The amount by which the air impedes the energy flow is Is a measure of the resulting energy flow through this unit area in a a particular force applied over a unit of area: 1Pa = 1N/m 2), the air impedes the energy flow and the sound intensity (W/m 2) Basically, this is a measure of how much the air impedes the flow of energy through it. In) called the " characteristic acoustic impedance". Proportional to the square of the pressure divided by a particularĬharacteristic of the air (or whatever medium the sound is traveling that they are what are called plane waves) then the intensity is If we make certain assumptions about the nature of the sound waves The calculation for us and converts the sound pressure measurement intoģ.4 The relationship between dB SPL and dB IL Single microphone which responds to sound pressure - the SLM then does We use a sound level meter (SLM) to measure a sound, the device has a Thus the dB scale now becomes dB sound pressure level, or dB SPL given by: In practice we usually take p 0 as being a reference value approximating the sound pressure at threshold of hearing - 2x10 -5 Pa. That is, the dB scale is simply a comparison of two sound pressures. it is a logarithmic ratio of sound pressures referred to Practice, the logarithmic representation of sound pressure we useĪctually compares one value to another rather than giving and absolute This leads to Fechner's law which states that the actual response will be proportional to the logarithm of of the stimulus: Where δS is the change in stimulus and δR is the change in response - Equation 2.1 is an example of Weber's Law Weber suggested that the subjective response (R) is proportional to the fractional change of stimulus (S). That is not directly proportional to sound pressure, a logarithmic Large range of pressures (100 000 000 fold) and (c) responds in a way These are relatively small values compared to normal atmospheric pressure which is around 10 5Pa.īecause the human ear (a) responds to sound pressure, (b) responds to a Which the human ear is sensitive ranges from approximately 2x10 -5Pa to 200 Pa. Sound Pressure is measured in Pascals (Pa) - the pressure range to Some moment later they are spread out giving a low pressure. Point in time they are bunched up together giving a high pressure. The pressure fluctuations in the air that are associated with sound areĪ result of the air molecules vibrating backwards and forwards. Sound pressure is usually denotes by a small p, and the unit is the The sound pressure is simply theĪmount of force that is applied by the moving molecule per unit area. The moving molecules then collide withĪdjacent ones and pass on the energy. Them together (compression), as it moves back it leaves a region ofįewer molecules (rarefaction). Speaker moves outwards it applies a force to the air molecule pushing
When the speaker isn't moving the surrounding air will be atĪtmospheric pressure (with a certain density of molecules). Usually denoted by a capital I and the unit of intensity is W/m 2 or Wm -2 Intensity refers to the amount of sound energy that passes through a As the speaker plays a sound, the energy travels out in allĭirections and as it does so it is spread out over a larger area.
Power is usually denoted by a capital P, and the unit of How much energy is expended per second in moving the surrounding air Much energy is transfered in every second. Power is simply the "rate of expenditure of energy" - that is how Energy is usuallyĭenoted by a capital E and the unit of energy is the joule (J). The energy is transfered to the air molecules. If we think of a single speaker producing sound, as it moves itĭoes work in the process of moving the air molecules around itself. Energy is defined as "the ability or capacity of an object to do These are essential to our understanding,īut are often confused. We discuss sound we use terms like sound energy, sound power, sound