hreshold) {
        return (task, should) => {
          let context = new OfflineAudioContext(2, testFrames, sampleRate);

          // Use a constant (step function) as the source
          let buffer = createConstantBuffer(context, testFrames, 1);
          let source = context.createBufferSource();
          source.buffer = buffer;


          // Create the biquad.  Choose some rather arbitrary values for Q and
          // gain for the biquad so that the shelf filters aren't identical.
          let bi