![]() ![]() As you increase the frequency at which you move your finger up and down, the ball will respond by oscillating with increasing amplitude. If you move your finger up and down slowly, the ball will follow along without bouncing much on its own. At first you hold your finger steady, and the ball bounces up and down with a small amount of damping. Most of us have played with toys where an object bobs up and down on an elastic band, something like the paddle ball suspended from a finger in Figure 14.18. The phenomenon of driving a system with a frequency equal to its natural frequency is called resonance, and a system being driven at its natural frequency is said to resonate. The natural frequency is the frequency at which a system would oscillate if there were no driving and no damping force. Over time the energy dissipates, and the amplitude gradually reduces to zero- this is called damping. This is a good example of the fact that objects-in this case, piano strings-can be forced to oscillate but oscillate best at their natural frequency.Ī driving force (such as your voice in the example) puts energy into a system at a certain frequency, which is not necessarily the same as the natural frequency of the system. It will sing the same note back at you-the strings that have the same frequencies as your voice, are resonating in response to the forces from the sound waves that you sent to them. Sit in front of a piano sometime and sing a loud brief note at it while pushing down on the sustain pedal. This means there aren't other planes to be removed, so they cannot be polarised.Before the start of this section, it would be useful to review the properties of sound waves and how they are related to each other, standing waves, superposition and interference of waves. Longitudinal waves, like sound, oscillate parallel to (in the same direction as) energy propogation, so don't oscillate in a variety of planes. If all but one of these planes of oscillation is removed - so the light oscillates in one plane only - the light is polarised. Transverse waves oscillate perpendicular to the direction of energy transfer (propogation), so can oscillate in a variety of planes. This is because sound waves are longitudinal, not transverse. ![]() Not interference, but worth pointing out. However, they behave exactly the same as other waves. Obviously, since sound has a wavelength much longer than light, the slit spacing would be a lot greater than that for a typical double slit. #D# is the distance to the screen/wall in metres #w#=fringe spacing (distance between maxima) in metres The distance between the sound maxima, #w# will be: This also follows the equations we have for Young's double slit. If the path difference is a half number of wavelengths - phase difference of #lamda/2# - then the two waves will cancel out, so the intensity is minimum. If you walk along the wall (which acts like a screen for a double slit), you would hear these points as when the sound is loudest.īetween these points, there will be areas when the sound is quietest, effectively zero. They will interfere constructively, and this leads to the peaks in intensity. If the path difference - the extra distance one wave takes compared to the other - is equal to a whole number of wavelengths, then the two waves will be in phase. As the two waves diffract, they interfere with each other. When the sound waves go through a gap, they diffract, like other waves. If you can get a source of two coherent sound waves - same frequency and wavelength, and a constant phase difference - then they will interfere just like a double slit. This diagram shows the second harmonic there are three nodes and two antinodes, forming 3 "loops." The distance between two nodes is always half the wavelength, #lamda/2# - this explains why stationary waves only form when the distance is a half-interger multiple of wavelength. Just like with waves on ropes, or with light or microwaves, or water waves, or any other kind of wave, this forms harmonics. These points form antinodes with maximum displacement, marked as A. ![]() This is when we have maxima in the sound. Points with equal phase difference - #lamda, pi# will reinforce each other. Points with opposite phase difference of #1/2lamda, or #pi/2# - one wave in compression when the other is in rarefraction - will completely cancel out to form a point on the stationary wave with zero amplitude, a node, marked as N. If the speaker emits coherent sound waves (same frequency, same wavelength, constant phase difference), and the distance to the wall #l# is a half-integer multiple of the wavelength #lamda#, then the waves will interfere with themselves and the superposition forms a stationary wave.Īs you walk along from the speaker to the wall, you will hear the intensity of the sound increase to a maximum and decrease to a minimum. We can see (hear?) this with stationary sound waves.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |