Acoustics is a branch of physics. However it has wide connections with engineering such as mechanical engineering, electrical and electronic engineering, ocean engineering, civil engineering, geophysical engineering etc. Usually people would think of acoustics as engineering rather than physics. Acoustics is a huge and broad field and can be divided into many subfields such as : physical acoustics, audio and electroacoustics, speech and hearing, noise and vibration, industrial ultrasonics, biomedical ultrasonics, infrasonics, nonlinear acoustics, underwater acoustics, musical acoustics, building and architectural acoustics etc. The applications of acoustics are broad and important such as to nondestructive evaluation, materials research, study of cavitation, medical ultrasound imaging, application of ultrasound to drug delivery , and gene therapy, sonars, geophysical exploration, seismic studies, noise and vibration control, acoustical designs of theatres, opera houses, buildings etc. With the arrival of nanotechnology and biotechnology, the role of ultrasonics is gaining even more prominence.