Acoustic principle of sound barrier

1. Diffraction

The sound energy diffracted over the top of the sound barrier to the receiving point is smaller than the direct sound energy without the barrier. The difference in sound level between the direct sound and the diffracted sound is called the diffraction sound attenuation, which is denoted by the symbol ∆Ld and increases with the increase of the ∅ angle. The diffraction sound attenuation of the sound barrier is a function of the geometric relationship between the sound source, the receiving point and the sound barrier and the frequency. It is the main physical quantity determining the insertion loss of the sound barrier.

2. Transmission

The phenomenon that the sound wave emitted by the sound source propagates through the sound barrier to the receiving point. The sound energy penetrating the sound barrier depends on the surface density of the sound barrier, the angle of incidence and the frequency of the sound wave. The sound insulation ability of the sound barrier is evaluated by the transmission loss TL. When TL is large, the transmitted sound energy is small; when TL is small, the transmitted sound energy is large. The transmitted sound energy may reduce the insertion loss of the sound barrier. The reduction of the insertion loss caused by transmission is called the transmission sound correction amount, denoted by the symbol ∆Ld. Usually in acoustic design, it is required that TL - ∆Ld ≥ 10dB. At this time, the transmitted sound energy can be ignored, that is, ∆Lt ≈ 0.

3. Reflection

When sound barriers are built on both sides of the road and they are parallel, the sound wave will be reflected multiple times between the sound barriers and diffracted over the top of the sound barrier to the receiving point, which will reduce the insertion loss of the sound barrier. The reduction of the insertion loss caused by the reflected sound wave is called the reflected sound correction amount, denoted by the symbol ∆Lr.

To reduce the reflected sound, a sound - absorbing structure is generally added on the side of the sound barrier facing the road. The magnitude of the reflected sound energy depends on the sound absorption coefficient α of the sound - absorbing structure, which is a function of frequency. To evaluate the overall sound absorption effect of the sound - absorbing structure of the sound barrier, the noise reduction coefficient NRC is usually used.