Can the amplitude be plotted in dB?

Question

Taylor Gray 2019-3-12

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/449726-can-the-amplitude-be-plotted-in-db

评论： Star Strider 2020-7-21

Gun_Samples.zip

In the code and figure below I have located peaks in an audio signal using the frequency domain and I can't figure out how to have the amplitude plotted in dB. Could anyone help me please?

[audio_file, Fs] = audioread("Handgun.wav");
L = size(audio_file,1);
Ts = 1/Fs;                                                      % Sampling Interval
Fn = Fs/2;                                                      % Nyquist Frequency
FT_af = fft(audio_file)/L;                                      % Fourier Transform
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                             % Frequency Vector
Iv = 1:numel(Fv);                                               % Index Vector
[PksL,LocsL] = findpeaks(abs(FT_af(Iv,1))*2, 'MinPeakHeight',0.006, 'MinPeakDistance',20);
[PksR,LocsR] = findpeaks(abs(FT_af(Iv,2))*2, 'MinPeakHeight',0.006, 'MinPeakDistance',20);
figure(1);
semilogx(Fv, abs(FT_af(Iv,2))*2), xlabel('Frequency, Hz'),ylabel('magnitude');
hold on
plot(Fv(LocsR), PksR, '^r', 'MarkerFaceColor','r')
hold off
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Left Channel')

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Answer 1

Star Strider 2019-3-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/449726-can-the-amplitude-be-plotted-in-db#answer_365078

在 MATLAB Online 中打开

Try this:

semilogx(Fv, 20*log10(abs(FT_af(Iv,2))*2)), xlabel('Frequency, Hz'),ylabel('magnitude');

That will convert the amplitudes to dB. The y-axis will reflect those changes, however the tick marks will not be in integer powers-of-10 such as with your x-axis and the semilogx call. You will have to format the y-axis ticks yourself.

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Brad 2020-7-21

Hi Star

Why is the multiple by 2 there?

Isn't this dB simply 20 log10(y)? (as the function mag2db) is defined?

Thank you

Star Strider 2020-7-21

Brad — The fft function creates a symmetric result, with the second half being the complex-conjugate of the first half, the total signal energy being divided approximately evenly between the two halves. In a single-sided Fourier transform, it is then necessary to multiply the abs() of the result by 2 in order to approximate the original signal amplitude. Here, I multiply by 2 prior to the dB transformation.

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