Airborne And Structureborne Sound

Airborne sound originates in a space with any sound-producing source, and changes to structure-borne sound when the sound waves strike the room boundaries. The noise is still considered airborne, however, because it originated in the air. Structure-borne sound is energy delivered by a source that directly vibrates or hits the structure. In practice, all sound transmission involves both airborne and structure-borne sound.

When airborne sound hits a partition, it makes the partition vibrate, generating sound on the other side (Fig. 53-1). The sound will not pass through the partition unless an air path exists. If the partition is airtight, then the sound energy causes the structure itself to become a sound source by vibrating the partition. The partition vibrates mostly in the vertical plane, but also causes some energy to pass into the floor and ceiling, resulting in structure-borne sound.

When a mechanical contact vibrates or hits a struc-

Figure 53-1 Sound transmission between rooms.

ture, the sound travels along the structure causing vibrations, which then become airborne sound. Rigid wall-to-floor connections result in sounds that can be heard clearly through the building structure. A rigid structure with rigid connections offers a good sound path even in a massive concrete structure with masonry walls. The approach then becomes one of absorbing impacts with heavy carpet and resilient floor-wall connections.

When there is no air cushion between a noise source and the building's structure, high-intensity energy is introduced into the structure, where it travels at great speed with minimal attenuation. The sound in the structure is attenuated only by breaks in the structure. The structure must have structural integrity to carry loads, so breaking the structure to stop noise is complex and expensive.

With structure-borne sound (Fig. 53-2), the entire structure becomes a network of parallel paths for the sound. Partial solutions are useless, as sound finds flanking paths. The entire building structure must be soundproofed. Adding mass does not usually block structure-borne sound, especially in buildings with long spans. The floor becomes a diaphragm, improving structure-to-air noise transfer efficiency like a drumhead. Exposed structural ceilings further reduce the attenuation that would occur in a plenum. As most structure-borne sound is carried by floor structures, the sound radiates up and down into the rooms above and below.

Airborne sound is usually less disturbing than structure-borne sound. The initial energy is usually very small and attenuates rapidly at the room's boundaries. Air- Figure 53-2 Structure-borne sound.

borne sound changes directions (diffracts) easily. Low-frequency sounds are the most flexible, and can get around barriers.

Structure-borne sound has a higher initial energy level, and attenuates slowly through the structure, thereby disturbing large sections of the building. Structure-borne sound is magnified by the sounding board effect, like the handle of a tuning fork placed on a table. The sound appears to be amplified, although it actually is just a case of more efficient energy transfer from the tuning fork to your ear. Similarly, a vibrating pump may make little sound, but will transfer a large amount of energy to the structure, resulting in audible sound at each partition, floor, or wall rigidly coupled to the structure. Soft or damping connections prevent energy transfer, so less energy is transferred into the connecting efficient radiation surfaces.

Structure-borne sound travels much more rapidly than airborne sound. Sound traveling along a massive structure will radiate outward from the structure only minimally, though enough to be very annoying. The large mass minimizes vibration in the outward direction, but focuses the speed along the direction of the structural members.

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