 Hands-On-Physics WAVES

Human Perception of Sound

• Loudness - the strength of a sound. Synonym: volume

• Pitch - the highness or lowness of a sound.

• Quality - the characteristic distinguishing sounds from different sources. Synonym: timbre

Characteristics of Sound Waves

• Amplitude - In general, this is the maximum change in value of a parameter during the oscillation of a wave. In this unit, that parameter will usually be pressure. In practice, this is the distance between a peak or trough and the x-axis on a graph.

• Frequency - In general, this is defined as the number of vibrations, oscillations, or cycles in a repeating process occurring per unit time. In the context of sound, it is the number of compressions passing a fixed point of reference in one second. The resulting unit of frequency is called Hertz (Hz). Frequency is perceived as pitch.

• Intensity - The time rate at which sound energy flows through a defined area. Since the flow of energy is power, the dimensions of sound intensity are power/area. Usually, sound intensity is measured in watts/meter^2. Intensity is perceived as loudness.

• Interference - Generally, interference is a synonym for superposition.

• Constructive Interference - The amplitude of the combined wave created by superposition is greater than the amplitude of either component wave.

• Destructive Interference - The amplitude of the combined wave created by superposition is less than the amplitude of either component wave.

• Inverse Square Law - The decrease in the intensity of a sound wave is proportional to the inverse square of the distance from the source.

• Period - The time required for a single wavelength to pass a fixed point of reference. The period of a sound wave is the inverse of its frequency.

• Phase - The phase of a wave is an expression of how far through its cycle of oscillation it has progressed. Because the mathematical description of wave motion is similar to the mathematical description of motion in a circle, wave phase is expressed in degrees or radians. A wave completes its cycle in 360°, just as a circle is completed in 360°. Half a cycle is 180°, and a quarter cycle is 90°.

The relative phase of two similar waves is a measure of how synchronized they are.

• In phase - Similar waves whose peaks (maxima) coincide are said to be in phase. Their phase difference is 0°.

• Out of phase - Similar waves whose peaks (maxima) do not coincide are said to be out of phase. If the peaks of one coincide with troughs (minima) of the other, the two waves are said to be 180° out of phase.

• Speed - In general, speed is the ratio of distance travelled to time required, for example, the distance travelled in one second. The speed of sound is no different. Usually the speed of sound is thought of in terms of one cycle of a wave. In one cycle, sound travels a distance of one Wavelength. The time required for one cycle is the Period, so the speed is the ratio of the wavelength to the period. Dividing by the period is the same as multiplying by the frequency, so the speed of a wave is often expressed as the product of wavelength and frequency.

• Superposition - This concept describes the way in which sound waves, and waves more generally, interact. In essence, two waves passing through the same point in space at the same time combine in a linear fashion to create a single new wave. If the displacement from equilibrium caused by the first wave equals "a" and the displacement from equilibrium caused by the second wave equals "b", the resulting displacement from equilibrium for the combined wave will be "a + b". This is an algebraic addition. Since "a" and "b" can be positive, negative, or equal to zero, the resulting sum can also be positive, negative, or equal to zero.

• Wavelength - The distance between one peak or crest of a sound wave and the next corresponding peak or crest. The product of the wavelength and the frequency of a sound wave yields the velocity of that wave.

• Waveform - The detailed way in which a parameter changes during the oscillation of a wave. For sound, that parameter will usually be pressure. In practice, this is the shape of a wave on a graph. Waveform is perceived as timbre.

Previous Page || Up a Level || Index || Next Page