25.08.2014
The ability of buildings to insulate impact sound excitation (e.g. the walking around of human beings) is characterized metrologically in such a way that a small tapping machine is used as excitation and the generated sound pressure ("standard impact sound level") is determined in the respective room. However, this method ignores the mechanical interaction between the source and the excited structure and leads to untransparent – and often misleading – results. Replacing the standard impact sound level by a clearly defined impact sound reduction index can solve the problem..
The central question of sound insulation in buildings is: How successfully can components, e.g. walls and ceilings, reduce sound? In the case of airborne sound events (traffic noise, neighbours talking to each other), this is marked with the "sound reduction index", which is the level difference of the sound power which strikes the considered component and the sound power which is able to pass through it. However, in the case of impact sound events – which, by the way, also comprise objects falling to the ground and other types of direct impact noise excitation – another procedure has established itself: The ceiling to be marked is excited by means of a standardized small tapping machine, and the sound power arriving in the subjacent room (i.e. the so-called "standard impact noise level") is determined. As this noise has practically no similarity to the impact noises of everyday life, people try to convert the measured levels to other sources (e.g. persons walking around) by means of additive "corrections".
Although this procedure is impressingly easy, it provides untransparent and often misleading results as it ignores the physical fundamentals.
The fact of the matter is::
1) Only a difference of the sound powers in front of and behind the considered component – a "sound reduction index" (in acoustic terminology) – contains recognizable information about the effect of the component.
2) However, this difference depends just as much on the mechanical characteristics of the source (here: the source impedance) as on the mechanical characteristics of the examined component. It thus does not provide the desired parameter for the component alone. As a consequence, a sound reduction index of its own has to be defined for each source type, and this index can only be measured by means of this specific source type.
In the present work, the system of sound reduction indices was transferred from airborne sound to impact sound and as a start developed for two source types which frequently occur in residential buildings. These are: human beings walking around; and small, hard objects falling to the ground. A great advantage of this is that, for this purpose, two measurement sources can be used which have already been standardized and which are already generally employed: the standard tapping machine, and the so-called "modified" tapping machine – both in accordance with ISO 10140 [1]. Using the example of the improvement of two commonly used floor constructions (a heavy solid floor and a wooden-beam floor) by means of additionally applied dry screeds, we were thus able to show that the impact sound reduction index reflects the actual and aurally compensated level reduction correctly, whereas the usual standardized impact sound level overrates the improvement especially on the lighter wooden-beam floor in the case of soft sources (e.g. a person walking around barefooted or with soft shoes) by several orders of magnitude (approx. 20 dB) and is thus totally inappropriate for the assessment whether or not given requirements are met.
In short: By using a second measurement source – which consists of a tiny modification of the first measurement source -, by applying a slightly modified assessment as a sound reduction index, and by otherwise completely maintaining the existing measurement procedure, useful impact sound parameters can be generated – for the first time – for a large part of the practically occurring noise situations, not to mention the enormous advantage of a consistent treatment of airborne and impact sound in terms of sound reduction indices.
References:
[1] ISO 10140, Acoustics – Laboratory measurement of sound insulation of building elements – All parts (2010)
[2] Scholl, W.: Why not use impact sound reduction indices instead of impact sound pressure levels? Acta Acustica 3, 2011
[3] Scholl, W., Ciszewski, R., Wittstock, V.: Why not use impact sound reduction indices instead of impact sound pressure levels? Part 2: Application to different impact sources. Acta Acustica 6, 2013
Contact person:
Werner Scholl, Dept. 1.7, WG 1.72, e-mail: werner.scholl@ptb.de