Page 21 - Spring 2007

never showing a coincidence dip.
To improve transmission loss, many noise control engi-
neers use two panels, particularly in windows. Called ‘double glazing,’ the panels are separated by an air (or other gas) gap. Since transmission loss is additive, there is a substantial improvement, much more so than simply increasing the thickness of a single panel. However, most double panel sys- tems have different panel thicknesses so that the coincident dips of the two panels occur at different frequencies. While this leads to two pass-bands for incident sound, it is general- ly preferable to have two weak pass bands rather than one very strong one.
In most practical situations, acoustic waves impinging on a panel do not arrive from the same angle. Consider a window in an office building or a hotel. Sound waves arrive from all angles, with a random distribution of the angles over time (imagine incident sound from passing airplanes and automobiles, reflecting off of adjacent buildings). Therefore, statistical methods have been used to estimate a random angle of incidence transmission loss for low frequencies:
(25)
where TL0 is 10log10(1/τf=0), or the mass-law normal inci- dence transmission loss. Since transmission loss is maxi- mized (transmission coefficient is minimized) at normal incidence, the random angle of incidence transmission loss is a fraction of that at normal incidence.
Be careful using the above expression at high frequen-
Fig. 12. Incident and transmitted sound fields around an infinite 25 mm thick steel plate, 30 degree angle of incidence. Top–at 50% of coincidence, Bottom–at coinci- dence. Dark blue and red indicate high pressures, and green indicates low pressures.
G.R.A.S. Sound & Vibration cost-efficient systems for beam-forming and acoustic holography.
MICROPHONE ARRAY MODULES
Depending on the application and the nature of
the emitted sound field, the microphones are available as pressure- or free-field types, all with a built-in Transducer-Electronic-Data-Sheet (TEDS).
For multi-channel systems without TEDS options, we offer uni-gain versions with pre-adjusted gain.
The array modules for mounting the microphones are available in various spacing configurations depending on frequency requirements.
The amount of microphones and modules in the array is only limited by the channels of the analysis system.
As always, we are ready to act on request, if our standard solutions do not fit in with your requirements.
G.R.A.S. Sound & Vibration A/S · Skovlytoften 33 · 2840 Holte · Denmark
Tel.: +45 4566 4046 · Fax: +45 4566 4047 · E-mail: gras@gras.dk · www.gras.dk
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