Acoustics 101 Presented By Noise and Vibration Control, Inc. What Is Sound? Sound is defined as any pressure variation in air, water or other fluid medium which may be detected by the human ear. What Is Noise? Noise is unwanted sound which may be hazardous to health, interfere with speech and verbal communications or is otherwise disturbing, irritating or annoying. Enloe HS Case Study 4 Characteristics of Sound • Loudness (Intensity) • Pitch (Tonal Content) • Character (Cyclical Ramping) How Are These Characteristics Expressed? The frequency of sound is expressed in wavelengths per second or cycles per second (CPS). It is more commonly referred to as Hertz. Low frequency noise is <250 Hertz (Hz) Mid-frequency noise is >250 and <2000 Hz. High frequency noise is >2000 Hz The amplitude of sound is expressed in decibels (dB). This is a dimensionless unit using a logarithmic compressed scale dealing in powers of 10 where small increments in dB correspond to large changes in acoustic energy. Threshold of hearing = 10 4 = 4 dB Threshold of pain = 10 13 = 130 dB Threshold of pain is 1 Billion times the Threshold of hearing What Is The Decibel Scale? The decibel (dB) is a dimensionless unit calculated using the ratio of a measured value (p) to a reference value (ref). The values of sound pressure of most interest range from the threshold of hearing at about 20 µPa to a level of extreme danger at about 200 Pa. This range of variation translates to 7 orders of magnitude with the high level being 10,000,000 times that of the lower threshold. The use of a logarithmic scale compresses the unit of measure to a manageable range in order to simplify calculations, computations and quantitative manipulation of data. What Are Octave Bands? What Are Octave Band Center Frequency Wavelengths At 70 °F? The speed of sound in air at 70 °F is 1,130 feet per second. Wavelengths in feet for each center band frequency are listed below: Frequency (Hz) 63 125 250 500 1000 2000 4000 8000 Wavelength (Ft.) 17.9 9.03 4.51 2.26 1.13 0.56 0.28 0.14 Common Sound Envelopes Acceptance Criteria Tonal Noise is Sound at a More Discrete Frequency Sioux Falls, SD Municipal Code defines tone as: “any sound which can be distinctly heard as a single pitch or set of single pitches…. and shall exist of the one-third octave band sound pressure level in the band when the tone exceeds the arithmetic average of the sound levels of the two contiguous one-third octave bands by five dB for frequencies of 500 Hz and above, eight dB for frequencies between 160-400 Hz, and by 15 dB for frequencies less than or equal to 125 Hz.” Screw Chiller Frequency Signature Tone Gen 297 Hz 594 Hz 891 Hz 1188 Hz 70 60 Full Octave 50 Frequency 40 Signature 30 SoundMicrobar) Re0.0002(dB PressureLevel 20 10 63 125 250 500 1000 2000 4000 8000 Center Octave Band Frequency (Hz) 1/3 Octave Frequency Signature 70 60 50 40 30 Sound Pressure Level (dB Re 0.0002 Microbar) 20 10 12 16 20 25 32 40 50 63 80 100 125 Center Octave160 Band Frequency (Hz) 200 250 315 400 500 630 800 1k 1.25k 1.6k 2k 2.5k 3.15k 4k 5k 6.3k 8k 10k 12.5k 16k 20k Narrow Band Frequency Signature What Is The Difference Between dB And dBA? “A” Weighted Correction Factors Based on Human Frequency Response Octave Band Center Frequency, Hz 31.5 63 125 250 500 1000 2000 4000 8000 Adjustment Factor -39 -26 -16 -9 -3 0 +1 +1 -1 What is Sound Pressure? Decibels of sound pressure (Lp) have a universally accepted reference pressure of 2.0 x 10-5 Pascals (Pa). Lp = 20 log 10 [Root Mean Square (RMS) Sound Pressure ] 2.0 x 10 -5 Pa (Reference Pressure) What Is Sound Power? Decibels of sound power (Lw) have a universally accepted reference value of 10 -12 watts (1 picowatt). Lw = 10 log 10 Watts Sound Power 10 -12 Watts (Reference Value) PWL vs. SPL Power vs. Pressure Cause vs. Effect Power vs. Pressure Sound Power Watts (Watts) Sound Pressure Illumination (Pascal’s) (Lumens) 21 Specification Strategies to Manage Project Acoustics .