Sounds in apartment buildings, Part 2: Airborne and structure-borne sounds

I told Kozo about the "impact noise" my dad told me about. Apparently, at the meeting of his apartment building's management association, the impact noise was indeed the main topic of discussion. But they couldn't reach a conclusion. It's been postponed until next time. I also told Kozo about the community, and he said that at the meeting, besides impact noise, the sounds of washing machines and vacuum cleaners were also discussed.

In an apartment building, you can sometimes hear sounds from other units that you might not expect to hear outside. Things like the sound of washing machines and vacuum cleaners, the sound of toilets flushing, and the sound of ventilation fans. Roughly speaking, there are two types of sound. One is sound that travels through the air, like voices, television sounds, and stereo sounds, and this is called "airborne sound." The other is sound (vibration) that travels through concrete, pipes, and other materials, and this is called "structure-borne sound." It's hard to imagine sound traveling through concrete, but for example, have you ever heard loud noises like "grinding" or "roaring" when there's construction work going on nearby, like at a school? This is because the vibrations (small tremors) from machines pouring concrete or drilling holes travel through the concrete and the ground to the classroom, and you hear them as "sound" coming from the walls and ceiling.
Structure-borne sound is a complicated term, but it's basically sound that originates as vibrations that travel through the air, and finally the ceiling and walls of the classroom vibrate, resulting in the sound that is produced. This type of sound is called "radiated sound." This "radiated sound" then travels through the air as "airborne sound," and finally reaches everyone's ears in the classroom.

So, the "impact noise from the floor" you told us about the other day is the same thing as what we're just talking about, right?

That's right. The "vibration" from the thud on the floor above is transmitted as "solid-borne sound" through the flooring (wooden floor finish) or concrete (floor slab), radiated from the ceiling, and that's what you hear.

So, are the sounds of washing machines and vacuum cleaners the same?

That's right. Washing machines also have feet on the floor, so when they vibrate a lot, like during the spin cycle, the vibration is transmitted to the floor (excitation), and in that sense, it's the same thing because that sound travels through the floor as "solid sound." However, both washing machines and vacuum cleaners not only generate vibrations, but they also emit sound into the room where they are being used. This sound travels as "airborne sound." But "airborne sound" is blocked by walls and floors, so it's unlikely to travel to the next apartment as "airborne sound." The sound of a television also won't be heard in the next apartment unless it's turned up very high.

Oh yeah, it seems the sound of the toilets flushing also came up as an issue at yesterday's meeting.

This is also a problem of "structure-borne sound." The sound of toilet flushing can be heard on the lower floor because the pipes (drainage vertical pipes) run through the floors of both floors, so when the sound of water draining from the upper floor apartment flows through, it can be heard as a "gurgling" sound. The sound of water flowing inside the pipes is transmitted to the pipes as "structure-borne sound," which is then transmitted to the supports holding the pipes in place and the concrete in which the pipes are embedded, and in the same way, it is radiated from the walls and other surfaces.

I see, so there are various types of "structure-borne noise" inside an apartment building, and they tend to be more problematic than "airborne noise" like TV sounds or conversations.

That's right. An apartment building is one large building, and the floors, walls, and pipes are connected to other units, so solid-borne noise is especially likely to be a problem.

Is there any way to block out solid-borne sounds?

We talked the other day about how impact noise from floors can be dealt with by attaching soft materials like felt to chair legs or laying down carpet in certain areas. As for heavy impact noise from floors, the only solution is to avoid jumping around.
Also, when it comes to washing machines and vacuum cleaners, the first thing to do is to avoid using them late at night or early in the morning.

Is it because it's quiet at that time?

That's right. It would take a long time to explain the relationship between noise and quietness, so I'll tell you about it in detail another time, but during the day and in the evening, there are quite a lot of different sounds outside, such as the sound of cars and the sound of the TV, so the noise level inside the room is surprisingly high.

Noise level?

Simply put, it's a way of representing the loudness of sounds that humans can hear. If the noise level is high, you might not hear other sounds coming in. When you want to hear something properly, you say, "Shhh, be quiet!" It's the same principle.

I see, so late at night or early in the morning, there are fewer cars and the TV isn't on, so the noise level is lower and you can hear sounds coming from other areas more clearly.

And then, let's see. For washing machines, you can take measures to prevent vibrations from being transmitted by placing vibration-damping rubber under the feet. However, not just any vibration-damping rubber will work; you need to choose the appropriate vibration-damping rubber depending on the weight of the washing machine. Vibration in the pipes can also be mitigated with vibration damping. This is a design issue, and once the building is completed, the renovation work will be extremely expensive.

I see, that makes sense. But aren't there times when you can't tell where the sound is coming from?

Besides what I've already mentioned, structure-borne sound can have many other causes of noise problems, including elevators, automatic doors opening and closing, kitchen exhaust fans, machinery in machine rooms, pump rooms, toilet sounds, parking lots, and footsteps on steel staircases. That's why, in many cases, it's impossible to determine where the sound is coming from and what's causing it in a room where noise is a problem without measuring vibrations and noise levels.
First of all, I think you have to accept that living in an apartment building means that there will be some degree of noise. And since many families live in apartment buildings sharing concrete floors, walls, and pipes, you have to be mindful of the time of day and make efforts to avoid making noise for each other.