Lowpass.cs

namespace WpfSonarAnalyzer
{
    /// <summary>
    /// https://en.wikipedia.org/wiki/Low-pass_filter
    /// </summary>
    public class Lowpass
    {
        private double x, xPrev,
                       y, yPrev;

        public Lowpass()
        {
            x = xPrev = y = yPrev = double.NaN;
        }

        public double Next(double value, double dt, double cuttoff)
        {
            var alpha = dt / (1 / (2 * System.Math.PI * cuttoff) + dt);

            if (double.IsNaN(x))
            {
                x = xPrev = yPrev = value;
            }
            else
            {
                xPrev = x;
                yPrev = y;
            }

            x = value;
            y = yPrev + alpha * (x - yPrev);

            return y;
        }
    }
}

MainWindow.xaml

<Controls:MetroWindow x:Class="WpfSonarAnalyzer.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfSonarAnalyzer"
        mc:Ignorable="d"
        xmlns:Controls="clr-namespace:MahApps.Metro.Controls;assembly=MahApps.Metro"
        Title="MainWindow" Height="147" Width="525" BorderBrush="Black">
    <Grid>
        <ProgressBar x:Name="Progress_1" Maximum="200" HorizontalAlignment="Left" Height="10" Margin="10,10,0,0" VerticalAlignment="Top" Width="497"/>
        <ProgressBar x:Name="Progress_2" Maximum="200" HorizontalAlignment="Left" Height="10" Margin="10,25,0,0" VerticalAlignment="Top" Width="497"/>
        <ProgressBar x:Name="Progress_3" Maximum="200" HorizontalAlignment="Left" Height="10" Margin="10,40,0,0" VerticalAlignment="Top" Width="497"/>
        <Label x:Name="Label_Visitors" Content="Visitors: " Margin="10,75,115,0" VerticalAlignment="Top"/>
    </Grid>
</Controls:MetroWindow>

MainWindow.xaml.cs

using System;
using System.Windows.Threading;
using MahApps.Metro.Controls;

namespace WpfSonarAnalyzer
{
    public partial class MainWindow : MetroWindow
    {
        private const double TIMER_INTERVAL = 50;
        private DispatcherTimer _deviceTimer;
        private Robots.Models.Riddler_v10.SerialPortRobot _robot;
        private SonarAnalyzer _sonarAnalyzer;

        public MainWindow()
        {
            InitializeComponent();

            Robots.Communication.SerialPortConfiguration config = new Robots.Communication.SerialPortConfiguration("");
            config.Port = "COM8";
            config.Rate = new Robots.Communication.BaudRate("115200");
            _robot = new Robots.Models.Riddler_v10.SerialPortRobot(config);
            Robots.Models.Riddler_v10.Parts.Senses senses = ((Robots.Models.Riddler_v10.Parts.Senses)_robot.Senses);
            senses.OnSonarsUpdate += OnSonarsUpdate;

            _sonarAnalyzer = new SonarAnalyzer(100, 5);

            _deviceTimer = new DispatcherTimer();
            _deviceTimer.Interval = TimeSpan.FromMilliseconds(TIMER_INTERVAL);
            _deviceTimer.Tick += _deviceTimer_Tick;
            _deviceTimer.Start();
        }

        private void _deviceTimer_Tick(object sender, EventArgs e)
        {
            if (!_robot.Connected()) { _robot.Connect(); }
            Title = _robot.Connected().ToString();
            _robot.Senses.Update();
        }

        private void OnSonarsUpdate(Robots.Models.Riddler_v10.Parts.Senses senses)
        {
            var left = senses.Sonar_Left.Value;
            double newLeftValue = _sonarAnalyzer.SetValue(SonarPosition.Left, 1 / TIMER_INTERVAL, left);
            Progress_1.Value = newLeftValue;

            var front = senses.Sonar_Front.Value;
            double newFrontValue = _sonarAnalyzer.SetValue(SonarPosition.Front, 1 / TIMER_INTERVAL, front);
            Progress_2.Value = newFrontValue;

            var right = senses.Sonar_Right.Value;
            double newRightValue = _sonarAnalyzer.SetValue(SonarPosition.Right, 1 / TIMER_INTERVAL, right);
            Progress_3.Value = newRightValue;

            _sonarAnalyzer.Update();

            Label_Visitors.Content = string.Format("Visited: {0}, count: {1}", _sonarAnalyzer.IsVisitedNow(), _sonarAnalyzer.VisitorCount());
        }
    }
}

SonarAnalyzer.cs

namespace WpfSonarAnalyzer
{
    public enum SonarPosition
    {
        Left, Front, Right
    }

    public class SonarAnalyzer
    {
        private class SonarValue
        {
            private Lowpass _lowpass = new Lowpass();
            public double _lastValue;

            public double Get()
            {
                return _lastValue;
            }

            public double Set(double value, double dt, double cutoff)
            {
                _lastValue = _lowpass.Next(value, dt, cutoff);
                return _lastValue;
            }

            public void Reset()
            {
                _lastValue = 0;
            }
        }

        private bool _isVisited, _lastIsVisited;
        private int _visitorCount;

        private double _threshold,
                       _cutoff;

        private SonarValue _left = new SonarValue(),
                           _front = new SonarValue(),
                           _right = new SonarValue();

        public SonarAnalyzer(double threshold, double cutoff)
        {
            _threshold = threshold;
            _cutoff = cutoff;
            _visitorCount = 0;
        }

        public double SetValue(SonarPosition position, double dt, double value)
        {
            double result;
            switch (position)
            {
                case SonarPosition.Left: result = _left.Set(value, dt, _cutoff); break;
                case SonarPosition.Front: result = _front.Set(value, dt, _cutoff); break;
                case SonarPosition.Right: result = _right.Set(value, dt, _cutoff); break;
                default: result = double.NaN; break;
            }

            return result;
        }

        public void Update()
        {
            _isVisited = _left._lastValue < _threshold
                      || _right._lastValue < _threshold
                      || _front._lastValue < _threshold;

            if (_isVisited && !_lastIsVisited) { _visitorCount++; }

            _lastIsVisited = _isVisited;
        }

        public bool IsVisitedNow()
        {
            return _isVisited;
        }

        public int VisitorCount()
        {
            return _visitorCount;
        }
    }
}