An acoustic listening device used by the Dutch army for hearing enemy airplanes

He heard that

Yes it is dumb, so dumb in fact that the British built a big "listening device" out of concrete but it was never truly used.
https://youtu.be/04F5osXK4vw

If I recall correctly from the Tom Scott video on it, the concrete listening wall thing was pretty decent at its job, but radar was put into use soon after construction and rendered the wall effectively useless.