Principles of Acoustics

The subject of acoustics, often perceived as somewhat of a dark art, is a key consideration on any façade of significance. We have set out below a few basic principles and considerations.

Sound is measured in decibels (dB) on a logarithmic scale, which means that a sound of 50 dB is 100 times louder than a sound of 40 dB. The Human ear has a large audible range and at the threshold of pain, the dB is roughly 1 million times greater than at the threshold of hearing.

The main consideration when reading specifications and test results is that a sound difference of 3 dB or less is barely audible to the human ear, whereas a sound difference of 5 dB is clearly audible. It is easy to look at minor differences between specification and test results and whilst numerically they may not be aligned, in practice the difference is so small that it will not materially affect the end installation. This is where a pragmatic interpretation of test results is required rather than a theoretical or literal comparison. Due to this, if you are doing acoustic testing, it is worth considering taking the average result over a range of tests if you get a minor failure as subsequent tests with all the same inputs and the same test rig, can give different results.

Acoustic performance values are often provided in octave bands / frequencies (125,250,500,1000,2000 & 4000). The lower to middle figures in these octave bands tend to be sounds of lower pitch and are more discernible to the human ear. Traffic noise is a key consideration when evaluating the lower octave bands and is often represented in specifications as a ‘+Ctr’ value. Under specifying or failing to comply with the mid to lower octave bands performance requirements will be more irritating to a building user than a failure in the upper octave levels – occupier comfort is No.1. Be aware of the impact of the ‘+Ctr’ value on the performance of any façade – take advice if unsure!

In order to improve acoustic performance there are a number of items that can be considered. The initial gains on a façade construction are usually quite easy to achieve, it is the subsequent improvements that are usually more challenging

No.1 of the governing principles of acoustics is the Mass Law. Essentially one of the most effective ways in enhancing acoustic performance on a facade is to increase the mass or weight of its main surface components. Going by the Mass Law, for every doubling of weight of a material, it equates to a 6dB improvement. Therefore, a 12mm thick glass will give a 6dB improvement in performance, over a 6mm thick glass.

No.2 is separating the elements in a façade build up. The separating of the elements disrupts the pattern of the sound waves as they pass through the façade and makes it harder for them to reach the inside surface. Including different materials in the façade build up can be used as a method for controlling sound. Different materials will absorb and block different sounds, so a variety of materials can work together to achieve the required result.

No.3 is the glass specifications themselves, by using laminated glass, especially one with an acoustic interlayer, you can often achieve the marginal gains that are often required.

No.4, when using double glazed units on a project, consider using different thicknesses / compositions of glass for the inner and outer panes, as the different thicknesses will resonate at different frequencies.

So, what about my façade? Typically, the effect of insertion loss through a framing system consideration should be given to the rule of thumb that a glazing system will generally give a dB reduction of 37. Therefore, unless the required dB reduction is in excess of 37 it is not necessary to separately consider the frame performance. Consideration should be given however, to the size of the glazed panels with all glass test data being based on an approximate glass size of: 1.23m x 1.41m, then any significant increase to these sizes will result in performance loss.

We have not sought to address the topic of flanking sound transmission in this document however we trust this article is of use and keep a watch for our next release!