Low frequency sounds can be harmful. Human beings are normally able to detect sounds in the range of 20-20,000 Hz and it is well known that sounds within this range can damage the hearing. The volunteers were exposed to a frequency of 30 Hz for 90 seconds.
Infrasound, sometimes referred to as low-frequency sound, is sound that is lower in frequency than 20 Hz or cycles per second, the ‘normal’ limit of human hearing. The ear is the primary organ for sensing infrasound, but at higher intensities it is possible to feel infrasound vibrations in various parts of the body.
At TELCO ERC, new building had come up and the office was shifted in that building. In those days, air conditioning was not common. At ERC on the top portion of the side wall (near to roof) exhaust fans were mounted for efficient ventilation and it was really very effective. But the problem was noticed that, after shifting in new building the staff was suffering from severe headache. The company being very conscious about the health of the staff started investigating the issue.
After careful study, no problem was found. The management was requested to shift the staff to old premises for few days for observation. For a week it was observed that none of the staff has any head-ache. I asked to shift some 10-20 people again to new premises. Surprisingly it was noticed that again they started getting head-ache. Those people were sent back to old premises. Through the study of sound engineering, it was suspected the low frequency noise. But in India no instrument was available to measure LF intensity. The management was requested to procure an instrument for GMBH. The measurements were taken with various combination. It was noticed that as soon as the exhaust fans were turned ON, the low frequency noise was of about 13 to 17 db at around 6 to 7 Hz. The entire mounting panel holding about 22 fans was having vibration.
I requested them to modify the mounting of the fans with bushing pads and provide cross ribs to the panels. Again, when readings were taken, the LF noise was totally eliminated.
The main problem is non-availability of proper instrument to measure LF noise level. This LF being below 20 Hz. It is not audible to human ear can not be noticed in normal manner.
They produced a series of audible noises and low-frequency sound, which made staff sick. Even if one can’t see it, hear it or feel it, sound waves can still make one sick or kill.
Low frequency noise, the frequency range from about 10Hz to 200Hz, has been recognised as a special environmental noise problem, particularly, to people who are sensitive to low frequency of below 20 Hz which is non-audible. The staff can have a large number of laboratory measurements of annoyance by low frequency noise, each with different spectra and levels, making comparisons difficult, but the main conclusions are that annoyance of low frequencies increases rapidly with level. Additionally, the A-weighted level underestimates the effects of low frequency noises. There is always possibility of learned aversion to low frequency noise, leading to annoyance and stress associated with constant headache similar to migraine severe throbbing pain or a pulsing sensation, usually on just one side of the head. In particular, problems of the hum often remain unresolved. An approximate estimate is that about 2.5 per cent of the population may have a low frequency threshold which is at least 12dB more sensitive than the average threshold, corresponding to nearly 1,000,000 persons in the 50-59-year-old age group in the EU-15 countries.
Following methods will significantly block the waves and reduce the overall impact in a room.
- Use Bass Traps (Low-Frequency Sound Absorber)
- Install Sound Blocking Curtains.
- Add an Extra layer of a wall using Green Glue.
- Add Acoustic Panels (Drywall alternative)
- Add a Thick Carpet.
- Use proper mounting method for fans or FCU or Cassettes or ducting etc.
Infrasonic sound can have very unusual non-auditory effects on the body. Sound and vibration control are becoming more important in HVAC. Sound and vibration are very closely related. The relationship between the two has to do with the frequency at which a vibration or pulsation occurs. The lowest he deals with are centred around 63 Hz (cycles per second) and go up to 8,000 Hz. The lower frequencies, have been more difficult to deal with than those at the upper end.
It’s one of the biggest challenges in HVAC industry, to make products that don’t have low-frequency sound. Sound attenuators can’t attenuate 60 to 125-Hz sound.
Fan Designs
In the design of fans, two things are considered for noise and vibration:
- Low-frequency vibration.
- Most of the energy a fan creates has a tone at the blade pass frequency. (Blade pass frequency equals fan rpm multiplied by number of blades in the rotor divided by 60.)
Vane axial fans have lots of blades in there, they rotate at a high rpm and create 500-Hz tones. A lower-frequency fan would be a centrifugal fan, with fewer blades, slower rpm, and creating a lot of lower-frequency energy.
Noise Solutions
One of the first solutions is to break the transmission path of the sound or vibration problem. (Typical transmission paths have already been pointed out.) Economical solutions may include:
- Vibration isolators applied right to the fan.
- Sound attenuators applied within the duct themselves, very close to the fan.
- Sound barriers, typically fiberglass that changes vibration into heat and drywall, which prevents sound from going through. When there is duct vibration to be stopped, drywall may be attached right to the sheet metal duct, as long as there is little risk of duct condensation.
- Changes to equipment (i.e., replacing equipment, put in a new fan, changing fan rpm to get off of a bad frequency).
New fan technology can also help reduce noise and vibration during the design phase. Mixed-flow fans (for instance, the Greenheck Model QEI) offer an alternative to conventional vane axial and tubular-centrifugal inline fans. Centrifugal fans (Model QEP plenum fans) also offer high-efficiency/low-noise wheel designs,
In the HVAC industry, most sound or noise is generated via rotating equipment and air and fluid movement through ducts and pipes. This movement creates vibration, sound, or noise.
