Among the vast and often rapid changes that have taken place over the last century of audio recording, it is notable that there is one crucial audio device, invented at the start of the "Electrical Era", which has survived virtually unchanged since its introduction in 1925 – the electro-acoustic transducer, or loudspeaker. The most common form of electro-acoustic speaker the dynamic loudspeaker – which is effectively a dynamic microphone in reverse. This device typically consists of a flattened conical acoustic diaphragm (usually made of a stiff paper compound) suspended in a metal ring, at the apex of which a moving-coil magnet is attached. When an audio signal from a recording, a microphone, or an electrified instrument is fed through an amplifier to a loudspeaker, the electrical impulses drive the speaker magnet backward and forward, causing the speaker cone to vibrate, and this movement generates the audio-frequency pressure waves that travel through the air to our ears, which hear them as sound. Although there have been numerous refinements to the technology, and other related technologies have been introduced (e.g. the electrostatic loudspeaker), the basic design and function of the dynamic loudspeaker has not changed substantially in 90 years, and it remains overwhelmingly the most common, sonically accurate and reliable means of converting electronic audio signals back into audible sound.