Difference between acoustic absorption and sound insulation

Estos dos conceptos son utilizados indistintamente cuando no debería ser así.

In this post we will try to clarify these concepts and move them to everyday situations to help understand them better.

We begin with the sound absorption, this is a property that present all materials which absorb part of the acoustic energy index on them in the form of sound pressure waves, transforming it into other forms of energy (usually heat).

The following graph (obtained courtesy of World of acoustic) illustrates these concepts, part of the incident energy (Ei) is reflected (Er), another part is absorbed by the material (Ea) and some is transmitted (Et), fulfilling provided that Ei = Ea + Er + Et

Most of the materials identified with the label "absorbent material" acoustic property should this entrapped air within as small cells. The acoustic energy is converted to heat energy due to rubbing. The advantage of such materials is that their good properties as combine sound absorbers with a reduced weight due to its greater part are formed by air. Examples of such materials are rock wool and glass widely used in construction.

The manufacturers of absorbent materials apport the absorption coefficient curves in function of the frequency of the incident sound wave, ls usual values ​​of the absorption coefficient is in the order of 0.9 to 0.95 for medium and high frequencies. This means that 90% of the incident acoustic energy is dissipated by the material in this frequency range.

Although acoustic absorbing materials dissipate a high percentage of incident sound energy by themselves do not solve the problems of acoustic isolation

Sometimes absorbent materials are attached directly to the walls of the enclosures to be treated, this treatment improves acoustics barely inside the premises diminishing its reverb but does not improve the sound insulation of the fences. This is due to the nonlinear nature of the sound.

But what does it mean to "non-linear"?, I will try to explain as clearly as possible with an example. If you have a stereo emitting a sound level of 60 dB and bring another team to 60 dB emitting identical the result will be 120 dB (thank God) but will have only 63 dB.

If the same thing happens absorbent materials, dissipating much energy even fail to dissipate enough to get rid of the noise transmitted to the trouble (just cut a few decibels of noise incident).

Absorbent materials are useful in architectural acoustics, but as we will see always in combination with other materials.

With regard to acoustic insulation property is presented a material or set of materials that form a closure to prevent the sound passing through ensuring the comfort across the enclosure in which sound is generated.

The mechanism of action is as follows: part of incident sound wave is reflected by the material while another portion thereof is irradiated by the material in the form of vibration, this vibration causes starts moving the air in the side material opposite the sound. A fraction of the energy that is dissipated through the material.

The sound insulation of a material has a strong dependence on the mass because the heavier material greater fraction of incident sound energy is reflected.

The materials have different characteristics in terms of energy dissipation through them, resulting generally best for soundproofing those which combine a high mass with a high energy dissipation capability.

The best results in terms of soundproofing materials are combined with consequent high surface mass with absorbent materials.

Acoustic absorption coefficients (II)

For certain types of rooms used to be occupied by a large number of people in its design question arises how will its acoustic behavior in different conditions for optimal occupancy of the same design.

It is necessary to estimate the sound absorption provided by people and furniture from the corresponding sound absorption coefficients.

 

We complete the table acoustic absorption coefficients publishedwith the following table:

Materials	Coefficients
	125Hz	250Hz	500Hz	1000Hz	2000Hz	4000Hz
Area of empty chairs with upholstered high percentage of surface	0.72	0.79	0.83	0.84	0.83	0.79
Area of empty chairs with upholstered surface average percentage	0.56	0.64	0.70	0.72	0.68	0.62
Area of empty chairs with upholstered surface low percentage	0.35	0.45	0.57	0.61	0.59	0.55
Area occupied chairs with upholstered high percentage of surface	0.76	0.83	0.88	0.91	0.91	0.89
Area occupied chairs with upholstered surface average percentage	0.68	0.75	0.82	0.85	0.85	0.86
Area occupied chairs with upholstered surface low percentage	0.56	0.68	0.79	0.83	0.83	0.86
Wooden seat	0.01	0.02	0.03	0.04	0.06	0.08
Wooden seat occupied	0.34	0.39	0.44	0.54	0.56	0.56
Seat upholstered with leather or vinyl	0.10	0.15	0.25	0.25	0.25	0.25
Seat upholstered with plastic	0.20	0.20	0.25	0.30	0.30	0.30
Seat upholstered with velvet	0.30	0.32	0.27	0.30	0.33	0.33
Group of people standing	0.25	0.44	0.59	0.56	0.62	0.50

How to soundproof a room

Neighborhood noises are one of the main reasons for dissatisfaction and discord in neighboring communities.

Different customs, schedules and lifestyles of people living together with insufficient acoustic performance of buildings cause most of the noise nuisance.

In this post I will raise a solution for soundproofing a room in order to avoid disturbing the neighbors.

The main objective should be to achieve maximum possible dissipation of sound energy transmitted to adjacent enclosures.

