See also • vibration at work regulations • exposure action value • exposure limit values

**The UK Noise Regulations require action if the following levels are exceeded**

The **Lower Exposure Action Levels** = 80 dBA and a
peak sound pressure of 135 dBC ≈ LCpeak

Measurements should be made with a precision Leq sound level meter equipped with a Cpeak network. Alternatively noise dose meters may be used to check the levels bearing in mind these are usually Class 2 devices

If the Leq (equivalent continuous sound level) is measured over 8 hours, then the LAeq, the LEP,d and the LEX,8h will all be the same.

See also • exposure action values • exposure limit values

See also • the exposure action value • exposure limit value

The term often refers to the attenuation of sound in a structure owing to the internal sound-dissipating properties of the structure or to the addition of sound-dissipative materials.

The action of frictional or dissipative forces on a dynamic system causing the system to lose energy and reduce the amplitude of movement.

Removal of echoes and reverberation by the use of sound absorbing material. See reverberation time

**Damped Natural Frequency Definition** IEC 801-24-10, frequency of free oscillation of a damped linear system

**Damping Ratio Definition IEC 801-24-11,** ratio of the actual damping to the critical damping.

**Viscous Damping Definition IEC 801-24-22,** damping that occurs when a particle in an oscillating system is resisted by a force that has a magnitude proportional to the magnitude of the velocity of the particle and direction opposite to the direction of the particle

Day-evening-night noise level

Day-evening noise level

Day-night noise level

Day noise indicator

dBA • dBB • dBC • dBD • dB Flat • dB Linear and dBZ - sound level frequency weightings

dB Calculations

● Note 1 : the kind of level is indicated by use of a compound term such as sound power level or sound pressure level.

● Note 2 : the value of the reference quantity remains unchanged, whether the chosen quantity is Peak, the RMS, or otherwise.

● Note 3 : the base of the logarithm is indicated by use of a unit of level associated with that base.

**dBm and dB(mW)**, power relative to 1
milliwatt. No reference impedance is assumed, though 600 ohms is common in audio equipment.

*dB SIL* under sound intensity level

*dB SPL, dB(SPL), dBSPL and dBSPL*, variations of sound pressure level

*dB SWL* under sound power level

**dBV and dB (1 V RMS)**, voltage ratio with a reference voltage of Vo = 1.00 volt = 0 dBV, regardless of impedance.

**dBW or decibel watt**, is a unit for the measurement of the strength of a signal expressed in decibels relative to one watt. It is used because of its capability to express both very large and very small values of power in a short range of number, e.g. 1 watt = 0 dBW, 10 watts = 10 dBW, 100 watts = 20 dBW and 1,000,000 W = 60 dBW.

Power in dBW = 10 lg (Power in W)

**DC Coupling** the connection of a signal from one circuit to another in a manner that passes both AC and DC components.

See also • AC coupling.

**Decay Rate Definition IEC 801-31-08,** at a given frequency, rate at which the sound pressure level decreases with time, for example in a reverberant room

● Note : the unit of decay rate is the decibel per second.

**Decibels** compress a wide range of amplitude values to a more manageable set of numbers.

**Decibel Definition** IEC 801-22-03, one-tenth of the bel

● Note 1 : the decibel is more often used than the bel as a unit of level.

● Note 2 : the decibel can be defined as a unit of level of a power-like quantity when the base of the logarithm is the tenth root of ten. Also, the decibel is the unit of level of a field quantity when the base of the logarithm is the 20th root of ten.

Vibration Levels | Sound Levels | |||

Acceleration : La | Velocity : Lv | Particle Velocity : Lv | Pressure : Lp | |

dB | m/s^{2} | m/s | m/s | Pa, N/m^{2} |

0 dB | 1 x 10^{-5} | 1 x 10^{-9} | 5 x 10^{-8} | 2 x 10^{-5} |

20 dB | 1 x 10^{-4} | 1 x 10^{-8} | 5 x 10^{-7} | 2 x 10^{-4} |

40 dB | 1 x 10^{-3} | 1 x 10^{-7} | 5 x 10^{-6} | 2 x 10^{-3} |

60 dB | 1 x 10^{-2} | 1 x 10^{-6} | 5 x 10^{-5} | 2 x 10^{-2} |

80 dB | 0.1 | 1 x 10^{-5} | 5 x 10^{-4} | 0.2 |

100 dB | 1.0 | 1 x 10^{-4} | 5 x 10^{-3} | 2.0 |

Table Notes :

0 dB is the reference level for each parameter.

The parameters above use the 20 Log formulae for example 20 Log10 p/po) dB for sound pressure levels.

