Check the units of the pyplot specgram to determine if the output is in terms of the original unit or unit-squared.

gistfile1.py

"""
Check the units of the pyplot specgram to determine if the
output is in terms of the original unit or unit-squared.
"""

import numpy as np
import matplotlib.pyplot as plt


def find_nearest(array, value):
    idx = (np.abs(array - value)).argmin()
    return (idx, array[idx])


Fs = 10000.0        # Hz
duration = 10       # seconds
t = np.arange(0, duration, 1 / Fs)

frequency = 100.0    # Hz
amplitude = 4.0     # volts
y = amplitude * np.sin(2 * np.pi * frequency * t)

plt.subplot(311)
plt.plot(t, y)

plt.subplot(312)
nfft = 8192
Pxx, freqs, bins, im = plt.specgram(y, Fs=Fs, NFFT=nfft, pad_to=nfft)
plt.ylim([0, 300])

plt.subplot(313)
index, nearest_freq = find_nearest(freqs, frequency)
print "Nearest frequency: " + str(nearest_freq)
plt.plot(bins, Pxx[index, :])
plt.ylim([0, 5])

plt.tight_layout()
plt.show()