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What kind of sound is solid-state sound (sound of solid propagation)? How does it differ from airborne sound (airborne sound)?

Have you ever heard of solid-state sound? It is officially called solid sound propagation, but it may not be well known to the general public. In this issue of Tidbits, we will discuss solid sound and explain how it differs from airborne sound, which is another type of sound propagation, in an easy-to-understand manner.

What is solid sound [difference from air sound]?

Sound is classified into two types: airborne sound and solid sound. Airborne sound, officially called airborne (propagating) sound, is sound transmitted through the air as vibrations, including those transmitted through windows and walls. Solid-state sound, on the other hand, refers to sound that is transmitted as vibrations through a solid (such as the structure of a building) and radiated from a wall or other surface at the destination.

Solid and airborne sound in buildings

Figures 1 to 3 show the sound heard in a building, classified into solid sound and air sound.
Airborne noise includes human voices, machine motors, and external traffic noise. Airborne sound includes sounds that are directly transmitted through the air from a sound source as well as sounds that penetrate windows, walls, etc.

Solid-state sound, on the other hand, is the sound of vibrations transmitted to the structural parts of a building (columns, beams, slabs, etc.) and heard through the floor and wall surfaces. Solid sounds include footsteps on the floor above, children bouncing on the floor above, machine motors, and the sound of trains outside. Of these, the sound of machine motors and the sound of trains outside may be both air and solid sound coming into the building, but the sound radiated from the vibrations being transmitted is individual sound. Other likely sources of solid sound include door opening and closing vibrations, vibrations from percussion instruments such as pianos and drums, and vibrations emanating from outside construction and factories.

Incidentally, since propagation was originally written as propagation, we often see the notation "solid propagation sound," but as for the name of solid sound, the new edition of the Acoustic Dictionary describes it as solid propagation sound*.
Acoustical Society of Japan, ed. New Acoustic Dictionary. 2003. 127p. Corona Publishing Co. 2003. 127p.

建屋内の空気音と固体音例(人から発生する音)

Figure 1 Example of air and solid sounds in a building (sounds generated by people)

Figure 2 Example of air and solid sounds in a building (machine operating noise)

建屋内の空気音と固体音例(屋外交通騒音)

Figure 3 Example of air and solid sound inside a building (outdoor traffic noise)

Solid-state sound characteristics

Solid sound is characterized by the fact that vibrations travel through the structural parts of a building and are transmitted farther than is possible with air sound. Furthermore, the amount of vibration radiated as sound varies greatly depending on the material, shape, and method of fixing the surface radiating the sound. As a result, sound may or may not be heard from the same vibration source over the same distance, simply because the radiating surface is different. In addition, solid sound travels through the structure in various directions, so there are cases where sound is heard from the ceiling of the upper floor even though the vibration source is on the lower floor, making the propagation path complicated and the source of the vibration difficult to identify.

Solid Sound Countermeasures

Solid sound countermeasures have problems: the source of vibration is difficult to identify, there are many propagation paths, and it is necessary to decide which path to prioritize for countermeasures. Therefore, solid sound countermeasures require more know-how than air sound countermeasures. In addition, solid sound countermeasures require a different approach than air sound countermeasures. For example, effective measures against airborne sound, such as separating the sound from the distance or installing soundproofing walls, may not be effective for solid sound. The basis of solid sound control is to reduce vibration, not sound.
The basic concepts of solid sound control measures are 1~3. In general, it is necessary to plan the most appropriate countermeasures based on these concepts while taking into consideration the conditions of the building and other factors.

  1. Reduce the vibrating motion or force (excitation force) itself.
  2. Make the structural (solid) part where vibration is transmitted heavier and less prone to vibration.
  3. Use anti-vibration materials in the path of transmission to isolate and reduce vibration.

Incidentally, a common example of countermeasure failure is when air noise and solid sound are generated simultaneously, and countermeasures are taken only for the air noise, resulting in the air noise being reduced, but the solid sound remaining as it is. Although there is no single method for solid sound countermeasures, here is an example of a countermeasure we have taken to reduce the transmission of bouncing sound from the gymnasium floor to the floor below.
Example of gymnasium floor installation using vibration isolator (rubber) for heavy floor impact noise