Chemistry for Everyone
Auditory Risk of Exploding Hydrogen-Oxygen Balloons Kent L. Gee* and Julia A. Vernon Acoustics Research Group, Department of Physics and Astronomy, Brigham Young University, Provo, Utah 84602 *
[email protected] Jeffrey H. Macedone Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602
Visually impressive demonstrations greatly complement teaching concepts in the classroom. A common example is exploding balloons containing hydrogen and oxygen that visually demonstrate concepts such as stoichiometry, combustion, and kinetics. Hydrogen-oxygen balloon explosions are popular (1-4) but constitute an impulsive noise source with possible hearing damage risk to the presenter and the audience (5, 6). (Related to the balloons are exploding hydrogen-oxygen soap bubbles (7, 8), which likely merit a study similar to this one in the future.) Although these balloon demonstrations are widely used, quantitative documentation of potential auditory hazards is sparse. In addition, the studies that have been performed leave significant room for expansion, clarification, and correction. Experiments to quantify the sound levels created by exploding hydrogen-oxygen balloons were first reported by Battino et al. (5). In these experiments, the authors examined acoustical levels as a function of balloon size, stoichiometry of the H2:O2 mixture, balloon materials, distance from the balloon, and room volume. Unfortunately, there are fundamental issues with the investigation that call into question the validity of the results. We do not wish to dwell on the shortcomings of this previous study, but they include misapplication of acoustics-related terminology, use of equipment likely ill-suited for impulsive noise measurements, and a lack of understanding of sound propagation in enclosures. Despite these limitations, however, some of the conclusions drawn in the article appear to be valid, and we will revisit these as appropriate. After the Battino et al. study, McNaught (6) published a short note that suggested a balloon size and fuel mixture for a demonstration that was not an auditory hazard. The author suggested a maximum balloon volume of 5 L with an H2:O2 mixture of 1:2. However, there is insufficient detail regarding how and where sound levels were measured and an inappropriate use of a continuous noise hazard threshold, rather than one for impulsive noise. The main purpose of this article is to promote safe chemistry education by raising awareness among teachers regarding the possible auditory hazards of hydrogen-oxygen balloon explosions. In the article, we quantify risk of auditory damage from exploding hydrogen-oxygen balloons as a function of amount of hydrogen, H2:O2 ratio, angle, and distance. These results allow us to make recommendations regarding acoustically safe demonstrations. This article contains more results and detail than may be typical for this Journal to allow the reader to understand the justification for our recommendations.
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Understanding Auditory Risk Before considering the possibility of hearing damage from balloon explosions, fundamental aspects of sound, its measurement and analysis, and the function of the human ear must be understood. Sound waves travel as local compressions and expansions through the air and are most commonly measured as pressure fluctuations. Because the human ear responds to acoustic pressures over several orders of magnitude, acoustic pressure amplitude is expressed logarithmically as a sound pressure level. Sound pressure level (SPL), in decibels, may be written as ! p SPL ¼ 20 log 10 pref where p is the acoustic pressure amplitude and pref is the reference pressure, 20 μPa. This reference value places the average minimum audible threshold at 1 kHz for people with normal hearing at 0 dB. Another reason that decibels are commonly used is that a 10 dB increase in level corresponds approximately to a doubling in perceived loudness. In other words, a tone at 70 dB may be perceived by a listener to be approximately twice as loud as a tone at 60 dB. Because the human ear does not respond uniformly as a function of frequency, weighting curves have been developed for the measurement and analysis of acoustic signals. These weighting curves (e.g., A and C weighting) essentially filter the acoustic signal according to its frequency content, thus, mimicking human auditory response to a disturbance. A-weighted sound pressure levels, expressed as dBA, are the foundation for many auditory risk and noise criteria for continuous noise. For example, OSHA requires that workers' noise exposure be less than 90 dBA averaged over an 8 h period (9). The short duration of impulsive noise (such as a gun firing or a balloon exploding) relative to continuous noise causes greater levels to be tolerable to the human ear. For impulse noise, the generally accepted (9, 10) maximum SPL to which the unprotected ear can be exposed is 140 dB because of the potential for mechanical stress on the ear. (Note that this exposure level is not A-weighted.) This threshold has been set because even a single exposure to SPLs greater than 140 dB can potentially result in temporary or permanent hearing loss. For impulses below 140 dB, cumulative auditory risk from multiple exposures comes from metabolic fatigue processes and can be determined by calculating an equivalent continuous noise dosage based on, for
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 10 October 2010 10.1021/ed100439h Published on Web 08/12/2010
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Chemistry for Everyone
Figure 1. (Left) Anechoic chamber with balloon and microphone tripods. (Right) Large hydrogen balloon explosion. All personnel are wearing double hearing protection.
