Slim sound transducers for active duct silencers

© Fraunhofer IBP

Sound transducer with curved surface element (disc spring), evacuated flat pack and electrodynamic drive.

Noise produced by ventilation systems and devices often cause annoyance. Classic examples for this are air conditioning systems in offices, cooker hoods in kitchens, exhausters in hotel bathrooms, and all kinds of exhaust air and extraction systems of machines and laboratories. Usually, porous sound absorbers are used to attenuate the noise. Wrapped in a trickle protection and sometimes also in foils, these materials reduce the ventilating noise. However, if there is only limited space available and if particularly low frequencies are annoying, porous materials reach their limits.

In these cases, resonators and activated resonance silencers, with significantly increased efficiency, offer a good solution to achieve high attenuation for low frequencies in short ducts. They have proven their advantages thousands of times, for example when being used in ventilating systems. However, active silencers require a large and deep casing to allow the enclosed air to be compressed by the loudspeaker diaphragm.

As with the loudspeaker at home, a large volume helps to improve the low frequency sound generation. What seems to be impressive at home though, presents a practical impediment for active silencers. This is why recent research and development work is aiming to find solutions on how to achieve the same results with reduced casings. A common way is to fill the casing with sound absorbing material. Thus, the casing volume can be reduced by 10 to 15 percent without any acoustical losses. This effect can be enhanced by so-called adsorbers usually used for air purification and dehumidification. The volume can be reduced by up to 75 percent by help of a special powder based on activated carbon. When evacuating the cavity, even smaller casings can be achieved. In this case, the loudspeaker diaphragm is a kind of disc spring, i.e. a closed and curved surface element. If dimensioned correctly, it takes an almost even shape at low pressure so that the connected electrodynamic drive only requires little energy for the deflection and sound generation respectively. This “slimming diet” for active silencers includes another enduring benefit: Slim silencers do not only save space but they also reduce the energy consumption of the fan. The acoustically damped duct cross section of the silencer may be larger so that the air does not have to be pressed through a narrow silencer gap.