Vibration assessment
In the summer of 2022, Festival Republic commissioned acoustics consultancy Vanguardia (a Buro Happold company) to undertake an independent scientific study of the vibration reported in the homes of some local residents during recent Wireless Festivals at Finsbury Park and Crystal Palace.
The findings of that study relate to vibration that is felt by local residents rather than heard. It has very particular characteristics that makes it stand out in a mix of other vibration from local sources in a home such as footsteps, a washing machine spinning or the closing of doors.
It is at a very low frequency, close to 2 Hz (it is a pattern of vibration which repeats every half second) and it is sensed in some local homes at the upper floors of taller buildings as a swaying motion.
This ‘2 Hz vibration’ was identified using sophisticated remote vibration monitoring techniques undertaken in 12 local homes over the two Wireless 2022 weekends. Homes are naturally full of vibration as people go about their daily routines so the analysis looked at pairs of monitors in different homes to identify which segments of vibration entered from outside the buildings and then compared this to vibration measurements undertaken on the Wireless sites themselves.
This 2 Hz vibration was identified as matching the reports from some local residents where they have experienced a swaying motion in their homes and noticed clothes hung in wardrobes swing on their hangers or letterboxes flap. Such phenomena are already familiar to residents in very tall buildings on particularly windy days but are unusual enough in these areas of London to attract the attention of people who experience them.
For residents living on the upper floors of taller buildings that are particularly near to the Wireless Festival site, the 2 Hz vibration data quantifies the effects that they report. However, the monitoring also shows that the amplitude of the 2 Hz vibration reduces down the two buildings being monitored. The amplitude for middle floors is between one-third and two-thirds of that seen at upper floors. At the base of the building, the characteristic vibration was found to be around one-eighth of that found at the upper floors. The reduction is different between buildings reflecting their differing constructions.
The 2 Hz vibration only happens during some songs and not during others. It is connected with the jumping of the crowd in time to the music but it is not caused by the sound system directly. This is proven by vibration measurements taken under the Wireless stages and under the front of house mixing desks. For example, vibration between 20-60 Hz is seen in the ground and this is attributable to the sub-woofers but this is not seen in the vibration measured in homes.
When the characteristic 2 Hz vibration happens, it can last several seconds but there can be minutes or sometimes hours between vibration events. During a particular song, it can happen in one building but not the next as all buildings respond differently to vibration in the ground.
The amplitude of vibration measured in homes remains below the international-standard levels at which vibration-induced damage to buildings has been credibly demonstrated. However, vibration levels above the threshold of human perception have been found in the data which corroborate the reports from local residents.
The longest duration of the 2 Hz vibration effect in a home is around 20 seconds. But there can be long periods of time when the effect is totally absent. To make a systematic assessment of this, the whole Wireless weekend can be divided
into 1 minute segments by the monitoring system. There are nearly 2,000 minutes of music at a Wireless Festival. The 2 Hz vibration effect occurs in 10 to 140 of those one-minute segments depending on monitoring location. It is important to note that these vibration events lasted for only a few seconds in each one-minute segment and the amplitude varied during that time.
Studies of the ground conditions and local topography reveal that that there is nothing in the ground to suggest that either Finsbury Park or Crystal Palace is not suitable for music festival events. The vibration phenomenon reported by residents occurs in short bursts and appears to vary according to song more than artist or genre of music. It is an occasional not continuous phenomenon which is hard to predict so simple things like altering music levels when the crowd jumps won’t control the jumping fast enough to be effective. Changes to staging and artist line-ups for 2023 are likely to reduce the occurrence of the 2Hz vibration effect.