Acoustics is derived from the Greek word ακούειν, meaning “to hear” (Turner and Pretlove in Acoustics for engineers, Macmillan Education, 1991). Acoustics is a branch of physics, but closely related to engineering and technology. It is a discipline that studies the generation, propagation, reception and effects of sound and other issues of science. The propagation of sound is essentially the propagation process of sound waves. Sound waves act on the human ear caused by the feeling called sound. Visible sound waves are the essence of sound propagation. In a narrow sense, the sound is what the human ear can hear. From a broader sense, the sound is the mechanical disturbance propagating through an elastic medium, such as pressure, density, mass displacement, mass velocity and other changes. Before the seventeenth century, the study of acoustics was mainly concerned with music and musical instruments. In the seventeenth–nineteenth centuries, classical acoustics has been systematically studied and summarised, including the principles of vibration of objects and generation of sound waves, propagation and radiation of sound waves, standing waves and reflections, and diffraction. In classical acoustics, sound, tone and music all refer to phenomena that can be heard. Sound refers to sound waves, but also to the sensations caused by sound waves acting on the human ear. Sound waves are therefore understood to be synonymous with “audible sound”. Since the 1920s, with the emergence of electronics and the progress of electronic products, acoustics has also been rapid development. Some acoustic equipment produce sound source frequency that are “inaudible sound” to the human ear. Sound waves can be classified into infrasound, audible sound and ultrasound according to their frequency range. The frequency range of audible sound is 20 Hz–20 kHz, the frequency of infrasound extends downwards from 20 to 10−4 Hz, sound waves extend upwards from 20 kHz to 5 × 108 Hz for ultrasound, and upwards to 1013 Hz for hyper-ultrasound. The division of sound waves by frequency is based on the frequency range of human acceptance as a reference, and people as a source of sound can be vocalised in the frequency range of 85–5000 Hz. People and animals are both vocalisers and receivers of sound, but the frequency range of their vocalisations and receipts are not the same. With the development of electronic technology, some sound-generating and receiving devices are widely used in physics, engineering, medicine, art and other human production practices. In essence acoustics is also the science of sound; that is to say, everything related to sound is within the scope of acoustic research. Sound emitted from various sources, propagated through different media, and received and perceived by organs that can hear sound (e.g., the ear, measurement transducers), is related to acoustics in every aspect of the series of processes. In terms of human hearing, the sound emitted by a sound source (vibration of an object) is divided into two aspects: firstly, how the sound is transmitted from human hearing to the human ear, thus making the tympanic membrane of the human ear vibrate; secondly, how the vibration of the tympanic membrane is made to be subjectively perceived as sound. In terms of sound measurement, the sound emitted by the sound source is also divided into two aspects: first, how the sound is transmitted from the source to the measurement point (measurement point). The second is how the sound is measured by the transducer. Whether from human hearing or sound measurement, the first part of the content is the same, that is, the transmission of sound in the medium is the same, but the second part of the obvious difference, that is, the human ear and microphone have a fundamental difference (Xiangjun in Learn NVH from here: an introduction and advancement guide of noise, vibration, and modal analysis, Machinery Industry Press, Beijing, 2018).