Measures to Alleviate Fume Hood Noise

Thursday, March 22, 2018

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Measures to Alleviate Fume Hood Noise

Indoor environmental quality (IEQ) is a growing concern, increasing the need for indoor health and safety monitoring.1-3 Previous studies on IEQ covered a wide range of indoor determinants such as thermal, visual, acoustic, and chemical.4-7 Elevated noise levels in the laboratory can result in noise pollution, can have an adverse effect on normal communication among operators, and can lead to potential health issues.8 Even if noise in the workplace is not loud enough to cause hearing loss, a poor acoustic environment can contribute to voice strain and vocal cord problems, while annoying or distracting noise has been associated with stress-related health effects and complaints about noise pollution.9,10 There is evidence that noise pollution leads to increased stress, adversely affecting worker safety, health, and efficiency. Positive correlation between acoustic comfort and occupant productivity in commercial buildings has received much attention.11-13 With the increasing amount of time workers stay indoors and are exposed to ototoxic chemicals in the laboratory, the problems associated with noise exposure are exacerbated.14-16

Acoustic issues in the laboratory may emanate from indoor equipment noise, structure- or airborne noise from nearby sources, and outdoor noise.17 Laboratory equipment that typically generate considerable noise include chemical fume hoods,biosafety cabinets (BSCs), electromagnetic stirrers, ventilation blowers, and refrigerators.

Despite being recognized as an important parameter in IEQ, noise produced by the laboratory fume hood is not considered a high priority, because in most cases it is well below the threshold set by OSHA.18 Nevertheless, even when the time-weighted average (TWA) value is only about 55 dB(A), excess noise can still significantly affect an operator’s cognition and ability to communicate.19

Combined exposure: noise and ototoxic chemicals

The complex interactions between noise and ototoxic chemicals in the laboratory environment have been studied.20-24 Findings show that exposure to ototoxic chemicals—organic solvents (e.g., carbon disulfide and xylene), metals (e.g., lead, mercury, and manganese), asphyxiants (e.g., carbon monoxide and acrylonitrile), and pesticides and herbicides (e.g., organophosphates and paraquat)—is detrimental to hearing capacity.8,25 Single exposure to a particular chemical in a quiet environment may not elicit a toxic response, yet the same exposure in a noisy environment can create the potential for hearing loss.26 These studies suggest that the risk of hearing loss is much greater for groups exposed to ototoxic chemicals.

It is difficult to establish a standard for combined exposure to noise and ototoxic substances, mainly because the dose-response relationships are unclear.27 It is impossible to completely avoid exposure to ototoxic chemicals; sometimes the lower capture efficiency or containment performance of local exhaust ventilation (LEV) equipment such as fume hoods may lead to severe leakage of chemical vapors in the laboratory. Therefore, noise-abatement measures are warranted even at relatively low levels (e.g., noise generated from fume hoods) in an ototoxic chemical exposure environment.

Some recommendations in regard to noise and ototoxic chemical exposure have been adopted worldwide. The Australian National Code of Practice mandates that periodic audiometric testing be conducted for workers exposed to ototoxic chemicals at a concentration greater than 50% of the occupational exposure limits (OELs), regardless of noise exposure level.28 Similar actions are recommended by the American Conference of Governmental Industrial Hygienists (ACGIH), which requires a periodic audiometry report for potential combined exposure.8,29

Other recommended standards have been established to create a healthy indoor acoustic environment. For example, as specified by the China State Bureau of Technical Supervision (CSBTS) and OSHA, respectively,19,30 the TWA value should be lower than 50 dB(A) and 55 dB(A). Sufficient evidence has demonstrated that annoyance is induced when the eight-hour averaged noise level is higher than 55 dB(A) in an office environment.31 Too high a noise level in the laboratory can make it difficult for operators to hear critical information such as safety instructions and emergency alarms.

While previous studies reported the importance of controlling chemical exposure, this paper presents a noise reduction study for a typical fume hood using an experimental approach. The purpose of the study was to investigate a potential noise-abatement solution and its ability to improve laboratory indoor environmental quality and protect operator health and safety. Noise-abatement solutions were reviewed based on the currently available noise-control technologies, after which proper acoustic absorbent materials were determined. Finally, noise spectrum analysis was done to demonstrate improved noise control.

Fume hood noise abatement

Recognized sources of noise from a chemical fume hood include sound that is generated aerodynamically by airflow friction with untreated solid walls, turbulence caused by improper treatment of duct turns and partially closed built-in dampers, and assorted debris left in the duct line during construction.32 The latter two are classified as preventive maintenance issues or beyond the design of the hood, and thus were not considered in this study.

Excessive fume hood noise can be reduced by treating the source (fume hood), sound transmission path, receiver (laboratory staff), or any combination of these. Making modifications to the blower or ductwork for noise abatement after fume hoods are in service is expensive and is thus not a viable solution. Similarly, moving noise-producing equipment such as fume hoods from the laboratory to an equipment room, or simply wearing hearing protection devices is not always practical and effective. Reducing aerodynamically generated fume hood noise is investigated in this paper.

Previous computational fluid dynamics (k-εturbulent with BNS modeling) simulation and physical measurement suggest that the inside wall of the fume hood is primarily responsible for high aerodynamic noise generation.15 The sampling point of sound pressure level (SPL) measurement was 15 cm away from the center of the hood sash and 1.10 m above the floor to simulate the location of an operator’s ear when performing experiments while seated. Noise spectrum analysis was performed following National Occupational Health Code GBZ/T 189.8-2007 Measurement of Noise in a Workplace.33 The average noise level monitored using a calibrated noise spectrum analyzer was 69.3 dB(A), much higher than the 55 dB(A) or 50 dB(A) recommended by OSHA and CSBTS, respectively.

