Frequently Asked Questions - Noise Control

Frequently Asked Questions - Noise control

1 What is Rw?

The Rw figure is a measure of the performance of the glass across a range of frequencies adjusted for the response of the human ear.


What is C and Ctr?

C and Ctr are the spectrum adjustments, which are the values added to Rw to take account of the characteristics of particular sound spectra.
Typical noise sources for each spectrum adaptation terms are given below.

Relevant spectrum adaptation term C

Type of noise source:
• Living activities (talking, music, radio, TV)
• Children playing
• Railway traffic at medium and high speed
• Jet traffic, short distance away
• Motorway traffic >50 mph
• Factories emitting mainly medium and high frequency noise

Relevant spectrum adaptation term Ctr

Type of noise source:
• Urban road traffic
• Railway traffic at low speeds
• Aircraft, propeller driven
• Jet aircraft, long distance away
• Music with low frequency bass sounds
• Factory emitting mainly low and medium frequency noise

3 What is Rtra?

The Rtra figure is a measure of the performance of the glass across a range of frequencies where there is increased sound pressure from lower frequencies. This makes this scale more appropriate for noise sources like road traffic.


What is LAeq?

This is an averaged measurement of a noise source over a period used by acoustic consultants to typically arrive at an exterior noise level for use in the design of the building.


How is the glass performance expressed?

The ability of glass to reduce noise is expressed in a number of ways. It can be shown by frequency in third octave bands or (more commonly) octave bands, i.e. the ability of glass to reduce noise at each frequency. It can also be shown as single figures like Rw with adjustments for other noise sources such as road traffic.


How is glass performance measured?

A sample of glass is glazed in an acoustic test house where it is subjected to various frequencies. Sensitive microphones compare the source side with the receiving side to produce a plot and measurement of the effect of the glass at each frequency. The chamber is calibrated to ensure that it is the glass performance only that is being measured. We use the data to present the performance by frequency band and by the shortforms such as Rw and Rtra. The figures show the attenuation.

7 How do I deal with aircraft noise?

Not all aircraft make the same noise. Propeller driven planes are different to military jets and commercial jets are different again. An approximation is possible for adjusting the sound of commercial jets sound to equate to road traffic. In some cases a professional survey is the best way to determine the noise levels at the site.


How do I deal with road noise?

Use the Rtra scale to arrive at an acceptable internal noise level. Note that a level of background noise is desirable and you should not aim for zero noise inside.


How do I deal with rail noise?

Rail traffic has a high frequency element when compared to road traffic. An approximation can be made in the absence of a proper noise survey.


Does distance have an effect?

Yes, noise decays with distance. This can be calculated and factored into a calculation. The surrounding buildings can influence the noise level and road traffic noise can be a nuisance in tall buildings due to channeling effects and the incidence of the sound to the facade.


Is double glazing an answer?

Not necessarily. If the glass panes are of the same thickness then probably not. Most people claim an improvement but it is due to better seals around the windows rather than having two panes of glass. Double glazed insulating units does allow you to mix the glass thickness and to use specialist acoustic products like Pilkington Optiphon™ to get substantial improvements.


How do special laminates work?

Acoustic laminates tend to have a softer interlayer that changes the resonant frequency of the glass and decouples the two panes in the laminate slightly.


Does the gas in an IGU make any difference?

Air, krypton and argon make no difference.


Does cavity size make a difference?

For the range of common insulating glass cavities available then, no.
Large gaps (60 mm plus) between glass and secondary glazing do have an effect.


What are third octave and octave bands?

Noise is measured by frequency band. The gap between the bands can be a third of an octave or a complete octave.


How quiet should a room be?

This depends on the use of the room. You would expect a background noise in a library to be around 30 dB. If you make a room too quiet then you begin to listen for other noise sources that you would not have picked up before. Go for a noise level appropriate to the use of the room. No noise is an eerie sensation.


Who can measure noise levels?

Anyone with a noise meter. There is an association of noise consultants and its members will have the knowledge to do a noise survey and present the information in a proper and useful way. Inexperienced use of a meter will not produce information that can be relied upon.


What are dBA?

This is an adjusted measurement of noise that takes into account the human sensitivity to the perception of the nuisance of noise by frequency. Certain frequencies irritate more than others. As an example think of fingernails dragging across a blackboard.

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