/// <summary>
        /// Triggers a note on event for the Helm instance(s) this points to.
        /// Instead of a midi note, uses a frequency measured in hertz.
        /// You must trigger a note off event later for this note by calling NoteOff.
        /// </summary>
        /// <param name="frequency">The frequency in hertz to play.</param>
        /// <param name="velocity">How hard you hit the key. [0.0, 1.0]</param>
        public void FrequencyOn(float frequency, float velocity = 1.0f)
        {
            int number = 0;

            pressedFrequencies.TryGetValue(frequency, out number);
            pressedFrequencies[frequency] = number + 1;
            Native.HelmFrequencyOn(channel, frequency, velocity);
        }