Brief Introduction To Vibro-Acoustics

Sound is always created by vibration. Vibration can be felt whenever we touch the surface of any stereo speaker or any rotating machine. In some cases, the object or structure must be vibrating in order for the sound to be produced.

Sound travels. It can be heard through a solid structure, (such as your teenager’s bedroom walls…) water or of course, air. When one of these objects vibrates in air, the result is a disturbance in local air molecules. That disturbance causes the molecules to bump into one another. This results to a small change in the instantaneous pressure, which we then hear as sound.

How Sound Is Propagated

To understand the way sound actually moves, you need to look at the air molecules themselves. If you look in a microscope at one sound-excited air molecule, you would notice that the molecule solely oscillates about its equilibrium position. In other words, it only moves in back and forth motion. However, this movement also allows it to strike the neighboring molecules, which propagates the disturbance.

The affected neighboring molecules, which are now excited by the disturbance, would then oscillate about their center points. This so-called exchange of energy among the molecules, which is referred to as the tiny fluctuations in pressure brought by molecular motion, is what is known as sound. So then quite literally, at a molecular level, sound is created by the molecules in the air around us throwing a giant dance party!

The Concept Of Frequency

Frequency refers to the number of times a structure in vibration moves back and forth within one second. It is usually measured in Hertz. A huge majority of sounds that occur in everyday life come in the range of 80 to 5000 Hertz. This range encompasses the fundamental tones found in most musical instruments. Cutting out frequencies, whether they are above 5,000 or 12,000 Hertz, tend to only reduce the orchestra’s subtle presence while most of its audio content remains present.

The Concept Of Amplitude

In vibration, amplitude is the maximum motion that is being moved by a structure while it vibrates. This motion can be defined in various terms such as acceleration (g’s), velocity (inches per second), or displacement (inches).

In sound waves, amplitude is related to the levels of the sound wave’s maximum pressure fluctuations. This sound pressure is represented by the base unit known as a Pascal. One Pascal is equivalent to around 0.000145 pounds for every square inch (PSI). Some examples of sound pressure as measured in Pascal include a typical conversational voice (around 0.063 Pascals); a whisper (0.0011 Pascals); and a 747 jet while it’s on a take-off (6,500 Pascals).