Building Acoustics

Multilayer elements 313

8.4.2.1 Lightweight primary floor

In all examples shown relating to the improvement gained from floating floors, we have
tacitly assumed that the primary floor is infinitely stiffer than the floating top floor. For
lightweight primary floors such as wood joist floors, normally comprising lightweight
panels both on top and on the underside, the impedance of the hammers will however
influence the impact sound level of the primary construction as well, i.e. the
measurement being the base for determining the reduction in transmitted sound. This
problem, which was mentioned in the introduction to section 8.4, will be illustrated by
measured data for the impact sound improvement of a lightweight floating floor
combined with both a heavy and a lightweight primary floor. However, up to now we
have not specifically looked into the impact sound insulation of such lightweight primary
floor, partly due to the complexity in modelling, partly as they normally cannot offer
sufficient impact sound insulation without being combined with a floating floor and/or a
suspended ceiling.

63 125 250 500 1000 2000 4000^

Frequency (Hz)

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No

rmalized impact sound level

Ln

(dB)

Predicted

Measured

Figure 8.30 Normalized impact sound level of wood joist floor. Predicted results are mean values using 15
tapping positions. After Brunskog and Hammer (1999b).

Brunskog and Hammer (1999b) have presented a literature survey (see also
Brunskog and Hammer (2000)), on the different approaches to the modelling and also
presented their own prediction model taking the periodicity of the beam-plate system into
account. Their approach is somewhat analogous to the one used by Lee and Kim (2002)
treating airborne sound insulation (see section 8.2.2), as both start out from the