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AS mad science: understanding burble. Place for discussion and amateur science.


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Data!

What does it tell me? Mostly not to stick the recorder in the energetic gas stream of the exhaust if you want to see the pressure pulses as opposed to the buffeting of the atmosphere. 

However, there is something to be understood:

The top is @mat_the_cat s Rover V8 in his Honda Stella burbling away at what i assumed to be about 1000 revs, this seems reasonable given the pressure peaks.

The bottom is my 1.9 vagshite pd at 1000 rpm.

The neat pressure peaks on my 4cyl are at every 180°. So one per combustion event, which is odd as there is a fundamental harmonic obviously a lot lower than it it would be given a naive understanding, which is wot I haz.

These rpm numbers are calculated simply by taking rpm and dividing by 60, then comparing this to the timing  which is in seconds on the software at the scale I'm looking at. 

Given this fact, what to make of the Stella exhaust, apart from that I'm a twonk for not placing my phone out of the air turbulence? I think the sample i used here is free from turbulence effects on the mic, not 100% sure, but it seems like there is constructive and destructive interference by the time the individual firing pulses reach the tip.

Mat's is a single tip, getting recordings of twin tips might be interesting. 

I also did a 2000rpm run on my vagPDshite but the increased exhaust turbulence caused buffeting similar to the Stellar recording. I want to know how rotational speed affects the grouping of firing pulses, as well as the firing order.

I gave up for tonight.

Then there's 5cyl, 10 cyl, 3 cyl, 6 and 12 cyl to wonder about.

This experiment may take some time to complete!!!

20210508_213617.jpg

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6 minutes ago, FakeConcern said:

So will you be able to explain why 5 cylinder engines sound so agreeable?

Hopefully. 

My hypothesis is that there are either implied fundamentals or some other interference going on.

I'm not a physicist, but I think we can at least get some kind of understanding of what's going on.

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1 minute ago, Crackers said:

You already have a Rover V8 sample, but since the thread is called Burble, you gave me a legal obligation to post this. Again. 

That's about 700rpm iirc. Do with this information what you will. 

It sounds clean, it may well be useful. Tomorrow...

For now though, I have a clean sample from @mat_the_cat and mine with times and good peaks...

They both work out at about 36 peaks per second. Interesting. 

20210508_224827.jpg

20210508_224711.jpg

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20 minutes ago, Tim_E said:

 what to make of the Stella exhaust, apart from that I'm a twonk for not placing my phone out of the air turbulence? I think the sample i used here is free from turbulence effects on the mic, not 100% sure, but it seems like there is constructive and destructive interference by the time the individual firing pulses reach the tip.

Mat's is a single tip, getting recordings of twin tips might be interesting. 

I also did a 2000rpm run on my vagPDshite but the increased exhaust turbulence caused buffeting similar to the Stellar recording. I want to know how rotational speed affects the grouping of firing pulses, as well as the firing order.

I gave up for tonight.

Then there's 5cyl, 10 cyl, 3 cyl, 6 and 12 cyl to wonder about.

This experiment may take some time to complete!!!

20210508_213617.jpg

This is very interesting! It does have a fairly lumpy idle, and off-idle, as a result of the cam I think - and the limitations of carb fuelling. But I think it had smoothed off mostly at the speed you were recording at.

Can I blame you for the sudden loss of coolant 20 minutes later though? 😛

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Looks like you’re using audacity to look at the waveforms. I think in audacity you can view a spectrum analysis- rather than just seeing the waveform you’ll be able to see how energy occurs across the frequency spectrum- that’s gonna be vital in getting to the bottom of this :) 

 

edit: here you go https://manual.audacityteam.org/man/plot_spectrum.html

You could also use REW software to generate a waterfall plot which will tell you how the acoustics of the environment are affecting the spectrum analysis too 

https://www.roomeqwizard.com/help/help_en-GB/html/graph_waterfall.html

Really interesting idea tho- I studied this kinda thing and do it for some of my day job, but never really thought about applying it to engines much other than idly thinking about 2 stroke expansion chambers. My rudimentary understanding of it is that the burble is to do with the firing order manifesting in some weird manifold/ exhaust pressures patterns. 

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31 minutes ago, mat_the_cat said:

This is very interesting! It does have a fairly lumpy idle, and off-idle, as a result of the cam I think - and the limitations of carb fuelling. But I think it had smoothed off mostly at the speed you were recording at.

Can I blame you for the sudden loss of coolant 20 minutes later though? 😛

No because you turned up in a steam cloud, unless you are suggesting some quantum eraser kind of effect with your coolant pipe and my phone. 

Allow me to suggest that's unlikely...😋

But yes on the rev up to 1000 rpm or so it came out clean. N/a Crossplane V8s are always going to suffer from uneven fuelling etc due to uneven exhaust scavenging. 

I'd like to get more samples with more understanding of what I'm doing 😁

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37 minutes ago, FakeConcern said:

I do and I could...

IMG_7872.thumb.JPG.4969fd871c6bb929b73da2cca8862f2d.JPG

I don't have any sophisticated recording gear though...

 

Phone is sophistimacated enough for now. 

Position it beneath the tip of the exhaust on something but not in the draught, get in and rev and hold at 1000, then 2000, then maybe 4000 then I can really get nerdy.

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I have a very limited understanding of noise and vibration from my day job, could interpret a waterfall diagram etc.

What I would suggest is seeing what has already been written on the subject, some journals etc are free on science direct and other places.

As so much work has been done in recent years on things like switchable exhausts that go from low to ASBO with the movement of a flap, the vehicle industry knows for sure how to do burble, or not, depending on the application and they often write papers to show off.

