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Terminologies in Acoustics

Terminologies in Acoustics. Building Utilities 3 M.S.Chua. Terminologies. Absorption The changing of sound energy to heat. Terminologies. Absorption coefficient

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Terminologies in Acoustics

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  1. Terminologies in Acoustics Building Utilities 3 M.S.Chua

  2. Terminologies • Absorption • The changing of sound energy to heat.

  3. Terminologies • Absorption coefficient • The fraction of sound energy that is absorbed at any surface. It has a value between 0 and 1 and varies with the frequency and angle of incidence of the sound. Image Source: http://www.acousticalsurfaces.com/acoustic_IOI/images/101x6_1.gif

  4. Terminologies • Acoustic barrier • Solid walls or partitions, solid fences, earth mounds, earth berms, buildings, etc used to reduce noise, without eliminating it. Image Source: http://www.acoustics.org/press/133rd/rose2.gif

  5. Terminologies • Air-borne noise • This refers to noise which is fundamentally transmitted by way of the air and can be attenuated by the use of barriers and walls placed physically between the noise and the receiver.

  6. Terminologies Ambient noise The composite of airborne sound from many sources near and far associated with a given environment. No particular sound is singled out for interest. Image Source: http://www.ssc.education.ed.ac.uk/courses/pictures/dmay1023.jpg

  7. Terminologies • Anechoic chamber • A room designed to suppress internal sound reflections. Used for acoustical measurements. Image Source: https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcRpgJwOnmNY-y26aHtQbsc-QYuLQtwLcWqAD3oAC42jzfKT1ftUMw

  8. Terminologies • Attenuate • To reduce the level of an electrical or acoustical signal.

  9. Terminologies • Audible range • The limits of frequency which are audible or heard as sound. The normal ear in young adults detects sound having frequencies in the region 20Hz to 20kHz, although it is possible for some people to detect frequencies outside these limits.

  10. Terminologies • Background noise • Background noise is the term used to describe the noise measured in the absence of the noise under investigation. It is described as the average of the minimum noise levels measured on a sound level meter and is measured statistically as the A-weighted noise level exceeded for ninety percent of a sample period. This is represented as the L90 noise level • Noise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structureborne, and instrument noise.

  11. Terminologies • Baffle • A movable barrier used to achieve separation of signals from different sources. The surface or board upon which a loudspeaker is mounted. Image Source: https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcSPDmf-kMlKjpE5-5d9yP57i_sTyBvLPUwut2IAtnkdjus0CsFJyQ

  12. Terminologies • Damp • To cause a loss or dissipation of the oscillatory or vibrational energy of an electrical or mechanical system.

  13. Terminologies • Decibel [dB] • The term used to identify ten times the common logarithm of the ratio of two like quantities proportional to power or energy. (See level, sound transmission loss.) Thus, one decibel corresponds to a power ratio of 100.1.

  14. Terminologies • Decibel [dB] • The range of pressure variations associated with everyday living may span over a range of a million to one. On the top range, may be the sound of a jet engine and on the bottom of the range may be the sound of a pin dropping. • Instead of expressing pressure in units ranging from a million to one, it is found convenient to condense this range to a scale 0 to 20 and give it the units of decibels. The following are examples of the decibel readings of every day sounds;

  15. Terminologies • Decibel [dB] • 0dB the faintest sound we can hear • 30dB a quiet library or in a quiet location in the country • 45dB typical office space. Ambience in the city at night • 60dB Martin Place at lunch time • 70dB the sound of a car passing on the street • 80dB loud music played at home • 90dB the sound of a truck passing on the street • 100dB the sound of a rock band • 115dB limit of sound permitted in industry • 120dB deafening

  16. Terminologies • Diffraction • The distortion around solid obstacles of waves travelling past.

  17. Terminologies • Far Field • That part of the sound field in which sound pressure decreases inversely with distance from the source. This corresponds to a reduction of approximately 6 dB in level for each doubling distance.

  18. Terminologies • Fluctuating noise • Noise that varies continuously and to an appreciable extent over the period of observation. It can also include intermittent noise. As a guide, when the level varies noticeably by more than 5 dB over a period of less than one minute, the noise is considered to be fluctuating.

  19. Terminologies • Free field • An environment in which a sound wave may propagate in all directions without obstructions or reflections. Anechoic rooms can produce such an environment under controlled conditions.

