Audio-frequency tube is an electronic component that amplifies or modifies audio signals. It is also known as a vacuum tube or valve. Other synonyms for audio-frequency tube include thermionic valve, electron tube, and tube amplifier. Vacuum tube is often used to describe audio-frequency tubes because they are typically vacuum-sealed and rely on the flow of electrons in a vacuum. Electron tube is another synonym, as it describes the device's use of electrons to control the flow of current and amplify or modify audio signals. Finally, tube amplifier refers to the fact that these devices are often used to amplify or modify audio signals before they are delivered to a speaker.