binaural effect

father

Today, I will talk about the 'both ears effect' that I touched on briefly the other day.

Oto-kun

You told me before about how the time difference of sounds entering both ears and the shape of the head are related. Then, when you are equidistant from the two speakers of a stereo system, the sound is localized in the center, and when you move closer to one speaker, the localization of the sound source shifts to the nearer speaker.

father

Even if there are several sound sources, perceiving the position of the sound source from the sound that reaches the ear first is called the "preceding sound effect," and it might be better to say that the direction can be localized due to the "binaural effect." (Figure 1)
Also, the sensation of feeling the spatial expansion is perceived because the ears are located on both sides of the head, and this is also referred to as the "binaural effect."

Oto-kun

Are you talking about the acoustics in a concert hall when you mention the expansion of space?

father

The resonance of space can still be felt even if one ear is blocked, right? It gets a bit technical, but it is understood that there are two types of perception regarding the "sense of expansion" in space. One is the "feeling of being enveloped by sound," which is closer to the image of spatial expansion. The other is the sensation of the sound source itself being expanded, which is referred to as the "apparent width of the sound source." Both perceptions are influenced by the structure of the reflected sound that arrives after the direct sound (its direction, time, and magnitude). (Figure 2)

Oto-kun

It's kind of difficult, isn't it? Since we have two ears, it's easy to imagine the direction from which sound comes.

father

That's right. Then, let's consider a situation with a single sound source in a room with no echoes (anechoic chamber). When the sound comes from directly in front, there is no temporal gap in the sound waves entering both ears, so it feels like the sound source is directly in front. However, the same applies when the sound comes from directly behind, so the lack of time difference means that sounds from behind can often be mistaken for sounds coming from the front.

Oto-kun

Oh, really? I don't think I've ever experienced mistaking a sound coming from directly behind me for one coming from the front.

father

In a normal room, one reason is that reflected sounds from walls and ceilings, other than sounds coming directly from behind, contribute to the information about sounds coming from the back. Another reason is that sounds almost always carry meaning, so when we hear a sound, we immediately assess it in conjunction with the information about our surrounding environment that we have grasped in our daily lives.

Oto-kun

An experiment that presents sound in a laboratory without any information other than sound, and where there are no reflected sounds from walls or ceilings, is indeed different from what we usually experience.

father

That's right. However, in experiments like this, we need to eliminate uncertain factors as much as possible and keep things simple, otherwise we won't be able to understand the truth. It's necessary to integrate what has been clarified in each experiment to explain what is happening in real-world environments.

Oto-kun

I see. How about outside of directly facing it?

father

"That's right. Otoha, how well do you think people can distinguish the direction of sound based on the angle?"

Oto-kun

I don't really understand, but maybe around 10 degrees?

father

In fact, in directions close to the front, there is a resolution of about 1 to 3 degrees depending on the frequency. This is quite remarkable, as the time difference of sound reaching both ears is only about 0.0001 seconds (10 μs).

Oto-kun

You can distinguish such fine angles, huh? What happens when you move away from the front?

father

When the angle is about 60 degrees from the front, the accuracy of directional localization significantly decreases, and from the side, it can only detect within a width of about 40 degrees.

Oto-kun

Wow, it really changes that much from the front to the side.

father

The reason is that the lateral differences in the head-related transfer function, which I explained earlier, become larger when the sound is coming from directly to the side. Therefore, even if the angle of incidence changes slightly, the difference in lateral perception becomes smaller compared to when the sound is coming from directly in front. Additionally, when a person tries to determine the direction of a sound, they tend to turn towards it, which may naturally increase sensitivity to sounds coming from the front. An important point is that, although I haven't discussed it until now, the cues for distinguishing direction are both time differences and sound pressure differences.

Oto-kun

Is the difference in sound pressure due to the different ways sound waves diffract on the left and right sides, which causes a difference in sound pressure?

father

That's right. The two clues are related to frequency, and it is said that the time difference is effective below 1500 Hz, while the sound pressure difference is effective above 1500 Hz.

Oto-kun

It's complicated, isn't it? As expected, it's necessary to gradually clarify things in a place like a soundproof room.

father

That's right. We talked about the cocktail party effect before, didn't we? This also contributes to selective hearing, as the ability to determine direction with both ears plays a role. Research on this kind of directional localization has a wide range of applications, including in robotics.