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Sound Reduction Explained

Posted By Avalon Construction & Design on November 9th, 2012 in category.

Sound is measured in decibels (Db)

… the scale of audible sound from one extreme to the next might be pictorially represented below

Sound Reduction Explained

It should be made clear that the scale that is used is not linear it is logarithmic. In simplified terms this means that one sound that is 10db louder than another is in fact TWICE as loud. Similarly a difference of 15db is THREE times as loud.

When dealing with sound reduction achieving over 10 db in sound reduction will have a very material affect on the sound one hears. So that if a sound reduction of 10db is achieved then the sound will appear half as loud as before.

Sound Reduction Index ( SRI )

In practice trying to describe soundproofing and what the difference between the sound you hear pre and post soundproofing is difficult to describe so it can be thought of using a different scale this is known as the sound reduction index scale. So looking at the table below you might determine where you are now on the scale. For example you might be able to distinguish loud speech from next door in your living room this would equate to possibly -35db on the scale. Then by adding wall solution 1 to this you might improve the situation by 13db taking you to nearer 50db privacy level, now loud speech or shouting could be heard with great difficulty.

Throughout the site we talk in terms of decibel improvement . Which is not to be confused with a figure which is often quoted as the total overall soundproofing of the entire structure post soundproofing. In the soundproofing world a new term SRI is used to describe the quality of sound proofing materials. The greater the SRI, the greater the acoustic privacy that is achieved.

The chart below demonstrates how this acoustic privacy works in layman’s terms.

So Soundstop wall solutions aim to increase the SRI number by 13db+ ( depending on the thickness of the board) on top of the original level of insulation (40-50 db).

While Soundstop floor and ceiling solutions aim to increase SRI by around 17-20db on top of the original level of insulation (35 db).

When one talks about SRI values there are a couple of other factors to bear in mind:

Sound Frequency

Sound is made up of a whole range of frequencies. Different frequencies travel through different transmission barriers with different degrees of ease. So any SRI figure given is known as the weighted or average figure (also known as Rw). Low frequency sounds are of a longer wavelength and tougher to soundproof against.

Site Conditions

The Rw quoted in most literature is obtained in sound testing laboratories. In practice different buildings have different sound insulation properties. This is due to the different materials used in construction and the different styles of construction. So when adding sound insulation the exact results will vary. This variance is mainly due to flanking transmission

Flanking transmission – Particularly Breeze Block Constructions

Flanking sound transmission is the term used to describe the movement of sound through the structures that flank the barrier that your trying to sound proof. In the case of wall insulation the flanking structures are the floors and ceilings. In the case of soundproofing floors and ceilings, its the walls.

In designing for sound insulation care should be taken to ensure that flanking transmission via the associated structure does not downgrade the performance of the partition or wall to a level below that required in use. In practice we know that breeze block partitions and breeze block structures that are attached to the party walls will carry sound very easily. Where breeze block exists soundproofing of all breeze block surfaces must be considered as simply blocking the offending wall may not block the passage of sound. Brick structures do not typically suffer from this problem.

Attention to the detail

When applying any soundproofing solution think of the analogy that sounds flows like water. It is absolutely imperative that you remember to fill any hole that sound can flow through. It will leak through tiny holes just the same way water would. Think airtight! Even a hole 1% of the area you are trying to soundproof can reduce the efficiency of the soundproofing by up to 10%. For this reason sunken spotlights can completely degrade any soundproofing work within a ceiling. Acoustic hoods should then be considered.

Impact sound and Airborne sound

To make life complicated the measurement of airborne and impact sound is measured in different ways:

Airborne sound – when we talk about airborne sound the higher the Rw figure achieved the better the sound insulation. In the wall example we are trying to achieve a figure over 50db.

Impact Sound – when we talk about impact sound the lower the Lw figure achieved the better the sound insulation

By Avalon Construction & Design: November 9th, 2012 in category.