This article describes the various terminologies we tend to use when measuring sound. Knowledge about these terminologies will help us to better understand sound measurement.
Before we proceed on with the various common terminologies we hear during sound measurement. Let us know more about the types of areas or fields where the sound measurements are bound to be taken. It is something like before we venture into a new zone or place we tend to have a map of the place which will tend to help us if we get lost. It is in the same way we plan to know more about the “SOUND FIELDS" which will help us to decide on the type of instrumentation to be use.
An area which possesses sound can be called as a sound field. This area will always tend to have a boundary. The sound fields basically describe the way in which sound waves travel in this particular environment. The various “SOUND FIELDS" are as follows
- The Free Field: This is classified as an area where the sound waves are not reflected back. A good example for the free field would be open air (which is quite far away from the ground as well as other obstacles) and an anechoic room where all the sound generated tends to get absorbed. In this particular environment, there is a drop in the sound pressure level by about 6 dB for each time the measurement distance is doubled from its source.
- The Diffused Field: This is just the opposite of a free field. Here the noise in the field tends to be reflected back and back many times that it tends to travel in all directions with the same magnitude. A good example for this would be a completely reverberant room, which is practically impossible as the sound tends to die after a certain period of time by loosing its energy for every reflection.
- Active Sound Field: We hear sound due to its flow, or we can say that there is a flow of sound in the form of energy. An active sound field is one in which there is a flow of sound energy. Here the sound energy produced just tends to keep flowing from the source and never returns back. It is just like the water falling from a waterfall cannot climb up the water fall.
- Reactive Sound Field: This is a form of an enclosed system in which the sound energy flow is enclosed within the system. The energy does not flow out of the field and also does not flow into the field. This can be compared in analogous to the spring wherein the energy used for compressing the spring is used again by the spring to bounce back to its original shape.
- The near field of a source: This is the area very close to the source of the noise. The air in this field tends to act as a spring-mass system having the capability to store energy. The sound energy circulates in this region without propagating. This field is very important for taking sound intensity measurements for calculating the sound power due to the non propagation of sound energy. The area where the sound energy tends to propagate is called as far field.