Wednesday, November 11, 2009

Audio: noise

I have been continuing some digital audio experiments.

I find red noise to sound calmer and more natural than white noise. (Red noise can be produced by integrating white noise). The way I produce red noise is to start with a single value, add a small random (positive or negative) sample to it, and then continue. In white noise, the samples are unconnected leaps, but for red noise, the samples do not change as drastically.Let's say one takes a small chunk of red noise, 220 samples long. If one plays this chunk repeatedly, a 200hz tone is heard. The tone is colored by the frequencies present in the initial chunk of noise.

Now let's create 15 different chunks of red noise, each of length 220. If we play one of the chunks ten times, then the next ten times, then the next ten times, and so on, eventually repeating,the result is a nice "warble" at 200hz. The upper frequencies present change abruptly about 20 times a second, causing a robotic sound. This is a fun sound that can be used for various purposes.

out_r.wav

However, what if our goal is to create a more natural sound? As mentioned before, a problem is that the frequency content changes abruptly. We want to create more subtle changes, while still having the presence of buzzing red noise.

The solution I came up with was inspired by how the red noise was created in the first place. I still begin with a chunk of red noise. After playing it ten times, instead of switching to an entirely different chunk, I instead slightly alter the chunk: by adding small random (positive or negative) values to each of the 220 samples. In this way, the frequency content does not change suddenly, and yet it changes in an unpredictable and subtle way. I find the resulting tone to still be harsh and noise-like, but surprisingly natural and musical. In the past it has only been in the frequency domain that I've been able to produce natural sounds.

Two examples of this "softened noise":


softened.wav


pulsedrop.wav

The second is seeded initially with a sine wave, and higher frequencies emerge over time.

I've been using my C audio library, "bcaudio", for these experiments. Writing psuedo-object-oriented C isn't that bad. It's fun to write code that will run very quickly.



bcaudio.tgz


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