Vibration in its simplest form can be considered an oscillation or repetitive motion of an object around an equilibrium position. In the HVAC industry, sound is not generated without some form of vibration from equipment.
Therefore, the best way to reduce sound is to limit the vibration produced by mechanical equipment. Examples are rotating shafts or gears, thermal processes such as combustion, or fluid dynamic means such as airflow through a duct or fan interactions with air.
Understanding vibration and sound
Control of HVAC system sound and vibration are of equal importance, but measurement of vibration is often not necessary to determine sources or transmission paths of unwanted sound or noise. Because vibration is the source of noise from HVAC systems, management of those conditions is imperative to a quiet design. System design that neglects to properly address vibration may result in malfunctioning components, noise, and, in some cases, catastrophic failure.
There are two facets of vibration management: isolation and damping. Isolation is the prevention of vibration from entering the system and dissipating it by changing kinetic energy of vibration into a different form of energy, such as heat.
Vibration isolation systems for mechanical components require some amount of damping. Damping dissipates mechanical energy from the system and attenuates vibrations more quickly. Without damping, these systems may vibrate for some time before coming to rest. The fluid in automotive shock absorbers is a kind of damper, as is the inherent damping in elastomeric (rubber) equipment mounts described below. This energy is converted to heat in the shock absorber or rubber mounts. There are also pads made of neoprene or cork used in equipment mounting that can be identified as damping devices.
A- and C-weighted sound pressure
A-weighted sound pressure (measured in decibels; dBA) has been used for 60 years as a single-number measure of the relative loudness of noise, specifically for outdoor environmental noise standards. It is popular because it is a single number that most sound meters include. A-weighted is corrected to more closely resemble the hearing characteristics of the human ear. The C-weighted curve (dBC), which is more sensitive to low-frequency sound, contributes less to the overall sound level than dBA. The human ear has a relatively poor sensitivity to low-frequency sound in the 20- to 30-dBA range.
When attenuating sound for an outdoor installation of an air-cooled chiller, the manufacturer-supplied decibel rating would be compared to design decibel level at the design distance from the source. If the design level is exceeded, attenuation will be required. Acoustic screen walls or manufacturer-supplied attenuation enclosures can be added, or relocating the chiller far may be the answer.
Sound pressure and sound power
The sounds are caused by sound pressure. It is due to fluctuation in atmospheric pressure that acts on the human eardrum. Sound pressure is dependent on the surroundings, therefore, making it difficult to measure sound levels of the equipment, or sound power.
The sound power and sound pressure relationship can be described with an example. Consider a 5-hp fan motor. A 5-hp motor is a definable measure that can be used to compare one motor against another. This is the equivalent of sound power. However, it is not possible to know whether a 5-hp fan is sufficient to provide cooling or ventilation unless the temperature of the space is known. The temperature of the space is equivalent of sound pressure. If the fan is providing ventilation or cooling for a 2,000 square feet building, it will most likely be large enough to provide a comfortable temperature. If the fan with the 5-hp motor is used in a 130,000 square feet space, it is unlikely to provide comfort to the occupants. In each situation, the same size fan (sound power) provides very different results (sound pressure level).
Room criteria (RC)
ASHRAE’s favoured method for determining sound levels is by room criterion, or RC, curves. The RC curve system was developed to overcome the shortcomings of the noise criteria (NC) system. The RC system adds the 16.5 and 31 Hz bands to deal with low-frequency sound and the 800-Hz octave band is dropped from the NC curves. The RC curve accounts for acoustically produced vibration in light building construction. In this article, we will address noise with the RC method. (see Figure 2).
Noise criteria (NC)
Noise criteria, or NC, curves are the most common standard in the United States for rating indoor noise from HVAC systems. NC curves were developed to take into account human response to sound pressure levels at different octave bands. NC curves are based on the 63 to 8,000 Hz octave-band values. The criteria curves define the limits of the octave band that must not be exceeded to meet occupant acceptance in a space. One issue with the NC method is that it does not evaluate low-frequency sounds below 63 Hz, which can be the most troublesome and most difficult to attenuate.
Basic acoustical design techniques
Based on the 2007 ASHRAE Handbook–HVAC Applications, when selecting fans, pumps, and other related mechanical equipment and when designing air-distribution systems to minimise sounds transmitted from system components to occupied spaces, engineers should consider the following:
- Design the air-distribution system to minimise flow resistance and turbulence. High flow resistance increases required fan pressure, which results in high staff noise being generated by the fan, especially, at low frequencies. Turbulence also increases flow noise generated by duct fittings and dampers, especially, at low frequencies.
- Select a fan to operate as near as possible to its rated peak efficiency when handling the required airflow and static pressure. Also, select a fan that generates the lowest possible noise at required design conditions. Using an oversized or undersized fan that does not operate at or near rated peak efficiency can substantially increase noise levels.
