# Matlab Spectrogram: How to convert the dB values to SPL?

조회 수: 15 (최근 30일)
Darren Ramsingh . 2015년 3월 18일
답변: Mathieu NOE . 2023년 9월 22일
Guys,
So I have to plot this spectrogram, but of course when I did the magnitude scale came out in -dB. Is there some way to change this to SPL, Pa?
The following is the code used;
if true
[x, Fs] = wavread ('C:\Users\User\Desktop\3020 Recordings_Final\C#\Straight Legs\Jawari Design 1 (Staright Legs)\Apple Ma\No Fret_d1am');%Reads file into Matlab
D = x(:,1); % Extracting channel 1 data
% plotting of the spectrogram
figure (21)
spectrogram(D, 1024, 3/4*1024, [], Fs, 'yaxis')
h = colorbar;
set(h, 'FontName', 'Times New Roman', 'FontSize', 14)
ylabel(h, 'Magnitude (dB');
xlabel('Time, s', 'fontsize', 16)
ylabel('Frequency, Hz', 'fontsize', 16)
title('Spectrogram of the signal', 'fontsize', 20)
end
Also I have the calibration factor from using a pistonphone; 85.91Pa
My problem is, I don't know what Matlab is doing within that function when it calculates the dB value.
Any help will be much appreciated, thanks!
##### 댓글 수: 1없음 표시없음 숨기기
Joaquin 2023년 9월 21일
Hello,
I have exactly the same question. If you have managed to solve it, I would appreciate it if you could tell me how, because I can't get the colorbar in the spectrogram to reflect SPL in Pa or dB.
Thank you so much,
Joaquin

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### 답변 (1개)

Mathieu NOE 2023년 9월 22일
hello
I admit it's not always evident to know how n normalize spectrogram data , especially when now in the new matlab releases the main code is hidden.
I ended up doing my own spectrogram code where I know what I do . I was just a bit lazzy so I took only one window (hanning) by default and it's corresponding amplitude correction coefficient
the code belows simply compute the spectrogram data and you can correct for the calibration factor , assuming you have first read the calibration wav file (pistonphone), then once the scale _factor is known (see to the code ) you must hard code that value for the next wav files
here for example a created a sine wave with 0.23 amplitude (this could be what you have in your wav calibration file) , that is supposed to correspond to the 85.91Pa level
therefore here the scale factor is obtained in this line : scale_factor = 85.91./max(abs(D));
full code :
%%%%%%%%%%%%%%%%%%%%%%%
% dummy data
Fs = 2e3;
dt = 1/Fs;
samples = 10000;
t = (0:samples-1)'*dt;
x = 0.23*sin(2*pi*100*t); % sine wave of amplitude whatever , obtained by calibration (pistonphone)
%%%%%%%%%%%%%%%%%%%%%%%
D = x(:,1); % Extracting channel 1 data
% scale factor : max amplitude read from wav file correspond to : pistonphone; 85.91Pa
scale_factor = 85.91./max(abs(D));
% plotting of the spectrogram
nfft = 1024;
overlap = 3/4;
[S,F,T] = myspecgram(D, Fs, nfft, overlap);
% apply scale factor
S = S*scale_factor;
figure (21)
imagesc(T,F,S)
set(gca,'YDir','normal')
h = colorbar;
set(h, 'FontName', 'Times New Roman', 'FontSize', 14)
ylabel(h, 'Magnitude');
xlabel('Time, s', 'fontsize', 16)
ylabel('Frequency, Hz', 'fontsize', 16)
title('Spectrogram of the signal', 'fontsize', 20)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [fft_specgram,freq_vector,time] = myspecgram(signal, Fs, nfft, Overlap)
% FFT peak spectrogram of signal (example sinus amplitude 1 = 0 dB after fft).
% signal - input signal,
% Fs - Sampling frequency (Hz).
% nfft - FFT window size
% Overlap - buffer overlap % (between 0 and 0.95)
signal = signal(:);
samples = length(signal);
% fill signal with zeros if its length is lower than nfft
if samples<nfft
s_tmp = zeros(nfft,1);
s_tmp((1:samples),:) = signal;
signal = s_tmp;
samples = nfft;
end
% window : hanning
window = hanning(nfft);
window = window(:);
% compute fft with overlap
offset = fix((1-Overlap)*nfft);
spectnum = 1+ fix((samples-nfft)/offset); % Number of windows
% % for info is equivalent to :
% noverlap = Overlap*nfft;
% spectnum = fix((samples-noverlap)/(nfft-noverlap)); % Number of windows
% main loop
fft_specgram = [];
for ci=1:spectnum
start = (ci-1)*offset;
sw = signal((1+start):(start+nfft)).*window;
fft_specgram = [fft_specgram abs(fft(sw))*4/nfft]; % X=fft(x.*hanning(N))*4/N; % hanning only
end
% one sidded fft spectrum % Select first half
if rem(nfft,2) % nfft odd
select = (1:(nfft+1)/2)';
else
select = (1:nfft/2+1)';
end
fft_specgram = fft_specgram(select,:);
freq_vector = (select - 1)*Fs/nfft;
% time vector
% time stamps are defined in the middle of the buffer
time = ((0:spectnum-1)*offset + round(nfft/2))/Fs;
end

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