Many interesting audio effects are achieved by combining a sound with a delayed (and possibly altered) version of itself. To delay a sound, one needs to store it for a certain amount of time till (a delayed copy of) it is needed. That storage has to be constantly ongoing when we’re dealing with realtime audio processing, yet we usually also want to dispose of the delayed audio data once it’s no longer needed. Realtime delay of audio is therefore most often achieved by storing the sound in what’s commonly called a ring buffer or a circular buffer.

In preparation for the upcoming discussion of delay-based effects, study these examples from a previous class.
Simple delay of audio signal
Delay with tempo-relative timing
Simple flanging
Delay with feedback

Study the MSP Tutorials that deal with delayed sound. [These links are to the web version of the documentation, but you’ll probably prefer to use the Max Documentation within Max so that you can try out the tutorial patches while you read about them.]
Delay Lines
Delay Lines with Feedback
Flanging
Chorus
Comb Filter

If you just can’t get enough of examples of delay, check out these other examples from a past class.
Change of delay time may cause clicks
Continuous change of delay time causes a pitch shift
Ducking when changing delay time
Abstraction for crossfading between delay times
Demonstration of crossfading delay

You can also read about digital filtering. Filtering is a special case of delay, using extremely short delay times to create interference between a sound and a slightly delayed version of itself, which causes certain frequencies in the sound to be attenuated (lessened in strength) or resonated (increased in strength), changing the sound’s timbre. [These are links to the web version of two MSP tutorials; you may prefer to read them in the Max Documentation within the Max application.]
Simple filters
Variable type filters

Here’s a very thorough tutorial on filters in MSP written by Peter Elsea.

Here are some filter examples from a past class.
Bandpass filter swept with a LFO
A variable-mode filter: biquad~
Smooth filter changes