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Noise may originate from natural sources as well as through anthropogenic activities. The natural sources are cyclone, thunder, roaring of sea, volcanic eruption, etc.

The science of human hearing sound is called human acoustics. Human acoustics specifically indicates the different processes that are involved in the perception of some sound by our ears.

A musical sound has three main characteristics namely loudness, pitch and quality.

In physical processes, the solid and liquid components can be separated through sedimentation of the solid in a sedimentation tank, followed by filtration for solids which do not settle and float.

The process of burning municipal solid fasted in a properly designed furnace under suitable temperature (850° to 1100°C) and operating conditions is known as incineration. The modern municipal incinerator is of continuously burning type.

The waste decomposes aerobically, until the oxygen that was present in the freshly-placed fill is used up by the aerobic microorganisms.

The present day management follows: (1) collection (2) storage (3) transportation (4) recycling (5) disposal.

Rocks can be classified according to their mode of origin and chemi­cal composition. The three such groups are: (1) Igneous rock (2) Sedi­mentary rock

Water pollution cannot be abated fully but can be controlled through awareness in different sections.

The relative potential of a substance or combination of different substances in causing injury or harm to living organisms is called toxicity. If chemical substances are responsible for toxicity then it is called chemical toxicity and the chemical substances are known as chemical toxicants, or toxins.

Water which cannot form lather easily with soap is known as hard water and this characteristic is called hardness. On the other hand, water which forms lather with soap is called soft water.

The main purpose of the tertiary treatment is to remove dissolved nutrients, especially nitrogen and phosphorus which have survived the primary and secondary treatment.

Waste water contains a lot of chemical pollutants as well as dissolved solids and suspended solids. The major contributor of pollutants is industrial waste water.

Parallel steel bars followed by wire mesh screen with small openings, are generally used to remove relatively large floating and suspended materials.

The number of intermolecular spaces between the soil particles is quite high and water percolates down through a region called unsaturated zone (the region is not fully saturated with water but contains lot of air with water).

One of the most important criteria for a river’s health is dissolved oxygen (DO). With decrease in DO, survival of life becomes difficult.