In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids. Often referred to as a quasiparticle, it represents an excited state in the quantum mechanical quantization of the modes of vibrations of elastic structures of interacting particles.

Phonons play a major role in many of the physical properties of solids, including a material's thermal and electrical conductivities. The study of phonons is an important part of solid state physics.

The concept of phonons was introduced in 1932 by Russian physicist Igor Tamm. The name phonon comes from the Greek word φωνή (phonē), which translates as sound or voice because long-wavelength phonons give rise to sound.

A phonon is a quantum mechanical description of a special type of vibrational motion, in which a lattice uniformly oscillates at the same frequency. In classical mechanics this is known as the normal mode. The normal mode is important because any arbitrary lattice vibration can be considered as a superposition of these elementary vibrations (cf. Fourier analysis). While normal modes are wave-like phenomena in classical mechanics, they have particle-like properties due to the wave–particle duality of quantum mechanics.

Susumu Hirasawa (平沢進, Hirasawa Susumu^?, born April 2, 1954 in Tokyo, Kantō region) is a Japanese electropop artist and composer.

In 1972, he enrolled at Tokyo Designer Gakuin College. From 1972 to 1978, he performed in his first band Mandrake, a progressive rock group influenced by King Crimson and Yes. In 1979 he formed a New Wave synth-rock & techno-pop band called P-Model, along with two former members of Mandrake. They released a string of albums through the 1980s, and in 1989, Hirasawa began releasing solo work, while also continuing to work with the reactivated P-Model beginning in 1992. The P-Model project continued until 1999; in 2004 Hirasawa started a new unit known as Kaku P-Model, which is effectively a solo continuation of P-Model.

Hirasawa uses Amiga computers extensively in his work, using applications such as Scala, Bars and Pipes, and OctaMED in the production of his music. One of the factors that set him apart from other Japanese electronic artists is ever-changing production techniques. For 2001's Solar Ray album, his recording studio was powered completely by solar energy.