D level difference: the difference in sound levels in two spaces separated by a partition, measured as part of a sound insulation test according to BS EN ISO 140.

Dnc, Dne, Dnf,: standardized level difference values for small elements (e.g. ventilators), Dne, suspended ceiling, Dnc, and raised floors, Dnf, defined in parts 10,12 and 9 respectively of BS EN ISO 140.

Dn normalised level difference: a measurement of airborne sound insulation, corrected according to BS EN ISO 140-4 for receiving room characteristics (sound absorption); a complete set of measurements consists of 16 third octave band values, from 100 Hz to 3150 Hz.

DnT standardised level difference: a measurement of airborne sound insulation, corrected according to BS EN ISO 140-4 for receiving room characteristics (reverberation times); a complete set of measurements consists of 16 third octave band values, from 100 Hz to 3150 Hz.

DnT,w weighted standardised level difference: a single-figure value of airborne sound insulation performance, derived according to procedures in BS EN ISO 717-1 used for rating and comparing partitions and based on the values of D at different frequencies; values of DnT,w are specified in the Building Regulations.

Damping: a process whereby vibrational energy is converted into heat through some frictional mechanism, thus, causing the level of vibration to decrease.

Damping ratio: the ratio of the amount of damping in a vibration system to the amount of damping when critical.

Day-evening-night level, Lden: the LAeq over the period 00.00-24.00, but with the evening values (19.00-23.00) weighted by the addition of 5 dBA, and the night values (23.00-07.00) weighted by the addition of 10 dBA.

Day-night level: Ldn an index of environmental noise which is a 24 h Leq, but with a 10 dB weighting added to the night-time noise levels (22.00 to 07.00) to allow for increased sensitivity to noise during the night-time.

dBA: the A-weighted sound pressure level; see under A-weighting.

Decade: a range of ten to one, e.g. from 100 Hz to 1000 Hz.

Decibel (dB): the decibel scale is a scale for comparing the ratios of two powers, or of quantities related to power, such as sound intensity; on the decibel scale the difference in level between two powers, W1 and W2 is N dB, where N = 10log10 (W1/W2); the decibel scale may also be used to compare quantities, whose squared values may be related to powers, including sound pressure, vibration displacement, velocity or acceleration, voltage and microphone sensitivity; in these cases the difference in level between two signals, of magnitude S1 and S2 is given by N = 20 log10 (S2/S1); the decibel scale may be used to measure absolute levels of quantities by specifying reference values which fix one point in the scale (0 dB) in absolute terms; a decibel is one tenth of a bel.

Degrees of freedom: the number of degrees of freedom of a mass-spring model of a vibrating system is the minimum number of coordinates required to specify all the different possible modes of vibration of the system.

Deterministic: a deterministic signal is one whose value can be predicted with certainty from a knowledge of its behavior at previous times, as opposed to a random signal, where this is not possible.

Dielectric: a material which is an electrical non-conductor or insulator; it is used between the plates of a capacitor.

Diffraction: the process whereby an acoustic wave is disturbed and its energy redistributed in space as a result of an obstacle in its path; the relative sizes of the sound wavelength and the object are always important in diffraction; reflection may be considered to be a special case of diffraction when the size of the obstacle is very large compared to the wavelength; the combined effects of diffraction from an irregular array of objects in the path of the sound is also known as scattering; diffraction theory deals with all aspects of the interactions between matter (i.e. obstacles) and waves, so it also determines the directional patterns of sound radiation from vibrating objects.

Diffuse reflection: reflection of sound at a rough irregular profiled surface, which scatters sound in different directions (as opposed to specular reflection).

Diffuse sound field: a sound field of statistically uniform energy density in which the directions of propagation of waves are random from point to point.

Diffuser: an object or surface profile designed to scatter sound in random directions and so to minimize specular reflection (according to the law of reflection).

Diffusion: the scattering of sound wave in many different directions by an object or a surface.

Digital signal: a signal having a discrete number of values, which can be represented as a sequence of numbers; see also analogue-to-digital converter and digital-to-analogue converter.

Digital audio tape recorder (DAT): a tape recorder which includes an ADC (and a DAC) and which records analogue signals on tape in coded digital form.

Digital-to-analogue converter (DAC): an electronic device, which converts digital signals into analogue signals.

Dipole: A sound source, which has the characteristics of two monopole sources of equal amplitude but opposite phase a short distance apart. An example is the sound radiated by an unbaffled loudspeaker cone, which approximates to a dipole source.

Directivity factor: the ratio of the sound intensity at a given distance from the source, in a specified direction, to the average intensity over all directions, at the same distance.

Directivity index: the directivity factor (DF) of a source expressed in decibels, i.e. 10 log10 (DF).

Direct sound: sound which arrives at the receiver having travelled directly from the source, without reflection.

Direct sound field: that part of the sound field produced by the source where the effects of reflections may be neglected.

Distortion: a lack of faithfulness in a signal, such as the introduction of harmonics into the frequency spectrum, introduced, for example, because of non-linearity or of overload of some component of the measurement system.

Disturbance: an effect of noise which may be indicated by some change of behaviour, e.g. closing windows, interruption to speech, moving bedrooms; the objectively measurable effect of noise on the performance of a task or activity, such as listening to speech, or getting to sleep (as compared with annoyance which can only be measured by asking people questions, e.g. via a questionnaire).

Doppler effect: the change in the observed frequency of a wave caused by relative motion between source and receiver.

Dose-response: the relationship between the human response to noise or vibration and the received exposure (or dose) of noise or vibration received.

Dynamic magnification factor (Q factor): a quantity which is a measure of the sharpness of resonance of an oscillating system (either mechanical or electrical); it is related to the amount of damping in the system.

Dynamic range: the range of magnitudes of a signal which a measuring system, or component of a system, can faithfully record, process or measure, from highest to lowest; usually expressed in decibels.

Dynamic stiffness: the ratio of change of force to change of displacement in a vibrating system; it may be different from the static stiffness of the system.