Go Back   Physics Help Forum > High School and Pre-University Physics Help > Waves and Sound

Waves and Sound Waves and Sound Physics Help Forum

LinkBack Thread Tools Display Modes
Old Jan 17th 2015, 08:07 AM   #1
Junior Member
Join Date: Jan 2015
Posts: 1
Finding the frequency of a wave.

A siren rings with 1200Hz frequency. A wind blows with 55m/s from the siren toward a man 1km away. In what frequency will the man hear it if the speed of sound is 330m/s? I found the wavelength first. Then the speed which is sound + wind velocity. Next I calculated the frequency which is 1400Hz. Is my answer correct and why was the distance 1km given?
Sohaib03 is offline   Reply With Quote
Old Nov 25th 2015, 12:18 AM   #2
Junior Member
Join Date: Nov 2015
Posts: 3
If both the siren and the man aren't moving, shouldn't the frequencies be equal since there is no Doppler's effect?
Lexadis is offline   Reply With Quote
Old Nov 25th 2015, 02:30 PM   #3
Senior Member
MBW's Avatar
Join Date: Apr 2008
Location: Bedford, England
Posts: 668
It looks to me like one of those nasty questions specifically designed to confuse.
I agree that the 1km distance seems to be superfluous information,
(perhaps it is part of a subsequent follow up question).

My thinking is as follows:
The frequency of the siren will be transferred to the moving air,
the velocity of the sound in the moving air with respect to the siren will be greater than the velocity of sound in still air, thus while the frequency will be the same, the wavelength will be different.
However, when it gets to the man, the effect will be reversed, thus the man will still hear 1200hz, the sound will just reach him quicker.
Effectively there is a Doppler shift between the siren and the moving wind, but then a reverse Doppler shift between the wind and the stationary man, which cancel each other out.

When presented by a series of questions it is often useful to at least scan the subsequent questions because they can sometimes provide a clue to the sort of answer required.
If the answer to the early question clearly does not fit with the later questions, there is probably an issue with the early answer...
You have GOT to Laugh !

Last edited by MBW; Nov 26th 2015 at 02:03 AM.
MBW is offline   Reply With Quote

  Physics Help Forum > High School and Pre-University Physics Help > Waves and Sound

finding, frequency, sound, velocity, wave

Search tags for this page
Click on a term to search for related topics.
Thread Tools
Display Modes

Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
Can the wavelength, and frequency of any EM wave noabsortion1 Light and Optics 0 Apr 24th 2016 02:30 PM
Apparent frequency of reflected wave ling233 Waves and Sound 0 Aug 3rd 2014 10:46 AM
Finding net force and frequency of an oscillation due to charges. mattyc33 Electricity and Magnetism 1 Sep 28th 2012 11:02 AM
Finding the frequency bobo Periodic and Circular Motion 10 Nov 2nd 2010 09:32 AM
Finding Amplitude of a Wave strgrl Waves and Sound 1 Jun 27th 2009 02:56 PM

Facebook Twitter Google+ RSS Feed