Sound. By definition, sound comes in two major categories: Music and Noise. These are the two shades of the same thing.
Music has consistent vibrations, while Noise is random vibrations.
Order and Chaos.
Music being Order, Noise playing the role of Chaos.

FM-X Considerations
When a whole integer frequency modulates another whole integer frequency, the result is consistent (musical) tone.
When the modulator is not a whole integer tuning, but is a fraction, when it modulates a whole integer frequency the result is described as a “bell tone”. We can pretty much agree on these general terms — they are broad and a large % of humans agree on these descriptions.
We cannot really know what someone else sees or hears, but we generally agree what “red” is or what sounds like a “bell”.

__ When an Operator that is a Modulator is set to Coarse Tune 1.00 and it is set to interact with an Operator as Carrier - also tuned to 1.00, the result is a Waveform with every harmonic pretty much getting softer and softer as the harmonic series goes higher and higher. This is the Sawtooth Wave family (called this because the energy output resembles the teeth of a saw tool).
__ If the Modulator is 2.00 and the Carrier is 1.00 the result will be the Pulse wave Family (a perfect 2:1 = a Square wave — the sound is either On or Off… you will hear every other harmonic as it climbs the series... Just the odd numbered harmonics are heard.

The more you increase the Output Level of the Modulator the more character (edge) the sound will exhibit. Once you go beyond a certain point the sound starts to over modulate and dissonance/distortion is possible. If you know the harmonic series, the intervals between harmonics gets increasing small until every pitch is represented (soft, but represented).

If you feedback a Modulator on itself, then feed it into a Carrier, you will begin to go beyond what we describe as dissonance/distortion, to where the result includes nothing but “chaos”… this chaos is less musical, it includes not only every harmonic it includes every frequency.
All frequencies mixed together defines Noise.
__ A Modulator fed back on itself, then fed to a Carrier will start to produce the chaos of frequencies we describe as noise__

All colors of paint mixed together makes what we describe as the color black.
All colors of light mixed together makes light that we describe as white.
This is because they are two shades of the same color, like Sound is noise and music and two shades of the same thing.

Obviously, not all Noise is the same, just like not all musical pitches are the same. They can have unique character.
White noise, Pink Noise are terms used to describe noise quality… either all frequencies with the same output, or the energy being equal per octave.

Speaking is a combination of musical tones and noise. Vowels are your musical equivalents (a, e, i, o, u) while the consonants are the elegant noises that make speech intelligible. You can only *sing* vowel sounds — what I mean by that is, consonants resist pitch change. When you sing, pitch is applied and controlled via the vowels… if you sing the word “smooth” it is the vowels that are pitch-important. The sound of “s” for your voice is fairly a fixed pitch of noise. Singing the word “squeeze” - the “s” sound would be a fixed frequency component of your voice.

Your “s” sound will remain fairly the same in every word you say or sing. It is the “oo” in smooth and “ee” in squeeze that have and you can control its music pitch. If you sing “oo” as a “Bb”, you can also sing it as a “F#”, but the consonant sound before it remain exactly the same in both instances.

“S” (written “ess” ) cannot support melody, but if removed or changed to another noise/consonant, this can entirely change what is being communicated. Take the words: Sing, fling, ring, ding, bling without the elegant (consonant) noise out front we don’t know what is being said.
Point: how it starts is critical. Analyze whatever sound you are designing by pitched element components and fixed components.. they can each last a differing amount of time. This where in FM you make use of multiple ‘stacks’ — the beginning of the sound maybe made by a stack of 2 Operators, the body of the sound might be made from a different stack of Operators.

Noise shaping is the next Part of the discussion. A “plosive” is a syllable that, literally, explodes into the microphone.
_ “Petunia” is word with a plosive beginning — place your microphone carefully with singers (mic their nose or their chin) to avoid that puff of air molecules we use to create plosives in our speech.
_ “Freedom” and “sizzle” are words that the noise component comes up out of the silence.
A percussive sound is like a plosive - abrupt beginning
A string pad sound might have an exaggerated slow attack, that gradually brings the sound in.

Your backfire sound… is going to be more of a Noise-based creation (vs. musical pitch-based) and be more on the plosive side of attack.
Distance from the sound source is something to be considered — because it is also instantly calculated by the ear/brain when we seek to recognize a sound.
We had a huge thunderstorm last night, here in NYC, the closer the thunder, the more clearly you hear the plosive attack, a more gradual attack, adds distance for our ear/brain’s perception. Also don’t forget Insertion Effects in MONTAGE are apart of the instrument and can create placement, time delays and movement.

Okay, we seemingly were all of the place but these all have to do with Sound Designing.

Generating Noise - there are samples in the factory ROM of both White and Pink Noise, we discussed how FM creates noise. Suggestion listen to sounds in the Sound FX Category, Natural SubCategory…. Particularly ocean (sea shore) wave sounds, thunder, etc.
The difference between it sounding like a steady rain, and it sounding the tide coming in, is really shaping the sound with envelopes, filters, effects, etc.

