The bell of a brass wind instrument emits sound into a first enclosure providing, at the bell, a relatively small open cavity containing a microphone, the remainder of the enclosure being filled with highly absorptive acoustical material. A highly resistive or absorptive long and narrow acoustical path exists around the outside of the first enclosure, extending from the small open front cavity to a rear opening to the atmosphere, and is formed by a second enclosure spaced from and surrounding the first enclosure.