Microphones in Action
Excerpted from PCB Piezotronics’ Microphone Handbook
Acoustical testing is performed for a variety of reasons, ranging from designing new products, to monitoring products, to predictive maintenance functions, to personal protection. Some typical applications for acoustical studies that require microphones may include:
Research and Product Design – Excessive sound pressure can cause damage to products or human hearing. Microphones are used to measure the pressure level exerted on a surface. Sound pressure can shake plaster off walls or cause damage to an airplane wing. Sound measurement is used in a variety of applications including: the study of door slams, clutch engagements, starter impact and sunroof noise. The automotive market will utilize high precision microphones for “Squeak and Rattle” tests in order to provide a quieter ride. Analysis of engine noise in a cabin or car interior, or sound exhibited from consumer appliances are tested to extend the lifespan of the product and keep the external noise minimal, for the comfort of the user.
Preventive Maintenance – Increased sound levels or changes in frequency can indicate that a product is not working to itscapacity.Motors, gears, fans, bearings, blades, or other industrial components can all experience changes in decibel level or frequency shift when not working properly. High precision microphones can be utilized to confirm that a product is experiencing a problem, or can be used to predict failure of a component.
Audiometric Calibration – Universities, governments and independent companies have audio testing equipment to perform hearing tests and research projects. Microphones are used to test and calibrate the systems to ensure the accuracy of the test equipment.
Compliance – Microphone tests can be performed and recorded for verification of pressure levels on products, and can be utilized in legal situations. Companies will use high precision microphone tests for proof of sound pressure levels during design. Microphones are used on sound level meters to ensure compliance with national standards for shop noise.
Environmental Noise Analysis – There are certain sound pressure levels that the human ear can be subjected to for specific amounts of time before ear damage can occur (dose) as suggested by OSHA (Occupational Safety and Health Administration). Exposure to high amplitude sound pressure, for example loud machinery, industrial noise, airport noise, highway noise, etc., can have adverse effects on the human hearing. These effects can range from annoying noise to permanent loss of hearing. Acoustic testing is performed so that a better understanding of the sound levels that are experienced in these surroundings is achieved, and the necessary adjustments can be made in order to provide greater personal protection.
Noise Source Identification – Acoustic holography, beamfoaming, and other pressure mapping techniques are areas where microphone use has been increasing. Grids consisting of multiple microphones, sometimes used in conjunction with acoustic cameras,can be set-up to tell the difference in the sound pressure at different points around an engine or a car tire well. Calculations can be made per zone or spectrum. Some applications include seismic activity monitoring, satellite tracking, automotive and industrial noise source identification. Microphones can be utilized to transform 2-dimensional complex sound pressure information into 3-dimensional acoustic fields using basic wave equations to indicate surface intensity and radiation patterns.
Array microphones are Free Field Type microphones, which are designed to offer a cost-effective solution for multiple channel sound measurement. This makes Nearfield Acoustic Holography (NAH) measurements practical. Grids can be constructed to take 2D mapping measurements. The 130E20, 130E21 and 130E22 have an integrated Microphone and Preamplifier. The 130 series utilizes the Prepolarized microphone design and incorporate ICP® type circuitry powered by a constant current signal conditioner. The 130 series provide an inexpensive alternative to the 377 series. The 130 series are accurate for frequency responses and great for trending, but are more sensitive to changes in temperature and less accurate than the 377 series of high precision condenser microphones, when the most accurate decibel amplitude measurements need to be made.