A comb filter is a visual representation of a frequency response that is characterized by a series of consistent and regularly spaced spikes over a period of time. This frequency response is caused by the combination of a sound and its duplicate, which is delayed. The display of this response consists of dips and peaks due to phase interference, and appears somewhat like the teeth of a comb. The deep, narrow notches are used to show the attenuated signals, and all these characteristics are the reason why this representation was given its name. The effect of comb filtering can create many problems for live recording of sounds or music.
Recording in a sound studio, especially one that is small with walls close together, can yield the effect of a comb filter. This gives the recording a hollow sound, similar to the effect of a flanger or phaser. Small rooms also increase the likelihood of the filter because the short echoes, which can turn into larger echoes, may thicken the recording sound. Echoes are not received by the recording device as an echo, but rather, as a sound that is meant to be recorded.
The comb filter effect also occurs as a result of boundaries within a room or recording space. Surfaces that are reflective create echoes and should generally be kept to a minimum within a room or recording space to prevent any comb filter effects. Other objects in a room, such as a chair or furniture, can disrupt the sound and create an echo that will affect the quality of the recording. Objects like these, therefore, are often kept to a minimum within a recording space, with only the essential recording equipment remaining. This usually only includes a microphone and possibly a stand to hold sheet music.
Preventing the comb filter effect from occurring requires applying highly sound-absorbing surfaces to the walls. The surface can also be diffusive and applied directly to the ceiling of the room. Common materials to use for this type of solution include rigid fiberglass or foam that ranges in thickness between 1 and 4 inches (2.54 and 10.16 cm). Floors, however, are not covered in any type of materials to absorb or diffuse echoes to prevent the comb filter from occurring. Usually, sound studios and even auditoriums have a floor made of linoleum or hardwood to allow a lifelike sound.