and interactions to determine the location and direction of sound, which in turn creates a realistic sound experience. This is why stereo music played through earphones gives you a sense of space, like you’re in a live performance.
