Acoustic and Vibration Terms - Research Paper Example

Acoustic and Vibration Terms 1. Absorption Loss As electromagnetic waves pass through a medium, their amplitudes exponentially decrease. This declension is induced by currents in the shield to incur ohmic losses and heating of the material. The interaction of waves with the material medium results to transmission loss because of the dissipation or conversion of electromagnetic, acoustic, or electrical energy into other forms of energy (Ott and Ott, 2009, p. 245). This energy loss due to wave and material medium interaction is called absorption loss. 2. Acoustics Acoustics is the science of sound, including its effects, production, and transmission. Acoustics is differentiated from optics in the notion that sound is a mechanical, rather than an electromagnetic wave (Everest and Pohlmann, 2009, p. 1). That sound serves as a means of transmitting information, regardless of the receiver’s hearing ability. 3. Baseline Spectrum The baseline spectrum of a machine, generally, is the average of the recorded spectra of several machines of the same type (Norton and Karczub, 2003, p. 541). It is also known as the reference spectrum because it is commonly used as the basis for comparison of spectra taken during machine operation. 4. Baffle A baffle is a large, rigid, flat surface on which all portions of the source of sound waves move together with uniform particle velocity normal to the baffle itself (Harris and Block, 2010, p. 24). Line θ = 0 is the acoustic axis, on which the principally generated sound wave propagates, while the major spreading beam of the sound is the major lobe of the radiated pressure field. Figure 1. Directivity patterns for a rigid circular piston mounted on an infinite baffle for different values of ka, where a is the radius of the piston and k is the wave number. (a) ka = 3. (b) ka = 5. (c) ka = 10. 5. Band Frequency A band frequency is a selected range of sound frequencies (Rossing, 2007, p. 214). A sound can have various frequencies. When characterizing the sound level, the weighted average of the sound over all the recorded frequencies can be used. The energy distributed over a specific range of frequencies can also be used as a basis. By using acoustic or electrical filters, the different frequency bands across the entire frequency spectrum can be isolated. 6. Beat A beat is the interference between two sounds with slightly different frequencies (Howard and Angus, 2009, p. 143). This interference is perceived as periodic variations in volume whose rate is dependent on the difference between the two involved frequencies. 7. Concord Concord is the harmonious combination of sounds (Broadhouse, 2009, p. 274). For instance, if two chords frequently vibrate together, they produce a concord. Such concord is more pleafing as the coincidence of vibration becomes more frequent. 8. Concha Concha is a part of the external ear, along with the pinna, eardrum, and ear canal. The concha generates resonance effects and influences sounds through the direction of the sound source (Sandlin, 2000, p. 394). The concha has two open cavities, the concha cavum and the concha cymba, that are closely connected to each other. Figure 2. The External Ear (Sandlin, 2000, p. 394). 9. Damping Material Damping material is used to eliminate or reduce noise in structural, ergonomic, electronic, or industrial applications caused by resonance and vibration. A damping material changes the natural frequency of a vibration in the vibrating surface, thereby lessening the radiated noise while increasing the material’s transmission loss (Berger and AIHA, 2003, p. 307). 10. Diffraction Diffraction is the spreading out or bending of sound waves in a rather unusual way as they come in contact with edges of objects (Haughton, 2002, p. 76). The intensity of diffraction is highly dependent on the size of the object and wavelength of sound. Figure 3. Sound Diffraction. 11. Diffuse Sound A diffuse sound possesses a uniform energy density in a particular region, making all energy flux directions over the region equally probable (Fahy, 2001, p. 328). It is also called as non-correlated sound because it no longer possesses the coherency of a reflected sound as various sound wavelengths egress with various reflection angles. 12. Digital Filter A digital filter is an electrical filter that responds to and performs numerical information on inputs (Winder and Winder, 2002, p. 31). The calculation generally involves multiplication of inputs and constants and addition of products. 13. Digital signal Digital signal is the output of the analog-to-digital signal conversion (Chitode, 2009, p. 1-2). Signal inputs are typically detected by analog processor. The analog outputs are converted to digital signals, which are then process by digital signal processor. Figure 4. The Signal Processing and Conversion. 