The Millenium Bridge over the Thames was opened on 10 June 2000, but was closed three days later after complaints of people feeling “sea sick”. The problem was caused by the people walking synchronously. When people walked across the lightweight bridge it vibrated at its resonance frequency and people subconsciously adjusted their walking pace so that it matched the resonance frequency of the bridge, as this causes the least resistance to a person walking. However when several hundred people are all exciting the bridge at the resonance frequency, it causes the bridge to sway. Engineers at ARUP tested the bridge and analysed the data. The remedy was to install dampers to reduce the vibration.

Read more about this story on the ARUP web site.

Read the article on the BBC web site.

The hard disks inside every desktop and laptop are designed to emit low noise and generate, and also to be relatively insensitive to vibration.

Two important designs features that enable this are fluidic bearings and winglets on the head of the arm.

Hard disks were once made with tiny rolling element ball bearings, however nowadays almost all hard disks are made with fluidic (journal) bearings. These bearings are more reliable, quieter, generate less vibration, and cheaper than the ball bearings.

Another important design feature are the winglets on the end of the read / write head. The hard disks spin at 4200rpm or higher which causes the air to move. Small wings attached to the sides of the hard disk head enables the head to “fly” precisely above the rotating disk. This design makes the hard disk less sensitive to vibration.

BBC news has an article about the effects of low frequency sound. Church organs emit infra-sound which causes mood changes in the congregation. Elephants use infra-sound to communicate.

Read the story on the BBC news web site.