example, the A-weighted scale. Although cumulative exposure may be of relevance to chemistry educators who present exploding balloon demonstrations several times a day, auditory risk of multiple exposures to impulse noise less than 140 dB is beyond the scope of this article. The auditory risk encountered due to impulsive noise depends on the distance from the source and reflecting surfaces. Without any obstructions nearby, geometric spreading from a spherical source will cause the SPL to decrease by 6 dB for each doubling of distance from the source. An order-of-magnitude increase in distance results in a 20 dB reduction in level. In a classroom, however, reflections off walls, floor, and ceiling may cause the peak SPL decay to be less than for a free-field environment because of constructive interference between the incident and reflected waves. A person standing very near a hard wall (the wall-to-ear distance being small relative to a wavelength) will experience a near pressure doubling, which represents a 6 dB increase. At a hard interior corner (a junction between two hard walls and a hard ceiling), the SPL could theoretically increase by 18 dB because the pressure doubling essentially happens three times. However, the increase in peak SPL at typical locations within a room will likely range from 0 to 10 dB. Experiment Summary For our experiments, balloons with different amounts of hydrogen and different ratios of oxygen and hydrogen were examined. Without sophisticated equipment such as mass flow controllers, accurate delivery of gases into a vessel is difficult. For practical purposes, gases were added to balloons until the balloon diameter matched a calculated value: the balloon was inflated with hydrogen until the circumference matched the inner circumference of a polyethylene-tubing ring and then oxygen added until the balloon circumference matched the inner circumference of a larger polyethylene-tubing ring. Ring circumferences were calculated taking into account that molar volume is not 22.4 L/mol in Utah, as the average pressure is about 650 Torr (0.86 atm). (The measured atmospheric pressure during experiments was 647 Torr, as measured by a mercury-filled barometer.) One possible source of error with this balloon-filling technique might be that typical “party” balloons are not spherical. The balloons we used were manufactured to be more spherical (Figure 1) under tension than typical tear-shaped party balloons (the spherical balloons are intended to demonstrate the volume of 1 mol). Another potential source of error might be 1040
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Table 1. Composition of the Balloons Used in the Test Total Volume (at 647 Torr)/L
No. of Balloons
Triala
H2/Mol
AA
0.070
0
1.98
4
AB
0.070
0.018
2.48
4
AC
0.070
0.035
2.97
3
BA
0.220
0
6.23
3
BB
0.220
0.055
7.78
3
BC
0.220
0.110
9.34
3
CA
0.370
0
10.47
4
CB
0.370
0.930
13.09
4
CC
0.370
0.185
15.71
3
DA
0.520
0
14.72
5
DB
0.520
0.130
18.40
3
DC
0.520
0.260
22.08
3
O2/Mol
a
The first letter (A-D) denotes amount of hydrogen and the second letter (A-C) denotes amount of oxygen.
differing tension of the balloon material as a function of volume. Balloon tension was assumed to be negligible, as the measured difference in pressure between the inside and outside of a fully filled balloon was about 2%. Gas mixtures were prepared assuming the chemical reaction 2H2 ðgÞ þ O2 ðgÞ f 2H2 OðgÞ For convenience in referring to a particular gas mixture, a two-letter code was assigned. The first letter (A-D) referred to a particular amount of hydrogen, whereas the second letter (A-C) referred to the relative amount of oxygen added to the balloon. The test conditions, with four different amounts of hydrogen and three different ratios of oxygen, which are 0, 50, and 100% of a stoichiometric mix, are shown in Table 1. Prior to igniting the balloons, they were attached to a metal laboratory clamp to which the microphones were aligned. The balloon was then ignited with a blowtorch attached to a 4 ft wand and aimed at the base of the balloon (Figure 1). The presenter was approximately 5-6 ft from the balloon at all times. All participants wore hearing and some form of eye protection during the experiments. The experiments were conducted in a large anechoic chamber. High-fidelity acoustical recording equipment was employed for the test. Data were recorded with National Instruments 24-bit
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Chemistry for Everyone
Figure 2. Overhead representation of 10 microphone layout at radii of 3, 6, and 12 ft. Microphones at 6 ft were located, relative to the þx axis, at 30-180°. There was also a large array of microphones near (6,0) but their data are not included in the analysis presented in this article.
