Acoustic parameter for timber buildings (part 1)

Impact sound insulation: how it is measured and a quick glance at its underlaying physics.

Mathias Barbagallo, PhD and senior consultant, Brekke & Strand Akustik AB,

Impact sound is one of the typical acoustic quantities that is investigated in a standardised way in building acoustics. Impact sound relates to the insulating properties of building elements or building systems in respect to a specific kind of impact. Impact sound is interesting for timber constructions because of the intrinsic acoustic challenge it represents for these structures. A holistic approach within acoustics is needed in order to consider all aspects relevant to it – how it is measured, how it is investigated in order to mitigate it, how it relates to regulations and acoustic comfort. These aspects are covered in a three-article series. We start by understanding how impact sound is measured and generated.

Impact sound insulation is evaluated by injecting power into a structure, e.g. a floor plate, by means of a structure-borne sound source, and then measuring the impact sound pressure level in the receiving room – plus a correction accounting for volume and internal losses of the receiving room (see ISO 16283-2 and Figure 1). Subject to the same input force, concrete floors will generally vibrate less than wooden ones – concrete has lower input mobility than timber. To overcome these intrinsic differences, the standardization approach has been to standardise the excitation source. The ISO tapping machine is the primary standardized source of choice. It is a machine with five equally spaced hammers, each weighing in at 0.5 kg. Hammers are ideally free-falling masses on the floor, but not in practice due to practical constraints, with a drop-height of 0.04 m. Impacts from the hammers happen every 100 ms with 10 impacts on the floor per second, see Figure 2.

This is a very specific source, not as general as a loudspeaker emitting pink or white noise. In fact, one does not speak of standard structure-borne measurements, but rather of standard impact sound measurement. A small but important difference. Adults walking, children running, falling objects, moving of furniture, drilling, vibrating machineries are all phenomena that will generate structure-borne sound but that are represented by the ISO tapping machine with varying success. There exists another standardized way to measure impact sound insulation i.e. via a standardised rubber ball that is dropped from 1 meter. It has shown to be better correlated to walking noise, but its results are generally trickier to judge due to lack of common and standardized acoustic regulations (correlation to real-life noises will be covered in the third article).

Especially at low frequencies, timber floors will vibrate more than concrete floors when subject to the same input force because timber has lower mass per unit area and lower bending stiffness – i.e. timber has higher input mobility than concrete. The tapping machine will therefore cause high vibration levels especially at low frequencies, say below 500 Hz, where mainly mass per unit area determines the response. At higher frequencies, on the other hand, timber is softer than concrete, thus dampening the strokes by the hammers of the tapping machine: naked timber has better high-frequency impact sound insulation than naked concrete, see Figure 3.

For concrete floors, it is quite straightforward to improve impact sound insulation at higher frequencies by using impact sound mats, whose effect is well-documented by the industry. On the other hand, impact sound mats will not improve the impact sound insulation of a timber floor where most critical, i.e. at low frequencies, because they will not add a considerable mass to it, see again Figure 3.

In the next article we will look at how impact sound may be investigated in order to be improved.

Brekke & Strand Akustik is an independent specialist company with acoustics, noise and vibrations as its field of expertise. Our continued focus on technical expertise, independence, cost-effectiveness and good customer service ensures that we can contribute to the largest and most prestigious acoustic tasks in Sweden and Norway where we operate. We are more than 100 specialists within acoustics, with a master’s degree in engineering, a doctoral degree or equivalent. Together we have accumulated more than 1000 years of experience in the field. If you would like to get in touch, just contact Mathias Barbagallo for acoustics of timber constructions () or Johanna Carpelan for other inquiries ().