Noise absorption has proven effective for controlling noise. The main principle of noise absorption is to control the sound propagation path and reduce the buildup of sound in the reverberant field where sound waves reflect off untreated hard surfaces, such as inside the walls of the fume hood.34 Key to the success of this method is proper selection of the sound-absorbing materials (SAMs) and verification of noise attenuation effectiveness. Upon reaching the hood’s inside walls, some sound waves are reflected to their surroundings, while the rest penetrate through sound-absorbing materials.35 Incident sound energy is partly transferred to heat via vibration and friction with SAMs, reducing the total noise level at the receiver. Moreover, the incident sound waves are reflected multiple times via chain reactions inside the SAMs.

In this study, fiberglass (Owens Corning, Toledo, OH) was selected as the sound-absorbing material because it is more durable and safer than mineral wool and foamed plastic materials. The density and thickness of the fiberglass was 48 kg/m3 and 50 mm, respectively; diameter was 5~6 μm and large-length scale of the single fiber stem was 15~20 cm. Sound absorption coefficients at different frequencies are shown in Figure 1. Because water or solvent vapor can be generated in the fume hood during operation, fiberglass with a film cover was used. This can prevent pores inside sound-absorbing material from blockage by fine droplets, and maintain its ability to absorb noise in moist environments. Further, fiberglass with a film cover significantly reduces potential chemical accumulation on its surface.

Figure 1 – Sound absorption coefficient of acoustic fiberglass at 100 to ~5 kHz.

Installation of the hollow interlayer on the inside wall of the fume hood is not considered in this study because it is difficult and impractical to make significant modifications to the internal structure of the hood. However, an air interlayer structure of inside walls of the hood integrated with SAMs was expected not only to reduce material consumption, but also improve the total noise attenuation capability, especially at medium and high frequencies.34 Hence, it is recommended that fume hood manufacturers consider this during the design and production phase to make products intrinsically quieter. A fume hood with sound-absorbing materials installed is shown in Figure 2.

Figure 2 – Laboratory fume hood with acoustic fiberglass placed inside.

Figure 3 shows the process of on-site fume hood noise monitoring. The spectrum analyzer was recalibrated with a 1-kHz calibrator tone, nominal level 94 dB(A) ± 1 dB. While the sampling location and instrumentation remained the same, the monitored noise level dropped down from 69.3 dB(A) to 64.8 dB(A) after acoustic fiberglass was installed inside the hood. Noise level at the receiver was reduced dramatically.

Figure 3 – Demonstration of on-site fume hood noise monitoring in the laboratory.

The majority of medium- and high-frequency noises were reduced successfully (Figure 4), important because human ear sensitivity is greatest from 2000 to 5000 Hz.35 As for the higher-frequency bands from 400 to 4 kHz, the sound pressure level was reduced even more than 5 dB(A) at each band center frequency. The amount of noise reduction reached its peak value as 7.5 dB(A) at 4-kHz center frequency, which is the frequency band most sensitive to workers. The human ear can clearly distinguish a sound pressure level change of about 5 decibels.36 Therefore, use of acoustic fiberglass is an effective method to reduce fume hood noise. This has been concluded not only by theoretical analysis, but also by subjective responses. While overwhelming evidence has verified that cognitive impairment is induced by laboratory noise, the improvement in this study, to some extent, avoids disturbance to laboratory operators and enhances laboratory indoor environmental quality.

Figure 4 – Noise spectrum before and after fume hood was insulated with acoustic fiberglass.

Conclusion

The following challenges in the present study will be addressed in future work:

  1. Aerodynamically generated noise from fume hood and blower noise may interact. The focus of this study was to reduce aerodynamic noise generated from the fume hood itself, which, from an engineering standpoint, is a relatively convenient and effective way to control fume hood noise. It may not be cost-effective to make significant improvements to blowers, fans, or ductwork, although they, too, are related to the overall noise level, especially when laboratory ventilation systems are in use. Nevertheless, analyzing and evaluating duct-borne exhaust fan noise can be beneficial at the initial phase of lab ventilation system design.
  2. It may be useful to explore low-frequency (LF) noise reduction since LF noise is proven to generate whole-body vibration.8 Some bands of low-frequency sounds at 50~60 Hz in the chest resonance are the main cause of whole-body vibration. This effect can lead to negative physiological and/or psychological effects such as hypertension, annoyance, and discomfort. Although in some circumstances it may be possible to reduce this noise by moving the dominant sound energy to lower-frequency bands,35 the sound pressure level at those bands should be reduced until symptoms disappear or are as low as reasonably practical (ALARP).
  3. Finally, excessive noise levels in the laboratory can pose a risk if safety information is misheard.

References

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Additional reading

  1. Noise Control Design Guide, Owens Corning Corp., 2004.
  2. Brown, J.J.; Brummett, R.E. et al. Arch. Otolaryngology 1980, 106(12), 744–50.

The authors are with the School of Mechanical and Power Engineering, East China University of Science and Technology, 200237, Shanghai, China; e-mail: [email protected]; [email protected]; http://www.ecust.edu.cn/. The authors thank Ideal Industries, Inc.; Owens Corning Corp.; ALS Global; and ESIS, Inc., for providing technical guidance and for supplying the fume hood, sound-absorbing material, and other testing equipment.

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