Metering with the Web Audio API·∞
When working with audio, there are times when it can be useful to see visualizations of the underlying data.
The most common visualizations tell us how loud the signal is in various ways. A basic peak meter tells us the instantaneous signal level relative to limit of what the system can handle. Originally taking the form of a VU meter, this has been useful for as long as we have been recording and broadcasting sound for level setting.
The web audio api has lots of cool features enabling lots of cool things, from creating synthesizers and drum machines to spatial processing for immersive games. The thing that excited me the most, though, about the web audio API is that it gives us access to the raw buffer data, allowing us to make visualizations with it.
The peak meter shown below has been designed to feel familiar to anyone who has worked with audio before. It has a bar for each channel and its size oscillates with the strength of the signal in the channel. It has a latching peak feature, in which the number at the right tells us the volume in dB of the loudest peak since we last refreshed it. Clicking on the meter refreshes the latches. Inspired by Dorrough meters, the meters also transition from green to yellow to red as the signal level approaches the system’s maximum volume.
Another audio visualization used in this audio player is a waveform. The waveform is just the volume data of the recording plotted as a function of time. This is useful for finding a spot in the recording where things get louder or softer. We can also get a sense of dynamic range from this visualization. The term “Stick of butter audio” has been used to describe audio that has been compressed so severely as to look like a solid rectangle in a waveform visualization. In the figure below, the waveform of each audio example is shown behind the slider for controlling playback location.
Volume-based visualizations need not be limited to just the peak meter and waveform, however. One lesser-used visualization is the goniometer, better known as a vectorscope. In a goniometer, two-channel audio is visualized on an x-y axis, often tilted 45 degrees. This visualization tells us the correlation between the left and right channels, something which is important to know if your two-channel audio might ever be played back in mono.
The first example in the figure below shows an audio file with a basic sine wave on the left channel and silence on the right channel.
The second example shows a sine wave on the right channel and silence on the left.
The next example shows the exact same sine wave on both channels. This audio could be described as “perfectly correlated”; it could also be described as mono, because if this file only contained one channel of data, the resulting sound would be the same.
This next example shows sine waves in the left and right channels with their polarity reversed. One would get the same result by delaying one channel by the period of the wave, so that the two channels would be 180 degrees out of phase with each other. When mixing stereo audio that might someday be folded down to mono, this is exactly what you don’t want to see in your vectorscope.
My personal favorite visualization using the goniometer is when you have two sine waves that are 90 degrees out of phase with each other. I can’t think of any reason why this would happen in the real world, but I find this example cool, so I’m including it.
Finally, the last two examples are of real-world audio visualized in a goniometer. The first is an excerpt from a song by Chvrches titled “The mother we share”. I chose it because it features a ping-pong panning effect that I think shows off the meter.
The last example is an excerpt from the Marine Chamber Orchestra performing Brahms Serenade No. 1 in D, Op. 11.