Quote:
Originally Posted by Dave IV
Acoustic waves DO travel in straight lines in air. Acoustic waves in air travel by compression and rarefaction of the air molecules in the wave's direction of travel. With respect to a vehicle's exhaust the waves emanate from a pipe (aka waveguide) that gives the waves direction. The waves DO NOT make a U-turn when they leave the (tail)pipe.
Anecdotal evidence of this principal is someone cupping their hands around their mouth to direct their speech (sound waves) toward another person who is out of normal hearing distance. This would not work if some of the energy in the sound waves made a u-turn as it exited the cupped hands (short pipe, waveguide) and traveled behind the caller. A more elegant example are the waveguides placed on stadium loudspeakers to direct the sound in the desired direction (for reference search EIRP- Equivalent Isotropic Radiating Power).
The only way sound waves will get redirected from their course of straight line travel (they actually travel in a conical pattern, which is determined by the aperture of the source) is by being reflected at boundaries of differing acoustical impedances.
Next, the source of the sound is irrelevant; be it a 2 cylinder motorcycle engine, an 8 cylinder car engine, an audio loudspeaker, or any other transducer. In any of these cases a mechanical device is putting air molecules in motion and as described above; acoustic waves in air travel by compression and rarefaction of the air molecules. The air molecules don't care what mechanical source put them in motion.
Maybe you are mistaking identifying a sound wave's source relative to its frequency and waves magically making u-turns in free space. As frequencies become lower they become more omnidirectional, meaning it becomes more difficult to identify the point of the source.
So, The short answer here is:
1) You are wrong.
2) Acoustic waves in air DO travel in straight lines.
3) Acoustic waves in air DO NOT magically make u-turns in free space.
4) Acoustic waves in air do not change direction until the encounter a boundary of a differing acoustical impedance.
If you disagree, add something meaningful, not just a " nah, nah, cause my mother said so". Oh, and calling me names is not going to have any effect, it only helps to discredit you.
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Classic example of subterfuge. Sound waves can be made to travel in straight lines if the source is very small and positioned at the focal point of parabolic reflector. Hardly the conditions of a Harley exhaust. Otherwise, they are omnidirectional. How can you even argue this point?
If what you are stating was correct or even partially correct, we could hardly hear a motorcycle that was facing us. Completely to the contrary, the damn things are obnoxiously loud no matter where you stand in relation to them. Yes, they are louder directly behind, but you stated it was due to reflection that we hear them anyplace but directly in the path of the exhaust. Really??????
Tell us again how you manage a group of engineers at a defense contractor. Not a huge surprise as I encountered plenty of clueless managers at Lockheed during my short stint watching them destroy a viable company they absorbed.
By the way, the majority of sound waves are refracted at boundaries of differing acoustical impedance, not reflected. Please stop trying to confuse us with jargon.