First indicate that the solution to such problems is to treat all surfaces of the room and not only shared with the site/s to which you want to avoid transmitting sound.

A possible proposal for soundproofing a room is as follows:

Acoustic treatment room ceiling

For the treatment room ceiling we propose the implementation of a ceiling with  gypsum plaster boards. This solution will decrease by at least 15 cm the room's height.

The construction details of the proposed solution is as follows:

1- Gypsum plasterboards.

2- Absorbent material (mineral wool 40 mm thick).

3- Metal fork

4- Multilayer panel.

5- Wrought

The steps for implementation are as follows:

Covering the surface of the original roof with panels that combine a multilayer composite foil which combine a viscoelastic sheet  with mineral wool at least 4 cm in thickness. These panels will be placed to the side of viscoelastic sheet seen. I leave two links of such commercial products:

• ChovACUSTIC 65 LR 70/4
• Acustidan 16/2

Through this type of solution the sound reduction index of a unidirectional wrought based 30 cm beams can be improved by 15 dB , vaults and compression layer of reinforced concrete.

Then run the metal frame for fixing plasterboard plates following these steps:

• Attach the threaded rods of the roof joists.
• Place the metal brackets for attaching the metal profiles.
• Place the metal sections that join the gypsum plasterboards.



Aliminium profiles structure

I leave some links of such commercial products:

• Catálogo Pladur
• THU

Screw plasterboard plates to metal profiles seal the joint between plates.

Plasterboard plates bolted to metal profiles

Place mineral wool thickness of 40-50 mm on plaster boards and metal profiles.

Sound absorbing material

I leave some links of such commercial products:

• URSA Glasswool
• Rockwool Rockcalm E-211
• Knauf Ultracoustic
• Isover 40 y 40R

Acoustical treatment of walls

When a room is to soundproof the walls of the enclosure require acoustic treatment to prevent the transmission of noise to adjacent enclosures.

A common mistake is to consider that only other premises adjoining enclosures require treatment.

Here we propose a cladding with the following elements:

• Viscolelastic sheet.
• Mineral wool.
• Plasterboard plates bolted to aluminum profiles.

The following figure shows a construction detail:

Cladding detail

1. Viscoelastic sheet.
2. Mineral wool.
3. Plasterboard plates.

The implementation process is as follows:

First you must remove the baseboard and then set the sheet with adhesive on the surface of the treated wall.

These films improve insulation to airborne sound especially in the low frequency spectrum. Sheets are recommended with a minimum mass of 6 kg/m2.

The sheets are supplied in rolls, usually 1 m wide, which may present the adhesive already incorporated.

The installation mode is recommended from the bottom up, occupying the full height of the wall. The different sections should overlap each other by about 2 cm.

I leave two links of such commercial products:

• VISCOLAM 65
• Membrana acústica M.A.D. 4

This is followed by placing the metal profiles, which must be removed about 2 cm of the sheet, to enable the creation of a small air chamber. Profiles must be assembled with a modulation of 60 cm.



This is followed by the insertion of the acoustic absorption material between the uprights. It can be used mineral wool thickness of 40 mm and a density of 35-40 kg/m3. Its function is to dissipate the sound waves that are stationary in the air. To perform this function should allow air flow through its interior, so that the sound energy is converted into heat energy by the effect of friction. The use of higher density mineral wool does not provide better performance. Thereof may be employed absorbent materials suitable for the roof.

Finally we proceed to the junction of the plates by screwing plasterboard to metal profiles.

Floor acoustic treatment

Last but not least, you should proceed to the acoustic treatment of the room floor.

The aim is to prevent the transmission of noise to the walls of the room. This requires executing a floating floor avoiding any rigid connection with the walls.

We propose as a platform floating placed directly on battens:



1- Wrought.

2- Absorbent material 40 mm thick.

3- Polyethylene sheet.

4- Battens.

5- Prints laminated flooring.

This constructive solution can be run directly on the floor or on the existent slab. The only thing to keep in mind is that the area that serves as support does'nt present slopes greater than 1 cm.

The steps in implementation are the following::

First play must verify that the supporting surface is clean, free of defects and with a height of less than 1 cm.

Adhere to the two faces of each batten crosslinked polyethylene layer, uniformly distributed over the surface battens spaced about 60 cm and fix the cleat and lag wrought by each 90 cm.

I leave two links of such commercial products:

• Chova Elastoband 50
• Danosa Bandas Fonodan

Fixing the battens to the slab

Then absorbing material is placed between the strips. Thereof may be employed absorbent materials suitable for the roof and walls.

Acoustic absorption material

Finally set the stage blades on battens, in the encounter with the walls should be placed polyethylene band which is placed on the baseboard.

Floorboards on battens

Constructive solutions described allow a substantial improvement in airborne sound insulation in walls of a room without a significant loss of usable space in the same.

