Intensity, Loudness Forced Vibration And Resonance Sound Part 2 Intensity, Loudness Forced Vibration And Resonance

Intensity The Intensity of a Sound wave is the amount of energy it delivers per second (power) to a given area. (Copy Formula, on board and p 414) As the sound moves away from the source the energy is spread over a larger area so intensity drops according to the Inverse Square Law Intensity is measures in W/m²

Loudness Loudness is the human perception of the intensity of sound. The least intense sound (1.0 x 10-12 W/m²) we can hear is called the “threshold of hearing” is measured as 0 decibels or 0dB The most intense sound we can tolerate is 1.0 W/m² or 120 db The “threshold of pain”. Our hearing is logarithmic, 40 db is 1000x as intense as 10 dB! Loudness = 10 log ( Intensity/ 1.0 x 10-12W/m²) That is why sound doesn’t seem to us to obey the Inverse square law

Natural frequencies Every object has a natural frequency at which it will vibrate determined by what length of standing wave are possible. If the object is struck it will vibrate at this “natural frequency” Once vibrating it will continue with the amplitude And thus intensity slowly decreasing

Forced Vibration An object can be forced to vibrate at a different frequency if put in contact with a naturally vibrating object Like a string forcing the body of a harp to vibrate this can increase the intensity of the sound. If the object is taken out of contact the forced vibration will immediately stop.

Resonance If an object is put into contact with a object or sound vibrating at its natural frequency it will vibrate easily, or Resonate If it remains in contact the amplitude of vibration may increase until the structure fails. This is why it is possible break a glass with your voice You need to produce a loud, perfectly steady note of exactly the right frequency Gusts of wind caused a resonance in the Tacoma Narrow Bridge…leading to collapse in 1940