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Natural Science Forum / Physics / Acoustics / June 2004SPL of acoustic source in hydrogen
Here's one for the gang: All other things being equal, how much louder (or quieter) would a given acoustic source be in an atmosphere of pure hydrogen? This is not as strange a question as it might seem. My current work deals with noise control of large electric power generators, which are often cooled by internal fans blowing hydrogen around a closed duct system. The relevant parameters I believe are air: rho*c=415 raysl, c=343 m/s; and H2: rho*c=109 rayls, c=1310 m/s. I get, alternately, -5.9 dB or -17.4 dB depending on whether I'm thinking about 1D plane wave propagation or spherical propagation from a small source. Is either one of these correct? Neither? Both? Am I overlooking something important? Can the type of field propagating into possibly matter? Any hints or discussion would be appreciated. Robert A. Hedeen GE Global Research Niskayuna, NY > noise control of large electric power generators, which are often > cooled by internal fans blowing hydrogen around a closed duct system. Depends on the multipole order of the source. Piston=monopole; free loudspeaker= dipole; bell (vibrating plate - higher order modes) =quadrupole; small fan= quadrupole or octupole. The higher the order, the higher your ratio. In addition, standing waves in the enclosure will be different, since the interior is fewer wavelengths across. Ang. C. > > noise control of large electric power generators, which are often > > cooled by internal fans blowing hydrogen around a closed duct system. [quoted text clipped - 4 lines] > the higher your ratio. In addition, standing waves in the enclosure will > be different, since the interior is fewer wavelengths across. Also I think you need to think about how the source will behave. Fans take less power when the air is less dense. I have heard that fans for buildings are noisier in summer when they are blowing colder air, I don't know if that's true but it seems reasonable if they are using more electrical power. If fans have to be tested in hydrogen to work out what gas flow they will produce it would seem sensible to do a noise test at the same time.  SignatureTony W My e-mail address has no hypen - but please don't use it, reply to the group. > Also I think you need to think about how the source will behave. Fans take > less power when the air is less dense. I have heard that fans for buildings > are noisier in summer when they are blowing colder air, I don't know if > that's true but it seems reasonable if they are using more electrical power. > If fans have to be tested in hydrogen to work out what gas flow they will > produce it would seem sensible to do a noise test at the same time. I need to hear from Bob Hadeen as to the nature of the source, then we can make some progress in answering his question. Ang. C. Signature--------- www.CampanellaAcoustics.com --------- "I have simply studied carefully whatever I've undertaken, and tried to hold a reserve that would carry me through." - Charles A. Lindbergh. "As for background noise level; 35 dBA is a good classroom; 45 dBA is a sound masking system!" - Anthony K. Hoover >> Also I think you need to think about how the source will behave. Fans take >> less power when the air is less dense. I have heard that fans for buildings [quoted text clipped - 7 lines] > >Ang. C. It is a 28 bladed axial flow fan, radius about 18" (stubby blades have a 6" length and a 3" chord on a 12" rad. disk), rotating at a speed of 3000 or 3600 rpm in a plenum of H2 pressurized to 50 psi. So I suppose it is fundamentally a dipole, if the type of source makes a difference to the original question. It discharges into a closed recirculating system of ducting and heat exchangers. All else equal, how much louder (or quieter) would the fan be in an air-filled system? The more I think about it, the more I think +17.4 dB is correct, but I would appreciate any suggestions or insights. It should be immaterial how louder or quieter is defined, but ultimately I am interested in the delta sound power from air to hydrogen. TIA; also, please no smoking! Robert A. Hedeen GE Global Research Niskayuna, NY > It is a 28 bladed axial flow fan, radius about 18" (stubby blades have > a 6" length and a 3" chord on a 12" rad. disk), rotating at a speed > of 3000 or 3600 rpm in a plenum of H2 pressurized to 50 psi. So I My field data indicates the SPL amplitude of a fan to be proportional to pressure, so 50 psi means 3.3 more amplitude, or about 10 dB more on account of pressure alone. The velocity of sound figures into the higher order mutipoles as in proportion to the source size in wavelengths. Higher sound velocity means relatively "smaller" fan dimensions in terms of wavelengths spanned, decreasing the amount of radiated sound. This implies a discount from the 10 dB, maybe even canceling it out altogether. It sounds like a fairy tale, so you had best run a pilot test at atmospheric pressure with a typical fan in an analogous enclosure. You can measure the wavelength effect directly as the contrast in SPL/air vs SPL/H2 to referee my hypothesis. > suppose it is fundamentally a dipole, if the type of source makes a > difference to the original question. It discharges into a closed > recirculating system of ducting and heat exchangers. My guess is a quadrupole. > All else equal, how much louder (or quieter) would the fan be in an > air-filled system? The more I think about it, the more I think +17.4 > dB is correct, but I would appreciate any suggestions or insights. My guess is quieter in hydrogen, even when pressurized. Angelo Campanella > > suppose it is fundamentally a dipole, > My guess is a quadrupole. No doube I am being very slow, but I would appreciate some explanation of how you arrive at dipole or quadrupole. SignatureTony Woolf Tony Woolf Acoustics 17 Canfield Gardens, London NW6 3JP Tel. 020 7624 2512 Fax 020 7372 3054 > Here's one for the gang: > All other things being equal, how much louder (or quieter) would a [quoted text