In typical installations, subwoofers are physically separated from the rest of the speaker cabinets. Because of propagation delay, their output may be somewhat out of phase from another subwoofer (on another channel) or slightly out of phase with the rest of the sound. Consequently, a subwoofer's power amp often has a phase-delay adjustment (approximately 1 ms of delay is required for each additional foot of separation from the listener) which may improve performance of the system as a whole at subwoofer frequencies (and perhaps an octave or so above the crossover point). However, the influence of room resonances (sometimes called standing waves) is typically so large that such issues are secondary in practice. Subwoofers are widely used in large concert and mid-sized venue sound reinforcement systems. Subwoofer cabinets are often built with a bass reflex port (i.e., a hole cut into the cabinet with a tube attached to it), a design feature which if properly engineered improves bass performance and increases efficiency.