Here are the matplotlib.specgram parameters
matplotlib.mlab.specgram(x,
NFFT=256,
Fs=2,
detrend=<function detrend_none at 0x1dd6410>,
window=<function window_hanning at 0x1e0b1b8>,
noverlap=128,
pad_to=None,
sides='default',
scale_by_freq=None)
The parameters provided in the question description need to be converted to comparable mpl.specgram
parameters. The following is an example of the mapping:
View range: 0-8000Hz Fs=16000
Window length: 0.005s NFFT = int(Fs*0.005) = 80
noverlap = int(Fs*0.0025) = 40
Dynamic range: 70dB n/a
Time steps: 1000 n/a
Frequency steps: 250
Window shape: Gaussian default window is hanning change to gaussian
If you use 8ms you will get a power of 2 FFT (128). The following is the description of the Praat settings from their website
View range (Hz) :
the range of frequencies to display. The standard is 0 Hz at the bottom and 5000 Hz at
the top. If this maximum frequency is higher than the Nyquist frequency of the Sound
(which is half its sampling frequency), some values in the spectrogram will be zero,
and the higher frequencies will be drawn in white. You can see this if you record a
Sound at 44100 Hz and set the view range from 0 Hz to 25000 Hz.
Window length :
the duration of the analysis window. If this is 0.005 seconds (the standard),
Praat uses for each frame the part of the sound that lies between 0.0025 seconds
before and 0.0025 seconds after the centre of that frame (for Gaussian windows,
Praat actually uses a bit more than that). The window length determines the
bandwidth of the spectral analysis, i.e. the width of the horizontal line in
the spectrogram of a pure sine wave (see below). For a Gaussian window, the -3 dB
bandwidth is 2*sqrt(6*ln(2))/(π*Window length), or 1.2982804 / Window length.
To get a broad-band' spectrogram (bandwidth 260 Hz), keep the standard window
length of 5 ms; to get anarrow-band' spectrogram (bandwidth 43 Hz), set it to
30 ms (0.03 seconds). The other window shapes give slightly different values.
Dynamic range (dB) :
All values that are more than Dynamic range dB below the maximum (perhaps after
dynamic compression, see Advanced spectrogram settings...) will be drawn in white.
Values in-between have appropriate shades of grey. Thus, if the highest peak in
the spectrogram has a height of 30 dB/Hz, and the dynamic range is 50 dB (which
is the standard value), then values below -20 dB/Hz will be drawn in white, and
values between -20 dB/Hz and 30 dB/Hz will be drawn in various shades of grey.
Link to Praat settings
The OP's question might be concerning the contrast difference between the Praat
specgram and the mpl (matplotlib) specgram. Praat has a Dynamic Range setting
which affects the contrast. The mpl function does not have a similar setting/parameter. The mpl.specgram does return the 2D array of power levels (the spectrogram) the
dynamic range could be applied to the return array and re-plotted.
The following is a code snippet to create the plots below. The example is ~1m15s speech with a chirp from 20Hz-8000Hz.
import numpy
import pylab
import wave
import array
pylab.close('all')
w1 = wave.open('example_no_noise.wav')
w2 = wave.open('example_noise.wav')
# hmmm, probably a better way to do this, scipy.io function?
x1 = numpy.array(array.array('h', w1.readframes(w1.getnframes())))
x2 = numpy.array(array.array('h', w2.readframes(w2.getnframes())))
x1 = x1 / (2.**(16-1)) # normalize
x2 = x2 / (2.**(16-1)) # normalize
Fs = 16000.
NFFT = int(Fs*0.005) # 5ms window
noverlap = int(Fs*0.0025)
pylab.figure(1)
pylab.specgram(x1, NFFT=NFFT, Fs=Fs, noverlap=noverlap,
cmap=pylab.get_cmap('Greys'))
pylab.title('Full 1m15s example min noise')
pylab.figure(2)
pylab.specgram(x2, NFFT=NFFT, Fs=Fs, noverlap=noverlap,
cmap=pylab.get_cmap('Greys'))
pylab.title('Full 1m15s example more noise')
pylab.figure(3); n=2100*176;
pylab.specgram(x2[n:n+256*256], NFFT=NFFT, Fs=Fs, noverlap=noverlap,
cmap=pylab.get_cmap('Greys'))
pylab.title('Full ~4s example min noise')
pylab.figure(4); pylab.plot(x1[n:n+256*256])