14. Dynamic mass Dynamic mass is the ratio of input force to output acceleration during a simple harmonic motion (Moses, 2009, p. 164). Masses of appliances and other machines are not always considered as compact masses. These masses can be subjected to elastic deformations with resonance characteristics. The moving or dynamic mass, substantial for the resonance properties of bearings, can possibly far smaller than the total static or mass at rest. 15. Dynamic modulus Dynamic modulus refers to the stress-to-strain ratio under vibrations (Malhotra and Carino, 2004, p. 7-2). It can be measured by determining the complete hysteresis loop during an isolated system during vibration, the natural frequency of vibration, or the natural frequency of an isolated system under sustained near-resonant vibrations of a freely decaying vibrating system, on which the specimen is the only unknown deflecting member or spring. 16. Dynamic range A dynamic range is the ratio of the loudest and softest sounds one can wind up getting with a system (McFarland and Gerhard, 2007, p. 556). The figure shows a mismatch between the tape and console deck. The console will overload prior to deck, getting 5 dB of dynamics. This can be avoided by proper adjustment of the input control of the tape deck. Figure 5. The Dynamics of the Recording Chain. 17. Eddy current Eddy current is the current induced in the small coils on a large conductor (Raj, Jayakumar, and Thavasimuthu, 2002, p. 27). Eddy currents induced in the core of a power transformer represent a power loss. They are commonly used to generate heat enough for cooking and metallurgical applications. In the figure below, an alternating current is allowed to flow in a coil, which in turn generates an alternating magnetic field. When the coil is brought near to the electrically conducting surface of a material, the electromagnetic induction instigates an eddy current. Figure 6. Electromagnetic Induction and Eddy Currents (Raj, Jayakumar, and Thavasimuthu, 2002, p. 28) 18. Excitation Excitation in acoustics is triggering vibrations in a material through sound waves (Castro, 2010, p. 18). In the case of excitation of cantilever-tip oscillations, in the excitation mode, a piezoelectric actuator is connected below the material containing the cantilever-tip ensemble. Applying oscillating voltage to the actuator generates vibrations, which in turn, triggers the cantilever oscillations. Figure 7. Acoustic excitation of cantilever-tip oscillations (Castro, 2010, p. 18). 19. Fenestra Ovalis Fenestra ovalis is an oval opening on the medial wall of the tympanic cavity, heading to vestibule, through which the ossicles of the ear transmit sound vibrations to cochlea (Buck, 2008, 148). It is connected to the Tympanum by a series of 3 small bones, called stapes, and the Anvil, or Incus. Vibrations in the Tympanum are transmitted to the membrane cover of the Fenestra ovalis by the stapes. Thus, Fenestra ovalis serves as the window of the inner ear that communicates with the stapes. Figure 8. The Middle Ear. 20. Gamnitude A gamnitude is tantamount to the amplitude of a generalized rahmonic and proportional to the power of all harmonics in a given series (Zakhezin and Malysheva, 2010, p. 40). List of References Berger, E. H. and American Industrial Hygiene Association. (2003) The noise manual. Fairfax, VA: American Industrial Hygiene Association Broadhouse, J. (2009) The Phenomenon of Sound as Connected with Music. London: General Books Buck, P.C. (2008) Acoustics for Musicians. Tennessee: Mason Press Castro, G. R. (2010) Amplitude modulation atomic force microscopy. Weinheim: Wiley-VCH Chitode, J.S. (2009) Digital Signal Processing. India: Technical Publications Pune Everest, F. A. and Pohlmann, K. C. (2009) Master handbook of acoustics, 5th ed. New York: McGraw-Hill Fahy, F. (2001) Foundations of engineering acoustics. San Diego, Calif: Academic Harris, J. G. and Block, G. I. (2010) Elastic waves at high frequencies: Techniques for radiation and diffraction of elastic and surface waves. Cambridge: Cambridge University Press Haughton, P. M. (2002) Acoustics for audiologists. San Diego, Calif: Academic Press Howard, D. and Angus, J. (2009) Acoustic and psychoacoustics, 4th ed. Oxford: Focal Press Malhotra, V.M. and Carino, N.J. Eds (2004) Handbook on Nondestructive Testing. Pennsylvania: CRC Press McFarland, J. and Gerhard, M. (2007) Mastering Autodesk VIZ 2008. Indianapolis, Ind: Wiley Publishing Möser, M. (2009) Engineering acoustics: An introduction to noise control. Dordrecht: Springer Norton, M. P. and Karczub, D. G. (2003) Fundamentals of noise and vibration analysis for engineers. Cambridge: Cambridge Univ. Press Ott, H. W., and Ott, H. W. (2009) Electromagnetic compatibility engineering. Hoboken, N.J: John Wiley & Sons Raj, B., Jayakumar, T., and Thavasimuthu, M. (2002) Practical non-destructive testing. Cambridge: Woodhead Publ Rossing, T. D. (2007) Springer handbook of acoustics. New York, N.Y: Springer. Sandlin, R.E. (2000) The textbook of hearing aid amplification, 2nd ed. California: Singular Thomson Learning Winder, S. and Winder, S. (2002) Analog and digital filter design. Amsterdam: Newnes Zakhezin, A.M. and Malysheva, T.V. (2010) ‘Vibration Diagnostics of Fatigue Cracks.’ Journal of Machinery and Reliability 39 (2), 185-190 Read More