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The doppler effect.

Discussion in 'General Motorcycling Discussion' started by VTRAffair, Jan 11, 2009.

  1. Saw a post claiming that loud pipes make no difference in regards to the ability of other road users to hear you due to the Doppler effect.

    My stance is that the Doppler effect primarily effects frequency more so than amplitude.

  2. Yes, it affects frequency more than volume. However, frequency is equally important.

    This is why a cager is, at best, unlikely to hear you coming until you're abreast (or rather until the rear edge of your exhaust is abreast) and begin to pass. If your pipes were pointed forward, this would be a different case, but I haven't seen anyone try that particular configuration, nor do I have any idea what the added backpressure would do to performance (then again, people buying Harleys probably aren't too concerned with performance :LOL: )

    Furthermore, our right to make ourselves noticed ENDS at the point where we become physically painful to hear. Lasers directed at the eyes of other drivers would for damned sure make them see us, but at what cost?

    Lastly, the whole "noticing" thing is contingent on a few things: The driver hearing you (obviously), the driver making the association with the sound and a motorcyclist in the vicinity, the driver being able to ascertain your location based on sound, and the driver giving half a crap about your well being.

    Knowing what I do about people, I'm not willing to bet much on any of those.
  3. yeh its more the fact that the pipes are pointed backward rather than doppler effect. its like being behind a speaker in an open environment then moving infront of it. theres a huge difference in volume.
  4. Doppler effect, my ass.

    It's because pipes aren't pointing forward, and modern car cabin is designed to be quiet.
  5. I seem to be able to hear obnoxiously loud harleys and loud twins coming up from behind... :?
  6. I think your better off with high beams that can be spotted in rear view mirrors than loud pipes.
  7. Are you people telling me that my twin with aftermarket pipes purchase was in vain!!!

    This is why i let cars tailgate me. They can see AND HEAR ME!!!!

  8. I could be wrong about the Doppler effect being contributory, I'll be the first to admit it.

    However, I stand by my opinion that "Loud pipes save lives" is a bullshit excuse to have pipes so loud that they're downright painful to be around.
  9. Don't turn it back into that argument please, the last thread got locked.

    I'm just looking to see if anyone understands the behavior of sound and the Doppler effect a little better than I do already, and can steer us towards the correct answer.

    I'm quite positive I have heard motorcycles coming up from behind my car many times in the past, I'm wondering if anyone has a distinct memory of how long before, that they could hear the motorcycle.
  10. I wasn't sure what happened to the last thread, actually. It was just suddenly gone with no explanation.

    You can definitely hear them coming, don't get me wrong. However, the perceived intensity of the sound from ahead of them is much much much lower. Furthermore, most cagers simply will not act upon hearing a bike.
  11. The doppler effect "compresses" the sound waves coming towards you, and "streches" them out when leaving you. It should only effect the frequency (tone) of the sound, not the volume.
  12. Grey Gentry has it right up to a point. The doppler effect does compress the sound wave travelling in the direction of the sound source, and stretch it out in the opposite direction. That is all the doppler effect does.

    But it is also true that low frequency sounds travel further than high frequency sounds. So at any given moment the bike will be audible at a greater distance behind the sound source and in front of the sound source. This would be true even if the muffler were not pointed backwards.
  13. I'll take any excuse for a nice pipe.
  14. Grue, on what basis do you say that loud pipes don't save lives? I've seen pissant drivers literally jump when a loud harley coming up from behind gets close enough that the car is now in it's noise zone.

    There's no argument that the volume in front is much lower than behind.
  15. In LIMITED circumstances they can help (urban environments are helpful), but by the time they're in the blast zone if you will, the bike is abreast and probably passing. On the highway? Highly unlikely to help.

    and yeah, I've been caught off guard by Harleys before. The funny thing is that I've generally wished ill upon them for hurting my ears.
  16. I find that loud pipes make a motorcycle get noticed before a bike is visible mostly in slow speed and stop-go traffic. This is the time that cars are likely to turn right in front of your, lane change without looking, close a gap you are filtering through and so on. Not only do loud pipes provide the greatest benefits at these speeds, but the Doppler effect is minimal.

    At highway speeds where the Doppler effect is greatest, being heard before being seen is much less important, since fast, erratic movements of vehicles are much less likely, unless they are crashing. :grin:

    So I think the benefit of loud pipes is achieved mostly where they are most noticable: In traffic.
  17. Grue, you wished them ill... :?

    You're not a very tolerant person are you?
  18. Wishing ill will on any rider is very uncool, and makes you look like an a55hole! May your pipes rust out and the noise assault your ears!

  19. I'm a snap anger person. About 10 seconds later I'll realize I was being childish, but for those 10 seconds I'm generally hoping they go deaf. Not that they crash or anything :?
  20. Grey Gentry here is right.. it primarily affects the frequency of the sound.