‘Vibration’is any periodic movement.Vibrationalmotionhas a frequencyandan amplitude.
Frequencytellsyouhowfastsomething isvibrating, andamplitude ishow large themovement is.Generally it takesmoreenergy todrivehigher frequencies, sohigher
frequencies typicallyproducesmallervibrations inpractice. Frequencyis usuallymeasured incyclesper second, or Hertz (Hz).For example, aclock pendulumwhichswings fromoneside to theotherandback four timesasecond hasa frequencyof
four cyclesper second. Inarchery, frequencies range froma few Hertz tomany thousandHertz. (Thehumanear issensitive to frequenciesbetween about 25Hzand20,000Hz
- thehigher the frequency, thehigher thenoteyouhear).
The frequencyof avibratingsystemdependson twomain factors;themassbeingmovedand the forcewhichreturns it to its rest position.The‘restoring force’is usuallythe ‘strength’orstiffnessofaspringor,inarchery,thebowlimborstabiliser
rod. Thereare twosimple rulestoremember:
“ASTIFFER or SHORTER spring gives a HIGHER frequency”
“AHEAVIER weight gives a LOWER frequency”
So if weneed tochangeastabiliservibration frequency, for example(we’ll come
to whylater) either astiffer rodor alighter weight willincrease the frequencyandvice versa.
But what causesdifferent frequenciesandvibrations?