Decibel

Summary

The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a power ratio of 101/10 (approximately 1.26) or root-power ratio of 10 (approximately 1.12). The unit expresses a relative change or an absolute value. In the latter case, the numeric value expresses the ratio of a value to a fixed reference value; when used in this way, the unit symbol is often suffixed with letter codes that indicate the reference value. For example, for the reference value of 1 volt, a common suffix is "V" (e.g., "20 dBV"). Two principal types of scaling of the decibel are in common use. When expressing a power ratio, it is defined as ten times the logarithm in base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level. When expressing root-power quantities, a

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https://en.wikipedia.org/wiki/Decibel

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Comparison of low-frequency background noise cancellation systems for room acoustics

François Aballéa, Hervé Lissek, Pierre-Jean René

Low frequency background noise (below 200 Hz) is perceptible in many situations such as in room acoustics or industrials issues. Generate by numerous type of source (traffic noise, aircraft, industrial plants, electricity transformer,… ), this type of noise is under evaluate by regulations laws due to the used of the A-weighted decibel (dB(A)). In spite of that low frequency background noise can reach high levels which produce poor speech intelligibility and annoyance feeling. Unfortunately traditional acoustic treatments have little impact to decrease this kind of disturbance. New approaches have to be developed to give new efficient techniques in order to overcome the problem. Three noise cancellation systems dedicated to low frequencies are studied here. The first one is based on an electromechanical transducers loaded passively to get an optimal damping around the resonance frequency of the disposal. In the second system the passive load is substituted by an active control to enhance acoustic properties of device. The last strategy consists on a control of the first modal frequencies of a room to decrease the low frequency background noise level. This paper aims at presenting those three noise cancelation systems. Their advantages and drawbacks will also be discussed.

2008

We describe a high-efficiency grating coupler (GC) fabricated on a silicon-on-insulator wafer with 220 nm top silicon layer. One single 60 nm shallow etch is required to define the diffractive gratings with a minimum lithographic feature size of 180 nm, which is within the limitation of 248 nm deep ultraviolet lithography. The measured average insertion loss is 3.1 ± 0.2 dB ~1550 nm with a 1 dB bandwidth of 41 ± 4 nm for TE polarization, whereas the best device exhibits 2.7 dB loss. The measured GC backreflection loss is better than 17 dB across the wafer. Cross-wafer data shows good uniformity and tolerance to fabrication variations. This is the best result reported for the commonly used 220 nm thickness Si that uses only a shallow etch step.

Institute of Electrical and Electronics Engineers2013

Shunt loudspeakers for modal control in rooms

Romain Boulandet, Hervé Lissek, Pierre-Jean René

Engineers dealing with noise reduction in habitations close to transportation traffic or indus-trial facilities encounter several problems to decrease noise level in rooms at low frequencies. Passive materials and current building construction knowledge enable to avoid noise trans-mission in habitations at medium and high frequencies and the regulations based on the dBA scale can often be respected. But these regulations do not take into account the real an-noyance of noise for the inhabitants who are still disturbed by low frequency noise. Because of the modal behavior of rooms, air-borne and structure-borne noise generate high sound pressure level at the first modal frequencies, even with small amount of energy. In order to lower the annoyance induced by low-frequency behavior in rooms, huge compliant wooden panels can be used, though with quite low performances in terms of resonances damping. An alternative can be found with bass-traps, comprising a mechanical resonator and a closed-box, with fixed acoustic performance at specific frequency. Enhanced performances can even be obtained with closed-box shunt loudspeakers, where the acoustic impedance of the louds-peaker's diaphragm around its resonance can be modified (in terms of magnitude and also in frequency) by way of a simple passive electrical device, providing the requested complemen-tary complex acoustic impedance for perfect absorption at the first modal frequencies. The present work describes the design of a small array of low-frequency shunt loudspeakers, ded-icated to damp a certain amount of modes within a known room. Assessment in laboratory conditions (impedance tube, reverberant chamber) will then be presented and compared to simulations, leading to concluding remarks on practical issues on the applicability of the con-cept of shunt loudspeakers for low-frequency noise reduction in habitations.

2009