Differences between Pitch and Noise… often FM generated noise has a lot of pitch in it. (FM noise has its own character - some folks complain about it. I say vive la difference! and I go ahead and use what it has to offer. And mostly this only gets noticed when mapped to an Equal Temperament Keyboard… if it’s a sound effect, play it like it’s one.

Shape detail is a key. How it comes in, what it does while it is sounding, how long it lasts, and how it goes away.
Other details deal with if there are multiple instances in a row, the spacing, and the relative volume of each becomes an important factor.

Alternatives
Find the sound, record it. 44.1kHz, 24-bit .wav
Load it to the MONTAGE
Create a MONTAGE Waveform
Build it into a Part/Performance

😀 😀 😀 😀
Hey, hey...it is a Merlin 27-litre, liquid-cooled V12 engine, producing a power output of 2000 horsepower that saw the Battle of England airborne while bringing order over the World.

So let there be noise, music and backfires.

You simply just listen carefully for a couple of hours and certainly by the time you run out of fuel you will surely have discovered the hidden beautiful beat.

You then understand 2/2, 3/4, 4/4, 5/7 and so on are just nothing but oversimplified backfires for people in a rush.

Oh, dear, I have just being told Montage lacks a pack of engines backfiring !!!!

Cannot be true. This is my dreamed machine.

Bad Mister, you legend.

Thank you, you've given me more than enough confidence that I'll figure this out.

i've been playing with ways to make noise, with FM, but it feels like I'm a ways off the nature of a backfire's various qualities.

As you wisely suggest, have also been experimenting with effects. It's likely in "randomising" effect values via Motion control that I'll get the variance required to make unique backfires.

Sometimes I feel like I'm close. It is kind fo fun to work on this sort of atonal sound design with FM, as it liberates me from most considerations of getting a 'tone" and can just experiment and explore.

The effects make a HUGE difference. Particularly distortions and very rapid delays.

Can you think of any way to make 'random' firings of the backfire? ie the spacings between each to be random, and the number of them to be random. I'm not fussed if this isn't possible, as I can program some MIDI to do this, but am just generally curious if this is possible onboard.

Again, thank you!

To @Fernando - I love you.

Envelope follower is a way for one Part to have an influence on another. I sometimes use this for pseudo-random.

@Jason, sorry to bother you, you've got a particular talent that might be pertinent to this problem, that I do not have, at all.

If you listen to a Subaru WRX backfire, or that of a Rotary engined RX7, or whatever is your fav kind of car backfire, what do you hear that sound being made up of?

I am unable to ascertain what sorts of pitches and noises and envelopes are what make these things so distinctively recognisable and enjoyable to hear.

I'm not sure what the entirety of the goal is. That is, I don't know the "devil in the detail" where one specific idea would not yield the desired result or would satisfy.

My impression is that sampling a number of backfires then devising a way to play one of the samples at a time would be a way to vary backfires. This may be easier to create or obtain samples vs. constructing them with FM-X.

Agreed. If I had a project where I needed to replicate the backfire sound of a specific vehicle… the first thing I would do is take my handheld recorder and record several different backfires. Because replicating any sound with synthesis require A/B comparisons (lots and lots of them).

Back in the day, we would tackle making sound effects with FM - listen, tweak, listen, tweak, it’s a process.
If you get good recording of the backfire… load it as a .wav to your MONTAGE.

In either case, a specific sound like you’re attempting will certainly require comparisons with the real thing — doing it from memory is simply the most difficult way to proceed.

If after you have the sample, you still want to mimic it with FM-X …that would be a plan. It depends on your goal.

You're both getting distracted by the examples (PLURAL) which were only used to (hopefully) trigger you to think of the backfires YOU recognise.

This was so that you'd have a mental model to consider what makes a backfire characteristic ie obviously not a gunshot, obviously not some kind of explosion... noticeably a backfire.

Pick anyone that YOU recognise, and then think about what makes it distinctively a backfire.

Here's why I say this: car backfires, when you're close to them, have a quality that's distinctively "of a car".

I don't have the audio discernment to know what's making them so distinctively "car-ish" but I suspect it's the bellow effect of the exhaust system.

If you're not into cars or bikes enough to have this kind of recollection of these sounds, try youtubing for some backfires. There's millions of clips of them, because folks enjoy these sounds, for some strange reason...

As to sampling, I'm not trying to make a specific backfire, I'm trying to get an understanding of what makes backfires so distinctively backfire sounds, and then hoping to be able to make many, many variations of them, having figured out the base recipe.

Have a listen to some of the diversity they have, but are still, even recorded with things like phones, obviously backfires:

https://youtu.be/b5Zi-8zwIZo

When I listen to the video I would say that what makes a backfire sound like it comes from a car (and not weapons or a helicopter) is the surrounding context. The preceding and following engine sounds heard. The backfires outside of this context don't sound like backfires to me.

