Expert: Paul Soderman - 4/23/2007

Question
QUESTION: Why, when an aircraft approaches (far overhead), does the engine noise suddenly become audible, and with a fast falling pitch which then levels out within about 1-2s to the 'normal' engine pitch and vol.? The same thing in reverse happens with a retreating 'plane. I cannot verify that this ALWAYS happens, only that I have never noticed it not happening. I understand that the amplitudinal fade in/out could be caused by doppler-like forshortening of the natural distance fade, but I can't understand how that becomes coupled with the pitch variation, or how it can happen in reverse. I am stuck!
ANSWER: Mark - I am not sure I understand your description of events perfectly.  I believe you are saying that as the aircraft approaches it generates a certain frequency that decreases as it passes overhead.  This is the Doppler effect, which is not related to amplitude, but describes the change in frequency of any periodic noise as the source approaches or recedes from the listener.  Doppler showed that motion of an approaching noise source will cause the wave lengths to shorten, which to the listener sounds like a higher frequency than would be heard from a stationary source, and the opposite happens on a receding source.  The classic example is a train bell which has a high frequency sound as the train approaches and changes to a lower frequency when the train recedes. So it is with aircraft.  A stationary propeller makes approximately the same frequency sound in all directions, but in flight sounds much different to stationary listeners upstream or downstream.  The amplitude of propeller noise, however, always varies depending on direction - the greatest sound is heard 30-40 degrees from the aft direction.  This is also where engine noise is loud.  The fact that an approaching aircraft is suddenly audible is related to the emergence of the sound from the background noise.  Hope this makes sense.  You can do a Google search on Doppler shift to find a thorough explanation of the phenomenon with equations.
Paul

---------- FOLLOW-UP ----------

QUESTION: Hi, Paul - sorry for my poor explanation of events! I understand the Doppler shift effect very well, I will describe what my ears believe is happening, and maybe you can tell me what is rally happening. sensually, there is a sudden beginning to the noise - the amplitude jumps from a magnitude of zero to an unchanging value. As the sound starts, the Frequency drops from a relatively high pitch to the 'normal' aircraft sound, over about 1-2 secs. The plane passes overhead with no noticeable change in amplitude or frequency, until at some apparently arbitrary point, the pitch suddenly rises for about 1-2 seconds, and then the amplitude is abruptly reduced to zero. This is what I hear. I have mentioned it to other people, and I am not alone in hearing this with nearly every aeroplane that passes overhead. the effect is very pronounced and appears unrelated to background noise, or any simple application of Doppler's rules (I studied physics at university). I hope this clarifies the situation! Thanks for your time,
Mark

Answer
Mark
You haven't told me what kind of aircraft this is.  If it is a propeller aircraft, many of them have an engine exhaust that points down or somewhat aft.  Perhaps you are hearing the propeller on approach, which has a higher pitch than the engine noise.  As the aircraft passes overhead, the engine noise might dominate until the aft propeller noise takes over.  

If it is jet noise you are talking about, I have no explanation.  I haven't noticed the phenomenon you describe.  It is common, however, for a distant aircraft to be inaudible until the noise lobe passes over the listener and the sound seems to appear suddenly.  The reverse can happen as it flys away.  Thermal inversion layers and wind can also cause sound to do tricks like that via refraction.
Paul