Sound Level, Energy-Equivalent Continuos Sound Level and Rating Level
Changes in physical pressure perceivable to the human ear are called sound. The velocity of sound propagation depends on the medium. Thus, the propagation velocity measures up to 5000 m/s in solids (structure-borne sound"), aprox. 1500 m/s in liquids (water) and aprox. 340 m/s in the air.The static air pressure is at 980-1030 Hekto-Pascal (hPa). The perceivable sound pressure fluctuations are distinctly lower, but cover a very large range from 20 µPa (the auditory threshold) to 100 Pa (the pain threshold). So the normal human ear is sensitive enough to register sound pressure fluctuations on a scale from 1 to almost a hundred millions. Consequentially, were sound pressure fluctuations measured on a linear scale (in Pa), the values would be very unwieldy. Furthermore, the ear reacts rather more logarithmically than linearly to acoustic stimuli. For this reason, acoustic sizes are indicated as the logarithmic relationship of the measured value to a reference value (the auditory threshold of 20 µPa). The sound pressure level LP is thus computed as follows:
The number of pressure fluctuations per second as measured in the physical unit of Hertz (Hz) is called sound frequency. The normal hearing of a healthy young person can perceive frequencies in a range from 20 Hz to 20 000 Hz (20 kHz). The ability to register very high frequencies diminishes with higher age. Contrarily to good microphones, the sensitivity of the human hearing is not the same at all frequencies. At the same sound pressure, it perceives low notes below 1 kHz and high notes above 4 kHz as fainter than it does at the middle frequencies. In order to be able to take this into consideration, assessment filters are used. The customary means of frequency weighting at this time is the A-Weighting, which approximately emulates the sensitivity of the human ear. The use of this weighting filter is indicated in the unit by the specification dB(A).
Typical sound level values of ambient noise:
Energy-Equivalent Continuous Noise Level Leq
Generally speaking, the permanent noise levels of the noises to be judged are subject to temporal fluctuations. In order to describe these noises, a temporal average value is introduced, which is called Equivalent Continuous Noise Level Leq following the international standardization.
T being the average time.
With different kinds of traffic, in Germany the continuous noise level Leqm is calculated using m=3 and T=24h; with aircraft noise, however, m=4 and T=6 months. This leads to an averaging out of e.g. peak days, operation changes and weekend breaks (e.g. at military airports).
Rating Level LAr
The rating level LAr is formed from the A-weighted equivalent continuous sound level (average level) Leqm and if necessary from added values for tone and information (3 or 6 dB(A)), impulse (consideration of sudden changes in level, 2-4 dB(A)) and for times of day with higher sensitivity (7-10 pm, 6-7 am, 6 dB(A)) (German directive "TA-Lärm").
Disturbing, troubling or endangering sound is called noise. So, noise is not a physical, but a psychological-medical phenomenon that can be influenced strongly by subjective impressions. Because of this, noise cannot be measured using physical devices, as individual feelings elude objective measurement procedures.
To be able to at least partially assess noise, we have to realize that a strict distinction between physical quantities (e.g. sound level, frequency) and the corresponding acoustic perceptions (e.g. loudness, tone pitch) is necessary. The human ear doesn't behave like a physical measurement device (see e.g. A-weighted filters). The scientific discipline of psychoacoustics tries to describe the relations between physically defined sound stimuli and the acoustic perceptions caused by them quantitatively. For example: if you raise the sound level of a 1kHz tone from 60 dB to 70 dB, this corresponds to doubling the perceived volume (loudness), but to multiplying the physical unit sound power by ten. For air traffic, this means that a decrease of sound power due to improved engines or lower flow noises are not perceived to the same extent as decreased loudness is by the human ear. Further psycho-acoustic variables that can be used to determine the annoyance of noise are roughness (=perceivable modulations in the noise, like e.g. clattering or whirring) or the sharpness (=percentage of high frequencies).
