Case Studies in the Absorption of Low Frequency Sound in Music Rooms

Room design for music is a special and highly valued skill. The spectrum, timbre and dynamics of music, coupled with our keen sense for musical sounds make these rooms a particular challenge for the acoustician. The goal is to create an acoustic environment whereby the sound field within the space supports the instruments and enhances their tone, with unfavourable colourations and effects minimised. Room dimensions, construction materials and specialised treatments
must be chosen carefully to achieve the perfect musical balance for the performance.
Architectural Acoustics is the field which brings together art and science to address the challenge of creating an environment suitable for music. The most challenging aspect is to achieve the required spectral balance, particularly at low frequency. This thesis presents a literature review of current knowledge of architectural acoustics for music and traditional acoustics treatments. This is followed by three case studies which each investigate a novel solution to improve the acoustic environment for musicians.
The first case study investigated room boundaries as potential absorbers. The study arose at a music college who wanted an evaluation of a beautiful new recital hall. Professional musicians had expressed concern about the unusual acoustic in the hall. A room acoustic assessment is described, and results analysed and compared to standards and guidance. Multiple studies including modal analysis, physical experiments and wave-based computer simulation were undertaken to investigate the mechanism for the low frequency dip in the room response. It was determined that the false walls in the hall were acting as quarter wavelength resonators but for multiple low frequencies thus creating an effective absorber. This absorber design could be repurposed to reduce the boom commonly found in modern music venues.
The second case study explored pneumatic absorbers. This investigation arose when a London orchestral rehearsal space wanted an inexpensive, quick to deploy, flexible acoustic which would reduce the low frequency boom in the space thereby balancing the room response. A variety of airbeds were selected for the study and tested in the laboratory with different levels of inflation. Finally, the position and number of the airbeds was investigated for optimisation purposes. The airbeds were installed in Henry Wood Hall and the room response measured. It was found the room response was flattened thus improving the acoustic quality. Conductors from world leading orchestras responded positively to the new condition of the hall.
The third case study focused on the Targeted Energy Transfer method as a means of creating a novel low frequency absorber. This approach transferred knowledge from the field of vibration control to architectural acoustics. A test rig was built to measure the vibrational damping in two types of hyperelastic latex materials using a laser vibrometer. A small low frequency non-linear response was found but there was not enough evidence to pursue the research further.