In this blog post, we’ll explore how stereo music played through earbuds creates a sense of realistic spatial depth, examining the principles of audio technology that utilize sound directionality and auditory cues.
When listening to stereo music through earbuds, the slight difference in sound reaching each ear creates a sense of space, as if a concert hall were unfolding right in front of you. What principle underlies this effect? This sense of space is the result of artificially recreating the directionality and sense of distance of sound—which we naturally perceive in daily life—through earbuds.
The human ear detects the frequency distribution to identify the type of sound source. This goes beyond simply perceiving sound volume and timbre; it provides the ability to determine where a sound is coming from and how far away it is through various acoustic cues. However, the ear does not detect direct information that allows it to pinpoint the exact location of the sound source. To compensate for this, the human auditory system determines the location of a sound source by utilizing various cues derived from the interaction between the two ears and between each ear and the sides of the head. This process is highly complex, and through it, our brain creates a three-dimensional and vivid acoustic environment.
The location of a sound source is perceived using the horizontal and vertical directions from which the sound comes and the distance to the source; the accuracy of this perception varies depending on the location and type of the sound source, and there are significant individual differences. For example, while some people are highly sensitive to sounds coming from a specific direction, others may struggle to distinguish sounds from the same direction. This varies depending on various factors, such as auditory experience, age, and hearing status. Additionally, the distance to the sound source is estimated by utilizing the correlation between the volume and distance of familiar sounds, such as a human voice. This is based on our ability to unconsciously associate sound volume with distance in everyday life.
If the sound source is directly in front of the listener, the distance from the source to both ears is the same, so there is no time difference between the sounds reaching the two ears. This makes us perceive the sound as coming directly from in front of us. On the other hand, if the sound source is offset to the listener’s right, the sound reaches the right ear first, creating a time difference between the two ears. The greater the offset, the larger the time difference. The order of arrival and the time difference serve as important clues for determining the horizontal direction of the sound source. This principle plays a crucial role in helping us identify the locations of various sounds we hear in our surroundings.
If the sound source is at the level of the listener’s right ear, the sound will be faint in the left ear due to the obstruction of the head. This phenomenon is called “sound shadow” and occurs primarily in the high-frequency range. This is because, while high-frequency sounds are blocked by the head and do not reach the left ear well, low-frequency sounds pass over the head and reach the left ear effectively. This characteristic helps us understand how sound properties vary with frequency when we perceive the location of a sound source. The sound shadow effect is clearly evident in high-frequency sounds above 1,000 Hz but is almost nonexistent in low-frequency sounds below that threshold. This phenomenon serves as a particularly important clue for determining the horizontal direction of a high-frequency sound source.
Meanwhile, before reaching the ear canal, sound is reflected in various directions due to the interaction between the side of the head and the contours of the auricle, and these reflected sounds interfere with one another. This process plays a crucial role not only in determining the direction of sound but also in assessing distance and the height of the sound source. Even for the same sound, the effects of these interactions vary depending on the direction from which the sound reaches the ear; differences in both horizontal and vertical directions have an impact. These interactions cause a distortion in the frequency distribution, as interference causes certain frequencies to become weaker while others become stronger. This also serves as a crucial clue for determining the direction of the sound source. Furthermore, by integrating all these complex processes, our brain reconstructs the surrounding acoustic environment in three dimensions, which plays a crucial role not only in music appreciation but also in our overall experience of sound in daily life.
As such, the human auditory system is a highly sophisticated and complex system that determines the position and direction of sounds through various cues and interactions, enabling us to experience a realistic acoustic environment. It is thanks to this principle that stereo music heard through headphones provides a sense of space, making it feel as if you are in a live concert hall.
