Sound intensity
The sound intensity (in watts per square meter) is given by
where
- is sound pressure in pascals
- is particle velocity in meters per second
A sound intensity probe is a probe capable of measuring sound intensity. The sound intensity is the product of sound pressure and particle velocity. Common probes measure the sound intensity using two microphones. Another type measures the sound intensity using a microphone and a particle velocity probe.
The sound intensity (in watts per square meter) is given by
where
A p-p type of sound intensity probe measures the sound intensity using two phase-matched microphones. These microphones are usually positioned face-to-face and are used to determine a pressure gradient. From this pressure gradient it is possible to calculate the particle velocity. The sound pressure is determined from the average from both microphones output.
A p-u type of sound intensity probe measured both the sound pressure and the particle velocity directly. Sound pressure is measured using a microphone and particle velocity using a particle velocity probe
Sound intensity probes are used for several applications. A common application is to determine the sound power of an object. Another application is locating a source of noise.
The use of sound intensity rather than sound pressure to determine sound power means that measurements can be made in situ, with steady background noise and in the near field of machines. The sound power is the average normal intensity over a surface enclosing the source, multiplied by the surface area. [1]
p-p probe
p-u probe
A pitottube, also known as pitot probe, is a flow measurement device used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy. It is widely used to determine the airspeed of an aircraft, water speed of a boat, and to measure liquid, air and gas flow velocities in certain industrial applications.
A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potential when subjected to a pressure change, such as a sound wave. Some piezoelectric transducers can also serve as a sound projector, but not all have this capability, and some may be destroyed if used in such a manner.
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At 20 °C (68 °F), the speed of sound in air is about 343 metres per second, or a kilometre in 2.9 s or a mile in 4.7 s. It depends strongly on temperature as well as the medium through which a sound wave is propagating.
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Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone. The SI unit of sound pressure is the pascal (Pa).
Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area. The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m2). One application is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.
Sound power or acoustic power is the rate at which sound energy is emitted, reflected, transmitted or received, per unit time. It is defined as "through a surface, the product of the sound pressure, and the component of the particle velocity, at a point on the surface in the direction normal to the surface, integrated over that surface." The SI unit of sound power is the watt (W). It relates to the power of the sound force on a surface enclosing a sound source, in air. For a sound source, unlike sound pressure, sound power is neither room-dependent nor distance-dependent. Sound pressure is a property of the field at a point in space, while sound power is a property of a sound source, equal to the total power emitted by that source in all directions. Sound power passing through an area is sometimes called sound flux or acoustic flux through that area.
Particle velocity is the velocity of a particle in a medium as it transmits a wave. The SI unit of particle velocity is the metre per second (m/s). In many cases this is a longitudinal wave of pressure as with sound, but it can also be a transverse wave as with the vibration of a taut string.
Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum.
In acoustics, the dummy head recording is a method of recording used to generate binaural recordings. The tracks are then listened to through headphones allowing for the listener to hear from the dummy’s perspective. The dummy head is designed to record multiple sounds at the same time enabling it to be exceptional at recording music as well as in other industries where multiple sound sources are involved.
A sound level meter is used for acoustic measurements. It is commonly a hand-held instrument with a microphone. The best type of microphone for sound level meters is the condenser microphone, which combines precision with stability and reliability. The diaphragm of the microphone responds to changes in air pressure caused by sound waves. That is why the instrument is sometimes referred to as a sound pressure level meter (SPL). This movement of the diaphragm, i.e. the sound pressure deviation, is converted into an electrical signal. While describing sound in terms of sound pressure metrics, such as Pascals, is possible a logarithmic conversion is usually applied and the sound pressure level is stated instead, with 0 dB SPL equal to 20 micropascals.
Acoustic location is the use of sound to determine the distance and direction of its source or reflector. Location can be done actively or passively, and can take place in gases, liquids, and in solids.
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Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water, its contents and its boundaries. The water may be in the ocean, a lake, a river or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and 1 MHz. The propagation of sound in the ocean at frequencies lower than 10 Hz is usually not possible without penetrating deep into the seabed, whereas frequencies above 1 MHz are rarely used because they are absorbed very quickly. Underwater acoustics is sometimes known as hydroacoustics.
A noise dosimeter or noise dosemeter is a specialized sound level meter intended specifically to measure the noise exposure of a person integrated over a period of time; usually to comply with Health and Safety regulations such as the Occupational Safety and Health (OSHA) 29 CFR 1910.95 Occupational Noise Exposure Standard or EU Directive 2003/10/EC.
The photoacoustic Doppler effect, as its name implies, is one specific kind of Doppler effect, which occurs when an intensely modulated light wave induces a photoacoustic wave on moving particles with a specific frequency. The observed frequency shift is a good indicator of the velocity of the illuminated moving particles. A potential biomedical application is measuring blood flow.
Brüel & Kjær is a Danish multinational engineering and electronics company headquartered in Nærum, near Copenhagen. It is the world's largest manufacturer and supplier of sound and vibration measurement equipment, systems and solutions. Brüel & Kjær is a subsidiary of Spectris.
In order to take a scientific measurement with a microphone, its precise sensitivity must be known. Since this may change over the lifetime of the device, it is necessary to regularly calibrate measurement microphones. This service is offered by some microphone manufacturers and by independent testing laboratories. Microphone calibration by certified laboratories should ultimately be traceable to primary standards a (National) Measurement Institute that is a signatory to International Laboratory Accreditation Cooperation. These could include the National Physical Laboratory in the UK, PTB in Germany, NIST in the USA and the National Measurement Institute, Australia, where the reciprocity calibration is the internationally recognised means of realising the primary standard. Laboratory standard microphones calibrated using this method are used in-turn to calibrate other microphones using comparison calibration techniques, referencing the output of the ‘test’ microphone against that of the reference laboratory standard microphone.
An acoustic camera is an imaging device used to locate sound sources and to characterize them. It consists of a group of microphones, also called a microphone array, from which signals are simultaneously collected and processed to form a representation of the location of the sound sources.
Per Vilhelm Brüel was a Danish physicist and engineer who pioneered and made fundamental contributions to the development of the physics of sound and vibration. He also formed and founded the world's largest manufacturer and supplier of sound and vibration measurement equipment, systems and solutions, Brüel & Kjær. Brüel was a close friend of Niels Bohr, and despite danger Brüel traveled from Sweden to Denmark during the German occupation with important documents of Bohr's work. Brüel was fluent in Danish, German, English, Swedish, and spoke French and Italian.
