Controlling the Transmission of Impact Sound through Floors

qinsdau

C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o.3 5
Impact sounds,such as those created by
footsteps,the dropping of an object or the
moving of furniture,can be a source of
great annoyance in residential buildings.
While there are no requirements in the
National Building Code for impact sound
attenuation,some degree of control is nec-
essary for occupant comfort and satisfaction.
Although there is some commonality among
the factors influencing the attenuation of
airborne sound2 and impact sound,the
latter is by far the more complicated to
measure,rate and control.
The character and level of impact noise
generated in the living space below depend
on the object striking the floor,on the struc
ture of the floor assembly,and on the floor
covering.Recently IRC conducted an
extensive study to measure the attenuation
of impact sound that can be achieved with
different types of floor assemblies.For each
floor assembly,IRC measured the impact
noise level and calculated a rating called th
Impact Insulation Class(IIC).The higher
the IIC,the better the attenuation of impact
sound,with 50 usually considered the
minimum rating for occupant satisfaction
in residential buildings.
Controlling the
Transmission of Impact
Sound through Floors
by A.C.C.Warnock
This Update presents guidelines for controlling the transmission of impac
sound through concrete-slab and wood-joist floor systems in multi-family
dwellings.It is based primarily on industry-supported research conducted
by IRC.1
Impact Insulation Class
The IIC of a particular floor assembly is derived using a standard tapping machine,as stipulated
in ASTM method E492.3 This machine incorporates five steel-faced hammers that strike the test
floor and generate noise in a room below.The noise levels are measured and used to calculate
the impact insulation class(IIC),following ASTM method E989.4
The calculation covers a frequency range of 100 to 3150 Hz.The measured noise levels in
16 standard frequency bands spanning this range are compared to a reference contour.The
contour is adjusted until the levels above the contour(the most audible levels,in principle)
satisfy criteria specified in ASTM classification E989.
The IIC rating and the ASTM tests have some limitations that have different implications for
different types of floors.The tapping machine,for instance,does not accurately simulate the
sound of people walking.This is especially true in the case of the low-frequency sound(less
than 100 Hz)that characterizes the“thumping”of someone walking on a lightweight joist
floor.Even though a joist floor has a good IIC rating,footstep sound with a frequency of less
than 100 Hz can still be annoying to the people below.