The best way to reduce any unwanted noises in a home is to insulate all internal spaces. This ensures that the living and working spaces are comfortable and helps increase concentration and relaxation.
Materials such as window or glass panels are tested for their sound reduction values through the sound frequencies 100Hz to 3150Hz as a standard. However, further frequencies can be tested if required.
When you are looking to ‘sound proof’ a space what you are actually doing is filtering the types of noise (frequencies) that can travel through the external materials of that space to ensure that any frequencies that do penetrate into the space are extremely difficult for the human ear to detect.
Understanding Acoustic Insulation in Glass
Insulating internal spaces against unwanted noise is important part of ensuring internal living and working spaces are comfortable and promote concentration and relaxation.
Understanding acoustic power, intensity and how to effectively insulate against unwanted noise can be complicated and difficult to understand.
First of all, the typical unit of measurement for sound is decibels (dB) which the measurement of acoustic power, or acoustic pressure. It is measured by a standard formula using the logarithmic scale and the I0 and P0 measurements of threshold hearing.
(Sound intensity level = L1 = 10 log l/I0, Sound pressure level = L1 = 10 log P2/ P02= 20 log P/P0)
It is important to note that dB is the measurement of acoustic power, not acoustic intensity. The acoustic intensity of more than one noise sources increases by addition. The same does not apply to acoustic power or pressure.
(eg. 2 trumpets each producing a level of 80 dB together produce 83 dB, and not 160 dB.)
The transmittance of ‘sound’ is actually the transmittance of vibrations from the sound source. These vibrations can travel through gas, solids and liquids. The number of vibrations from a sound source per second is known as the frequency and is measured in Hertz.
The human ear can here frequencies between 16Hz to 20,000Hz. When we are looking at building design and architecture the range of frequencies that are considered to be ‘important’ are the frequencies between 50Hz and 5000Hz.
Testing Materials for Sound Insulation
The sound insulation of a material is denoted as R. This value is tested under strict laboratory conditions under the testing method EN ISO 140. It tests the material, such as a window or glass panel, for its sound reduction values through the sound frequencies 100Hz to 3150Hz as standard. Further frequencies can be tested if required.
This is where sound reduction and insulation becomes a little more complicated, as materials (such as glass) do not perform or insulate at the same value for all frequencies or types of noise.
Through the standard EN ISO 717-1, the acoustic performance of a material or product is standardised and also adapted for two defining ‘types’ of noise. This creates the most commonly used and most useful sound reduction index:
C = the weighted correction of insulation for ‘Pink Noise’ which is generally used as the correction for outside background noise.
Ctr = the weighted correction of insulation for traffic noise
The minimal windows 4+ slim sliding door has been tested for noise reduction and determined that the difference in noise level inside to outside is 43dB. Therefore the R index for the sliding glass door is 43dB.
However according to EN ISO 717-1 the weighted noise reduction for the sliding glass door is Rw(C;Ctr) 43 dB(1-;-5).
This means that the sound insulation value for the window is 43dB and is reduced by 5dB for traffic noise and 1dB for ‘pink noise’. So the difference in traffic noise level inside to outside is 36dB and the difference in ‘pink noise’ inside to outside is 42dB.
These values are sometimes broken down into two separate acoustic insulation values for materials to RA and RA,tr. For example, if we were to use the example above, the RA value of the minimal windows 4+ would be 42dB and the RA,tr value for the sliding glass door would be 36dB.
The use of these weighted calculations for acoustic reduction allow architects to specify glass and windows that insulate against specific types of noise and chose the optimum specification for each individual project situation. This reduces the need to over specify the glass and improves comfort levels internally in a more effective way.
Materials insulate against noise differently at different frequencies, glass is no different. The frequency at which glass performs the worst (ie has the lowest acoustic reduction) is known as the Critical Frequency (or coincidence resonance). This also needs to be included in the consideration of glass specification for noise reduction.
Part of engineering glass for acoustic reduction is ensuring that this low-performance point (the Critical Frequency) is at a lower end of the frequency scale (ie is at a low Hertz value) and preferably below 100 Hz. This ensures that the glass unit insulates well across the higher frequencies which are likely to cause most discomfort for internal occupants.
For a 4mm thick piece of toughened glass the critical frequency is 3000Hz, this means noises at this frequency travel through the glass very easily.
To move the critical frequency of a single piece of toughened glass to below 100Hz the glass sheet would have to be 1200mm thick…. Not something that is feasible for most building projects.
This is why laminated glass is used for acoustic reduction, especially glass laminated with specially designed acoustic interlayers. By using these specifically designed acoustic laminates, coupled with using multiple glass units (double and triple glazed units) and glass of differing thicknesses, you can specify glass with an extremely low Critical Frequency which improves the acoustic reduction of the glass and improves the corrected (Rw(C;Ctr)) acoustic insulation values of the glass.
You can view the typical acoustic values of some standard glass specifications here. When specifying noise reduction glass you must also consider that the acoustic value of a whole window installation will depend on the frame performance as well as the glass to create an overall Rw value.
Sound Proof Glass
To create a ‘sound proof’ material you would want the critical frequency to be below 100 Hz. For a 4mm thick piece of toughened glass the critical frequency is 3000Hz, noises at this frequency are able to pass through the glass very easily. For the critical frequency to be below 100Hz, the toughened glass would have to be 1200mm thick. To prevent making glass with this thickness, laminated glass is used for acoustic reduction. The glass can be laminated with specially designed acoustic interlayers which will reduce the critical frequency even further.
By using these specially designed acoustic laminates, joined with double or tripled glazed units, you can specify glass with an extremely low Critical Frequency which improves the acoustic reduction of the glass.
To create sound proof glass the specification of glass will require to be quite thick, normally constructed using up to three layers of glass at different thicknesses and with different cavity depths. If any glass doors or opening elements are required then the Rw value for the glass and frame will be required and a good acoustic seal will be required.
Contact the team at IQ Glass if you have any more questions about choosing a sound reducing glass unit for your project.
01494 722 880