What do I measure with pyAudio?

Question

I am recording sound using the python pyAudio library, and plotting it with matplotlib. Here I'm recording the volume and want to know in which value it is and how many Dezibel this are.

Currently I think that it's recorded in PCM , but I'm not sure.

In this part I'm setting up a stream as described in the docs :

# constants
CHUNK = 1024 * 2             # samples per frame
FORMAT = pyaudio.paInt16     # audio format (bytes per sample?)
CHANNELS = 1                 # single channel for microphone
RATE = 44100                 # samples per second

# pyaudio class instance
mic = pyaudio.PyAudio()

# stream object to get data from microphone
stream = mic.open(
    format=FORMAT,
    channels=CHANNELS,
    rate=RATE,
    input=True,
    output=True,
    frames_per_buffer=CHUNK
)

This is the part where I'm measuring the audio-signal:

def measure():
    # binary data
    data = stream.read(CHUNK)  

    # convert data to integers, make np array, then offset it by 127
    data_int = struct.unpack(str(2 * CHUNK) + 'B', data)

    # create np array and offset by 128
    data_np = np.array(data_int, dtype='b')[::2]
    data_np = [i+127 for i in data_np]

This is the whole code:

import pyaudio      #for capturing the audio-signal
import struct       #for converting the binary-data from the signal to integer
import matplotlib.pyplot as plt     #for displaying the audio-signal

import numpy as np
import time

#functions
def plot_setup():
    # create matplotlib figure and axes
    fig=plt.figure()
    ax=fig.add_subplot(111)

    # variable for plotting
    x = np.arange(0, 2 * CHUNK, 2)

    # create a line object with random data
    line, = ax.plot(x, [128 for i in range(2048)], '-')

    # basic formatting for the axes
    ax.set_title('AUDIO WAVEFORM')
    ax.set_xlabel('samples')
    ax.set_ylabel('volume')
    ax.set_ylim(0, 255)
    ax.set_xlim(0, 2 * CHUNK)
    plt.xticks([0, CHUNK, 2 * CHUNK])
    plt.yticks([0, 128, 255])
    # show the plot
    plt.show(block=False)
    return fig, line

def measure():
    # binary data
    data = stream.read(CHUNK)  

    # convert data to integers, make np array, then offset it by 127
    data_int = struct.unpack(str(2 * CHUNK) + 'B', data)

    # create np array and offset by 128
    data_np = np.array(data_int, dtype='b')[::2]
    data_np = [i+127 for i in data_np]

    line.set_ydata(data_np)
    try:
        fig.canvas.draw()
        fig.canvas.flush_events()
    except:
        return 0

# constants
CHUNK = 1024 * 2             # samples per frame
FORMAT = pyaudio.paInt16     # audio format (bytes per sample?)
CHANNELS = 1                 # single channel for microphone
RATE = 44100                 # samples per second

# pyaudio class instance
mic = pyaudio.PyAudio()

# stream object to get data from microphone
stream = mic.open(
    format=FORMAT,
    channels=CHANNELS,
    rate=RATE,
    input=True,
    output=True,
    frames_per_buffer=CHUNK
)

if __name__=="__main__":
    fig, line=plot_setup()
    while True:
        m=measure()
        if m==0:
            break

This is the current output(plot):

On the y-axis is a value of 255 and I want to know the unit of it and convert it to Dezibel.

Answer 1

The Y-axis unit is the absolute volume level. If you want to convert to decibel dB , the equation is

Volume(dB) = 20*log10(v1/v0) where v0 is base reference level.

Below urls have lot of useful concepts related to sound, amplification, etc.

http://www.sengpielaudio.com/calculator-soundvalues.htm Above has table which relates dB vs level of sound.

http://www.sengpielaudio.com/calculator-FactorRatioLevelDecibel.htm