| The conventional open-pore aluminum foams with different pore structure, aluminum foams with mixed pore size, aluminum foams with wide pore size range, aluminum foams with gradient pore structure and aluminum foams with periodic pore structure were prepared by infiltration method and the Al-Si12 alloy as raw material; the mufflers with different core materials were prepared by using the prepared aluminum foam samples or the traditional sound-absorbing materials; the sound absorption coefficient of the mufflers with different core materials were measured and studied by using home-made performance test equipment of muffler, as following conclusions were given:(1) 4# aluminum foam core muffler with the biggest porosity and mixed pore structure has the best integrated sound absorption property in 30mm aluminum foam core mufflers, 8# aluminum foam core muffler with gradient interval pore size has the second best sound absorption property (5000Hz above is best), the third is 3# aluminum foam core muffler with the smallest pore size and the second biggest porosity,1# aluminum foam core muffler with wide pore size structure and the biggest pore size is slightly worse than 3# aluminum foam core muffler, and 2# aluminum foam core muffler with narrow pore size range and the bigger pore size has the worst performance, the other aluminum foam core mufflers are placed in the middle.(2) The sound absorption property of aluminum foam core muffler improves with the decreasing of pore size, the increasing of porosity, the growth of length of aluminum foam core and the increasing of sound frequency; the sound absorption property of aluminum foam core muffler with wider pore size range are better than these with narrow pore size range; the sound absorption property of 4# aluminum foam core muffler with mixed pore size structure is superior to these with only a single continuous pore size structure, but the number of pore size of aluminum foam has little effect on noise reduction property of aluminum foam core mufflers;(3) The performance of aluminum foam core mufflers with gradient pore structure is superior to aluminum foam core muffler with conventional mixed pore structure. Aluminum foam core muffler with gradient interval pore size structure is much better than the same porosity these with conventional mixed pore structure, compared with 30mm aluminum foam core muffler with conventional mixed pore structure, the sound absorption coefficient of 30mm aluminum foam core muffler with gradient interval pore size pore structure was increased by 0.32 and 0.54 and get to 0.78 and 0.82 when frequency is 5000Hz and 6000Hz, the performance of aluminum foam core muffler with gradient continuous pore size is slightly better than these with the conventional pore structure, namely the steeper the pore size gradient of aluminum foam, the better the sound absorption property of the corresponding muffler. In addition, the sound absorption property of aluminum foam core muffler with gradient pore structure is related to noise spread direction, the performance of muffler when noise spreads from big pore to small pore is more superior than the noise spreads from small into big pore, compared with the latter, the sound absorption coefficient of the former increases by 0.22 and 0.35 when frequency is 5000Hz and 6000Hz,.(4) Aluminum foam core muffler with periodic pore structure can improve the performance of the muffler in high frequency region, but it reduces its performance in low frequency region. In addition, the performance of aluminum foam core muffler with periodic pore structure increases slightly with the adding of periodic number;(5) Being superior to centrifugal glass wool core muffler, the sound absorption property of 4# aluminum foam core muffler with mixed pore structure (big pore (vol.80%) and small pore (vol.20%)) is as good as polyurethane foam core muffler, its sound absorption coefficient arrive to 0.9 hardly when frenquency is 5000Hz. More importantly, aluminum foam has a better overall performance, and especially fits in noise control in hot, wet, high pressure and open-air environments.(6) Theoretical analysis of the sound absorption principles of aluminum foam has been studied in this paper, the results of theoretical analysis supportes the experimental results, as following conclusions:the more helmholtz resonators in aluminum foam, the better the performance of aluminum foam core muffler; the sound absorption property of aluminum foam is correlated to flow resistance, in certain pore size range, the bigger the flow resistance, the better the sound absorption property; the sound absorption property of aluminum foam muffler is related to expansion chamber, the more expansion room and the greater expansion ratio, the better the sound absorption property; the performance of aluminum foam core muffler has relationship with the non-elastic collision mechanisms, the stronger the inelastic collision and the more the collisions number, the better the sound absorption property; the greater the acoustic impedance of aluminum foam, the better the sound absorption property.When the porosity of aluminum foams is same, the smaller the pore size and the longer the length, the greater the surface area of aluminum foam, the more and longer curved channel in aluminum foam, lead to the greater the flow resistance and acoustic impedance, the inelastic collisions and the passed through expansion rooms will increase when it passes through aluminum foam, and lead to the vibration bettween pore walls and sound save enhances, then the function of sound energy changes into heat will enhance, so that energy consumption increases, result in the better the property will be achieved. In addition, the flow resistance and the acoustic impedance of aluminum foam increases with the increasing of sound frequency, the sound absorption property of aluminum foam improves, and that due to resonance effects, the best sound absorption property of muffler will be achieved when the sound wave frequency match to the natural frequency of aluminum foams. |
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