Sound absorption coefficients for various materials

The following table shows the sound absorption coefficients as a function of frequency for various materials commonly employed in construction:

Materials	Coefficients
	125Hz	250Hz	500Hz	1000Hz	2000Hz	4000Hz
Brick, unplastered	0.03	0.03	0.03	0.04	0.05	0.07
Brick, without plastering, painting	0.01	0.01	0.02	0.02	0.02	0.03
Lime and sand plaster	0.04	0.05	0.06	0.08	0.04	0.06
Plasterboard	0.29	0.10	0.05	0.04	0.07	0.09
Carpet on concrete	0.02	0.06	0.14	0.37	0.60	0.65
Porous lightweight concrete block	0.36	0.44	0.31	0.29	0.39	0.25
Painted concrete block	0.10	0.05	0.06	0.07	0.09	0.08
Concrete or terrazzo flooring	0.01	0.01	0.015	0.02	0.02	0.02
Marble tiles	0.01	0.01	0.01	0.01	0.02	0.02
Wood	0.15	0.11	0.10	0.07	0.06	0.07
Plywood panel thickness of 1 cm	0.28	0.22	0.17	0.09	0.10	0.11
Chipboard panel	0.47	0.52	0.50	0.55	0.58	0.63
Hardwood	0.04	0.04	0.07	0.06	0.06	0.07
Hardwood on concrete	0.04	0.04	0.07	0.06	0.06	0.07
On hardwood slats	0.20	0.15	0.12	0.10	0.10	0.07
Rubber carpet 0.5 cm thick	0.04	0.04	0.08	0.12	0.03	0.10
Curtain 475 g/m2	0.07	0.31	0.49	0.75	0.70	0.60
Polyurethane foam 35 mm (Fonac)	0.11	0.14	0.36	0.82	0.90	0.97
Polyurethane foam 50 mm (Fonac)	0.15	0.25	0.50	0.94	0.92	0.99
Polyurethane foam 75 mm (Fonac)	0.17	0.44	0.99	1.00	1.00	1.00
Polyurethane foam 35 mm (Sonex)	0.06	0.20	0.45	0.71	0.95	0.89
Polyurethane foam50 mm (Sonex)	0.07	0.32	0.72	0.88	0.97	1.00
Polyurethane foam 75 mm (Sonex)	0.13	0.53	0.90	1.00	1.00	1.00
Glass wool 14 kg/m3 and thickness 25 mm	0.15	0.25	0.40	0.50	0.65	0.70
Glass wool 14 kg/m3 and thickness 50 mm	0.25	0.45	0.70	0.80	0.85	0.85
Glass wool of 35 kg/m3 and 25 mm thick	0.20	0.40	0.80	0.90	1.00	1.00
Glass wool of 35 kg/m3 and 50 mm thick	0.30	0.75	1.00	1.00	1.00	1.00
Ordinary window glass	0.35	0.25	0.18	0.12	0.07	0.04
Brick wall plastered with gypsum	0.013	0.015	0.02	0.03	0.04	0.05
Swimming pool surface	0.008	0.008	0.013	0.15	0.020	0.25
Doors and windows opened	1.00	1.00	1.00	1.00	1.00	1.00

Absorption materials

One of the properties that have different sound absorption materials is, defined as the ability to transform sound energy into other energy.

This property is used as materials for insulation and acoustic enclosures for conditioning.

It defines the sound absorption coefficient as the ratio of energy incident on the surface and the energy absorbed by it. A totally reflecting surface would have a sound abortionists coefficient equal to 0 and a totally absorbing surface equal to 1.

The sound absorption properties depend on the frequency of the incident sound wave, usually given as curves of absorption:

We distinguish the following types of materials according to its absorption:

• Resonant materials present their maximum absorption at a given frequency (natural frequency of the material).
• Porous materials: have a higher absorption coefficient with increasing frequency.
• Absorbents as a membrane or panel: convert mechanical sound energy in the deformed wave to be excited by sound. The maximum absorbance for low frequencies.
• Helmholtz resonance: dissipate only a few certain frequencies for which they were designed.

Materials resonant

Are often used as plates and are used in cases where special treatment should be performed at low frequencies and there is a small space.

Its configuration is in the form of sheet or plate which vibrates on a cushion of air.

The absorption coefficient depends on the internal losses of the plate material and frictional losses in mounts. The absorption can be increased by filling the air cavity with absorbent materials.

Porous materials

Such materials have a structure formed by a series of air cavities linked together.

At inicidir the sound wave on the material, a significant portion of its energy penetrates into the interstices, causing the fiber movement and converting the sound energy into kinetic energy. The air enters the cavities occupied by brushing motion with the moving fiber and converting the kinetic energy into heat energy.

Rock wool and glass wool are examples of such materials. Used in combination with rigid materials.

