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Acoustic & Vibration Fundamentals

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Acoustic & Vibration Fundamentals ACOUSTICS AND UMM… VIBRATION FUNDAMENTALS JEFF BOLDT | 2019-09-09 Introduction PRESENTATION INCLUDES • Physics of Sound • Understanding Decibels • Psychoacoustics • Acoustic Scales • Sound Power and Pressure PRESENTER Jeff Boldt, PE, FASHRAE, LEED® AP, HBDP, FPE Managing Principal – Director of Innovation & Quality at IMEG Corp. (>1,200 staff) Chair ASHRAE SSPC-90.1 MSC Former Chair ASHRAE 189.1 Acoustics Discussion Group Author AEDG Small Healthcare Facilities & AEDG Large Hospitals Member WHEA Code Committee Member SSPC 3.6, TC2.6, 5.2, 8.6 Search for “Jeff Boldt Engineering Nerd” Acoustical Background Two years graduate study through Heriot-Watt University in Acoustics, Vibration, and Noise Control Emphasis in building systems and noise control HVAC noise, generator enclosures, property line noise, Architectural Acoustics Has read many acoustical texts PHYSICS OF SOUND Physics of Sound Compression - Rarefraction Speed & Wave Equation Speed in air ~ 1100 fps ~ 750 mph Speed depends on • Material - mostly density (e.g. faster in metals) • Temperature V=f*L Velocity = frequency*wave length Frequency Audible Range ~ 16 Hz – 20 kHz (20,000 Hz) Audible Length ~ 68 ft – 5/8” 1,000 Hz ~ 1.1 feet Speech ~ 500 – 5,000 Hz • 2 to 0.2 feet Middle C = 262 Hz Grand Piano 28 – 4,186 Hz How People Describe Sounds Voice Frequencies Please put this into Excel and reprint the charts. Please only show the “normal” curves and include both in one graph. Volume Range HUMAN EAR HEARS OVER A VOLUME RANGE OF: • 140 dB (120 starts pain region) • 10^14 to 1 • 100 trillion to 1 • Accuracy of ~ +/- 5 dB • Comparable to a scale that weighs small ants (1 milligram) and the largest (110,000 ton) aircraft carriers, with reasonable USS Gerald Ford on the James River accuracy Reflection Specular = follows mirror rule Diffuse = random reflections Reflection is specular if surface irregularities are much smaller than the wave length Dissipative Absorption Friction converts noise to heat Typically fibrous materials Should be ~1/4 wavelength deep or held away from rigid surfaces May need covering for: • Cleanliness • Durability • Vapor barrier Reactive Absorption Tuned frequencies Very narrow band if not dampened Best for narrow frequencies Vehicle mufflers Perforated panels – small holes Can’t equal absorptive performance Gypsum board - Transformers Reactive Absorption Mufflers Dissipative (“packed” muffler) Reactive muffler Active Absorption Active • Measure and counter noise • Needs to be a controlled situation • Digisonix was the last manufacturer I know of • Works best at low frequencies • Absorptive silencers were cheaper, except maybe at very low frequencies DECIBELS Decibels Used to handle large ranges Decibel is a ratio, not a quantity Alexander Graham Bell invented the Bel Notation is dB, dBA, etc. Sound Power & Pressure Lw = SwL = Sound Power = energy • Lw = 10*log(watts/10^-12W) dB • Lw = 10*log(watts) + 120 dB • Like Lumens Lp = SpL = Sound Pressure • Lp = 10*log(p^2/Pref^2) dB • Lp = 20*log(p/Pref) dB • Like Footcandles At 1 kHz 0 dB is audible (youngsters) At 1 kHz 120 dB is painful More Decibels Original energy = Original dB 1 Double energy = +3 dB 2 4X energy = +6 dB 4 10X energy = +10 dB 10 100X energy = +20 dB 100 Etc. Adding each 10 dB adds a zero • 120 dB is 1 followed by 12 zeros • Them zeros ain’t nothing • Jethro Bodine understood… Adding Decibels Divide each number by 10 Take the anti-log (10^X) of each number Add the numbers Take the 10-log of the sum Multiply by 10 Spreadsheet has this feature Create in Excel so we own © to this Decibel Addition Graph Sound Pressure Sound Pressure Level Sound Source (Decibels, A-weighted) Saturn Rocket 194 Ram Jet 160 Propeller Aircraft 140 Threshold of Pain 135 Riveter 120 Heavy Truck 100 Noisy Office 80 Heavy Traffic 80 Conversational Speech 60 Private Office 50 Quite Resident 40 Recording Studio 30 Leaves Rustling 20 Hearing threshold good ears at frequency of maximum sensitivity 10 Hearing threshold excellent ears at frequency of maximum response 0 PSYCHOACOUSTICS Psychoacoustics Octave = double frequency Preferred center frequencies • 31.5 Hz • 63 Hz • 125 Hz • 250 Hz • 500 Hz • 1K, 2K, 4K, 8K • 16K Psychoacoustics Hyperacusis = poor ability to accommodate large volume ranges • Most common in elderly and young Sense of direction is mostly from phase effects, but also from shadowing • Concert hall design • Vertical symmetry – Dolby Psychoacoustics Hearing loss is either: • Conductivity – mechanical failure • All solvable • Can you make a graphic of a ME ruling the Universe? • Sensory-Neuro – electrical failure • Sensory-Neuro is more common • Sensory-Neuro is irreparable • Can you make a graphic of a despondent EE? Psychoacoustics Hearing loss is either: • Conductivity – mechanical failure • All can be repaired • Sensory-Neuro – electrical failure • Sensory-Neuro is more common • Sensory-Neuro is irreparable Psychoacoustics Hearing loss is either: • Conductivity – mechanical failure • All can be repaired • Sensory-Neuro – electrical failure • Sensory-Neuro is more common • Sensory-Neuro is irreparable • Clearly demonstrates the superiority of mechanical engineers Hearing Loss – Women 60 yrs 4,000 Hz -15 dB Hearing Loss – Men 60 yrs 4,000 Hz -32 dB Honey I can barely hear you Voice Frequencies Recreate graph Information Most information is in consonants Consonants are high frequency Vowels add volume and tone Honey I can hear you – but all I get is WA-WA- ACOUSTIC SCALES Acoustic Scales Everybody’s got one • Beranek has the most… Phons Phons = equal loudness contours Single number is 1K perceived loudness Decibel scale Phons and Sones Sensitivity much flatter for loud noise Most people consider +10 dB double volume 1 Sone = 40 Phons 2 Sones = 50 Phons Etc. Wikipedia = best Sone definition Pink and White Noise Pink noise has equal power per octave White noise has equal power per Hertz 80 70 60 Pink Noise 50 White Noise 40 dB 30 20 10 0 63 125 250 500 1000 2000 4000 8000 Hz Weighing Scales dB(A) is the most common one-number acoustic value • dBA resembles 40 Phon contour • OSHA uses dBA dB(B) is for louder sounds • Resembles the 80 Phon contour • Never used dB(C) is for even louder sounds • dBC is nearly unweighted • C and “flat” are often the same on meters dBA, B, and C Weighting Flat weights all frequencies equally dBC slightly filters high and low frequencies to simulate the sensitivity of the (young) human ear at high vlolumes. dB(B) is between A&C, but is seldom used. It simulates the human ear at moderate volumes. dBA filters high and especially low frequencies to simulate the sensitivity of the (young) human ear at low volumes. dBA Sensitivity Curve 30 25 20 dBA Curve 15 10 5 0 -5 63 125 250 500 1000 2000 4000 8000 RTA App NC (Noise Criteria) Another one-number index Proposed by Beranek Based on the worst octave Emphasizes annoying high frequencies Used by AMCA for noise ratings of grilles, registers, and diffusers Used by AHRI to rate VAV boxes NC Curves Worst case determines rating PNC & NCB Criteria PNC = Preferred NC • Improvement over NC • Very seldom used NCB • Lowered 4K & 8K limits • Added 32 and 63 Hz criteria • Proposed by Beranek in 1989 NCB Curves RC (Room Criteria) Developed by ASHRAE Not as common as dBA or NC Gives information about character of sound • RC-35R = Rumble • RC-35H = Hiss RC Curves RC Mark II Latest ASHRAE noise index Improves on RC, but is more complex Very strict criteria for “Normal” frequency distribution E.g. RC-35 (N) Spreadsheet Capabilities Calculates the following: • Unweighted dB • dBA • NC • RC Mark II • Outdoor property line noise – very fast Spreadsheet Example 1 SOUND CALCULATIONS Octave Center Frequency (Hertz) 6 2 1 2 5 2 5 0 5 0 0 1 K 2 K 4 K 8 K Sound Pressure at Chosen Location (Lp) 69.0 46.0 46.0 4 4.0 40.0 37.0 30.0 20.0 Total dB at Selected Location 69.1 dB dBA Calculations A-W eighting Conversion -2 6 .2 -1 6 .1 - 8 .6 - 3 .2 0 . 0 1 .2 1 . 0 - 1 . 1 A-W eighted Sound Pressure (LpA) 4 2 .8 2 9 .9 3 7 .4 4 0 .8 4 0 .0 3 8 .2 3 1 .0 1 8 .9 dBA at Chosen Location (Perceived Volum e) 47.4 dBA RC M ark II Calculations Conversion to RC Contour -20 -15 -10 -5 0 5 10 15 O ctave Band RC Ratings 49 31 36 39 40 42 40 15 D eviation from R C R ating 9 -9 -4 -1 0 2 0 -25 Spectral Deviation factor ranges LF MF HF Spectral D eviations 8 .7 -3.9 0.4 Q AI (Q uality Assessm ent Index) 1 2 .6 Q AI > 10 is likely to cause com plaints at m arginal noise Descriptions of RC abbreviations: 49.0 (N) denotes Neutral character sound, which is pr eferred. Q AI <5 is considered neutral. (LF) denotes that the sound is dom inated by Low Frequency Rum ble. (M F) denotes that the sound is dom inated by M id- Frequency Roar. (HF) denotes that the sound is dom inated by High -Frequency Hiss. LFB denotes likely m oderate but perceptible soun d-induced ceiling/wall vibration. LFA denotes likely noticeable sound-induced ceiling/w all vibration. RC M ark II Rating (per ASHRAE 2003 Applications - 47.28) RC 40 LF LFB Noise Criteria (NC) Calculations Peak NC curve each octave band is below 50 30 40 40 40 40 35 25 NC is not an appropriate rating scale for values ov er N C 50. N oise C riteria R ating = NC 50 OUTDOOR NOISE Temperature Refraction Make better graphic Directivity Directivity Directivity = Q • Q - Multiplier Application • 1 +0 dB Spherical • 2 +3 dB Ground (normal) • 4 +6 dB One wall • 8 +9 dB Two walls • 12 +12 dB roof also = megaphone Outdoor Shadowing Outdoor Barrier Effect Barriers • Noise reduction depends on extra distance divided by wavelength • NR = 10*log(3+20* DL*freq/1100) Title 35 Emergency Generator Other Outdoor Effects Humidity – low RH attenuates (slightly) Foliage – negligible Ground absorption Wind and temperature Humidity vs.
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