Matlab signal processing removing white noise from signal What is the way to address the removal of white noise from a signal?

Extra Details 
Hi I also agree the details are as follow:                                                                                    The signal is a acoustic or a sound wave. The the power associated with the frequency of the noise is distributed evenly across the whole frequency range (white noise) The actual signal characteristic is not known as its a inverse problem. But the actual frequency of the signal is a part of the total frequency. Power associated with the signal is higher than the noise can be seen in a spectogram.
Actuallty I also cant figure out how to extract the coefficients of the power/frequency  (db/Hz) from the spectogram. Can you suggest a way?
 see the graphs in the link

http://www.mathworks.com/help/signal/ref/spectrogram.html?refresh=true

Actual problem is to find the time of arrival of the actual signal
Without further details on the signal and the type of processing you want to apply, the two most basic approaches are:


signal enhancement via linear filtering (filter or filtfilt),
Wiener filtering, assuming a known stationary signal and noise spectra in an additive noise (matlab code).


If your signal is non-stationary, a time-frequency (spectrogram) or time-scale (wavelet) decompositions might help. In their domain, signal and noise are often better separated, and you can there apply Wiener filering, or alternative forms of thresholding, or more involved source separation techniques.

Wiener filtering in the wavelet domain are generated a lot of works, and codes, that you can test.