U6 – BUILDING PHYSICS AND SUSTAINABILITY
Session Plan U6-S4: Acoustics
INFO S4
U6
values to the airborne sound or impact sound insulation measure if the frequency range covered has been extended or if sound other than customary noise has to be soundproofed. Basic principles of acoustics With some knowledge of the following fundamental principles of acoustics, the various manifestations of building acoustics and constructs can be better understood and unnecessary errors avoided. Berger’s Law of Mass for Sound Insulation It is substantial for heavy solid walls and solid ceilings from a surface mass m 'of approx. 100 kg/m². According to this law, the air-sound insulation measure increases as the surface mass, ie the thickness of the components, increases. This results in the solid construction of the known thickness dimensioning of separating components for usual standard requirements, e.g. the required surface area m'= 400kg/m² (approx. 20 cm concrete wall thickness) for apartment walls. The lower density of wood construction materials would make a wall thickness of about 2 to 3 times the thickness necessary for walls and ceilings. Consequently, in wood construction, the required sound-absorbing dimensions must largely be produced using a different acoustic law, namely the resonance phenomenon. Resonance Phenomenon and Coincidence Effect This phenomenon pervades the entire building and room acoustics with its law. It is based on the fact that each spring-loaded mass has a system resonance with a defined natural frequency. Partly well-known examples of such oscillatory systems are e.g. elevator machine sets resting on elastic rubber elements. Such "spring / mass systems" behave like a weight attached to a coil spring. If you then let the weight vibrate vertically, it does so with the typical natural frequency, the resonant frequency per second in Hertz. At this natural frequency and in its vicinity, large, excessive oscillation widths are produced with only a slight impulse. It comes to a " resonance amplification " of vibrations. However, if the oscillation number of the excitation force, e.g. the engine speed per second, is well above the natural frequency, then the swinging is suppressed intensively, the vibrations will be transmitted in a much more reduced way to the supporting surface. The vibration suppression is already very pronounced when the excitation frequency is three times greater than the natural frequency.
Particularly problematic is the structure-borne sound energy, which - once penetrated into the timber structure - can experience significant reinforcements by resonance enhancements on various structural parts. Their propagation must be prevented already at the vibration source. Plate-shaped separating components are also excited by the impact of airborne sound to bending vibrations, which propagate in this component as water surface waves and on the other side of the component lead to a sound radiation. Now, if the airborne sound wave and the bending wave caused thereby run parallel on a wall at the same speed, then coincidence arises at a certain frequency, the coincidence limit frequency fG . About above the triple fG again the Berger mass law is decisive. It is important to keep this strong reduction in sound insulation outside the building acoustics relevant frequency range. This means that the coincidence limit frequency fG should be 31 50 Hz or higher or 100 Hz or less.
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