Noise is unwanted sound which may be hazardous to health, interfere with speech and verbal communications or is otherwise disturbing, irritating or annoying.
Sound is defined as any pressure variation in air, water or other fluid medium which may be detected by the human ear.
The two most important characteristics which must be known in order to evaluate the sound or noise are its amplitude and frequency. The amplitude or height of the sound wave from peak to valley determines the loudness or intensity. The wavelength determines the frequency, pitch or tone of the sound. The third characteristic of sound is character. Character describes the cyclical changes of sound patterns generated by a given source.
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) and below. High frequency noise is 2000 Hz and above. Mid-frequency noise falls between 250 and 2000 Hz.
The amplitude of sound is expressed in decibels (dB). This is a logarithmic compressed scale dealing in powers of 10 where small increments in dB correspond to large changes in acoustic energy.
Sound wavelengths are the linear measurement of one full cycle of displacement where the motion of air molecules is first compressed and then rarefied or expanded. The wavelength is determined by the ratio of the speed of sound to the frequency.
Standardized octave bands are groups of frequencies named by the center frequency where the upper limit is always twice the lower limit of the range. Test data for performance of acoustical materials is standardized for easy comparison at the center frequencies. Equipment noise levels and measurement devices (dB meters) also follow the preferred octave bands.
The decibel (dB) is a dimensionless unit calculated using the ratio of a measured value (p) to a reference value (pre). The values of sound pressure of most interest range from the threshold of hearing at about 1 x 10-9 psi to the threshold of damage at about 15 psi. This range of energy variation translates to 10 orders of magnitude with the high threshold at a level 1,000,000,000 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.
Decibels of sound pressure (Lp) have a universally accepted reference pressure of 2.0 x 10-5 Pascals (Pa). Sound Pressure is measured in decibels (dB) and its value reflects environmental influences such as distance, room construction, air temperature, density and humidity, etc.
Decibels of sound power (Lw) have a universally accepted reference value of 10-12 watts (1 picowatt). Sound Power is the baseline sound output of a source at the acoustic center of the equipment independent of all space and environmental influences.
No! While both sound power levels (Lw) and sound pressure levels (Lp) are both expressed in decibels, the referenced standards for each are different. More importantly, the sound power level is the total acoustic energy output of a noise source independent of environment. Sound pressure levels are dependent on environmental factors such as the distance from the source, the presence of reflective surfaces and other characteristics of the room/building/ area hosting the source. Actual sound pressure levels will always be lower than sound power levels.
dB sound pressure levels are unweighted. dB(A) levels are “A” weighted according to the weighting curves that approximate the way the human ear hears. For example, a 100 dB level at 100 Hz will be perceived to have a loudness equal to only 80 dB at 1000 Hz. The dB(A) scale is based on a child’s hearing and was originally documented based on actual hearing tests to characterize the human ear’s relative response to noise.
Broadband noise has a frequency spectrum or signature where there are no discrete or dominant tones. Sound pressure fluctuations of broadband noise are non-periodic in nature with relatively random phase and amplitude. Although devoid of discrete frequencies, the acoustical energy of broadband noise may still be largely concentrated in one or more areas of the spectrum.
Tonal noise is commonly referred to as discrete frequency noise and is characterized by spectral tones that are pure tone in nature. Pure tones are wave forms that occur at a single frequency. Tonal noise is generated by rotating equipment at a predictable frequency relating to the rotational speed of the shaft and the number of compressor vanes, fan blades, engine pistons, gear teeth, etc. The fundamental tone (F) may also manifest itself at progressively lower intensity levels at integer harmonic multiples (2F, 3F, etc.). Tolerance levels for tonal noise are generally at a lower threshold.
Objective criteria are black and white such as maximum allowable day or night dBA levels listed in town, city or state ordinances for residential and commercial property lines. Subjective criteria are just that: subjective to the individual receiver. Subjective considerations are a moving target influenced by noise intensity, tonal content, ambient background sound levels and more. Screw chillers have prominent tones at 297 Hz, 594 Hz and 891Hz that humans are very sensitive too. Subjective reaction to the tones or compressor “whine” may be very strong despite intensity levels that are below objective ordinance criteria.
This is done considering maximum baseline noise level ratings using the equipment Sound Power published as per industry standard AHRI test procedures. Qualified acoustical companies can accurately convert from the AHRI baseline data to property line sound pressure considering reflecting planes, directionality factors, site specific spreading loss, ambient background contribution and environmental influences.
If ambient background sound is too close to the target level, it could add to the overall equipment sound pressure. If ambient is very low, the mitigated equipment level may still represent a large rise over ambient value. Rise over ambient is another indicator of subjective tolerance. Not knowing ambient background levels compromises accurate modeling and expected results.
The measure of transmission loss of a panel is STC per test procedure ASTM E 90-10. Panel performance unfortunately has little relationship to barrier wall performance. There is no lab test to show a barrier wall system performance. Barrier wall performance is a calculated Insertion Loss (IL) value that considers the exact configuration of the barrier in relation to the equipment, the exact community location under consideration and relative elevations of equipment and receiver. Other considerations for flanking sound affect the calculated Barrier Wall System Insertion Loss such as nearby reflecting planes and environmental influences such as wind and temperature gradients.
Air-cooled chillers, coolers, condensers and cooling towers use low static pressure condenser fans for cooling. Barrier wall enclosures impede needed airflow for the cooling fans. The fans are very sensitive to pressure loss associated with surrounding obstructions such as barrier walls. All offers for barrier wall solutions should be evaluated for their impact on operating efficiency and power consumption. Exceeding the manufacturer’s guidelines for air flow restrictions could severely de-rate efficiency and result in nuisance AC trips. Barrier walls require features to mitigate conditions that could facilitate short cycling of warm condenser fan exhaust air.
There is a static and dynamic load to consider. The dynamic overturning wind load transfer at tie-in points to the existing structures is usually most critical. Calculated loads should be required of the barrier wall manufacturer’s structural engineer to be used by the building engineer of record to assure structural compatibility. For remedial projects, additional structural consideration must be given as to how a new barrier wall system can be cantilevered at certain locations to provide needed air flow clearances and tie back to the equipment dunnage.
The design/construction team should secure written approval by the equipment manufacturer for the treatment to be installed so that there is not a disconnect which later could compromise warranties, operational/service access and efficiency.