Saturday, 16 February 2013

Discover how one can improve noise free comfort at home………..








Noise is increasing day-by-day, especially in metros, and is adversely contributing to the health of people. From hearing loss to sudden change in behavioral pattern, doctors say, noise pollution is just adding to the existing stress levels.And those who gets it a lot, doctors say suffer from fatigue, lack of concentration, irritability, tinnitus (constant ringing sounds), and gradual hearing loss among other health problem. “Over- exposure to loud sound can result in temporary or permanent hearing loss is gradual, it leads to headache and fatigue. As the person is not to able to hear properly, he has to strain more to hear which often leads to headache and lack of concentration, “said Dr. Atul Mittal Senior consultant surgeon, ENT, Max Healthcare. (Source : Times of India : New Delhi Edition published : 7 April 2007)

However with proper glass & window combination you can protect your home from this disturbance and reduce noise pollution thanks to  uPVC window! At here, working with different specialists creates many opportunities to learn from each other. Sometimes one forgets that engineers, architects & consultants who know everything about dark matters as wind loads in glass or intricacies of structural elements may not have a clue about the acoustic performance of a window.
That’s why these questions keep coming to my desk (remember I’m a professional in the Glazing world): what effect does glass thickness have in the acoustics of a double glass unit? Or what matters more in the acoustical performance of insulated glass: the thickness in a monolithic pane, the effect of lamination or the dimension of the cavity? Here you will find some answers to these questions. As usual a number of hidden surprises will come out from the data mining.
Noise explained
Sound, a form of energy, is caused by molecules vibrating in a gas, liquid or solid. These vibrations are known as sound waves.The frequency or pitch of a noise, measured in hertz, is the number of sound waves emitted per second. High pitched sounds are carried by short sound waves and low pitched noises by longer waves. The actual level or intensity of noise is measured in decibels (dB).
Improving acoustic insulation
Acoustic insulation works by reducing a sound wave’s energy with proper combination of glass & window. The acoustic insulation properties of a window are measured with the ‘R’ sound reduction index. For example, a window with an R of 20 decibels should reduce a 60 dB outside traffic noise level to 40 dB within the room.
Noise pollution can be in any combination of low, medium or high frequency sounds. Some types of frequencies are easier to block or reduce. High pitched sounds (carried by short sound waves) are easier to absorb but reducing low frequency noise such as traffic can be more difficult. It is important to select the acoustic insulation properties of the window required according to the frequency or pitch of the noise to be blocked, as well as the desired number of decibels to be reduced.

Improving the sound insulation of a window can be achieved by:
  • Having the widest possible cavity between panes of glass
  • Using thicker glass
  • Differing the thicknesses of the two glass panes used
  • Using an efficient insulating window frame
  • Using specially laminated acoustic glass

Acoustic glass is a sandwich of two or more sheets of glass, heat or pressure bonded together with one or more acoustic polyvinyl butyral (PVB) inter-layers The inter-layers act as a noise damper, weakening the energy of the sound waves as they travel through the acoustic glass.
Some useful values
Rw index: The Rw index or sound reduction index (expressed in decibels) measures, in just one number, the acoustic performance of a specific glass unit. The higher the Rw index, the better the level of acoustic insulation offered by that glass composition. The Rw index of ordinary double glazing is around 29 dB whereas a good acoustic inter-layer offers an Rw index of around 50 dB.
Rw is a single figure rating for the airborne sound insulation of building elements (not just glass). It includes a weighting for the human ear and measures actual sound transmittance. Rw is measured in a laboratory, not on site (the site-measured equivalent value has the Egyptian denomination of DnT,W). The Rw value is merely an average simplifying mutual comparison of various building components. That can be confusing some times. Two glass units can have the same Rw index while one of them performs well at low frequencies and bad at high ones, and the other one performs just the opposite.
C and Ctr factors: To slightly avoid this issue two spectrum adjustment factors: C and Ctr, have been added to modulate the Rw average. For sound waves featuring high frequencies, the factor C is added to the Rw value. For lower frequencies, factor Ctrneeds to be added. The acoustic behaviour of a building component is hence defined by three numbers: Rw (C, Ctr). A building component with the values Rw (C, Ctr) = 40 (-1, -4) provides an average insulation performance of 40 dB. For higher pitched sounds the sound insulation is lessened by 1 dB (39 dB) and for lower pitched sound sources it is lessened by 4 dB (36 dB).
The table below, extracted from Saint Gobain, helps showing how these three numbers apply to different laminated units with acoustic interlayers:
C takes into account medium and high frequency noise sources such as TV, music, loud conversations or aircraft noise a short distance away. Ctr takes into account medium and low frequency noise sources such as urban traffic noise or aircraft noise a long distance away.
Pink Noise:  Expressed in dB(A), this is an assessment of the sound insulating properties of a building material over specified standard frequencies, which represent general activity noise when equal levels of power are applied at each frequency. So, in pink noise each octave carries an equal amount of noise power. Funnily: the name arises from the pink appearance of visible light with this power spectrum.
Ra:  Ra is the abbreviation for the sound reduction index when the spectrum adaptation term C is applied to the single number weighted sound reduction index (Rw), using pink noise as a sound source.
Ra,tr:  Ra,tr is the abbreviation for the sound reduction index when the spectrum adaptation term Ctr is applied to the single number weighted sound reduction index (Rw)using pink noise as a sound source.

