In this blog post, we’ll explore the problem of constant noise in our surroundings, the acoustic materials used to reduce it, and transmission loss (TL), which indicates the performance of these materials.
There are young children living upstairs from my apartment. Their parents must both work, because these little ones are always home alone. The problem is that these energetic little ones run all over the house every evening, creating an enormous amount of noise. This noise problem doesn’t end there. Just step outside onto the street, and you’ll hear a variety of sounds coming from all directions. The noise continues both when the subway—the public transportation used by South Koreans—arrives and after you’ve ridden it. The noise inside the subway is also louder than you might think. Even when we aren’t aware of it, noise is constantly being generated around us. Just like the song “Christmas Is All Around” from the soundtrack of the movie ‘Love Actually’, which was released a long time ago, noise is always all around us. So, how can we escape this noise? The best solution would be to avoid creating noise in the first place, but there are times when noise is unavoidable. If we’re bothered by noise even at home—a place where we’re supposed to relax with peace of mind—then that home cannot be called a comfortable one. Engineers and architects conduct extensive research to solve these problems. Anyone who has visited a music room will notice that the walls are different from those in an ordinary room. These walls are made of special materials that prevent sound from inside the music room from leaking out. Materials designed to reduce noise in this way are called acoustic materials, and they are used not only in rooms and homes but also in various other fields to reduce or block noise. Thus, acoustic materials serve a role similar to that of a breakwater, which reduces the impact of noise—caused by waves rolling in from afar—on ears along the coast. In fact, when constructing apartment buildings, acoustic materials are incorporated into the building’s spatial structure, the interior of the walls, and even the interior design to minimize noise. So, what makes a good acoustic material? Since the purpose of acoustic materials is to reduce noise, it is clear that the more noise they reduce, the better the material is. The metric that indicates this “degree of noise reduction” is called TL. TL stands for “Transmission Loss.” Therefore, the higher the TL value, the better the acoustic material. Let’s take a brief look at the experiment used to measure TL. To measure TL, prepare a tube like the one shown below. This tube is used specifically for measuring TL and is called an impedance tube.
Point A is a speaker, from which sound is emitted. The acoustic material to be tested is placed at point C. Next, a microphone at point B measures the sound level before it passes through the tube, and the sound level at point D is measured after it has passed through. Since TL represents the degree of sound level reduction before and after the sound passes through the specimen, we can calculate TL in this way. In fact, because sound can leak out of the tube as it propagates, precise experiments are conducted in a “reverberation chamber”—a room where the sound reflection rate is nearly 100%. However, for simple experiments, an impedance tube is sufficient. Through these TL measurements, we select appropriate acoustic materials for specific types of noise. Yet, there is no such thing as a perfect acoustic material. This is because noise does not consist of a single tone but encompasses a wide range of frequencies, including high and low pitches. Pitch is proportional to sound frequency, and finding an acoustic material capable of blocking all these different frequencies is nearly impossible. Furthermore, the noise generated around us is produced at various frequencies in diverse environments. Nevertheless, through these engineering solutions, we are able to live with noise a little more comfortably.
