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Engineering Guide Noise Control

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SECTION M Engineering Guide Noise Control Please refer to the Price Engineer’s HVAC Handbook for more information on Noise Control. Noise Control Engineering Guide Terminology Acoustics when a sound source is located a large Near Field The science that is related to the production, distance from reflecting surfaces, or when The sound field immediately surrounding control, transmission, reception, and effects nearby surfaces are highly absorbent. the source where the sound pressure is of sound. Frequency influenced by the radiation characteristics Acoustic Media The number of times in one second that of the sound source. In this field the sound a passing wave repeats itself. The unit of pressure and the particle velocity are out The sound-absorbing material used inside of phase. absorptive type silencers and plenum walls. frequency is Hertz ( Hz), which corresponds to 1 cycle per second. Negative Air Flow Ambient Noise Fundamental Frequency See Reverse Air Flow. An all-encompassing noise that is present in a given environment and is comprised The lowest frequency in a harmonic series. Noise of many sounds from many sources, with Generated Noise Any unwanted sound or meaningless sound no particular sound being dominant. Also The sound power produced by air flowing of greater than usual volume. known as background noise. through a silencer at a given velocity and NR (Noise Reduction) A-Weighting direction (forward or reverse). The unit of A value that represents the difference in A weighting network that is widely used, measurement is decibels (dB). sound pressure level between any two often abbreviated dBA, dB(a). It has a Harmonic points along the path of sound propagation. prescribed frequency response that bears An integer multiple of the fundamental The unit of measurement is decibels (dB). a close relationship to loudness judgments frequency. NRC (Noise Reduction Coefficient) of the human ear. Hertz ( Hz) A measure of the acoustical absorption Background Noise The unit of measurement for frequency, or performance of a material, expressed as Noise sources other than the noise source the number or cycles per second. a single value. This value is calculated by that is of interest or being measured. Also averaging the material’s sound absorption known as ambient noise. High Transmission Loss (HTL) coefficients at 250, 500, 1000 and 2000 Hz Breakout Noise Occurs when a heavier material or and rounding to the nearest multiple of 0.05. combination of materials create a higher Octave Noise that travels through the wall of an resistance to sound transmitting through enclosure or duct work into an occupied them. The interval between two sounds having a space. frequency ratio of 2. There are 8 octaves on Insertion Loss (IL) Broadband Noise the keyboard of a standard piano. The decrease in sound pressure level or Octave Band Noise with components over a wide range sound intensity level measured at a receiver of frequencies. when the silencer or a sound-attenuating A segment of the frequency spectrum Decibel (dB) element is inserted into the path between separated by an octave. A unit for expressing the ratio of two the source and the receiver. The unit of Pascal (Pa) amounts of acoustic signal power equal to measurement is decibels (dB). A metric unit of measurement for pressure 10 times the common logarithm (to the base Intensity that is equivalent to one Newton per square 2 10) of this ratio. See Sound Intensity. meter (N/m ). Dynamic Insertion Loss Intensity Level Pink Noise The insertion loss at a given air flow direction See Sound Intensity Level. A sound with a frequency spectrum that and velocity. The insertion loss of a silencer is flat in logarithmic scale, and has equal varies depending on whether the sound is Media Protection power in bands that are proportionally traveling in the same or opposite direction A liner that is applied between the fiberglass wide. When compared with white noise, the as the air flow. Silencer performance media and the air stream of the silencer to power density decreases by 3 dB per octave. changes with the absolute duct velocity. protect the media from erosion due to high Pitch The unit of measurement is dB. velocity air flow and airborne contaminates. A subjective sound quality that is determined Flanking Module Factor by the frequency of a sound wave, or a Is when the walls of the ductwork on the A value that is used to define the width of combination of waves, that is placed on a upstream side of a silencer are readily a single module. scale extending from low to high. excited into vibration, which is then NOISE CONTROL NC (Noise Criteria) Positive Air Flow transmitted through the silencer walls and internal components and re-enters the A weighted value obtained from a series See Forward Air