- Design duct connections at both the fan inlet and outlet for uniform and straight airflow. Both turbulence (at fan inlet and outlet) and flow separation at the fan blades can significantly increase fan-generated noise. Also, turning vanes near fan outlets can also increase turbulence and noise, especially if airflow is not sufficiently uniform.
- Select duct silencers that do not significantly increase the required fan total static pressure. Selecting silencers with static-pressure losses of 0.35 in of water or less can minimise regenerated noise from silencer airflow.
- Place fan-powered mixing boxes associated with variable-volume air-distribution systems away from noise-sensitive areas.
- Minimise flow-generated noise by elbows or duct branch take-offs whenever possible by locating them at least 4 to 5 duct diameters from each other. For high-velocity systems, it may be necessary to increase this distance to up to 10 duct diameters in critical noise areas. Using flow straighteners or honeycomb grids, often called egg crates, in the necks of short-length take-offs that lead directly to grilles, registers, and diffusers is preferred to using volume extractors that protrude into the main duct airflow.
- Keep airflow velocity in ducts serving sound-sensitive spaces a low as possible by increasing the duct size to minimize turbulence and flow-generated noise.
- Duct transitions should not exceed an included expansion angle of 15 degree, or the resulting flow separation may produce rumble noise.
- Use turning vanes in large 90-degree rectangular elbows and branch take-offs. This provides a smoother directional transition, thus, reducing turbulence.
- Place grilles, diffusers, and registers into occupied spaces as far as possible from elbows and branch take-offs.
- Minimise use of volume dampers near grilles, diffusers, and registers in acoustically critical situations.
- Use barriers near outdoor equipment when noise associated with the equipment will disturb adjacent properties. In normal practice, barriers typically produce no more than 15 dB of sound attenuation in the medium frequency range. To be effective, the noise barriers must at least block the direct line of sight between the source and receiver.
- Vibration-isolate all reciprocating and rotating equipment connected to a structure. Also, it is usually necessary to isolate mechanical equipment in a basement, directly below a tenant space. It may be necessary to use flexible piping connections and flexible electrical conduit connections for pipes and ducts connected to the equipment.
- Vibration-isolate ducts and pipes using spring and or neoprene hangers for at least the first 50 feet from vibration-isolated equipment.
The World Health Organisation (WHO) recognises the special place of low frequency noise as an environmental problem. Its publication on Community Noise (Berglund et al., 2000) makes a number of references to low frequency noise, some of which are as follows:
- It should be noted that low frequency noise, for example, from ventilation systems can disturb rest and sleep even at low sound levels.
- For noise with a large proportion of low frequency sounds a still lower guideline (than 30dBA) is recommended.
- When prominent low frequency components are present, noise measures based on A-weighting are inappropriate.
- Since A-weighting underestimates the sound pressure level of noise with low frequency components, a better assessment of health effects would be to use C-weighting.
- It should be noted that a large proportion of low frequency components in a noise may increase considerably the adverse effects on health.
- The evidence on low frequency noise is sufficiently strong to warrant immediate concern.
Level variations
Holmberg et al (1997) investigated noise in workplaces, using the (dBC – dBA) difference as an indicator. Low frequency noise exposure was found in a group of 35 out of a total of 337 persons. Measurements of temporal variation of the levels of low frequency noise at the workplaces, averaged over 0.5, 1.0 or 2.0 seconds, was correlated with subjective annoyance. Significant correlation was found between the irregularity of the noise levels and annoyance.
Low frequency noise annoyance and stress
Stresses may be grouped into three broad types: cataclysmic stress, personal stress and background stress. Cataclysmic and personal stresses are evident occurrences, which are met with sympathy and support, whilst their impacts normally reduce with time. Background stresses are persistent events, which may become routine elements of life. Constant low frequency noise has been classified as a background stressor (Benton, 1997; Benton and Leventhall, 1994). Whilst it is acceptable, under the effects of cataclysmic and personal stress to withdraw from coping with normal daily demands, this is not permitted for low level background stresses. Inadequate reserves of coping ability then lead to the development of stress symptoms. In this way, chronic psychophysiological damage may result from long-term exposure to low-level low frequency noise.
Changes in behaviour also follow from long term exposure to low frequency noise. Those exposed may adopt protective strategies, such as sleeping in their garage if the noise is less disturbing there. Or they may sleep elsewhere, returning to their own homes only during the day. Others tense into the noise and, over time, may undergo character changes, particularly in relation to social orientation, consistent with their failure to recruit support and agreement from the regulatory authority that they do have a genuine noise problem. Their families, and the investigating officer, may also become part of their problem. The claim that their “lives have been ruined” by the noise is not an exaggeration, although their reaction to the noise might have been modifiable at an earlier stage.
Conclusion
Regulatory authorities must accept that annoyance by low frequency noise presents a real problem which is not addressed by the commonly used assessment methods. In particular, the A-weighted level is very inadequate, as are the NR and NC criterion curves. Assessment methods specific to low frequency noise are emerging, but a limitation of existing methods is that they do not give full assessment of fluctuations. It is possible that application of noise quality concepts, in particular fluctuation and roughness (Zwicker and Fastl, 1999), may be a way forward.
