# Toolbox spectrum - Forum - XBeach

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## Forum

### Toolbox spectrum

AW

##### Andy W, modified 4 Years ago.

#### Toolbox spectrum

Capillary Posts: 1 Join Date: 2/16/16 Recent Posts 00

Hi All,

I am relatively new to XBeach and the Matlab Toolbox. I am interested in the low frequency waves/ infragravity waves/bound long waves and therefore I am looking at the low frequency part of a spectrum. Therefore, I am doing a spectral analysis with the XBeach Matlab Toolbox.

Using the Matlab Toolbox and the command xb_get_spectrum, xb_plot_spectrum, I obtain 2 spectra in 1 plot, see attachment. The input I gave is a time serie (6 hours, every second a water level) of water level. Other settings I just leave it as default.

where ts is my time serie of water level (see attachment).

I do not understand what these 2 spectra indicate. The left one has very low energy over the whole frequency domain (which I do expect from my simulations). The right one has a large peak at the low frequency, but I have no idea where this comes from. Initially I thought it was a zoom-in of the left picture, but it looks like two different spectra.

Could someone tell me what is going on?

I greatly appreciate your help!

Cheers,

I am relatively new to XBeach and the Matlab Toolbox. I am interested in the low frequency waves/ infragravity waves/bound long waves and therefore I am looking at the low frequency part of a spectrum. Therefore, I am doing a spectral analysis with the XBeach Matlab Toolbox.

Using the Matlab Toolbox and the command xb_get_spectrum, xb_plot_spectrum, I obtain 2 spectra in 1 plot, see attachment. The input I gave is a time serie (6 hours, every second a water level) of water level. Other settings I just leave it as default.

spectrum1 = xb_get spectrum(ts); | |

xb_plot_spectrum(spectrum1); |

where ts is my time serie of water level (see attachment).

I do not understand what these 2 spectra indicate. The left one has very low energy over the whole frequency domain (which I do expect from my simulations). The right one has a large peak at the low frequency, but I have no idea where this comes from. Initially I thought it was a zoom-in of the left picture, but it looks like two different spectra.

Could someone tell me what is going on?

I greatly appreciate your help!

Cheers,

### Attachments:

##### Bas Hoonhout, modified 4 Years ago.

#### RE: Toolbox spectrum

Infra-gravity Posts: 362 Join Date: 5/20/11 Recent Posts 00

Hi Andy,

I think the first spectrum is the spectrum of time itself :-)

The function xb_get_spectrum assumes the time series has equidistant points in time. Therefore you do not need to supply time as a column in "ts". Each colum is treated as a separate time series and ends up as a subplot in the figure created by xb_plot_spectrum. You can set the sample frequency using the keyword "sfreq". So this is probably what you want:

Although the default of "sfreq" is 1, so you can skip that part.

Bas

I think the first spectrum is the spectrum of time itself :-)

The function xb_get_spectrum assumes the time series has equidistant points in time. Therefore you do not need to supply time as a column in "ts". Each colum is treated as a separate time series and ends up as a subplot in the figure created by xb_plot_spectrum. You can set the sample frequency using the keyword "sfreq". So this is probably what you want:

s = xb_get_spectrum(ts(:,2), 'sfreq', 1) |

Although the default of "sfreq" is 1, so you can skip that part.

Bas

SM

##### sweetz mwtey, modified 3 Months ago.

#### RE: Toolbox spectrum

Capillary Posts: 16 Join Date: 12/4/19 Recent Posts 00

WB

##### will burn, modified 3 Months ago.

#### RE: Toolbox spectrum

Capillary Posts: 1 Join Date: 12/23/19 Recent Posts 00

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