Air noise attenuation in ventilation shafts

The ventilation shafts (usually for bathrooms) are composed of architectural solutions coming to an end, but the care of the unpleasant and reciprocal noise transmissions between autonomous units should be monitored and combated, under penalty of causing a severe environmental discomfort.

If on one hand the openings (windows) are necessary to promote adequate ventilation, on the other hand they are composed of sound sources and/or receptors, once the sound is spread in all directions, as below:

Due to the differences in the paths of sound propagation, the discomfort is increased if you listen to the same sound in two or more different times: either directly or through multiple reflections within these amplifiers. This classic issue of compatibility of natural ventilation with air noise block, in volumes that make clear the pipes, is of delicate solution and must be properly applied two basic concepts for the attenuation of noise to acceptable levels: noise absorption and noise barriers.

The sound absorption proceeds according to the partial attenuation of sound energy incident on the walls. Attention should be paid to the fact that the acoustic absorption rates of the coatings to be used are related to sound energy and not to decibels (dB), as exemplified below:

The noise barriers are structures positioned between the noise source and the receiver. In this case attention should be paid on the principle of diffraction of sound that, touching a suitable acoustic barrier mass, passes to be heard at different distances from its other side, according to the frequency band.

As the case in question refers to air noises, we should look for medium and high frequencies (from 500Hz), as shown below:

The correct combination of these two concepts will promote noise attenuation at satisfactory levels according to each specific case:

We still have the option of making use of individual attenuators of noise with the scantlings, more commonly used when we have isolated interventions, without any prejudice of the ventilation:

Impact noise on reinforced concrete slabs

Any impact on any hard surface produces vibrations that generate noise perceptible to the human ear from the frequency 20Hz (low frequency).

Our object of study is, then, the attenuation of such effects. The vibratory process of a concrete slab promotes the transfer of vibrations to the supports. These supports, once connected to the walls of the immediately below ground, lead them to vibrate, becoming secondary sources of noise:

In these terms, the adoption of coating on the lower floor contributes little or nothing for the attenuation of such noises. And, depending on the distance of this coating from the slab, the system could become instrument of resonance (drum), whose function is to increase the noise:

To the extent that the opening between the slab supports increase and maintains its thickness, its stiffness is reduced, making it more prone to vibrate under small impacts (and to transmit). And the inability to substantially increase its mass, it is imperative to adopt elastic foundation under the floors:

In the lack of interest in the adoption of the above and possible interest of using ribbed slabs of any kind (small copings of concrete - the ribs only have structural function), it is imperative the use of floating floors:

High density Rockwool, high density glass wool, geotextile cloth, rubber, extruded polystyrene and expanded polyethylene and others, if sized properly, can become good elastic bases.

In the case of use of coating (usually plasterboard) on the slab, creating finishing and hiding potential electrical and/or hydraulic installations, it is vital that it is disconnected from the walls and to carry out a substantial acoustic absorption on it, convenient with the vibration process of the whole, otherwise the system becomes an echo chamber (emphasizing the noise).

For the discussion in the preceding paragraph, any material or low-frequency acoustic absorber fit, provided it is properly sized. Absorbent foams are not suitable in this case, since they are usually excellent absorbers mainly in medium and high frequencies, and the issue is of low frequency noises.

Air noise through walls

A sound pressure incident on a wall, for example, will induce a vibration process. And it is this vibration process that will generate, on the other side of that wall, a secondary source of noise. This is the beginning of the sounds/noises transmissions.

That said it is concluded that the mass of any wall is directly related to their capacity of acoustic insulation: the smaller the mass of a wall, the greater the chance of the vibration of it and subsequently to transmit. Then the opposite is also true.

At a frequency of 500Hz, the sound insulation based on their surface density is given by 20 x log10(δS).

An important aspect in relation to the ability of the material systems consists of generate empty spaces inside, or filled with noise absorbent material, as shown below:

Walls then composed just of two plasterboard plates (12,5mm x 2 = 25mm) with an air chamber between the plates, promote a sound insulation of the order of 29dB (500Hz), provided that these are completely sealed: insufficient in the case of a bedroom.

However, from the use of elastic material in it (mineral wool, preferably of high density), this insulation may reach a level of 35dB (500 Hz): sufficient, but the limit for bedrooms.

A little relevance factor that may be considered in the case consists of the attenuation of noise produced in the environments adjacent to the bedrooms. This will occur because of the substantial increase of its various absorbent surfaces (floor, walls, ceiling, furniture, fabrics, people, etc.). But this issue is object of an appropriate text.

Domestic noise

Long ago, since the second half of the last century, studies on issues of noise and its effects on humans are being conducted and published internationally, fueled by the World Health Organization.

The doctors, phonoaudiologysts, and other specialists already have an extensive literature on the damage caused by this by-product of the progress:

It doesn´t matter the source: if is the aircraft noise, urban noise, industrial noise, in large offices or shopping malls and even in isolated rooms (appliances in operation), the noise bothers and affect the health: this is an indisputable fact, widely studied and scientifically proven.

And at night, after a stressful day? At the moment we think we´re going to sleep, at which time the level of urban noise is low and usually all are sleeping in the apartment, we start to realize that there´s other noise sources, that according to the level of sensitivity of the receiver, are highly irritating accessories: the “knock knock” of the shoes of your upstairs neighbor, distasteful and inappropriate conversations that are herd through the bathroom windows in front of the ventilation shafts, the flow of the fluid from the discharge of a toilet, the drift made by a car in the next street and many others.

Arch-tec also concerned with these “minor” issues. Tests and studies were carried out in recent years, Conferences were uttered and none were dispute, and now develops its own technology. Usually, for inherent technical reasons in each specific case, one can´t completely block these disorders, but a satisfactory attenuation/noise decrease is achieved.


Projects and perfomances of acoustic insulation and conditioning:

The acoustic solutions adopted in projects involve both the use of industrial products (domestic and imported) as the other elements produced by Arch-Tec specifically for each case.