German Aircraft Noise Legislation
On the 30th of March 1971, the German Law for the Protection against Aircraft Noise for the protection of the population against dangers, substantial disadvantages and substantial annoyances by aircraft noise in the neighbourhood of airports was issued. The law consists of two sections.In the first section of this law, noise protection ranges are specified for all traffic airports with regular transport service and all military airports maintaining jet aircraft. The noise protection range comprises the area outside of the airport area in which the continuous sound level caused by aircraft noise amounts to more than 67 dB(A). This range is divided into two protected zones (protected zone 1: more than 75 dB(A), protected zone 2: 67-75 dB(A)). The noise protection range is established under consideration of the kind and extent of the predicted flight operations on the basis of the development of the airport (prognosis data) that is to be expected. Especially the noise emission data of the aircraft, the number of flight movements in the six most traffic-intensive months of the prognosis year, as well as the routings of the outgoing and incoming flights enter the calculations. The Law for the Protection against Aircraft Noise prescribes several restrictions against the structural use for the two protected zones of the noise protection range. In the protected zone 1, no dwellings may be established. In the protected zone 2, this is only permitted if certain structural sound-proofing measures are attached. In both protected zones, the construction of sensitive facilities (e.g. hospitals, schools) is not allowed. The law justifies reparation claims of property owners against airport authorities for costs concerning structural sound-proofing arrangements.
If a change in the lay-out or in the operation of the airport leads to a change in noise pollution of more than 4 dB(A), the noise protection areas are to be reviewed and, if necessary, to be newly determined. Apart from this, an examination of the noise-protected zones becomes necessary ten years after their fixation.
In the second section of the law, airport owners, aircraft owners and aircraft pilots are obligated to prevent avoidable noises and to reduce them to a minimum to protect the population from dangers, substantial disadvantages and substantial annoyances by noise. Furthermore, the law states that for all traffic airports for which noise protected zones are to be determined, aircraft noise comissions are to be set up.
Main Points of Criticism of the German Air Traffic Noise Act
The main points of criticism brought forward by experts concerning the 1971 German air traffic noise act are concerned with the limit values for the protected zones, which use the averaging level Leq4 instead of Leq3, which is used with other kinds of traffic, and the non-observance of single sound events. The German Federal Government is planning to amend the air traffic noise act. In an information booklet (May 2000) by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety the following points are mentioned as the basis for the planned amendment:
Acoustic Devices and Airplane Sounds Used in the Study
Calibrated Class-1 sound level meters by Cortex are used in the study. With these, it is possible to simultaneously determine and store the total levels including all usual time constants and frequency weightings (A, B, C, Lin and F, S, I, Peak), maximum, minimum and average value (Leq) of these total levels, the maximum tact levels, percentile values, third-octave spectra and the level time history according to the current DIN norms. Loudness can be determined here as well as a third, octave, or FFT analysis. Furthermore, the triggered audio signal recording of the noise is possible at a sampling rate of 12,24 or 48 kHz.
The airplane sounds (of airplane take-offs as well as landings) brought in via loudspeaker in the sleep laboratory studies have all been recorded near the Düsseldorf airport using a sampling rate of 48 kHz. As windows absorb the high frequency portions of aircraft noise more strongly than the low frequency portions, the recordings were made in dwellings with half-open windows to achieve a more realistic production of aircraft noise in the sleeping laboratory.
Stored in the data base are airplane sounds of different structures ranging from 50 to 80 dB(A) maximum level.
The acoustic workstation CF85 makes possible the post-processing and post-analysis of the recorded audio signals. It can also determine the psychoacoustic factors of roughness and sharpness of the signal.The workstation also serves as a pre-programmable play-back station during the sleep laboratory studies that plays the airplane signals controlled by a radio clock and thus allows a double study (neither the examiners nor the volunteers are aware if, when and which airplane sounds will be played).
Further Reading on "Acoustics" can be found here andHelpful Internet Links here