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.

  • $\begingroup$ noise per burned fuel rate would make 120db/2 kg/s = 60 db s/kg better than 100/1 db s/kg $\endgroup$ – L.Dutch - Reinstate Monica Jun 26 '19 at 8:49
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    $\begingroup$ @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. $\endgroup$ – Eth Jun 26 '19 at 8:52
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    $\begingroup$ @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! $\endgroup$ – Samwise Jun 26 '19 at 22:46

What defines a pulsejet as a jet is the speed of the exhaust. What makes it loud is the pulse.

The pulse firing in the engine accelerates the gasses out of the system and then new air enters more easily though the "intake" than the exhaust pipe. On a valveless pulsejet air also exits via the intake pipe, hence the "both pipes backwards" setup of many such engines you'll see. It works simply by virtue of having a lower air mass on the intake than the exhaust, so you're not just sucking all your exhaust gasses back in and suffocating the engine.

Given that your intent is to burn fuel rather than create propulsion, (my first question is why aren't you just burning it rather than using a setup that's specifically designed for propulsion) and using a valved engine you can happily baffle and silence the exhaust through the usual mechanisms on car exhausts. The exhaust gasses are expanded in a controlled environment and the pulses that come from the cylinders firing are damped through multiple rounds of diffusion and compression. This way you can easily have a quiet incinerator with no propulsive effect.

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  • $\begingroup$ They also could use an internal combustion engine, but a pulsejet has less moving parts and can more easily burn less-than-ideal fuel. And a pulsed deflagration happens to give more of that supernatural effect. This is totally not because pulsejet-wielding wizards are a fun idea. Nope, not at all. Purely practical here. $\endgroup$ – Eth Jun 28 '19 at 14:59
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    $\begingroup$ Witches also like pulsejets to "magically" power their brooms in flight... $\endgroup$ – Thucydides Jun 30 '19 at 5:04

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