A tap channel sets the number of repeated tones (only works for discrete tones, and recommended using with relative or simultaneous timing). Essentially, tap and duration works in a similar way. That is, a tap channel encodes a data value in a different scale from that of the time channel.

There are two tap channels: tapSpeed and tapCount.

tapSpeed usage pattern

The tapSpeed channel maps a data value to the number of tappings per second, given the same fixed duration of time (defined using band). When the polarity of the scale is positive, a higher value is mapped to more tap counts, and a lower value to fewer tap counts, where tappings are distributed evenly for the same time duration. For instance, when the scale’s domain is [0, 1] and range is [0, 5] for the band of 2 seconds, then a value of 0.5 is mapped to 5 tappings played over 2 seconds. For the tapSpeed channel, the pauseRate property in scale can adjust the ratio between the tapping sound and the following pause. When there is only one tapping sound (e.g., the output tapSpeed value is 0.1 for the band of 2), then the sound is played as determined by singleTappingPosition. The default singleTappingPosition value is 'middle' (i.e., pause > tap > pause). If it is 'start' then tap > pause; if it is 'end' then pause > tap. To cap the length of tapping sound, one can use maxTappingLength (unit: seconds; default: 0.3).

{
  ...
  "encoding" : {
    "tapSpeed": {
      "field": "density",
      "type": "quantitative",
      "scale": {
        "doamin": [0, 1], // optional
        "range": [0, 10],
        "band": 2, // unit: seconds
        "singleTappingPosition": "start" // or "middle" / "end"
      }
    }
  }
  ...
}

let stream = new Erie.Stream();
...
stream.encoding.tapSpeed.field("density", "quantitative");
stream.encoding.tapSpeed.scale("domain", [0, 1]); // optional
stream.encoding.tapSpeed.scale("range", [0, 10]); // optional
stream.encoding.tapSpeed.scale("band", 2); // unit: seconds
...

tapCount usage pattern

The tapCount channel directly maps a data value to the number of tap sounds with the same speed. The length of each tap sound is determined by band. For instance, when the scale’s domain is [0, 5] and range is [0, 10] for the band of 0.2 seconds, then a value of 2.5 is mapped to 5 tap sounds each of which lasts 0.2 seconds. For the tapCount channel, the pauseRate property in scale can adjust the ratio between the tapping sound and the pause following it. Alternatively, the pauseLength property in scale makes it possible to directly set the pause length (default unit: seconds).

{
  ...
  "encoding" : {
    "tapCount": {
      "field": "length",
      "type": "quantitative",
      "scale": {
        "doamin": [0, 5], // optional
        "range": [0, 10],
        "band": 0.2 // unit: seconds
      }
    }
  }
  ...
}

let stream = new Erie.Stream();
...
stream.encoding.tapCount.field("length", "quantitative");
stream.encoding.tapCount.scale("domain", [0, 5]); // optional
stream.encoding.tapCount.scale("range", [0, 10]); // optional
stream.encoding.tapCount.scale("band", 0.2); // unit: seconds
...

tapCount + tapSpeed

You can use tapCount and tapSpeed channels together. In this case, the band of tapSpeed (i.e., the duration), the pause of tapCount, and the singleTappingPosition of tapSpeed are all ignored. It is basically a tapCount channel with varying speeds.

{
  ...
  "encoding" : {
    "tapCount": {
      "field": "length",
      "type": "quantitative",
      "scale": {
        "doamin": [0, 7], // optional
        "range": [0, 7], // optional
        "band": 0.2 // unit: seconds
      }
    },
     "tapSpeed": {
      "field": "sparsity",
      "type": "quantitative",
      "scale": {
        "doamin": [0, 1], // optional
        "range": [0, 7], // optional
      }
    }
  }
  ...
}

let stream = new Erie.Stream();
...
stream.encoding.tapCount.field("length", "quantitative");
stream.encoding.tapCount.scale("domain", [0, 7]); // optional
stream.encoding.tapCount.scale("range", [0, 7]); // optional
stream.encoding.tapCount.scale("band", 0.2); // unit: seconds
stream.encoding.tapSpeed.field("sparsity", "quantitative");
stream.encoding.tapSpeed.scale("domain", [0, 1]); // optional
stream.encoding.tapSpeed.scale("range", [0, 7]); // optional
...

Notes

Excessive tappings (e.g., 30 taps within 0.5 seconds) may cause browser crash due to memory shortage (it is not really a good sonification design as well).

Each tapping sound and following pause’s duration has limited precision upto 2 decimal points. This may result in a situation where your intended band for tapSpeed (i.e., the length of each tap) is 2 seconds, but resulting sound may be 2.02.

Given that there is no concept of 0.5 tap sound, so the final tap counts are all rounded.