A few companies, including Victor Talking Machine Company and Pathe, produced record players using compressed-air loudspeakers. However, these designs were significantly limited by their poor sound quality and their inability to reproduce sound at low volume. Variants of the system were used for public address applications, and more recently other variations have been used to test space equipment resistance to the very loud sound and vibration levels that launching rockets produce.
To accurately reproduce very low bass notes without unwanted resonances (i.e., from cabinet panels), subwoofer systems must be solidly constructed and properly braced; good ones are typically heavy. Many subwoofer systems include power amplifiers and electronic filters, with additional controls relevant to low frequency reproduction. These variants are known as "active subwoofers". Passive subwoofers require external amplification.
