vumeter | A nixie tube audio meter

You're drawing a white square over your image with the fillRect call.

Each frame you start with a drawn canvas (the previous frame), you need to start clearing that one and then drawing the image on the empty canvas and last draw the needle over it:

To begin with to get values for a VU meter you want to use getByteTimeDomainData, not frequency.

The reason your analyser is not giving readings is because the audio is not being routed anywhere. A script processor node has an input and an output array, you have to copy the input array to the output array to keep the signal going. These are properties on the event object of onaudioprocess. I would recommend removing the script processor node as it will introduce a large amount of audio latency, at least 2048 samples in your example + script execution time. You can move the code inside onaudioprocess into a request animation frame loop, this will cause no audio latency. The only downside is that you are not processing every single sample so you could miss a clip, you should be able to get 60 fps though. If detecting clips is important to your application you could look for them in the audio data (all values > 1 or < -1 of the decoded audio data) Basically all the analyzer node is giving you is a tiny portion of the full audio data and converting it from float to byte format. Using bytetime domain data actually takes longer than float because internally its float and it has to convert it. Something you may want to consider is that VU meters use mainly either peak or Rms values, not average.