Soundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to reflect or absorb the energy of the sound waves, using damping structures such as sound baffles, or using active antinoise sound generators. There are 5 elements in sound reduction (absorption, damping, decoupling, distance, and adding mass). The absorption aspect in soundproofing should not be confused with sound-absorbing panels used in acoustic treatments. Absorption in this sense only refers to reducing a resonating frequency in a cavity by installing insulation between walls, ceilings or floors. Acoustic panels can play a role in treatment only after walls, ceilings, and floors have been soundproofed, reducing reflections that make the overall sound in the source room louder. Two distinct soundproofing problems may need to be considered when designing acoustic treatments—to improve the sound within a room (see reverberation), and reduce sound leakage to/from adjacent rooms or outdoors (see sound transmission class and sound reduction index). Acoustic quieting and noise control can be used to limit unwanted noise. Soundproofing can suppress unwanted indirect sound waves such as reflections that cause echoes and resonances that cause reverberation. Soundproofing can reduce the transmission of unwanted direct sound waves from the source to an involuntary listener through the use of distance and intervening objects in the sound path.