Direction of sound

Yesterday we had an evacuation drill at school. We were all gathered in the gymnasium when an announcement came on, and the teacher told us to evacuate in the direction from which the sound was coming. It was the first time for all of us, so we didn't really understand what was going on at first, but in the end we were able to split into two groups and evacuate from around the middle of the gymnasium, so it seems the drill was a success.

I heard that Oto's school has introduced an evacuation guidance system that uses pre-emptive sound effects, so that must have been a drill for that.

Preceding sound effect?

Humans perceive the direction of a sound source based on the direction of the first sound they hear when sounds are coming from various directions. The use of the lead sound effect in evacuation guidance systems is intended to help people evacuate to a safer location in a shorter time by guiding them to a safer place, especially when there are several exits and it's unclear which one is the safest, or when they are unsure of their direction.

So, even if there are several sounds, you perceive the direction of the first sound that arrives?

You probably don't know the term "localization" yet, but in stereo, when you're roughly equal in distance from the speakers, the sound seems to come from the center, right? This is because the sound is localized at the midpoint between both speakers. However, if you move a little closer to one speaker, the sound will localize towards the closer speaker. That's because the sound from the closer speaker arrives faster.

Does the volume of the sound not matter?

That's a good point. Actually, the volume of the sound is a factor, as is the time difference between the first and second sounds. Depending on the type of sound source, if the delay is within 30ms and the second sound is no more than 10dB louder than the first sound, this lead-to-sound effect will work.

So that means the speaker system in the school gymnasium has some kind of mechanism in place.

That makes sense. For induction to work properly, the conditions for precedent sound localization must be met, so the design must ensure that the level of the precedent sound, no matter which speaker it approaches, is never 10dB lower than the sound of that speaker.

During the drill, one of the students in the middle seemed unsure which way to go.

If there's only one exit, it's relatively easy to determine the conditions, but conversely, if everyone is concentrated in one place, evacuation can take longer. In the event of a fire, for example, it would be necessary to link it with a system that monitors in real time which exit is the safest.

I've heard that we can tell which direction a sound is coming from because we have two ears...

The direction in which a sound is localized depends on the time difference between the sounds entering both ears, and, although it's a bit complicated, also on the shape of the head, including the ears, which is called the head-related transfer function. A sound coming from the right will reach the right ear directly, but will reach the left ear after bending around the head. I explained diffraction to you before when we were talking about sound barriers, remember?

In the case of sound barriers, even if you can't directly see the sound source, lower frequencies can still reach you due to diffraction, right?

That's right. In the case of the head, the sound diffracts around it and reaches the ear, which is not directly visible, but the relationship between the ear and the head differs depending on the angle at which the sound is incident, so the effect of diffraction changes depending on the angle of incidence of the sound.

That's why humans can tell the direction from which a sound is coming, even with their eyes closed.

That's right. This is called the binaural effect, and there's an even more interesting story about it, but let's save that for next time.

Okay, then please tell me about the binaural effect another time. Are there any devices that can measure sound and determine where a sound is coming from?

There are several methods, and some are even used as measuring instruments. First, there's acoustic intensity, a system that measures the flow of sound energy (it measures the sound pressure of nearby microphones and calculates the intensity approximately). More recently, beamforming, a method that uses multiple microphones arranged in a plane for measurement, and acoustic holography are also being used.

There are all sorts of things, huh? Is that used to investigate where factory noise is coming from, or where sound is leaking from?

That's right. To reduce noise generated by machinery or to prevent sound leakage from building walls, it's necessary to pinpoint the source of the sound and identify areas with sound insulation defects. In such cases, the sound source is investigated using methods appropriate to the characteristics of the sound coming from the machinery in question (loudness, pitch, whether it's a steady or transient sound, etc.).

 

Take a look. This is the result of measuring, analyzing, and visualizing the parts of the sound generated when a car's tires contact the road surface while it's moving, using a technique called near-field acoustic holography with 100 microphones.

Are you going to use 100 microphones?

By using many microphones and taking measurements simultaneously, we can cover a certain area and analyze in detail which parts of the sound are loudest. Humans have excellent ears as sound sensors in some respects, and while sound is used in evacuation systems by utilizing properties such as the precedence effect, these kinds of devices are used at the same time to distinguish between complex and subtle sounds occurring in different locations.