What also helps in the video is the visual.

I'm not sure what you're trying to do with the backfires - so it's difficult for me to say.

If I painted an abstract painting with an orange splotch with some red lines and a blue background -- I would call the piece "66 Pontiac GTO Backfire". Here, the title would give my abstract painting it's clarity.

If I held a stapler up to the side of my head and nodded my head - you'd see a phone and not a stapler. The stapler wouldn't need to be much of the shape or color of any given cellphone model.

... now this is entirely different if you say you have 1 second to produce.a single sound that makes someone think "backfire". If that's the goal - then there would be little room for suggestive context.

If you could shut your eyes, and block out the other sounds, can you hear what's making up the sound of a backfire?

Sure, but which one? I would pick one and record it. Then I would listen to it while programming. Doesn’t change my methodology, not one bit.
Without context (the pictures and other surrounding engine revving noises or squealing tires) they are mostly exactly like gunshots (what people commonly think first, until they also notice the “context”, the tires, the engine, or something that says “lower the threat response, it’s not a gun…”

Without context, it could be fireworks, gunshots or backfires… so the smart programmer would also include the context in the result. The listener will always think GUN SHOTs! first… Without context not only are your “eyes shut” you cutoff most of your brain’s ability to figure out what it’s hearing.

Understanding reciprocating engine backfire...

The sound comes from inside an exhaust pipe.

The sound travels along the exhaust from a critical area.

The critical area of origin does move along the exhaust depending upon a ratio of fuel to air.

Too poor a ratio will not ignite and a proper ratio would have granted ignition prior to reaching the exhaust, so too rich a mixture is required for backfire to happen.

As the intially too rich mixture moves along the exhaust, the fuel in the air expands or deposits on the pipe, making a bit leaner the mixture along the flow and if the exhaust is too short, the mixture reaches the atmosphere and that is all; but if the exhaust is long enough, the mixture will approach the stoichiometric 1/15 ratio (fuel/air) and ignite thanks to the high temperature inside the pipe. Other different ratios such as 1/9, 1/10,1/11.... 1/14 can also achive combustion thanks to the inner turbulence in the chamber, a fact that gave way to the cleaver design of advanced piston heads far from being plain flat...

This first combustion will change the ambient conditions inside the pipe, increasing its temperature and thereby decreasing the density of the remaining air, and together will change the location of the next point or area originating the backfire to come, since the very same released mixture will need more travel along the previously heated pipe and the thinner air that adds richness (same fuel/less mass of air) to the new arrival...and the circle continues on and on till detonations are seen at the very end of the exhaust pipe. At this point the noise...music !!!! seems to lock or settle, since there are no changes in the location originating the sound, no Doppler effect....the woofer, the tweeter are not moving anymore.

So the sound has been changing thanks to varying loads of changing mixtures igniting at different points of a varying inner length pipe.

And as one calls into play another makers, the distinction from one type to another as different engines sing the song will be granted by materials (as in a pan flute, traverse flute), sizes (violin, cello, viola; piccolo, trumpet, trombone) ...tuning (A4@440 or A4@485?)...scale (D, G...).

It is a pipe organ using chaos at every single step.

And once you have had the chances to grasp them many times, it would be piece of cake to say Audi, Mercedes, Renault, Rolls-Royce, Stuka, Messerschmitt, Volvo... Yamaha's sound metallic, Steinway's sound woody.

Be aware that all this can have serious consequences for the integrity of the engine and human beings in close vecinity should the adequate mixture explode close to the engine itself and not far and along the exhaust pipe.

Backfires...? Safer with Montage.

.

Spot on, Fernando!

You poet.

Exhausts are man made, machine driven, didgeridoos. Booming with the resonances of themselves, from both the noise shoved into them and the air they corral and exhale. When some of that air gets another shot at going "boom", in the chambers of the exhaust rather than the engine itself, depending on where it's at in the pipes, what's going on with the engine's rate and push of more air/fuel, and the subsequent and prior air/fuel mix around it, there's extreme variations in resonant qualities, pitch, durations and volume.

At Bad Mister. They really do not sound like gunshots to those of us addicted to cars and bikes. If I know a car, I can tell it from all others, just by its backfire, from (quite literally) miles away, when that's the only sound from that vehicle making it to where I am. This is something to do with the exhaust system's intonation of the backfires and the way the engines tuned, and the type of engine.

But I'm not yet able to discern why they're so distinctive.

Yes, I get it, to the "untrained" ear, they might just sound like gunshots. And they might on youtube, too, due to the difficulty of recording such loud sounds with mere phones.

Which is why the initial example, so that those considering the question could think of a car they'd recognise when it backfires.

RX-7's and WRX's just happen to be two of the most distinctive, generally speaking.