The characteristics of how an electrical circuit or mechanical system responds to an impulse -- a short, sharp input of energy. Some types of percussion synthesis are done using the impulse response of resonant filter circuits; when the filter is fed a very short voltage spike, it "rings" in a manner characteristic of the circuit and its settings (e.g., cutoff frequency and resonance). This can produce sounds resembling a tom-toms, bongo, or marimba. Another use of impulse responses is for convolution reverb and effects; a response of an enclosed space is done by producing a short "crack" or "bang" noise in the space by means such as striking a piece of wood with a hammer, shattering glass, or sometimes a small explosive such as a firecracker or a starter's pistol. This is recorded and then used to convolve sound signals, impressing the spectral characteristics of the space on the sound signal.