The Foster Extension is a small tube-like device that is inserted into the end of the footjoint. It has two tone hole shaped openings that can be angled up and down or side to side. The extension’s purpose is to improve the harmonic capabilities of the flute by lengthening the flute’s resonating chamber. Many players comment that their tones are bigger and project better.
It is even possible to use two extensions simultaneously, one piggy-backed to the other for interesting results. This extension will work on B or C foot instruments.
Made by Gail Williams, the extensions are available in silver and gold. She feels they work better on Cooper Scale flutes, however excellent results have been heard on older instruments as well. Foster Extensions are available at Carolyn Nussbaum Music Company www.flute4u.com (Information current in 2008)
Foster Flute Extension on YouTube
Foster extension for flutes: Abstract
The Foster extension is a flute attachment which improves and enhances the harmonic capability of a flute by lengthening the resonant chamber of the standing wave. The instant invention ‘fine tunes’ the harmonic range of the flute by taking into account the end point of the fundamental length of the sounding oscillations of an air column without negating the frequency which has been predetermined by the original terminus of the physical scale length of the flute. The sound wave length within a flute terminates slightly beyond the physical length of the flute tube and this difference in length is known as the ‘end correction’. The instant invention captures the node which extends beyond the flute tubing and balances and reinforces the propagation of upper partials throughout the sounding range of the instrument.
U.S. Patent: https://patents.google.com/patent/US7476793B2/en