Writing wav file in Python with wavfile.write from SciPy

Question

I have this code:

import numpy as np
import scipy.io.wavfile
import math

rate, data = scipy.io.wavfile.read('xenencounter_23.wav')

data2 = []

for i in range(len(data)):
    data2.append([int(round(math.sin(data[i][0])*3000)), int(round(math.sin(data[i][1])*3000))])

data2 = np.asarray(data2)

print data2

scipy.io.wavfile.write('xenencounter_23sin3.wav',rate,data2)

This prints (truncated):

[[-2524  2728]
 [ -423 -2270]
 [ 2270   423]
 ..., 
 [-2524     0]
 [ 2524 -2728]
 [-2270   838]]

The wav file opens and plays in Windows Media Player, so at least its the proper format. However, when opening it with Audacity and looking at the individual samples, they're all 0, and concordantly the file plays no sound at all.

What I don't understand is how that numpy array listed above becomes all 0's. It should be below the maximum value for a sample (or above, if it's negative).

Answer 1

I found that scipy.io.wavfile.write() writes in 16-bit integer, which explains the larger file sizes when trying to use a 32-bit integer (the default) instead. While I couldn't find a way to change this in wavfile.write, I did find that by changing:

data2 = np.asarray(data2)

to

data2 = np.asarray(data2, dtype=np.int16)

I could write a working file.

Answer 2

In creating wav files through scipy.io.wavfile.write(), i found that the amplitude is very important. if you create a sine wave with amplitude 150, it sounds like silence when played in VLC. if the amplitude is 100, it sounds like a distorted sine wave, and if you make it 80, it starts to sound like a normal file.

Definitely have to be careful about the amplitude when creating wave files, but it's not clear to me right now what the maximum level is before it starts clipping or disappearing.

Answer 3

As you discovered by printing out the output at different points and re-saving what you originally loaded, the line data2.append([int(round(math.sin(data[i][0])*3000)), int(round(math.sin(data[i][1])*3000))]) is the source of the problem.

I suspect that 3000 is too large of an amplitude. Try 1.