# How quiet can a static, non-propulsive pulsejet be?

The wizards of Magitechia have discovered that the deflagration or detonation of certain chemical compounds have beneficial supernatural effects, that are useful for powering magical apparatuses. After some experimentation, they started using pulsejets. Those things have little to no moving parts, are relatively easy to build, and will run on almost anything flammable. This is especially useful for producing stable, effectively continuous effects, which is very handy for powering said apparatuses. Assume that for varied other reasons having to do with fast deflagration (much more efficient than simply burning the stuff), constant pulse rate, and ease of isolating the engine from outside magical interference, pulsejets are indeed the most practical solution they found.

The problem is that pulsejets are loud.

How quiet can a static pulsejet be made? It doesn't have to have any propulsive capability, the goal is to pulse-deflagrate the compound, not produce useful work. It is to be used statically, like an industrial system, so ideally it doesn't need air speed to work (if necessary it can be put in a wind tunnel, but it would be simpler without it). Think industrial heat source, not aircraft engine.

The main criterion is lowest possible noise per burned fuel rate. So an engine that can burn 1 kg/s of compound at 100 dB will be better than an engine that can burn 2 kg/s at 120 dB.

Secondary criteria are the mass of the engine per burned fuel rate, and ease of production and maintenance. So at equal noise, a more compact engine with less (or no) moving parts will be better. However, noise is always the primary concern, unless the difference is enormous.

The chemical compound can anything flammable for the needs of the question, from ethanol or kerosene to gunpowder, depending on what is the most practical.

Assume technological level is the same as ours, any supernatural effect is irrelevant to technology for the needs of the question. The goal is basically to burn as much fuel for as little noise as possible with a given fuel, but with the constraint of using a pulsejet.

• noise per burned fuel rate would make 120db/2 kg/s = 60 db s/kg better than 100/1 db s/kg – L.Dutch - Reinstate Monica Jun 26 '19 at 8:49
• @L.Dutch I'm pretty sure dB are not linear, and two side-by-side 100 dB emitters will make less noise than a single 120 dB emitter. – Eth Jun 26 '19 at 8:52
• @Eth That's correct, in fact a 120dB sound is 100x louder than a 100dB sound as it goes: $Power = 10^{dB/10}$. Fun fact: adding two 100dB sounds together only gets you a 103dB sound! – Samwise Jun 26 '19 at 22:46