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

The Noise at Work Directive 2003/10/EC defines the following limit and action values:

**Exposure Limit Values (LEX,8h)** = 87 dB(A) and LCpeak = 200 Pa respectively;

**Upper Exposure Action Values (LEX,8h)** = 85 dB(A) and LCpeak = 140 Pa respectively;

**Lower Exposure Action Values (LEX,8h)** = 80 dB(A) and LCpeak = 112 Pa respectively.

*LCpeak* is the maximum value of the
C-weighted, instantaneous sound pressure.

**LEX,8h** is the Leq (equivalent continuous sound level) corrected for the length of the working shift, in this case 8 hours

See also the Daily Personal Noise Exposure (LEP,d) below

**LEP,d** is calculated from the measured sound exposure the **exposure time** and a reference time of 8 h.

If the Leq is measured over 8 hours then Leq and LEP,d and LEX,8h would all be the same.

The European Directive recommends LEX,8h is adopted from 2006 onward.

See also • Leq (equivalent continuous sound level)

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

**Data Logging** on-going measurements, stored at regular intervals for downloading to a PC.

**Data Source**, the identity and location of the data that are used in an analysis.

Day-evening-night noise level

Day-evening noise level

Day-night noise level

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

● 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.

● 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.

● 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.

**The decibel is used extensively in acoustics** - so we have a more detailed description.

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 Lg 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

**Deltatron ®** trade name for IEPE (integrated electronics piezoelectric).

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

**Density of air**, air density decreases with increasing altitude, as does air pressure.

At sea level and at 20 °C, air has a density of approximately 1.2 kg/m^{3}

At sea level and at 0 °C, air has a density of approximately 1.3 kg/m^{3}

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** recording or storing information as series of the numbers 1 and 0, to show that a signal is present

See also • sampling.

Direct Sound

• 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.

**Discrete** with reference to a spectrum, discrete means consisting of separate distinct points, rather than continuous

See also • tonal assessment

**Discrete Fourier Transform (DFT)** mathematical calculation that converts or transforms a sampled and digitised waveform into a sampled spectrum. They reveal periodicities in input data as well as the relative strengths of any periodic components. The fast fourier transform, is an algorithm that allows a computer to calculate the *discrete fourier transform* very quickly.

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

**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

**DOD : Department of Defence - USA**

**Dodecahedron** a general dodecahedron is a polyhedron having 12 faces. Acoustic examples are dodecahedron and hemi-dodecahedron loudspeakers arrange to provide isotropic sound sources.

**Dose related subjects**

DOHR (noise dose per hour)

Dose - Noise

Dose%

Dose Badge

Dose per Hour

Dose - Vibration

Dosimeter

See also acoustic impedance and related topics

*DSP* ▷ Digital Signal Processing

Dual Input Room Acoustics Calculator

Duration of Shock Pulse

*Dw* ▷ weighted level difference

**Dynamic Range**, all audio systems are limited by inherent noise at low levels and by overload distortion at high levels. The usable region between these two extremes is the dynamic range of the system. Expressed in dB.

**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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