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Here's one I've yet to understand - why does a flat 4 cylinder engine (VW/Subaru) sound so distinctively different to an inline 4? Same number of cylinders and especially with the Scooby all exiting through a long single exhaust. 4 cylinders, common crank, why does the physical layout of the engine make it sound different? 

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https://blog.prosig.com/2011/08/09/how-to-measure-noise-vibration-in-rotating-machines/
 

This should help you Tim. When it gets to the end and is showing FFT vs rpm for a 4-cylinder engine, you can see the dominant order is the firing frequency, two combustion events per crank rotation, second order. That gives you a lot of the character.
Different cylinder counts have different primary firing orders, 3-cylinder engines fire 1.5 times per crank rotation etc...

That’s where a lot of the character comes from. 

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14 minutes ago, bunglebus said:

why does a flat 4 cylinder engine (VW/Subaru) sound so distinctively different to an inline 4?

I don’t know much about flat configurations, but I think this is to do with intake and exhaust manifold layout. They have an unequal header length, so by the time the sound wave pressure pulsation from each cylinder reaches the main exhaust they are phased differently. 

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9 hours ago, Fabergé Greggs said:

Looks like you’re using audacity to look at the waveforms. I think in audacity you can view a spectrum analysis- rather than just seeing the waveform you’ll be able to see how energy occurs across the frequency spectrum- that’s gonna be vital in getting to the bottom of this :) 

 

edit: here you go https://manual.audacityteam.org/man/plot_spectrum.html

You could also use REW software to generate a waterfall plot which will tell you how the acoustics of the environment are affecting the spectrum analysis too 

https://www.roomeqwizard.com/help/help_en-GB/html/graph_waterfall.html

Really interesting idea tho- I studied this kinda thing and do it for some of my day job, but never really thought about applying it to engines much other than idly thinking about 2 stroke expansion chambers. My rudimentary understanding of it is that the burble is to do with the firing order manifesting in some weird manifold/ exhaust pressures patterns. 

If you wanted to chip in and help that would be good, I'm not very good with the tech!

Yes I'm using Audacity because it's all a folk muso and a live jazz bassist needs... 😅

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1 hour ago, bunglebus said:

Here's one I've yet to understand - why does a flat 4 cylinder engine (VW/Subaru) sound so distinctively different to an inline 4? Same number of cylinders and especially with the Scooby all exiting through a long single exhaust. 4 cylinders, common crank, why does the physical layout of the engine make it sound different? 

As far as I know so far the headers play a big role in separating the two banks and essentially preventing an even hum from occurring, instead the pulses get a bit squashed together to create a rough splutter, even if the actual engine firing evenly.

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49 minutes ago, Lacquer Peel said:

Would you like samples of a straight 5 diesel and an indirect injection straight 4 diesel? It would be interesting to compare the latter with your VAG diesel, IDI has a much nicer sound than a direct injection diesel. 

I'd love that thanks.

Out of interest, I thought diesels always had direct injection even it was mechanical for most of history, as the injection is the event that times and initiated combustion?

Anyway, I'd love samples. Thanks 😊 

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9 hours ago, Tim_E said:

 

I'm not a physicist, but I think we can at least get some kind of understanding of what's going on.

Someone I worked with, had a wife. Who was a noise expert in the jet engine industry.   Charged RR £65 an hour.  In 2012. 

Had a degree in mechanical engineering, but had spent 20 years specifically becoming expert at using CFD to look at noise.  

I look forward to having you test my omega a 6000 rpm. 

 

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9 hours ago, Crackers said:

Also, if you've not seen this, have a scroll through the videos on this channel, it's a great audiovisual comparison of different cylinder counts and firing orders. 

 

This channel is awesome! Much time was lost spent  doing research last night.

He uses a slightly different pitch of click for different cylinder banks and successfully re-creates a lot of the engine sounds.

Maybe the length and position of header tunes the note on a car...?

More is to be learned.

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1 hour ago, bunglebus said:

Here's one I've yet to understand - why does a flat 4 cylinder engine (VW/Subaru) sound so distinctively different to an inline 4? Same number of cylinders and especially with the Scooby all exiting through a long single exhaust. 4 cylinders, common crank, why does the physical layout of the engine make it sound different? 

My understanding is that scoobies, have different length exhaust manifolds on one side to the other due to packaging constraints. And if they didn't, they d sound like any other 4 cylinder 

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  • Cord Forteener aka Tim_E changed the title to AS mad science: understanding burble. Place for discussion and amateur science.
9 minutes ago, New POD said:

My understanding is that scoobies, have different length exhaust manifolds on one side to the other due to packaging constraints. And if they didn't, they d sound like any other 4 cylinder 

This. I loved the sound of it on my old classic Impreza turbo. I ran a decat stainless exhaust on mine and found the soundtrack addictive. 

I do know some owners who went a step further and paid handsomely for even headers as an 'upgrade'. I assume it freed up a few horses, but the burble was gone. 

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OK, any recordings you may want to make please do.

I would recommend placing your phone underneath the exhaust or out of the way of the turbulence, revving your engine to 1000 rpm and holding, then 2000 rpm and holding then, if possible, 4000rpm and holding. Hold only so revs stay at the desired revs for a few seconds. 

Then send me clips, I'll cut and compare then l'll try to learn this spectrum analysis and waterfall stuff, but if anyone wants to contribute expertise (any knowledge is more than mine) or time I'll share all data.

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58 minutes ago, Tim_E said:

I'd love that thanks.

Out of interest, I thought diesels always had direct injection even it was mechanical for most of history, as the injection is the event that times and initiated combustion?

Anyway, I'd love samples. Thanks 😊 

Traditional or indirect injection CI engines have a pre-combustion chamber. Direct means literally into the combustion chamber. Great for outright power in high speed diesels, appalling for particulate and NoX creation. 

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