1.3 Acoustical Testing 21 time-varying signal are collectively called FFTs. If a broadband stimulus is used, the FFT can show the spectral response of the device under test (DUT). One such stimulus is the unit impulse, a signal of theoretically infinite amplitude and infinitely small time duration. The FFT of such a stimulus is a straight, horizontal line in the frequency domain. The time-honored hand clap test of a room is a crude but useful form of impulse response. The hand clap is useful for casual obser- vations, but more accurate and repeatable methods are usually required for serious audio work. The drawbacks of using impulsive stimuli to measure a sound system include: 1. Impulses can drive loudspeakers into nonlinear behavior. 2. Impulse responses have poor signal-to-noise ratios, since all of the energy enters the system at one time and is reacquired over a longer span of time along with the noise from the environment.