The optimal values of absorption (of the order of 99%) occur for thicknesses that match with 1 / 4 wavelength.

The thicknesses used in practice are constrained by the limitations of space and cost. Usually used thicknesses of 3-4 cm at densities of 70-80 kg/m3.

Hole resonators of Helmholtz

They come in plates as described above, except that it presents a series of perforations in its surface.

The cavities are filled with air in the enclosure through a narrow opening that is  resonator neck. By influencing the sound wave in the air in the cavity causes continuous compressions and rarefactions so that dissipates the energy of the sound wave.

The resonators have high sound absorption values in a narrow range of frequencies, which are used when you want to fix the sound absorption of a compound for these frequencies. In the case of filling the cavity with porous absorption materials lose some of its effectiveness to expand their design frequency range of effectiveness.

Thermoacoustic insulation: open-cell polyurethane

The polyurethane foam is used for its low density and good properties such as thermal and acoustic insulation.

The foam of open cell polyurethane materials are very porous and has an excellent performance regarding sound insulation.

Detail porosity open-cell polyurethane

Its main advantages are the following:

• Low birth weight (10-12 kg/m3).
• Low thermal conductivity (0,035-0.040 W / m K)
• High sealability.
• Easy implementation, possibility of application by projection.
• Good behavior in case of fire.
• Increased durability over other insulation systems.
• Good adhesion to different substrates: brick, wood, concrete, plaster laminate.

Laboratory testing shows that its acoustic features are similar to rock wool and glass wool commonly used, being easier to install and ensuring a better seal cracks and holes that form the bridge acoustic seal encounters with carpentry metal boxes and shutters.

Thermal insulation foam closed-cell polyurethane has better properties than open cell (conductivity 0.028 W / m K) being able to combine both to achieve better performance or Thermoacoustic.

In the next picture can be seen the mode of application:

Implementation of open cell polyurethane projection

But should not make the mistake of applying it to improve soundproofing enclosures already implemented without first analyzing transmisón pathways of sound. It is common to confuse the information provided by the manufacturer on possible improvements in sound reduction index with real improvement when applied in a run in a building enclosure.

In architectural acoustics applications may be employed in improving the sound insulation of facades and interior partitions as in improving the impact sound insulation.

Local soundproofing: acoustic ceilings

The first element of a place that should be considered when trying toimprove the sound insulation of a room is the roof of it.




The first step is to perform a repair sealed it for the purpose of filling the cracks and pores that can deposit the forged like the rest of the room walls to be treated.

Avoid tight junctions between the conduits and the various walls of the room, for elements that can be used as fasteners elastic rubber sheets ...

When performing a local acoustic conditioning must begin with the roof of it. The installation of an acoustic ceiling prevents the transmission of airborne noise through forged.

Good acoustic performance of these roofs is due to the following mechanisms of action:

• Law of mass: the soundproofing of a homogeneous element depends directly on its surface mass. In an acoustic ceiling elements that give mass to all gypsum boards are laminated.

• Elasticity: the sound waves cause vibration of the acoustic ceiling. Rigid connections between roof and structure contribute to the transmission of vibration to other enclosures. The vibrations cause the generation of sound waves in adjacent enclosures. To avoid this effect used so-called roof insulators, these elements are dampers that dissipate the energy of the incident sound waves allowing some movement of the roof.

• Sealing: The junction between the acoustic ceiling and walls of the enclosure should not be rigid, as it conveys the vibrations of the walls to the acoustic ceiling. To resolve this union used rubber bands around the perimeter. Thus ensuring flexible watertight joint between the different elements.

Among the plasterboard is usually placed bituminous foil which provides better insulation at low frequencies and avoids the effect of resonance effects of the plates.

In the cavity between the plasterboard and forged which is based should be placed sound-absorbing mineral wool to improve the sound insulation of the whole. These mineral wool favor the dissipation of waves that form in the air, transforming sound energy into heat energy by friction with the pores of the material.

The following details can be seen the various elements of acoustic ceiling:






The installation of the roof is done through a series of metal sections that are attached to supporting elements through the roof insulators. Plasterboard fixing these metal sections.

In cases that require very high performance can use a cladding for the roof of the local boards as plasterboard.

For the proper design of an acoustic ceiling special attention to the mass of all elements of it. Depending on the mass of the assembly and the type of roof insulators indicates the number of insulators. They must withstand a load as close as possible to its optimal design load provided by the manufacturer.

Insulators should be distributed evenly to avoid supporting various loads, and to prevent the occurrence of buckling of the plasterboard and metal structures that support them.

Once installed, the local acoustic ceiling will proceed to install the cladding on the walls of the enclosure and decorative ceiling is suspended acoustic ceiling of the room. The cavity between the two roofs will pass facilities (air conditioning ducts, electrical ...)