Monopole in a rigid walled rectangular duct:

• Fluid is air at ambiant conditions  (c=344 m/s)
• Frequency is 1000 Hz
• Source diameter is 10 cm
• Maximum positive pressure is blue, and minimum (negative) pressure is red
• zero pressure is blue/green (the B&K color!)

                    Low frequency                                                                        High frequency
    
Movie created using MATLAB (Registered Trademark)