| 摘要 |
PURPOSE:To prevent the interference between sound waves occurring between duct wall surfaces opposing to each other and to generate a muffling sound wave with sufficient acoustic power by radiating the muffling sound waves from plural muffling speakers respectively set up on the duct wall surfaces opposing to each other into the duct. CONSTITUTION:A muffling digital electric signal formed by an operation part 19 passes through a digital frequency cut-off filter 15 by a digital signal process with a short time delay and a steep frequency characteristic and becomes the muffling digital electric signal with a frequency area becoming an object of muffling. The digital electric signal is made analog by a digital-analog converter 12, and passes through an analog frequency cut-off filter 9, and becomes a muffling analog electric signal incorporating only the signal in a low frequency area becoming the object of muffling. The muffling analog electric signal is amplified by a power amplifier 6, and drives two muffling speakers 301, 302 provided on the duct 1 wall surfaces opposing to each other. The speakers 301, 302 are provided on the duct l wall surfaces opposing to each other, and generate the same muffling sound waves. |
