According to my back-of-the-envelope calculation, the flow velocity within an exhaust pipe of a standard sized car is a small fraction of the speed of sound in carbon dioxide.
This allows me to treat the exhaust pipe, for acoustic purposes, as if it were filled with static fluid.
Today’s invention is therefore a suppressor for combustion engine exhaust noise.
Each pulsewave emitted by the opening or closing of the exhaust valve is allowed to pass into the clockwise arm of the looped pipe at 1 o’clock *
(using a valve linked to the exhaust valve itself -not shown). Once the pulse travels around the loop, it is allowed to leave using another linked valve at the 11 o’clock position. By tuning the length of the loop to be (n+ 1/2)*the exhaust note wavelength, destructive interference can occur between the exhaust and loop waves, greatly reducing the engine noise emitted.
*I had to rethink this whole thing to avoid using the valves and fixed-length loop (thanks Andy, see below). Instead, a toroidal loop would be inserted in the exhaust as shown. A port would allow waves to enter and propagate around the loop. The loop itself would be spun, much faster than the wave speed, so as to position its port against the exhaust pipe. It would do this in such a way that if a compression were moving down the exhaust, the loop would inject a rarifaction and vice versa. In this way, pressure fluctuations could be eliminated within the exhaust, without introducing extra ones by the operation of conventional valves.