Speakers use drivers to turn voltage into sound. Although they only have a few parts, the physics behind them can be complex, and each part is important to the proper functioning of the speaker and must be engineered to precise specifications to work properly. This is especially true of the speaker baffle.
The baffle is the front part of the speaker, on which the drivers of a speaker, the tweeter, woofer and subwoofer are mounted. The purpose of the baffle is twofold: to hold up the drivers and to prevent the sound from the front and the back of the drivers from meeting and causing noise interference. This function is what gives the baffle its name, as it confuses frequencies enough to block interference and create a better sound.
A driver creates sound by vibrating the air at a specific frequency, creating waves of energy. This wave can be expressed as a sine wave. As sound waves reflect, they may change in timing enough that the exact opposite wave of the desired sound may be sent toward your ear. If that sound and the reverse wave meet at the same time, they add up mathematically to zero: It is as if no sound was there.
A baffle works by preventing reflection and absorbing sound energy broadcast from the driver, so only energy from the front of the driver--the desired sound--reaches the ears. The material of the baffle will absorb some of the energy of sound waves striking it and reflect the rest, and continue to do so as that wave returns, The effect is like watching ripples fade out in a pool of water; as the ripple strike the sides, they lose energy until the pool is calm.
A baffle can be made out of any material. Generally, baffles are made out of the same material as a speaker enclosure, such as metal, plastic or wood, but any solid material which will not vibrate when exposed to sound, called a resonator, will work just as well. The baffle may be thinner than the other walls of the enclosure, but any homemade baffles should be tested to ensure they absorb and reflect enough energy.
A baffle is not strictly necessary from a physics standpoint, depending on the power of your drivers. Very small or low-powered drivers, such as the ones found in standard headphones, do not generate enough power for cancellation to be an issue and often have their own noise-cancellation functions. A baffle is more important when you have multiple drivers generating a range of frequencies in a larger cabinet. Baffles can also protect against environmental damage, such as in car stereos.