  20. Terminologies • Frequency • Frequency is synonymous to pitch. Sounds have a pitch which is peculiar to the nature of the sound generator. For example, the sound of a tiny bell has a high pitch and the sound of a bass drum has a low pitch. Frequency or pitch can be measured on a scale in units of Hertz or Hz. • The measure of the rapidity of alterations of a periodic signal, expressed in cycles per second or Hz. Image Source: http://teacheratsea.files.wordpress.com/2012/07/frequency-wavelength.jpg

  21. Terminologies • Hertz • the unit of frequency, abbreviated Hz. The same as cycles per second. The name is in honor of Heinrich Hertz, an early German investigator of radio wave transmission.

  22. Terminologies • Impulsive noise • Having a high peak of short duration or a sequence of such peaks. A sequence of impulses in rapid succession is termed repetitive impulsive noise.

  23. Terminologies • Intermittent noise • The level suddenly drops to that of the background noise several times during the period of observation. The time during which the noise remains at levels different from that of the ambient is one second or more.

  24. Terminologies • Loudness • A rise of 0 dB in sound level corresponds approximately to a doubling of subjective loudness. That is, a sound of 85 dB is twice as loud as a sound of 75 dB which is twice as loud as a sound of 65 dB and so on. That is, the sound of 85 dB is 400% times the loudness of a sound of 65 dB.

  25. Terminologies • Low-frequency noise • Containing major components within the low frequency range (20Hz - 250Hz) of the frequency spectrum.

  26. Terminologies • Near field • Locations close to the sound source between the source and the far field. The near field is typically characterized by large sound pressure level variations with small changes in measurement position from the source.

  27. Terminologies • Noise • Sound which a listener does not wish to hear.

  28. Terminologies • Octave • An octave is a doubling or halving of frequency. 20Hz-40Hz is often considered the bottom octave. Each octave you add on the bottom requires that your speakers move four times as much air!

  29. Terminologies • Periodic • A signal that repeats the same pattern over time is called periodic, and the period is defined as the length of time encompassed by one cycle, or repetition. The period of a periodic waveform is the inverse of its fundamental frequency.

  30. Terminologies • Quality, acoustic • An attribute, characteristic or property of the noise, its duration, its time-varying characteristics or its frequency content. Examples are the "screech" of screaming, the "rumble" of an air conditioner, the "dripping" of a tap.

  31. Terminologies Reverberation The persistence of sound in an enclosed or partially enclosed space after the source of sound has stopped; by extension, in some contexts, the sound that so persists. Image Source: http://www9.dw-world.de/rtc/infotheque/sound_processors/REVERB1.gif

  32. Terminologies • Reflection • Sound wave changed in direction of propagation due to a solid object obscuring its path. Image Source: Link here

  33. Terminologies • Sabin • The unit of measure of sound absorption in the inch-pound system.

  34. Terminologies • Sound • A fluctuation of air pressure which is propagated as a wave through air.

  35. Terminologies • Sound absorption • The ability of a material to absorb sound energy through its conversion into thermal energy.

  36. Terminologies • Sound level meter • An instrument consisting of a microphone, amplifier and indicating device, having a declared performance and designed to measure sound pressure levels.

  37. Terminologies • Sound pressure level • The level of noise, usually expressed in decibels, as measured by a standard sound level meter with a microphone.

  38. Terminologies • Sound power level • Ten times the logarithm to the base 0 of the ratio of the sound power of the source to the reference sound power.

  39. Terminologies • Structure-borne noise • This refers to noise which is generated by vibrations induced in the ground and/or structure. These vibrations excite walls and slabs in buildings and cause them to radiate noise. This type of noise can not be attenuated by barriers or walls but requires the interposition of a resilient (neoprene, springs etc.) break between the source and the receiver.

  40. Terminologies • Timbre • The quality of a sound that distinguishes it from other sounds of the same pitch and volume. The distinctive tone of an instrument or a singing voice.

  41. Terminologies • Tonal noise • Containing a prominent frequency and characterized by a definite pitch.

  42. Terminologies • Wavelength • The distance the sound wave travels to complete one cycle. The distance between one peak or crest of a sine wave and the next corresponding peak or crest. The wavelength of any frequency may be found by dividing the speed of sound by the frequency. (speed of sound at sea level is 331.4 meters/second or 1087.42 feet/second). Image Source: http://img.tfd.com/cde/WAVELEN.GIF

  43. References • http://www.renzotonin.com.au/wizardsPDF/acousticTerminology.pdf • http://www.csupomona.edu/~dmgrasmick/mu108/acousticterms.pdf

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