Sound Attenuation Properties of Different Materials for Staircase Construction
Sound – Is vibrations in the air which the Human Ear is able to detect frequencies from 500 Hz (low-pitch) – 6,000 Hz (high-pitch). Artificially sound can also be measured and quantified using a Sound Level Meter.
Amplitude is the measurement of the intensity of the sound and it’s movement through the air and is measured in Decibels (dB). The Human Ear can hear ranges from 0 dB – 140 db (painful at 140 dB).
For the purpose of this article sound should be considered as a beautiful sweet melody the individual wishes to hear intently e.g. Susan Boyle singing ‘I Dreamed a Dream’.
Noise – Noise is generally considered to be unwanted-sound the individual would rather not be able to detect, e.g. traffic noise, neighbours arguing, or next door’s Dog aggressive, repetitive barking.
Acoustics (Sound) in Construction:
Acoustic Attenuation (Noise Reduction) is concerned with the with the transfer and decrease of sound through materials. With careful planning, knowledge, craftsmanship, expertise and different materials noise can be greatly reduced nowadays and is an important factor governed by Part E Building Regulations in the United Kingdom. Methods of measuring the effectiveness of Acoustic Attenuation is listed in BS EN ISO 16283-1:2014. Unfortunately SRIs, NRCs IICs prove illusive to obtain from credible sources.
Generally sound dissipates at a rate of 6 dB in relatively normal air each time the distance from the sound source is double. In the real world there are more things to consider; such as weather conditions, humidity, screening by buildings and trees, background noise etc.
Sound Absorption is the amount of sound a material can absorb and is measured by it’s Noise Reduction Co-Efficient (NRC).
Material Acoustic Properties:
Noise Reduction Co-Efficient (NRC): Generally soft, textured materials perform best and reduce the unwanted noise and have a rating of 1 (complete absorption). Hard, reflective surfaces reverberate (echo) the sound and have a rating of 0 (complete reflection and no absorption).
Sound Reduction Index (SRI): The effectiveness a material, or construction element such as a door or window, by the amount of airborne-sound detectable once the sound has passed through said material or element. The greater the ability to absorb sound the greater it’s rating. Generally the heavier the material the less the material vibrates, thus reducing the amount of sound passed through it and the better it’s SRI rating and able to absorb loud airborne sound e.g. a double-brick leafed wall with a cavity plastered internally has an SRI rating of 53 (average frequency 100 – 3,150 Hz). If you had a recorded sound at 2,000 Hz you could go outside and play your stereo at 53 dB and you would not be able to hear the sound. If you went to an adjacent room with a window of 10 mm single glazed glass with an SRI of 31 you would be able to hear 53 – 31 = 22 dB.
Obviously the materials perform better and worse at different frequencies. In the UK
SRIs are produced in a laboratory with no flanking sound paths possible. The Americans call this a Sound Transmission Class (STC) and their method of measurement is different.
Impact Insulation Class (IIC): The ability of the material to resist transferring impact noise. A rating of 70 almost completely soundproof and 39 or below loud speech and impact noise audible; so the higher the rating the better it is at resisting you walking up in your stilettos.
Concrete Stairs:
- SRI: 57 dB (fair-face) “Walls between dwellings: 18cm thick reinforced concrete wall featuring a sound reduction index of 57 dB” taken from Building Acoustics written by Asselineau page 212.
- IIC: 27 (fair-face)Kinetics Noise Control Test Report AT001049; courtesy of Kinetics Noise Control based upon testing a 152 mm bare concrete slab. It seems ridiculous to compile wall-ratings with slab-ratings though at the time of publishing acoustic data is extremely sparse. Clearly with concrete being one of the densest materials it’s SRI will be naturally quite high, however it’s hard reflective surface will naturally reverberate sound. To mitigate the transfer of impact-sound soft, textured surfaces are greatly beneficial to their IIC rating. Flanking is a concern and this should be considered when choosing a contractor as the workmanship will affect flanking. However it should be noted that the walls supporting the staircase will naturally be of a heavy composition; as they must be able to support the staircase safely. Compared to a timber staircase where the walls will not necessarily comprise of high SRI ratings.
Metallic Stairs:
- SRI: 36 dB Gathered from Haniso Steel trapezoid section 50/250-0.88 floor with insulation.
- IIC: Not Available. Again data is sparse and at this moment in time one can only think logically about how a steel staircase would perform. It cannot be argued that the NRC cannot be greatly high. Flanking could be a cause for concern with transmission from the fixings.
Timber Stairs –
- SRI: 25.3 dB (24 mm plywood) Gathered from HOM Loudspeakers Cabinets
- IIC: Not Available: Again data is sparse and at this moment in time one can only think logically about how a timber staircase would perform. Timber has a great disadvantage where mass is not present compared to concrete and metal and separation from the components would need to be carefully considered to come anywhere near close to concrete and metal.
Summary:
At the time of writing this article there was currently no data relating specifically to the sound performance of staircases and it has proven illusive to gather fair relevant data to each material and the best has been done to gather similar data; though unsuccessfully. It should be noted that contradictions have been noticed upon gathering data and ultimately the reader must come to their own conclusion using practical common sense.
Concrete has the advantage of great mass compared to steel and timber and shares it’s disadvantage of reverberation with steel, though when covered with softer textured finishes it seems unbeatable.
Construction quality is paramount and a mixture of using dense products with the latest technology and diligence will yield the best results.