clipped - 17 lines] > GE Global Research > Niskayuna, NY AFAIU the power from a point source would be: W=U^2*w^2/(4*pi) * rho0/c and the pressure : P=jwU/(4*pi*r) * rho0 So given a constant source volume flow U, the sound pressure would be proportional to the density of the medium. The power equation OTOH includes the velocity of sound c, which in turn is depending on the density rho0: c=sqrt(1.4*p0/rho0) for diatomic gases, p0 being the static pressure. This means that the power is proportional to rho0^(3/2). So it depends if you define "loud" as sound *pressure* or sound *power* (~intensity) Since the density of hydrogen is 0.07 of the density of air, the SPL difference would be 20*log(0.07) = -23dB, the SIL difference would be 10*log(0.07^(3/2))= -17 dB. Another thing to think of is that the higher propagation speed in hydrogen makes the source smaller, compared to the wavelength. Please check my equations, I just put them together so I might be wrong somewhere. > < ...snip.. > > [quoted text clipped - 3 lines] > Please check my equations, I just put them together so I might be > wrong somewhere. I find the entire discussion fascinating, yet somehow I would expect the SPL to be related to fan efficiency and thus associated with (say) the difference in turbulence for hydrogen vs air or some such. I'd expect propagation factors and the like to be applied after the fact. ...or am I missing something? [Me, I'm an experimentalist. I'd wanna measure stuff... ] ;-) Ron Capik -- > > < ...snip.. > > > [quoted text clipped - 14 lines] > Ron Capik > -- Probably true, too. I rarely deal with turbulence, so I just assumed a point source. It seems reasonable that a different viscosity in the two media would affect the amount of turbulence, and thus the amplitude of the generated sound. But don't ask me how... :-) Various wrote: >>>Another thing to think of is that the higher propagation speed in >>>hydrogen makes the source smaller, compared to the wavelength. >>I find the entire discussion fascinating, yet somehow I would expect >>the SPL to be related to fan efficiency and thus associated with (say) >>the difference in turbulence for hydrogen vs air or some such. I'd expect >>propagation factors and the like to be applied after the fact. >>[Me, I'm an experimentalist. I'd wanna measure stuff... ] ;-) > Probably true, too. I rarely deal with turbulence, so I just assumed a > point source. It seems reasonable that a different viscosity in the > two media would affect the amount of turbulence, and thus the > amplitude of the generated sound. But don't ask me how... :-) I have had some experience in producing aerodynamic reference sound sources (See RSS in my web page). I also ran a sound power vs altitude calibration test (the hard way, traveling to Kansas and Colorado). what I found was that the sound pressure varied directly with altitude, [20*log(p)], but that the sound power (pressure squared times air density) varied only as 10*log(p^2*density), or simply 10*log(p), since air is less dense at altitude. The wavelength effect came into play when calibrating in cold vs warm air, where the only way I could explain the values I actually measure, winter vs summer (0C-20C), was that the aerodynamic fan is behaving as a multipole source, where radiated power varies as the ratio of the fan size to the wavelength size raised to an order of the multipole. A monopole has no wavelength effect, while a dipole goes as the first power of that ratio, the quadrupole as the second power of that ratio, and an octupole (possible at high frequencies for fans generating considerable turbulence at those frequencies) the third power. Some of Lighthill's work touches on this wavelength dependence of turbulence sound emission. Consequently, when we consider a fan in hydrogen, where the wavelengths are about 4 time longer than in air, my empirical theory predicts very small sound emission, as compared to air. Along with the air/hydrogen(helium?) quick-test, one might also run an air/carbon dioxide test. For a first-run test chamber, I suggest a 55 gallon steel drum and a small table fan for starters. You may need more motor horsepower in CO2. Fix the sound level microphone anywhere inside the drum, preferably at a surface or in a corner, purge the drum, run the test. Use a bung hole window to run a strobo-tach to verify RPM. Measure SPL. Angelo Campanella Signature--------- www.CampanellaAcoustics.com --------- "I have simply studied carefully whatever I've undertaken, and tried to hold a reserve that would carry me through." - Charles A. Lindbergh. "As for background noise level; 35 dBA is a good classroom; 45 dBA is a sound masking system!" - Anthony K. Hoover > Along with the air/hydrogen(helium?) quick-test, one might also run an > air/carbon dioxide test. For a first-run test chamber, I suggest a 55 > gallon steel drum and a small table fan for starters. Come to think of it an even quicker test will be to run a TEFC (totally enclosed, fan-cooled) motor on the bench without load. The noise generated by the motion of the shrouded fan in those units dominates the motor sounds in that circumstance. The air inlet is a hole on the back end of the motor casing, and is not open to the motor armature and windings. Then fan is accordingly bathed only in this test gas, and will generate sound according to the rules we are discussing, at atmospheric pressure (but it can be also later tested inside a 55 gal drum). The cooling air(gas) outlet is a peripheral slot on fins attached to the motor case, and where the fan sound is emitted. One can easily feed hydrogen(helium), air or carbon dioxide into the inlet, and measure the resulting radiate noise near the peripheral outlet.... Angelo Campanella. --------- www.CampanellaAcoustics.com --------- "I have simply studied carefully whatever I've undertaken, and tried to hold a reserve that would carry me through." - Charles A. Lindbergh. "As for background noise level; 35 dBA is a good classroom; 45 dBA is a sound masking system!" - Anthony K. Hoover |
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