PXI-4462 cards at a sampling rate of 192 kHz. Type-1 6.35 mm diameter pressure microphones were set up at essentially balloon height at the locations shown in Figure 2. Note that this same hardware has been used in previous high-amplitude noise tests, with sources that include military jet aircraft (11), 30-mm Gatling guns (12), and a large solid rocket booster (13). In other words, its microphone-recorder characteristics are well-established and the experiment described here is within its capabilities. The use of a “sound level meter” (which, for acoustical measurements, is as vague as “thermocouple” is for temperature measurements) by Battino et al. (5) in high-amplitude noise measurements is problematic for two reasons. First, the bandwidth and temporal averaging of the meter was likely insufficient to capture relatively rapid rise and true value of the pressure peak. Second, the high SPLs probably caused the meter's electronics to saturate and the microphone to distort for some of the measurements. Displayed in Figure 1 are photographs from the test. The balloon is inside an anechoic chamber where the floor is a wire mesh that permits the sound waves to travel through it to the fiberglass wedges below. The microphones are mounted on wooden dowels extending out from tripods to reduce acoustic scattering. A large hydrogen balloon explosion is shown (Figure 1, right). Note that at and behind the balloon ( y e 0 in Figure 2), plywood panels were placed on the wire mesh floor opposite the microphones for ease of movement in and out of the chamber during the test. The locations of the panels relative to the microphones were such that the sound wave reflections off the panels did not arrive at the microphones. Results For the purpose of establishing auditory risk according to the 140 dB impulse noise criterion, the time-dependent pressure data were recorded for each balloon explosion and the peak SPL extracted from those data. The first analysis performed was to confirm that the geometric spreading of the noise from the balloon is essentially spherical (6 dB reduction per doubling of distance) so peak SPLs may be predicted at other distances than those at which there are microphones. To demonstrate spherical spreading, the SPLs from all 42 balloon explosions at the 3-12 ft microphones along 60° (relative to the þx axis in Figure 2) were
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Figure 3. Average change in sound pressure level, SPL, from 3-12 ft relative to the 6 ft level. This average was created by using all 42 balloon explosions for the microphones located at 60° relative to the þx axis. Also shown are error bars representing one standard deviation above and below the mean for 3-6 ft.
Figure 4. Sound pressure level, SPL, as a function of oxygen stoichiometry (see Table 1). The smaller magnitude error bars represent average standard deviation as a function of angle, whereas the larger bars represent average variation in SPL as a function of trial.
found. Next, the average change in SPL relative to the 6 ft measurement was calculated. This result is displayed in Figure 3, where the departure from spherical spreading is minor and falls within the error bars showing the measured standard deviation. The fact that the level decreases as approximately 6 dB per doubling of distance allows us to reasonably predict level as a function of distance from the source from our anechoic measurements prior to considering the possible effects of room reflections. Displayed in Figure 4 is the SPL at 6 ft for balloons containing the four amounts of hydrogen as a function of oxygen content. This SPL has been averaged over all trials and over all six microphones at that distance. For most cases, the peak SPL increases as a function of the amount of hydrogen and the relative amount of oxygen. To show the deviations as a function of angle and trial, two sets of error bars are displayed. Both variations are important in characterizing possible auditory risk. Without exception, the smaller error bar represents the standard deviation in level as a function of angle, averaged over all trials. The
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Chemistry for Everyone • Balloons filled only with hydrogen are well below 140 dB at 6 ft and do not constitute an acute risk. • As oxygen is added to the hydrogen, the peak SPL rapidly increases, with the rate of the increase slowing as a 2:1 mixture of H2:O2 is reached. • As the distance from the noise source is increased, the peak level decreases at 6 dB per doubling-of-distance in the absence of significant reflections. This suggests that the first protection against auditory damage is to ensure that the audience is sufficiently far away from the balloon.
Figure 5. Average data from Figure 4, where SPL at 6 ft is represented as an interpolated two-dimensional color map as functions of amount of hydrogen and oxygen stoichiometry. The 140, 146, and 152 dB contour lines are drawn.
larger error bar represents the standard deviation in level as a function of trial, averaged over all angles. These results show that the SPL changed little as a function of angle but that, from one balloon explosion to another, the peak SPL varied by as much as a few decibels. Note that the variations for a given amount of hydrogen typically decreased as the amount of oxygen was increased. Also, the results for the pure hydrogen case validate one of the conclusions of Battino et al. (5) that increasing the amount of hydrogen does not necessarily cause an increase in SPL. Finally, the smallest amount of hydrogen (0.070 mol) resulted in a different SPL-with-increasing-oxygen trend than the balloons with greater amounts of hydrogen. We have seen a similar trend in separate classroom experiments with balloons containing 0.15 mol of hydrogen. In those cases, 50% and 100% stoichiometric H2: O2 mixes yielded the same average level to within 1 dB. Figure 5 displays the same average results in Figure 4, but in a convenient color map. It helps to visualize approximate amounts of hydrogen and gas mixtures that produce
