Acoustics

Sound theory

Noise has a wide range of consequences for humans, from reducing personal comfort (such as lack of privacy or difficulty in talking / communication) to minor and even serious health problems.


Definition

Noise wave

Acoustics is the science that describes, measures, records and plays sound.

Sound is a physical phenomenon, caused by a pressure wave coming from a vibrating object (emission), and travelling through a given medium (air, water, ...) to be detected by the human ear (reception). Noise is the sum of one or more sounds. Loud and unwanted noise is basically objectionable to a human being, and is very often considered an inconvenience.


Noise wave to the ear

Noise is identified by a vibrating wave inside an elastic medium (gas, liquid, solid) and by a speed (m/s) peculiar to that medium (air, water, wood, glass - anything but a vacuum).

Whether the sound is structure-borne (impact sound), or Airborne sound depends on whether the medium through which it travels is a solid or a gas.

As the reception device for human hearing : the ear is the only sensory organ that stays alert continuously.

Tyhe human can generally perceive sounds starting at 0.00002 Pa.


Characteristics

Sound has a number of characteristics, the two main ones being:

  • The frequency, expressed in Hz (Hertz)
  • The level of pressure, expressed in dB (decibels)

The frequency

The number of pressure variations (cycles) per second is called the frequency and is measured in Hertz. The normal hearing range of a healthy young person extends from 20 Hz (or even less in a very quiet environment) to 20,000 Hz. As a comparison, a piano ranges from 27 Hz up to 4186 Hz. The perceived tone of a noise source is dependent upon its frequency. Thunder has a low frequency, a whistle has a high frequency.

Frequency

The pressure level

Pressure Level Formula

A pressure level (Lp) mesures the the level of a sound in dB. Acoustic pressure is measured in Pascals (Pa). However, the human ear is ultra-sensitive and detects sound levels ranging from 0.00002 Pa to 20 Pa. The use of a logarythmic scale, expressed in dB, allows the range to be narrowed to a useful scale.

Examples of acoustic pressures: 1 Pa = 1/100 000 atmosphere pressure

Pressure in Pa
Acoustic level in dB

20

120

2

100

0.2

80

0.02

60

0.002

40

0.0002

20

0.00002

0