message: setting exponent in test
  for additional coverage
diff: @@ -55,6 +55,8 @@ describe("FrequencyEnvelope", () => {
  expect(env1.attack).to.equal(0.1);
  expect(env1.decay).to.equal(0.2);
  expect(env1.sustain).to.equal(0.3);
  + env1.exponent = 2;
  + expect(env1.exponent).to.equal(2);
  env1.dispose();
  });
  