# Pink vs White Noise



## SamSpade (Jun 3, 2010)

Long before I began using the REW analysis, I used the white noise output from my Lexicon MC-12 sound processor for speaker setup. In a setup mode, the MC-12 sequentially sends about 30 seconds of white noise to each speaker connected to it. I use an SPL meter, C weighted, and adjusted the SPL of each speaker, at the listening position, until they all read approximately the same (i.e., + or – 2dB).

As I understand the REW program, a frequency swept sine wave is generated, but the amplitude is decreased in a logarithmic manner as the frequency increases (i.e., it generates pink noise).

I believe I understand the difference between white noise and pink noise, but I don’t understand when to use one or the other.

Thanks for your help.


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

The sine-wave sweep signal that REW uses to generate measurements is not the same thing as pink noise. Pink noise is a broadband signal – i.e. the entire frequency range generated all at the same time, in a single signal. Sine waves by their nature can’t do that, hence the need for a _time-lapsed_ sweeping signal to cover the entire frequency range.

Typically pink noise is used for tuning the in-room frequency response of a sound system (not to be confused with the level-setting exercise you mentioned, which could be performed with any number of test signals). For audio applications, the main use for white noise is determining the frequency response of electronics.

Regards,
Wayne


----------



## SamSpade (Jun 3, 2010)

Hi Wayne,

Thank you for the very clear explanation. 

The one remaining question I have relates to your comment that pink noise or white noise or even other signals can be used for level-setting. If I used pink noise for level setting and compared it to using white noise, would I get the same result? That is, would the SPL at my listening position be the same when using pink or white noise? If that is true, it seems odd to me given white noise has a constant power spectral density over the audio spectrum (in my MC-12) while the power spectral density of pink noise falls off as the frequency increases.


----------



## laser188139 (Sep 19, 2009)

SamSpade said:


> ... If I used pink noise for level setting and compared it to using white noise, would I get the same result? ...


If the room/system response is perfectly flat, and if the SPL meter reads flat over the range of the signal, then I would expect the values on the SPL meter to be the same, because the total sound energy across the bandwidth would be the same. If the room response is not flat, then the fact that white noise puts more energy in the higher frequencies should cause the reading to be different. And SPL meters are themselves weighted, consider the C-weighting curve, so if the test signal frequencies extend outside the midrange, I would expect a C-weighted SPL meter to read higher on pink noise than on white noise. 



SamSpade said:


> ... If that is true, it seems odd to me given white noise has a constant power spectral density over the audio spectrum (in my MC-12) while the power spectral density of pink noise falls off as the frequency increases. ...


You can see this effect with REW comparing spectrum measures to RTA measures. If you look at white noise as a spectrum, it will appear flat and pink noise will appear to fall off. If you collect the data as RTA, with buckets whose width are a constant fraction of an octave, pink noise will appear flat where white noise will appear to increase with frequency. 

Bill


----------



## gsmollin (Apr 25, 2006)

First some definitions:

White noise is a random signal that contains equal energy per unit bandwidth.

Pink noise is a random signal that contains equal energy per logarithmic-unit bandwidth, e.g., octave, 1/3 octave, 1/10 octave, etcetera.

White noise may be used to determine the bandwidth of an unknown system. The bandwidth of the system is the output energy divided by the energy in a 1 Hz bandwidth at the center of the response. Pink noise is convenient to to use when a constant percentage filter, such as 1/3 octave, is used to analyze a system. When the system has a flat frequency response, the constant percentage filter will pass equal energy across this frequency band wherever it is tuned, when excited by pink noise. Pink noise has a -3 dB /octave roll off when analyzed by a constant bandwidth filter, such as a spectrum analyzer. This response is impossible to synthesize exactly by filters with rational roots, so approximations are done with distributed-pole filters, or digital signal processing is employed where non-rational functions may be written.


----------

