Easy, make a pivoted arm that scans vertically through the near field signal to be mapped from your speaker or microwave horn/lens.
. Place microphone or detector diode on the tip of the free end of the arm. Koch used a neon lamp, but these days we have LEds.
Place the light just behind the detector on the wooden pivoting arm.
The above is your Y scanning axis, and drive the arm with a motor or a pully and string.
The X axis is the pivoted arm described above, mounted on a cart on a track.
The lamp is driven by a mixer, amplitude detector, or phase detector , depending on what you seek.
The signal source is a reasonably stable, coherent, oscillator split between the sending transducer and providing a phase reference to whatever drives the lamp.
Place a camera at right angles to the track in a darkened room.
Turn everything on. Set the camera on " bulb" if using film and open the shutter. These. Days you'd use "integrate, average, or time exposure" on a digital camera.
Leave the shutter open. Sweep the arm,
Move the cart back, sweep the arm, then repeat as needed.
Close shutter when done.
The wave will show up as a series of light and dark spaces proportional to amplitude and phase of the signal in your time exposure.
Makes really neat near field plots with short wavelengths.
If you Google "koch bell labs sound " in "Googlr images" something will pop up.
Steve