How to plot the spectrum of the sum of sine waves

I need to plot the spectrum of the sum of 3 sine waves (100 Hz & 20 dB SPL, 300 Hz& 30 dB SPL, and 400 Hz & 15 dB SPL). This is what I have so far, but the amplitude on the y axis seems wrong.

    % Sampling
    fs = 1000;     % Sampling rate [Hz]
    Ts = 1/fs;     % Sampling period [s]
    fNy = fs / 2;  % Nyquist frequency [Hz]
    duration = 10; % Duration [s]
    t = 0 : Ts : duration-Ts; % Time vector
    noSamples = length(t);    % Number of samples

    A_1 = 20;
    A_2 = 30;
    A_3 = 15;

    f_1 = 100;
    f_2 = 300;
    f_3 = 400;

    s_1 = A_1*sin((2*pi*f_1*t));
    s_2 = A_2*sin((2*pi*f_2*t));
    s_3 = A_3*sin((2*pi*f_3*t));

    % Contaminated signal
    xn = s_1+s_2+s_3;

    % Frequency analysis
    f = 0 : fs/noSamples : fs - fs/noSamples; % Frequency vector
    % FFT
    x_fft = abs(fft(x));
    xn_fft = abs(fft(xn));
    % Plot

    plot(f,xn_fft);
    xlim([0 fNy]);
    xlabel('Frequency (Hz)')
    ylabel('Amplitude (dB SPL)')
    title('{Spectrum}')

[Joyce]

Generate a composite signal by summing 2 sine waves of frequencies 100HZ and 300 HZ APPLY FFT components write matlab program to plot magnitude spectrum

[Voss]

@Joyce: You should post a new question about that.

https://www.mathworks.com/matlabcentral/answers/questions/new/

Answer 1

[Rick Rosson]

    x_fft = abs(fft(x)/noSamples);
    xn_fft = abs(fft(xn)/noSamples);