Four acoustic terms you need to be familiar with:
  • Reverberation
  • Reflections
  • Absorption – Noise Reduction Coefficient (NRC)
  • Isolation – Sound Transmission Class (STC)
Reverberation:
In an enclosed space, when a sound source stops emitting energy, it takes some time for the sound to become inaudible. This prolongation of the sound in the room caused by continued multiple reflections is called reverberation.
Reverberation time plays a crucial role in the quality of music and the ability to understand speech in a given space. When room surfaces are highly reflective, sound continues to reflect or reverberate. The effect of this condition is described as a live space with a long reverberation time. A high reverberation time will cause a build-up of the noise level in a space. The effects of reverberation time on a given space are crucial to musical conditions and understanding speech. It is difficult to choose an optimum reverberation time in a multi-function space, as different uses require different reverberation times. A reverberation time that is optimum for a music program could be disastrous to the intelligibility of the spoken word.
Conversely, a reverberation time that is excellent for speech can cause music
to sound dry and flat.

Reflections:
Reflected sound strikes a surface or several surfaces before reaching the receiver. These reflections can have unwanted
or even disastrous consequences. Although reverberation is due to continued multiple reflections, controlling the Reverberation Time in a space does not ensure the space will be free from problems from reflections.
Reflective corners or peaked ceilings can create a “megaphone” effect potentially causing annoying reflections and loud spaces. Reflective parallel surfaces lend themselves to a unique acoustical problem called standing waves, creating a “fluttering” of sound between the two surfaces.
Reflections can be attributed to the shape of the space as well as the material on the surfaces. Domes and concave surfaces cause reflections to be focused rather than dispersed which can cause annoying sound reflections. Absorptive surface treatments can help to eliminate both reverberation and reflection problems.
Noise Reduction Coefficient (NRC):
The Noise Reduction Coefficient (NRC) is a single-number index for rating how absorptive a particular material is. Although the standard is often abused, it is simply the average of the mid-frequency sound absorption coefficients (250, 500, 1000 and 2000 Hertz rounded to the nearest 5%). The NRC gives no information as to how absorptive a material is in the low and high frequencies, nor does it have anything to do with the material’s barrier effect (STC).
Sound Transmission Class (STC):

The Sound Transmission Class (STC) is a single-number rating of a material’s or assembly’s barrier effect. Higher STC values are more efficient for reducing sound transmission. For example, loud speech can
be understood fairly well through an STC 30 wall but should not be audible through an STC 60 wall. The rating assesses the airborne sound transmission performance at a range of frequencies from 125 Hertz to 4000 Hertz. This range is consistent with the frequency range of speech. The STC rating does not assess the low frequency sound transfer. Special consideration must be given to spaces where the noise transfer concern is other than speech, such as mechanical equipment or music.
Even with a high STC rating, any penetration, air-gap, or “flanking” path can seriously degrade the isolation quality of a wall. Flanking paths are the means for sound to transfer from one space to another other than through the wall. Sound can flank over, under, or around a wall. Sound can also travel through common ductwork, plumbing or corridors.
So far so good. Acoustic performance of glass & Window should now be less of a dark matter for us. But this is not all: remember that detailing to achieve a proper air tightness between glass and frame will always be required! Loose gaskets can severely harm the best glass selection for acoustics…

3 comments:

  1. Hi Shivendra,

    Need your expert advice. I am looking for sound proofing solution for my home in Delhi which is barely 10 meters away from busy street and there is traffic noise like honking, loud music. The below 2 UPVC options have been suggested to me:

    1. 12mm clear+16mm air+8mm clear Toughened bothsides. Total 36mm

    2. 6mm+1.5 PVB layer +6mm+10mm air+6mm. Total 29.5 mm

    Which option is better for reducing traffic noise?

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    Replies
    1. Dear Mr. Singh,

      As per my calculation, both glasses has same kind of noise insulation against traffic noise i.e. approx. 35 dB. Please note that it is estimated value & in-situ performance may vary.

      But this much reduction is good enough to perceive as 80% noise reduction depending also on Door/Window design. Pl go for casement windows rather than sliding for noise insulation & also multi point locking in all Door/window.(Performance characteristics of Door/window is covered in air permeability classes but unfortunately no MNC or, Indian company in uPVC Door/window is sharing these data which is best practice guidelines for Quality Door/Window systems.)

      Regards,

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