for https://www.reddit.com/r/dailyprogrammer/comments/4o74p3/20160615_challenge_271_intermediate_making_waves/

makingWaves.cs

using System;
using System.Collections.Generic;

namespace MakingWaves
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            WaveForm a = new WaveForm(8000, 300, "ABCDEFG_GFEDCBA", @"c:\waveyWave.wav");
        }
    }

    /// <summary>
    /// creates a .wav file using the sampleRate, duration of note, a string of notes, at the file path given.
    /// </summary>
    public class WaveForm
    {
        public WaveForm(double dblSampleRate, double dblDuration, string strNotes, string strFilePAth)
        {
            char[] chrArrNotes = strNotes.ToCharArray();
            int bytesPerNote = (int)Math.Floor(dblSampleRate * dblDuration / 1000.0);
            List<byte> bytsWaveForm = new List<byte>();

            foreach (char note in chrArrNotes)
            {
                byte byteCurrentAmp = 128;

                for (int i = 0; i < bytesPerNote; i++)
                {
                    byteCurrentAmp = (GetFrequencyFromNote(note) == 0.01) ? (byte)128 : (byte)Math.Round(Math.Sin(2 * Math.PI * i * GetFrequencyFromNote(note) / dblSampleRate));
                    bytsWaveForm.Add(byteCurrentAmp);
                }
            }
            int time = (int)Math.Ceiling(dblDuration * strNotes.Length / 1000.0);

            GetWAVFileFromByteString(bytsWaveForm, dblSampleRate, time, strFilePAth);
        }

        private double GetFrequencyFromNote(char chNote)
        {
            double freqReturn = 0.0;
            switch (chNote)
            {
                case 'A':
                    freqReturn = 220.00;
                    break;

                case 'B':
                    freqReturn = 246.94;
                    break;

                case 'C':
                    freqReturn = 261.63;
                    break;

                case 'D':
                    freqReturn = 293.66;
                    break;

                case 'E':
                    freqReturn = 329.63;
                    break;

                case 'F':
                    freqReturn = 349.23;
                    break;

                case 'G':
                    freqReturn = 392.00;
                    break;

                default:
                    freqReturn = 0.01;
                    break;
            }
            return freqReturn;
        }

        private void GetWAVFileFromByteString(List<byte> bytsWaveForm, double dblSampleRate, int intSeconds, string strFilePAth)
        {
            int intBitSize = 16;
            short intWaveType = 1;
            short intChannels = 1;
            int fileSize = (bytsWaveForm.Count) + 44;
            short intBitPerSample = 8;
            int intSampleRate = (int)Math.Floor(dblSampleRate);
            int intSampleSize = intSampleRate * intBitPerSample * intChannels / 8;
            short intBitChannels = (short)(intBitPerSample * intChannels / 8);

            using (System.IO.BinaryWriter w = new System.IO.BinaryWriter(System.IO.File.Open(strFilePAth, System.IO.FileMode.Create)))
            {
                byte[] arr = System.Text.Encoding.ASCII.GetBytes("RIFF");
                w.Write(arr);
                w.Write(fileSize - 8);
                arr = System.Text.Encoding.ASCII.GetBytes("WAVE");
                w.Write(arr);
                arr = System.Text.Encoding.ASCII.GetBytes("fmt ");
                w.Write(arr);
                w.Write(intBitSize);
                w.Write(intWaveType);
                w.Write(intChannels);
                w.Write(intSampleRate);
                w.Write(intSampleSize);
                w.Write(intBitChannels);
                w.Write(intBitPerSample);
                arr = System.Text.Encoding.ASCII.GetBytes("data");
                w.Write(arr);
                w.Write(fileSize - 44);

                for (int i = 0; i < bytsWaveForm.Count - 1; i++)
                {
                    w.Write(bytsWaveForm[i]);
                }
            }
        }
    }
}