This stem motion is very small, and difficult to feel if you place a finger tip at the bottom of the stem. However, the stem actually vibrates up and down at the fundamental frequency as well as at the second harmonic, 852 Hz - twice the frequency of the fundamental (even there is no vibrational mode of the fork at this frequency). When vibrating in the fundamental mode, it would appear that the stem of the fork is stationary. This is a symmetric mode, since the two tines are mirror images of each other.Ī video on my YouTube Channel shows the slow-motion oscillation (shot with a high speed camera at 1200fps) of a 125 Hz tuning fork vibrating in its fundamental mode of vibration. The two tines of the fork alternately move toward and away from each other, each bending like a cantilever beam, fixed at the stem and free at the other end. If you can do anything to reduce ambient noise, you will almost always get cleaner data.The fundamental mode of vibration is the mode most commonly associated with tuning forks it is the mode shape whose frequency is printed on the fork, which in this case is 426 Hz. We prefer a keyboard for demonstrations over tuning forks. This sidesteps the problem of bad tuning fork strikes. If the sound is bright to the ear, the strike was almost certainly a poor one that will give a waveform rich in undesired harmonics.Īn excellent alternative to using a tuning fork is to use an inexpensive electronic keyboard set to the flute sound. Other people prefer to strike the tuning fork on an object like their kneecap.īecause the waveform will be a function of how the fork is struck, there is an element of experimental skill involved in getting a clean waveform. Many people use something made of rubber, such as a shoe sole or rubber mallet. Strike it against an object that is not real hard. We suggest striking the fork about 2/3 of the way down the side towards the base. The method used to strike the tuning fork will directly affect how clean your waveform is. So, if you are using a 400 Hz tuning fork, you need to be collecting data at at least 2000 samples/second in order to get enough of the sine wave to see it clearly on the screen. You need to be sampling at about 5 times the rate of the highest frequency signal you want to detect. Keep in mind the relationship between the sampling frequency and the signal. This way, you know that the sound signal will already be reaching the microphone when your data run starts. Be sure to strike the tuning fork first, get it in position, THEN quickly start your data run. When sampling at a high frequency, you are usually only collecting data for a very short amount of time. Many times people try pointing the tuning fork at the microphone, but the side of the microphone will actually produce a cleaner signal.Īnother common problem is the timing of the experiment. One of the best things to do to improve the signal from a tuning fork is to hold it so that the outside of the tuning fork is parallel to the face of the microphone. Hold the tuning fork very close to the microphone, almost touching it. What is the best procedure to get a clean sine wave when using a tuning fork with the microphone?
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