k-Wave Toolbox |

## Modelling Sensor Directivity in 2D Example

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Overview Defining a large element detector Defining a collection of point sources Running simulations for each point source Plotting

### Overview

This example demonstrates how the sensitivity of a large single element detector varies with the angular position of a point-like source. It builds on Monopole Point Source In A Homogeneous Propagation Medium and Focussed Detector in 2D examples.

- open the file in the MATLAB Editor
- run the file in MATLAB

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### Defining a large element detector

A large area detector is defined as a line (as this example is in 2D) and stored in `sensor.mask`.

    % define a large area detector
    sensor.mask = zeros(Nx, Ny);
    sensor.mask(Nx/2+1, (Ny/2-9):(Ny/2+11)) = 1;

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### Defining a collection of point sources

Equi-spaced point sources are defined at a fixed distance from the centre of the detector face. To do this, the Cartesian coordinates of the points are calculated using `makeCartCircle` and then a binary source mask corresponding to these Cartesian points is calculated using `cart2grid`. The indices of the matrix elements for which the binary mask is equal to 1 (the source points) are found using `find`.

    % define equally spaced point sources lying on a circle centred at the
    % centre of the detector face
    circle = makeCartCircle(30*dx, 11, [0, 0], pi);

    % find the binary sensor mask most closely corresponding to the cartesian
    % points coordinates from makeCartCircle
    circle = cart2grid(kgrid,circle);

    % find the indices of the sources in the binary source mask
    source_positions = find(circle == 1);

A time varying pressure source is defined to drive the point sources.

    % define a time varying sinusoidal source
    source_freq = 0.25e6;
    source_mag = 1;
    source.p = source_mag*sin(2*pi*source_freq*kgrid.t_array);

    % smooth the source
    source.p = filterTimeSeries(kgrid, medium, source.p);

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### Running simulations for each point source

For each point source, a binary source mask is set to one at the point corresponding to that source, and the simulation is run. When each simulation has finished, the calculated time series (one for each grid point of the detector) are summed together to mimic the fact that in this example the grid points together define one single large measuring element. (This is in contrast to when each grid point is treated as a element of an array, see Steering A Linear Array Example.)

    for source_loop = 1:length(source_positions)

        % select a point source
        source.p_mask = zeros(Nx, Ny);
        source.p_mask(source_positions(source_loop)) = 1;

        % create a display mask to display the transducer
        display_mask = source.p_mask + sensor.mask;

        % run the simulation
        input_args = {'DisplayMask', display_mask, 'PlotScale', [-0.5 0.5]};
        sensor_data = kspaceFirstOrder2D(kgrid, medium, source, sensor, input_args{:});

        % average the data recorded for each grid point to simulate the
        % measured signal from a large aperture, single element, detector
        single_element_data(:, source_loop) = sum(sensor_data, 1);

    end

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### Plotting

Finally, time series corresponding to the different point sources are plotted, and their maxima are plotted as a function of the angle between the detector plane and a line joining the point source and the centre of the detector face. The directionality introduced by the large size of the detector (because it averages the incoming wave over its area) is clearly seen.

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© 2009, 2010, 2011 Bradley Treeby and Ben Cox.