Vibration (Structure-Borne) and Airborne Sound

There are two primary modes of sound travel in Architectural Acoustics – through air, and through solid components, such as walls, floors, beams, columns, and other rigid building elements. This is because the molecules of the material are so much closer together.

Sound travels much more quickly in hard materials than it does in air and has less transmission loss. Therefore, once a sound enters the structure of a building, through direct vibrations or through impact from high-decibel low-frequency airborne noise, it travels very far. This is why you can sometimes hear weights dropping or loud thuds several floors above where the noise source is located. It is also why isolating vibrations at the source is such a critical element of effective sound mitigation.

NOTE: Sound frequency and wavelength are related, and are inversely proportional. The speed of sound is entirely dependent on what it is traveling through. Higher frequencies result in shorter wavelengths, while lower frequencies result in longer wavelengths. Longer wavelengths are more difficult to reflect, and diffract more easily.

c = f/l

(c is speed of sound, f is frequency, and l is wavelength)

Take a look at a few unique noise sources here:

• Foghorn: a low frequency is used so it can travel far, helping those at sea avoid dangerous obstacles in heavy fog. Fog Horns are generally 125Hz (as seen in Figure 4), a low, roaring sound, and are emanated at 105 dBA, loud enough to be heard for miles.
• Trucks: are louder and lower frequency than cars, roadways with higher % of trucks will require special facades or treatment. It is not just because they are louder, but the lower frequency travels much further without attenuation.

Consider that damage may occur to your hearing after being exposed to loud noises over a period of time:

• Gym Noise: American Deafness Association notes that this is one of the major contributors to hearing loss today. Being exposed to 90 dBA for even 1 hour each day is unhealthy.
• OSHA Limits: The safe exposure limit for 8 hours or more is 85 dBA.

Diffuse Field vs Direct Field

There are two types of sound fields. From a point source, sound travels outward radially. As it impacts surfaces, it is reflected back into the space or absorbed. The amplitude of the sound you hear, as well as other properties, are determined by whether you are hearing the initial sound, or a reflected version of it.

The strongest noise source is the first, direct impact. If you are within the Line of Sight of a sound, you will be receiving its strongest impact. Therefore, by interrupting Line-of-Sight Propagation, you can greatly improve the sound attenuation between two objects.

When sound builds up within a space, such as a church during singing or an auditorium with cheering, you are said to be within a diffuse field. In this case, sound is being reflected off many surfaces and the amplitude of the sound is a combination of the all of the various point sources, rather than any direct noise source.

Sound Travel Mechanisms

When soundwaves contact a surface, there are several potential mechanisms with which it can react. The sound may transmit through the surface, reflect off of it, or be absorbed by it.

1. If the surface is thin and the sound wave has a low frequency (and thus, long wavelength), it is likely that the sound will transmit directly through the surface. On the other hand, a dense, thick surface being impacted by a high-frequency wave is much less likely to transmit.
2. If the wave is reflected, then it may do so in 1 of 2 ways. When the wave is reflected, it imparts an equal and opposite force on the wall, causing it to vibrate, although often-times this vibration is too subtle to notice with human touch.
   • There may be a basic reflection, in which the angle of the incoming wave equals the angle of the outgoing wave (like a laser bouncing off a mirror) – this is common if the surface is smooth and plain.
   • There may be diffuse reflection, in which the wave is scattered in many directions. This is the case with convex panels, complex staggered panels, or walls with many outcroppings. Diffusion is particularly effective with low-frequency sounds that are more difficult to absorb.
3. If a surface absorbs the sound wave, then it is not reflected, and instead, the energy is transferred into the surface, and transformed into heat.

1. Sound waves also have the ability to diffract over walls, which is to bend around them. Certain waves are able to diffract more easily – specifically low-frequency sounds are able to bend much more quickly. This is what makes low-frequency waves more “omni-directional”, or likely to spread in all directions, versus high frequencies which are more angled.
2. Lastly, sound may transfer from a solid surface back to the air by emanation. This is the process of the surface, having absorbed an inbound sound or reflected the sound and received its vibrational energy, vibrating against the nearby air particles and creating an airborne sound wave.