How to add noise to a mixture of signals?

Hello. I am now using Audio Systems Array Processing Toolbox ( http://www.engr.uky.edu/~donohue/audio/Arrays/MAToolbox.htm) to simulates a system like this: some speech sources are placed inside a room, the signal is obtained by a triangle microphone array. However I don't know how to add noise to the signal obtained from microphone array to simulate specific SNR of 10dB, 20dB etc. of environment. Can anyone tell me how to do it? Thank you very much!

[Daniel Shub]

http://www.mathworks.com/matlabcentral/answers/6200-tutorial-how-to-ask-a-question-on-answers-and-get-a-fast-answer

http://www.mathworks.com/matlabcentral/answers/8626-how-do-i-get-help-on-homework-questions-on-matlab-answers

[Daniel Shub]

+1 because that is a cool looking toolbox that I didn't know about.