WAV audio file handling in SciPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

What Python library and function do you use to read WAV audio files?

You use the scipy.io.wavfile.read function from the scipy library to read WAV audio files.

beginner

What two pieces of information does scipy.io.wavfile.read return when reading a WAV file?

It returns the sample rate (number of samples per second) and the audio data as a NumPy array.

beginner

How do you write audio data back to a WAV file using scipy?

Use scipy.io.wavfile.write(filename, rate, data) where filename is the output file name, rate is the sample rate, and data is the audio data array.

intermediate

What data type is the audio data returned by scipy.io.wavfile.read?

The audio data is a NumPy array, usually with integer types like int16 for 16-bit audio.

beginner

Why is the sample rate important when working with WAV files?

The sample rate tells you how many audio samples are recorded per second. It affects the playback speed and quality of the audio.

Which function reads a WAV file in scipy?

A scipy.io.wavfile.read

B scipy.io.wavfile.load

C scipy.audio.readwav

D scipy.soundfile.read

The correct function to read WAV files in scipy is scipy.io.wavfile.read.

What does the sample rate represent in a WAV file?

A Number of samples per second

B Number of audio channels

C Length of the audio in seconds

D Bit depth of the audio

Sample rate is the number of audio samples recorded per second.

What data type is typically used for 16-bit WAV audio data in scipy?

A int8

B int16

C float32

D uint8

16-bit audio data is usually stored as int16 integers.

Which function writes audio data to a WAV file in scipy?

A scipy.io.wavfile.save

B scipy.soundfile.write

C scipy.audio.writewav

D scipy.io.wavfile.write

Use scipy.io.wavfile.write to save audio data to a WAV file.

If you want to play back a WAV file correctly, which information is essential?

A File creation date

B File size

C Sample rate

D Number of channels only

The sample rate is essential to play back the audio at the correct speed and pitch.

Explain how to read a WAV file using scipy and what information you get from it.

Think about the two outputs from the read function.

Describe the steps to save audio data back to a WAV file using scipy.

Remember the order of arguments in the write function.

Practice

1. What does the function scipy.io.wavfile.read return when you load a WAV audio file?

easy

A. The file size and duration

B. Only the audio data as a list

C. The sample rate and the audio data as a NumPy array

D. The audio format and bit depth

Solution

1. Step 1: Understand the function purpose

   scipy.io.wavfile.read is designed to load WAV files and extract audio information.

2. Step 2: Identify the returned values

   It returns two things: the sample rate (how many samples per second) and the audio data as a NumPy array.

3. Final Answer:

   The sample rate and the audio data as a NumPy array -> Option C

4. Quick Check:

   read() returns (rate, data) [OK]

Hint: Remember read() gives rate and data array [OK]

Common Mistakes:

• Thinking it returns only audio data
• Confusing sample rate with file size
• Expecting metadata like format or bit depth

2. Which of the following is the correct way to import the WAV file reading function from scipy?

easy

A. from scipy.io import wavfile

B. import scipy.wavfile.read

C. from scipy import wavfile.read

D. import wavfile from scipy.io

Solution

1. Step 1: Recall correct import syntax

   Python imports use 'from module import function_or_submodule' format.

2. Step 2: Match with scipy structure

   The correct way is to import the wavfile submodule from scipy.io as from scipy.io import wavfile.

3. Final Answer:

   from scipy.io import wavfile -> Option A

4. Quick Check:

   Correct import syntax = from scipy.io import wavfile [OK]

Hint: Use 'from scipy.io import wavfile' to access read/write [OK]

Common Mistakes:

• Trying to import read directly
• Using dot notation incorrectly in import
• Swapping import order

3. What will be the output shape of the data array when you read a stereo WAV file with 44100 samples per channel using scipy.io.wavfile.read?

medium

A. (88200,)

B. (44100, 2)

C. (44100,)

D. (2, 44100)

Solution

1. Step 1: Understand stereo audio data shape

   Stereo audio has two channels, so data shape is (samples, channels).

2. Step 2: Calculate shape for 44100 samples

   With 44100 samples per channel and 2 channels, shape is (44100, 2).

3. Final Answer:

   (44100, 2) -> Option B

4. Quick Check:

   Stereo shape = (samples, 2) [OK]

Hint: Stereo data shape is (samples, 2) always [OK]

Common Mistakes:

• Confusing channels and samples order
• Assuming shape is (2, samples)
• Thinking stereo data is flattened

4. You try to save a NumPy array with float values using scipy.io.wavfile.write but get an error. What is the likely cause?

medium

A. The file path is incorrect

B. The sample rate is missing

C. The array shape is (samples, 2) instead of (2, samples)

D. The array must be integer type, not float

Solution

1. Step 1: Check data type requirements for write()

   scipy.io.wavfile.write expects integer arrays (e.g., int16) for audio data.

2. Step 2: Identify error cause

   Using float arrays causes errors because WAV format stores integers, so conversion is needed.

3. Final Answer:

   The array must be integer type, not float -> Option D

4. Quick Check:

   write() needs int arrays [OK]

Hint: Convert floats to int before writing WAV [OK]

Common Mistakes:

• Ignoring data type and writing floats directly
• Forgetting sample rate argument
• Misunderstanding array shape requirements

5. You want to double the speed of a WAV audio file using scipy.io.wavfile. Which approach correctly achieves this?

hard

A. Double the sample rate value and write the original data unchanged

B. Read the file, then write only every second sample to a new file

C. Halve the sample rate value and write the original data unchanged

D. Reverse the audio data array and write it with the original sample rate

Solution

1. Step 1: Understand speed and sample rate relation

   Speed changes by adjusting sample rate: doubling sample rate doubles playback speed.

2. Step 2: Apply correct method

   Keep audio data unchanged but write with double the original sample rate to speed up playback.

3. Final Answer:

   Double the sample rate value and write the original data unchanged -> Option A

4. Quick Check:

   Speed ∝ sample rate, double rate doubles speed [OK]

Hint: Change sample rate to speed up audio, not data length [OK]

Common Mistakes:

• Skipping samples instead of changing sample rate
• Halving sample rate to speed up (actually slows down)
• Reversing data does not change speed