the direction of sound
Oto-kun
Yesterday, there was a fire drill at school. When we were gathered in the gymnasium, a broadcast was made, and the teacher instructed us to evacuate in the direction of the sound. Since it was everyone's first time, we were a bit confused at first, but in the end, we were able to split into two groups and evacuate from the middle of the gym, so the drill seemed to be a success.
father
I heard that the school has introduced an evacuation guidance system that utilizes the precedence effect of sound, so that was the training.
Oto-kun
Preceding sound effect?
father
Humans perceive the direction of a sound source based on the direction of the first sound that arrives when sounds come from various directions. The use of the precedence effect in evacuation guidance systems is intended for situations where there are multiple exits and it is unclear which one is safe, or when the orientation is not clearly understood. By guiding people to a location through sound, it allows for quicker evacuation to a safer place.
Oto-kun
Even if there are several sounds, does it mean that we feel the direction of the sound based on the first sound that arrives?
father
I think you might not know the term "localization," but when you are at a position where the distance from both speakers in a stereo setup is almost equal, the sound seems to come from the center, right? This is called sound localization at the center position between both speakers. However, if you move a little closer to one of the speakers, the sound will localize to the nearer speaker. This happens because the sound from the closer speaker arrives sooner.
Oto-kun
Does the volume of sound not matter?
father
You noticed a good point. In fact, the volume of the sound is also related, as well as the time difference between the first sound and the next sound. Depending on the type of sound source, if the delay time is within 30ms and the next sound is not more than 10dB louder than the first sound, this precedence effect holds.
Oto-kun
That means the speaker system in the school gymnasium has such a mechanism.
father
That's right. For the guidance to function effectively, the conditions for leading sound localization must be met, meaning that the level of the leading sound must be designed so that it does not exceed 10dB below the sound from the speaker, regardless of how close it is to that speaker.
Oto-kun
During training, the student who was in the middle was unsure about which way to go.
father
If there is only one exit, it is relatively easy to set conditions, but conversely, concentrating on one exit can also lead to delays in evacuation. In the event of a fire, it seems necessary to have a system that monitors in real-time which exit is the safest.
Oto-kun
I've heard that you can tell which direction the sound is coming from because you have two ears...
father
The direction in which a sound is localized depends on the time difference of the sound entering both ears, and it's a bit complicated, but it also involves the head-related transfer function, which means the shape of the head, including the ears, contributes to it. A sound coming from the right reaches the right ear directly, but it will reach the left ear after wrapping around the head. I told you about diffraction in the context of soundproof walls, remember?
Oto-kun
In the case of soundproof walls, even if the sound source is not directly visible, it was mentioned that due to diffraction, lower frequencies are more likely to reach the listener.
father
That's right. In the case of the head, sound diffracts around it to reach the ears that cannot be seen directly, but the relationship between the ears and the head changes depending on the angle at which the sound enters, so the effect of diffraction varies with the angle of incidence.
Oto-kun
So humans can tell the direction of sound even with their eyes closed.
father
That's right. This story is called the 'binaural effect,' and there is a more interesting story about it, but let's save that for next time.
Oto-kun
Well, tell me about the both-ear effect next time. Are there devices that can measure sound and tell where it's coming from?
father
There are several methods, some of which are used as measuring instruments. First, there is a system called acoustic intensity, which measures the flow of sound energy (it approximates intensity by measuring the sound pressure of closely placed microphones). Recently, techniques such as beamforming, which involves arranging multiple microphones in a planar configuration for measurement, and acoustic holography have also been employed.
Oto-kun
There are various things, right? Is that used to investigate where the factory noise is coming from and where the sound is leaking from?
father
That's right, in order to reduce noise generated by machines and address sound leakage from building walls, it is necessary to determine where the sound is coming from, identifying the source and any soundproofing deficiencies. In such cases, depending on the characteristics of the sound from the target machine (such as volume, pitch, whether it is a steady sound or a transient sound, etc.), we conduct sound source exploration using suitable methods.
Look at this. This is the result of measuring, analyzing, and visualizing the sound generated by a car's tires in contact with the road using 100 microphones and a method called near-field acoustic holography.
Oto-kun
Do you use as many as 100 microphones?
father
By using many microphones and measuring at once, it is possible to cover a certain area and analyze in detail which parts have louder sounds. Humans have excellent ears that serve as sound sensors in some respects, utilizing characteristics like the precedence effect, while at the same time, devices like these are used to distinguish sounds that occur in complex and subtle ways.