Vacuum cleaner is a very noisy and an irritating appliance in any house hold. Design improvements have been done to improve the sound quality of the radiated noise from such vacuum cleaner. A wet and dry domestic vacuum cleaner was used for
noise control studies using jute derivatives. The physical dimensions and the weight of this dryer are 41.5 x 41.5 * 44.0 cm and 6 kg. The suction of the vacuum cleaner motor was 30 L/sec. From the sound intensity mapping using a two-microphone sound intensity probe the most noise producing component of the vacuum cleaner was found to be the exhaust pipe. A 2.5 cm lined dissipative muffler of 30 cm length consisting of jute fibers wrapped in jute textile was placed as shown in Fig. 6.18. An overall noise reduction of 8 dB was obtained by such a treatment with an improve­ment in its sound quality as well. The measured radiated sound power octave band spectrum of the vacuum cleaner with and without treatment is shown in Fig. 6.19. The overall radiated sound power of the vacuum cleaner was reduced to 57.1 dBA from 67.6 dBA with jute-based treatments59. The sound power measurements of the vacuum cleaner was done as per ISO 9614 standard60. It may be noted that the sound power of a sound radiating body is estimated by at first performing a normal sound intensity measurement over a surface area, and then summing up the product of the sound intensity times the surface area as given in Eq. (17). The sound intensity of the entire radiating area needs to be measured as a function of frequency. For a rectangular body, usually the intensity from the five radiating surfaces are measured, leaving aside the bottom surface, which may be placed against a sound radiating hard floor or an absorbing floor.

Sound Power, W = S IA (17)

where Ii is the measured sound intensity and A is the corresponding normal surface area. Usually these areas are predefined and marked before the sound intensity mea­surements are done61. The sound power thus calculated is a function of frequency, and is usually represented in octave frequency bands. The sound intensity being a vector quantity is very much dependent on the direction of measurement. Thus enough care needs to be taken while traversing the two-microphone probe over a product during sound intensity measurements so that no directional error is made.

FIGURE 6.18