Flow. downstream duct path as noise. of curves that covers a spectrum of octave Pressure Attenuation (Pa) Code bands with center frequencies ranging from Forward Air Flow 63 Hz to 8000 Hz. Sound levels are plotted A code used to define the % of free area A condition that exists when airborne at each of these frequency bands, with the of a silencer cross-section. A decrease in sound and air flow are moving in the same highest point on the NC curve, regardless of free area typically results in an increased direction. Also known as positive air flow frequency, being the NC rating for the piece insertion loss and increased pressure drop or supply air flow. of equipment. These values are commonly across the silencer. ENGINEERING GUIDE - Free Field used by manufactures to rate equipment Pressure Drop because this system factors in the human The sound field where only directly radiated The difference in static pressure from the sensitivity to each of the frequency bands. inlet to the outlet of a system component. sound waves moving away from the sound The single number value is known as the source are present. This condition exists NC level. For more information on additional imperial and metric sizes, © Copyright Price Industries Limited 2014. M-2 please visit www.priceindustries.com or contact your local Price representative. Noise Control Engineering Guide Terminology Pure Tone baffles. A silencer can have multiple properties of floors, walls, ceilings, A single frequency sound. A sound for which modules within one component. windows, doors, etc. the sound pressure is a simple sinusoidal Sone Static Pressure (Ps) function of the time. A subjective unit of measurement for The pressure exerted equally in all directions Random Noise loudness for an average listener that is on a system, regardless of air flow. This can An oscillation whose instantaneous equal to the loudness of a 1000 Hz sound also be defined as the difference between magnitude is not specified for any given that has an intensity 40 dB above the Total Pressure (Pt) and the Velocity Pressure instant of time. listener’s threshold of hearing. (Pv), expressed as Ps = Pt-Pv. RC (Room Criteria) Sound A value obtained from a series of curves (A) An oscillation in pressure, stress, Static Pressure Loss that cover a spectrum of octave bands with particle displacement, particle velocity, etc. The difference in static pressure from the center frequencies ranging from 16 Hz to in an elastic or partially elastic medium, inlet to the discharge of a duct element. or the superposition of such propagated 4000 Hz. This rating system was specifically Static Regain created to establish design goals for HVAC alterations. systems, as it provides guidance for (B) An auditory sensation evoked by the An increase in system static pressure that maintaining a certain level of background oscillation described above. occurs in correlation with a decrease in velocity pressure caused by an increase in sound for masking. The curves are used to Sound Intensity provide a rating of an occupied indoor space the cross-sectional area of the ductwork. with a single number value indicated as the The amount and direction of flow of acoustic Supply Air Flow energy at a given position (W/m2). RC level. See Forward Air Flow. Sound Intensity Level Return Air Flow System Effect See Reverse Air Flow. The ratio of sound intensity, which is obtained by taking ten times the common logarithm A deviation from the cataloged performance Reverberant Field of a given sound intensity in reference to for a piece of equipment. This is caused by The sound field that consists of both directly 10-12 W/m2. The unit of measurement is dB the difference between how the product radiated and reflected sound waves. re 10-12 W/m2. was tested and how the product is actually installed in the system. Reverse Air Flow Sound Power Tone A condition that exists when airborne sound The total sound energy radiated by a source and air flow are moving in the opposite per unit time. The unit of measurement is An intense sound that is concentrated at direction. Also known as return air flow or the watt (W). a single frequency and typically contains negative air flow. harmonics at multiples (2x, 3x, 4x, etc.) of Sound Power Level the fundamental frequency. Sabin Algebraically defined as LW=10log(W/Wref) Total Pressure (Pt) A unit of measurement for acoustic where typically Wref = 10-12 watts. This absorption in relation to the absorption equation indicates that sound power level The combined effect of velocity pressure by one square foot of a perfect absorptive is ten times the logarithm (to the base 10) and the static pressure. This can also be material. of the ratio of a given sound power to a defined as the algebraic sum of Static -12 Pressure (Ps) and Velocity Pressure (Pv), Pt SIL (Speech Interference Level) reference power (typically 10 watts).
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