Reverb

This effect adds a natural reverberation effect to the selected audio. When you are in a room, a hall, an auditorium, a stadium, or any other kind of enclosed chamber, the sounds you hear have some kind of reverberation because of the sound waves bouncing back and forth between the walls, the floor, and the ceiling. This effect is most noticeable in a large enclosed stadium, where the announcer's voice echoes through the stadium. You first hear the announcer's voices, and then you hear several, less distinct echoes of the announcer's voice. Usually you don't notice reverb because your ears are used to hearing it, but without it, the audio sounds flat, dry, and lacking in character. Our ears use reverb to define the size and shape of the room we're in.

Audio signals in the computer are often recorded without any reverb. If you record an instrument directly, or if you use a unidirectional microphone or one close to the sound source, you will get little or no reverb in the signal. To make the audio sound grander, we add reverb.

The "Room Size" controls how long the audio takes to bounce back and forth between the walls. The shape is fixed in this effect, but you can adjust the size, so you can simulate anything from a small room to a large stadium.

The "Decay Length" controls how long the reverberations can be heard bouncing between the walls. A short decay means that the reverberations die away quickly, while a long delay means that they can be heard longer. Generally, a bare room with hard surfaces like tile and stone reflect sound well, and will allow the reverberations to keep bouncing around longer. A room with carpets, drapes, and lots of soft furniture will cause the reverb to die away very quickly because all those soft surfaces absorb the sound.

The "Low-Pass Filter" simulates the effect that sound going thorough the air and bouncing off of softer surfaces will tend to lose their higher frequencies. This option can be adjusted to help color the sound of the reverb.

The "Level" controls how loud the reverbs are compared to the original sound. Typically, you want to keep this low so that the reverb doesn't overwhelm the original audio and make it a mess. You can set the level high if you want the effect of a bad PA system in large stadium where you can't tell what the announcer is saying.

The reverb filter uses six comb filters with low-pass filters and a simple delay. The "Room Size" parameter controls the delay length of each comb filter. The "Decay Length" parameter controls the amplitude of each comb filter. For stereo files, the effect is applied equally to both channels.

A comb filter is a type of filter where the input audio is delayed by a constant amount of time, and then fed back into itself. It is called a comb filter because it will cancel out those frequencies that are 180* out of phase when the delayed audio is fed back upon itself. When you look at the output of such a filter on a frequency graph, it will look like a comb.


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