Sound | Power Level, LW | Sound Intensity | Sound Energy Density |

dB | Watts | Watts/m^{2} | Joules/m^{3} |

0 dB | 1 x 10^{-12} | 1 x 10^{-12} | 1 x 10^{-12} |

20 dB | 1 x 10^{-10} | 1 x 10^{-10} | 1 x 10^{-10} |

40 dB | 1 x 10^{-8} | 1 x 10^{-8} | 1 x 10^{-8} |

**Degree °** a measure of angles. There are 360 degrees in a full rotation or circle and 90 degrees (90°) is a right angle. The symbol for degree is °

Although in common use, the degree is not part of the International System of Units (SI) the derived unit is the radian

See also • angles

ρ = m/V where ρ is the density, m is the mass and V is the volume.

SI units • kilogram per cubic metre, kg/m

At sea level and at 20 °C, air has a density of approximately 1.2 kg/m

At sea level and at 0 °C, air has a density of approximately 1.3 kg/m

See also bull; stationary signals • transients may also be deterministic.

*DI* ▷ Directivity Index

Diffraction

Diffraction Factor

*Diffuse Field Microphone* under random incidence microphone

Diffuse Field

Diffuse Field Distance

Diffuse Field Sensitivity

Diffusion

**Digital Filter Analyser** constant percentage (or relative) bandwidth resolution. This is often preferred for acoustic measurements because it best simulates the way in which the human ear perceives sound.

**Digital Signal Processing (DSP)** is the analysis of digital signal data. The original analogue signal is sampled at regular time intervals, and an Analogue to Digital converter converts the sampled amplitudes into a number series.

Direct Sound

**Directivity Factor (Q)**, if a noise source radiates uniformly in all directions, it has a Q = 1.

• Place the same source on the ground so the energy can only radiate hemi-spherically, then the Directivity Factor Q=2.

• Place the source on the ground next to a wall so the radiation is concentrated into a 1/4 of the sphere, then Q=4.

• If the source is placed on the floor in a corner, the sound energy is further concentrated into 1/8th of a sphere so Q = 8.

**Directivity Index (DI)** is 10 times the logarithm to the base 10 of the Directivity Factor Q. Say for example we buy a piece of equipment with a sound power rating of 80 dB and we install it in the corner of an empty building. The resultant sound power level radiated into the building would be 80 dB + 10·log(8) = 89 dB.

**Directivity Pattern**, the graphical description, usually in polar co-ordinates, of the response of the transducer as a function of the direction of the transmitted or incident sound waves in a specified plane and at a specified frequency.

Direct Sound Field

Directly Proportional

See also • tonal assessment

Displacement (ξ) = v/ω = a/ω^{2}, where v =
velocity, a =
acceleration and ω = 2·π·f = angular frequency.

If Displacement (s) = v/ω and Velocity v = a/ω

it follows that 10 m/s^{2} = 0.01 m/s = 10 μm at 159 Hz

This works for all frequencies, we just chose 159 Hz to keep the numbers simple. We also have a vibration nomogram for downloading.

See also • angular displacement • particle acceleration • particle displacement used in acoustic wave theory • particle velocity • standard reference levels table

**Dissipation Factor IEC 801-31-30,** ratio of sound energy dissipated as heat to the energy of the incident sound wave.

**Distance**, change in position of an object in metres - see also displacement.

● Note : Distortion may result from:

a) non-linear relation between input and output;

b) non-uniform transmission at different frequencies;

c) phase shift not proportional to frequency.

**Distortion** signal components not in the original signal due to non linearities in the system or transmission path.

● Note:

*Dn* ▷ normalized level difference

*DnT* ▷ standardized level difference

*DnT,w* ▷ weighted standardized field level difference

*DnT,w + Ctr* ▷ weighted standardized level difference with spectrum adaption term Ctr

**Dose related subjects**

DOHR (noise dose per hour)

Dose - Noise

Dose%

Dose Badge

Dose per Hour

Dose - Vibration

Dosimeter

**Driving Point Impedance Definition IEC 801-25-17,** quotient of a dynamic field quantity at one point in a system by the resulting kinematic field quantity at the same point

Dual Input Room Acoustics Calculator

Duration of Shock Pulse

*Dw* ▷ weighted level difference

**Dyne**, the force that will accelerate a 1 gram mass at the rate of 1 cm/s.

1 dyne = 1 g·cm/s^{2} = 10^{-5} kg·m/s^{2} = 10 μN

The Dyne was the old standard reference level for sound pressure (0.0002 dyne/cm²), the same reference level today is 20 micro Pascals, or 20 μPa

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