Audio Nerd: learning about and applying ways to visualize sound

The following is an assignment I complete for a Cousera Music Production MOOC. It examines some different means of sound visualization and possible applications.

For my illustrations I will be using a sine wave which has been increased in frequency over time (or “swept”) from 16hz to 20,000 hz. It was taken from a mono MP3 file at 64 kbps. To understand the three means of visualizing sound as detailed in the course, we will view the manifestation of the “swept” sine wave 3 ways. We will look at the oscilloscope, spectrum analyzer, and lastly the sonogram (or spectrogram).

First, we will use the oscilloscope, which measures the amplitude of the wave on the y (vertical) axis and time on the x (horizontal) axis. In this example, Audacity’s Y axis is simply a linear scale with 0 as no signal and 1 or -1 as a fully saturated signal. You can see in the illustration that, even in the small amount of time captured, the amplitude of the wave is decreasing over time, and that the frequency is growing higher because the distance between waves is shorter.

Oscilloscope

The oscilloscope, while quite good at displaying amplitude variations, is not so good at displaying frequency variations. In order to to discern the actual frequencies you would have to do a fairly complex mathematical calculation based on the length of each complete wave to determine how many times that wave would oscillate per second. It is however an extremely useful tool for quickly recognizing something like a transient peak in the DAW.

Next, we will use a spectrum analyzer, which measures changes in frequency on the x axis, and amplitude on the y axis. For this example, I used the Voxengo Span Plugin (a free FFT Spectrum Analyzer VST plugin) in my DAW, Reaper. I haven’t used a spectrum analyzer plugin before and was excited to try it out. FFT stands for Fast Fourier Transform, which is a mathematical means of transferring something from a time domain to a frequency domain. Here is what the plugin looks like, with the sweeping sine wave captured at 80hz. The x axis is in hz and the y axis is in decibels.

80hz Spectogram

Because we are now looking at a still image of the spectrum analyzer we don’t see the sweep. As the wave sweeps up to 20khz the image will move from left to right. Here is a snapshot at 1k

1k Spectogram

Notice that the spectrum analyzer doesn’t give us a sense of the time in terms of the x/y axis.  We do get a sense of time when viewing in the DAW in this example because what we are hearing moves from left to right on the frequency axis as time passes. But that sense of time exists only from experiencing the changes in real time, not from any graphical information we are receiving.

I was excited to use the Spectrum Analyzer in the DAW, as I can see many places where it would be a useful tool in postproduction. Looking for unpleasant frequencies jumping out during the mixing or mastering processes, I could find them quickly and then pinpoint them with EQ. It would also be helpful for identifying potential imbalances in my mix, particularly in the lower frequency range. These frequencies can be difficult to hear sometimes because of room modes or unfaithful reproduction in my current monitoring system.  Lastly, the spectrum analyzer can help me assess the frequency response of my room, which will help me treat the room acoustically.

The last thing we will look at is the sonogram, or spectrogram. The sonogram gives us time on the x axis, frequency on the y axis, and amplitude mapped by color change. It gives us the clearest picture of how our ears actually hear over time. In this sonogram (which is taken by the VST plugin SG-1 Sonagram used in Reaper) you can see that the amplitude of the wave is decreasing over time while the frequency is increasing. It provides a good (albeit somewhat counter intuitive) sense of frequency increasing as decibel level decreases.

Sonogram

 

The Audacity DAW native spectrogram feature reads a bit more intuitively with regards to the decibel level decreasing, although fails to provide a visible precise db measurement.sonogram audacity

The sonogram or spectrogram can be particularly useful in advanced mastering programs such as Izotope’s Ozone 5  (which can render a sonogram in 3d as opposed to the 2d “waterfall” image) to give a fuller snapshot of a mix.