The sonics of combustion–as a field of inquiry, if it has any properties worth noting. I now claim it as a band name, but it originally occurred to me while while lighting my cigar with a torch lighter in bright sunlight. The lighter is fiddly, and sometimes will spout gas without sparking, and it’s annoying, somehow, after puffing and not noticing any heat, until repeated flicking changes the sound instantly, and then I notice the difference between the sound made by the streaming gas alone and the sound when it is lit–before I can tell by sight (the flame now visible) and effect (heat and smoke).
So it’s a real thing, at least for me in that context.
Anyplace else? Has the roar of a spaceship when powered been figured out for crew comfort, or as some obscure sign of something in combustion engineering?
I don’t think it’s obscure… Anything that rapidly changes the pressure of a medium can create or alter sound. It would fall under normal acoustics. A lot of this kind of work has gone into things like making jet engines quieter. Modern jet engines may seem loud (and they are) but they’re nothing like the brutal roar of the old-school turbojets and.low-bypass turbofans.
Hmmm… I’m thinking chicken and egg, for combustion. Of course…pressure drop in combustion chamber.
Is that was engine knocking in a car means? (Please don’t laugh at me.)
So, when aircraft x is “too loud,” engineers can calculate the sound of a different fuel? (Leaving aside engine design, baffles, and the rest.) Not saying they should, just wondering if it’s doable from first principles.
pre-ignition: some “hot spot” elsewhere in the cylinder ignites the air:fuel charge before the spark plug can fire, and when the plug fires the two separate flame fronts collide
detonation: either the temperature in the combustion chamber is too high or the air:fuel mix is too lean, so when the spark plug ignites the air:fuel mix it detonates rather than deflagrates (burns.)
the fuel itself has nothing to do with it. it all depends on the velocity of the exhaust leaving the engine.
In fact, the principle has been used to build high-fidelity speakers for your home entertainment system. ISTM that I first read advertisements for systems like this back in the 1960’s or 1970’s.
These speakers, unencumbered by massive magnets or paper cones in their moving parts, are free of the physical mechanical limitations of more conventional speakers, and in particular they can reproduce the highest frequencies more faithfully.
See: Plasma speaker. The actual version that I saw advertised (I never actually saw one IRL) actually used a flame. Gas was cheaper in those days.
Well, the answer then is “yes, engine knock has two causes,” isn’t it? The collision changes the immediate pressure – now, I don’t know if you/one can hear two flames collide–which is part of the fun/interest of the OP, but the heat and subsequent pressure change is reflected in the response of the chamber metal, no?
Ditto “detonation/deflagration:” that it, to my understanding, a perfect example of pyroacoustics, in the minimal sense I was/am thinking about.
look, there’s no such thing as “pyroacoustics,” OK? it’s just acoustics. sound is sound, it doesn’t matter if it comes from a speaker, a person, an explosion, something burning, a cricket, or a kookaburra. something causes a pressure disruption in a medium, and it’s transferred through that medium via waves of compression and rarefaction. what that “something” is doesn’t matter.
“From BMW to Volkswagen, motor manufacturers are adding technology to their vehicles that deliberately exaggerates the noise of their engines. The aim is to make them sound like roaring beasts from the past rather than today’s purring pussycats.”
IMHO - this comes close to pyroacoustics - but not quite.
Okay - since you want to get technical, here are examples of flame acoustics currently used in the industry.
Pilot flame acoustic detection system : Cite .See page 13. An acoustic system is used to detect the flame on the pilot burner of flares.
Acoustic impact of flares : Combustion in flare systems cause noise that is regulated by environmental standards. There are software like this - see section 5 - that model the noise from a flare at a given distance.
Gasifiers react 95%+ Oxygen with Coal or Coal Slurry or Natural Gas or Liquids (typical temperature = 2500F and Pressure = 800psi). It is extremely difficult to measure temperature in the reactor due to the erosive/corrosive environment. One method is the acoustical measurement of temperature - cite.
When multiple burners are used in a gasifier, acoustic analysis is used to check if there is flame impingement on the sides of the gasifier or the burners themselves. cite.
Preignition by itself doesn’t result in audible knock, but it can result in detonation, which is where the sound comes from.
You made a distinction between detonation and deflagration, but it ought to be explained a bit more:
Deflagration is the normal mode of operation for a spark-ignited engine: the spark starts the combustion, and combustion then proceeds at a subsonic pace. Pressure changes over time, but it’s relatively constant over distance across the combustion chamber; relatively little acoustic noise is transferred to the combustion chamber walls in this mode.
Detonation is an abnormal mode of combustion. In this mode, the mixture has been at high temperature/pressure for too long and has become unstable. Instead of a flame front propagating by thermal means (as in deflagration), the reaction is now touchy enough to be triggered by sound, and the flame front accelerates to supersonic velocities. Pressure is not uniform across the combustion chamber: the flame front is coincident with a supersonic shock wave that exhibits extremely high local pressures and temperatures. When it slams into the combustion chamber wall, it makes an audible impact, and the temperatures/pressures can erode these surfaces over time.
The audibility of knock has led to the development of knock sensors that listen for the ringing as the shock waves bounces back and forth across the combustion chamber. When the sensor hears that signal, the ECU temporarily retards spark timing to reduce peak pressures/temperatures, thereby reducing the tendency to knock.
Engine exhaust noise is, for the most part, unrelated to how combustion takes place or what fuel is being used, because combustion is done by the time the exhaust valve opens. The exhaust noise is due almost entirely to “blow-down,” the sudden release of residual combustion chamber pressure when the exhaust valve opens. The harder you press on the accelerator pedal, the greater the residual pressure there will be at EVO, and so the greater the exhaust noise.
Naturally, plasma is not quite fire, but I could see a similar core design being applied to e.g. a flamethrower with a controllable nozzle. Or possibly an organ made out of different sizes and shapes of Bic lighters.
OP here, quoting am in this reanimation on “this industry” in the quote. (It also serves nicely because upthread jz hurt my feelings (OK, “was unduly harsh”) and I never got to give a “so there.”)
Is what OP [risibly?] calls “pyroacoustics,” and the examples in the quote, properly encompassed scientifically in photoacoustics, at its broadest? (I take it that use the word “pyroacoustics” is not a thing.)
Or is “pyroacoustics” (pace myself :)) in this thread on something one step removed? Heat or light?
I.e.,
A) is photoacoustics one concerned with infrared radiation (or any other electromagnetic field–the “photo”) creating acoustic force? “On” what? There’s no aether.
I know it’s right there in front of me in the Wiki, but I lost the dividing line in my head compared to
B) is it, rather, simply explained, as in this thead, how radiation initially interacts with another material (eg material heating, such as combusting air, or chemical transition in one’s retina) initially–after which the “interaction” with a medium provides the acoustics that is measured/measurable?
Every rocket must be tested for the Max Q point, which takes into account all the vibrations that occur due to engine noise, aerodynamic forces, etc. We used to test sub assemblies on large vibrating test beds. The guys would sometimes run music through them to make a giant speaker (without a test structure on board, of course).