The most basic methods of noise control, sound absorption and sound insulation, are explained in an easy-to-understand manner.
sound absorption
Sound absorption is a phenomenon in which sound energy is converted into thermal energy by a material, thereby reducing noise by reducing sound energy. Materials suitable for sound absorption are called sound-absorbing materials.
The performance of a sound absorbing material is expressed in terms of sound absorption coefficient per frequency. The sound absorption coefficient α is expressed as the ratio of the energy of sound incident on the material to the energy of sound not reflected back (Equation 1, Figure 1). Therefore, α is a value between 0 and 1. The closer the value is to 1, the higher the sound absorption performance.
The sound absorption coefficient α of incident sound from any direction is generally measured by the sound absorption coefficient measured by JIS A 1409: 1998 "Measurement method of sound absorption coefficient by reverberation room method".
Also, as can be seen from (Equation 1), if no reflections are returned, the sound absorption coefficient is 1. In other words, apertures can be regarded as having a sound absorption coefficient of 1.
α=Ii-IrIi=1-IrIi=1-R (Equation 1)
α Sound absorption coefficient
Ii Incident wave (W/㎡)
Ir Reflected wave (W/m2)
R Reflectance ( Ir/Ii )
Figure 1 Incident and reflected waves
Sound absorbing materials can be divided into three main categories according to their sound absorption mechanism: 1. Porous type Porous means a sponge-like material with many small pores. When sound enters such a material with continuous bubbles, part of the sound energy is converted into thermal energy by friction and vibration in the pores. By thickening the sound-absorbing material or adding a layer of air behind it, the sound-absorbing performance can be increased.Generally excellent at absorbing high frequency sound.When sound strikes a material that does not allow air to escape, such as thin plywood or canvas, plate vibration or membrane vibration occurs, and part of the sound energy is consumed by internal friction.Generally absorbs low frequency sound, but its sound absorption rate is not so good.3. for sounds in the frequency band close to the resonance frequency of the resonator-type resonator, the area around the hole vibrates violently, and sound energy is consumed as frictional heat.Generally, high sound absorption coefficient can be obtained only near the resonance frequency.
Sound absorbing materials used as building materials are either of these three types or a combination of them. By selecting a sound-absorbing material according to the frequency characteristics of the sound you want to absorb, you can achieve the appropriate effect.
sound insulation
Sound insulation is to block the propagation of sound by means of a material, and the material to be blocked is called a sound insulation material. As shown in Equation 2 and Figure 2, sound insulation performance is expressed as sound transmission loss TL (dB) per frequency based on sound transmission coefficient τ (ratio of energy of incident sound to that of transmitted sound through the material). The sound transmission loss (TL), like the sound absorption coefficient (α), depends on the incident sound condition to the material, so the TL for incident sound from any direction is generally obtained from the measurement specified in JISA1416:2000 "Method of measuring airborne sound insulation performance of building components in a laboratory.
TL=10 log10 IiIt=10 log10 1τ (Equation 2)
TL Sound Transmission Loss (dB)
Ii Incident wave (W/㎡)
Ir Reflected wave (W/m2)
It Transmitted wave (W/m2 )
τ Transmittance ( It/Ii )
Figure 2 Incident and transmitted waves
For single-layer (one-layer) walls, there is also a method for determining the sound transmission loss (TL) for incident sound (diffuse incident) from any direction using a practical formula called the mass law shown in Equation 3.
TL=18 log10 (mf)-44 (Equation 3)
- TL Sound transmission loss for diffuse incidence (dB)
- m Density of sound insulation material (kg/m2 )
- f Frequency of incident sound (Hz)
A common measure in construction is to surround the sound source with sound insulation material. By enclosing the sound source, the sound inside becomes denser and louder. By combining sound-absorbing materials inside the enclosure, it is possible to reduce the increase of sound inside the enclosure, thus providing a more effective noise reduction measure. The sound absorbent material is used as a sound absorbent material.
References: Pollution Prevention Techniques and Regulations Editorial Committee, New Pollution Prevention Techniques and Regulations 2023 Noise and Vibration Edition * Examples of noise reduction measures using sound-absorbing and sound-insulating materials (soundproof houses) At construction sites, there are machines and vehicles that generate loud noise. By enclosing these noise-producing machines with panels made of sound-absorbing or sound-insulating materials, the sound inside is reduced and transmitted outside. A soundproof box or a large building size is called a soundproof house (Figure 3). Soundproofing panels (Fig. 4), which combine steel plates as sound insulation material and rock wool as sound absorption material, are used as wall and ceiling materials to achieve high soundproofing performance.
Figure 3: Exterior and interior of soundproof house
Figure 4 Cross-section of soundproofing panel
For information on soundproofing panels, etc.this way (direction close to the speaker or towards the speaker)for more information.