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[Will] I have an expensive motorcycle - assume:) with a sound of the exhaust of the high-speed engine. How to slow down wave sound speed of motorcycle exhaust [High to Low Frequency] that will sound like a low-speed engine? It's hard and tricky question. For any help, I will be appreciated.

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    Let's check if I understood the question correctly. You want an engine running at 6000 RPM to sound like an engine running at 1000 RPM? And for what purpose, may I ask? I would categorize this as "can't be done", unless you manage to make the engine really quiet (e.g. electric motor) and then have loudspeakers playing a synthetic engine soundtrack. – juhist Feb 19 '18 at 16:32
  • For example - look at bose system electronic suspension (now another company buy this specific product, and develop a new version) - that can make car ride like flying. youtube.com/watch?v=3KPYIaks1UY – Adam Ł. Feb 19 '18 at 16:53
  • I want to make sound of exhaust like to be low RPM for people doesn't have enough money to buy a new motorcycle, but have some motorcycle look like much better than sound. – Adam Ł. Feb 19 '18 at 17:01
  • It’s called a Helmholtz Resonator like organ pipes where the resonant frequency and its harmonics are invert to pipe length from end to end by the 1/4 wave length. This length also adds more engine load from air turbulence so fatter pipes are used. So-called resonators in cars use a labyrinth with baffles and multiple path lengths to create multiple smaller resonances to reduce engine noise. – Tony Stewart Sunnyskyguy EE75 Jun 15 '18 at 4:46
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This is largely down to the design of the silencer and the length / diameter ratios to the speed of the exhaust gases and when resonance can occur.

This has been researched but I don’t have any to hand at the moment (from memory David Vizard : tuning Minis comes to mind), however tuning the exhaust can make a huge difference to the noise and the power produced ...

Edit: in addition David also showed that having a “step” or mismatch from the diameter of the exhaust port in the head to a larger manifold diameter would promote better scavenging of the exhaust gases - but this was very rpm dependant...

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  • Hmm... 6000 RPM is 100 Hz, 1000 RPM is 16.667 Hz. How do you produce 16.667 Hz from a 100 Hz source? I assume the higher harmonics (200 Hz, 300 Hz, ...) could perhaps be created, but the non-harmonic 16.667 Hz seems impossible to create according to my knowledge of physics and mathematics. Sure, with exhaust tricks you can create really loud sounds at 100 Hz, 200 Hz, 300 Hz, 400 Hz and so on, but it still doesn't make 6000 RPM engine sound like a 1000 RPM engine. A well-tuned ear will hear the difference. – juhist Feb 19 '18 at 16:43
  • Thanks for comment - this led me to directions what I need to search :) It will be hybrid system mechanic and electric to manipulate wave. This will convert from 7500 RPM to 9500 range to range like, will be 2500 RPM up to 3500 range. First, this must be simple. Second how to convert wave x Hz to wave X Hz. How to add to wave some kinda adjustment, interrupt for making it non-harmonic. – Adam Ł. Feb 19 '18 at 17:37
  • I have found solution. The Binaural beats. This make non harmonic low tone sound. Modulate high tone wave and beat of wave will sound like low tone. – Adam Ł. Apr 15 '18 at 20:18
  • We can play with interference sound of exhaust. Sound wave have delay, pitch and volume. Specifically delay of another wave generate by electronic can interference wave generate by exhaust. That we can make sound less loudness when pitch is the same. Volume is a high of wave, and pitch is width of wave. So now we can use delay, pitch, volume to modify exhaust - as mentioned this will be hybrid solution machanic and electronic. – Adam Ł. Apr 15 '18 at 20:44
  • After all, I found a solution to the problem. Solution: What is Convolution? – Adam Ł. May 29 '18 at 14:27
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Do like Subaru does and have unequal length exhaust headers installed. This will make your bike have a lower RPM sound (just like a Subaru I guess).

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Assuming you have a chain drive motorcycle, you could also try to change the sprocket sizes to change the final drive ratio of the bike (within limits). This will reduce the actual rpm of the engine at highway speed somewhat, which will make the bike quieter and more fuel efficient at the expense of acceleration. I doubt you can gear up more than 10 to 20% (reducing your rpm by roughly the same amount). Also, large capacity twins generally are torquier and run at lower rpm than four cylinder motorcycles, so the type of motorcycle will have an effect.

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