Joists

Both the physical characteristics of joists and their spacing in a floor construction can influence the impact sound pressure levels generated from a floor construction.

Joist Spacing & Floor Modal Response

Joist spacing or, for walls, stud spacing has traditionally been used in INSUL when modelling the performance of cavity constructions. The spacing of the joists is used to determine the total length of line connections which join the two sides of a cavity construction as required, for example, by Sharp's (1978) classic theory.

Observations from measured data suggest that the modal response of a section of the floor/roof effects measured sound levels. INSUL's calculations for impact sound and rain noise include effects from the influence of modal response of a section of the floor/roof layers which is bounded by the length of the floor and its joists. Therefore, the joist spacing becomes significant.

The frequencies of the primary and secondary modes of the floor section are calculated based on theory described by Warburton. At these frequencies, the predicted sound level in the receiving room increases moderately above the average response of the floor. Modal effects tend to occur in the mid-frequency region. INSUL does not allow for modal effects above the critical frequency of the floor layers.

Note

Modal effects on airborne sound reduction are considered separately.

Joist Properties

Impact sound and rain noise prediction routines include consideration of the joists which form part of the floor/roof construction. Theory from Mayr, Gibbs and Fischer (2008) is used to estimate the influence of joists on the impedance of the floor/roof construction in the low-frequency region.

The cross-sectional depth and width of the joists can be specificed from the Properties window. These parameters, along with assumed values for density, bending stiffness and damping are used to determine the impedance/admittance of the joists, which is relevant for lightweight floors and roofs. In particular, in the low-frequency region the impedance of the joists can be greater than that of the floor/roof layers and, given the large wavelengths of the sound in this frequency region, can control the performance of the overall construction.

Note

The joist length is automatically set to equal the floor length.

The low-frequency region where the joist impedance can be influential is dependent on the joist spacing as well as the bending wave speed of the floor/roof layers which in turn depends on the critical frequency of the roof/floor layers.

Joists properties are not significant for massive floor structures such as concrete floors.