Since the company was founded in 1951, the motto of TROX GmbH has been: "The human being is the yardstick and people's well-being is our goal." And healthy air is a basic prerequisite for our well-being. However, the ventilation and air conditioning of rooms is a complex subject area with many variables - such as acoustics. This is because ventilation and air-conditioning systems generate sound and thus noise through the transport and supply of air, which can be disturbing and even stressful.

Here we provide an insight into acoustics in order to better understand the generation and avoidance of sound and to facilitate planning. Here you can find out how noise can be reduced to a comfortable level and which TROX components are suitable for which case.


Learn more


Learn more


Learn more


Learn more


Learn more


Learn more



If the ventilation system is too loud, this can lead to discomfort, stress, reduced concentration and sleep disturbances - for example in hospitals, hotels, office buildings and schools or other sensitive environments. People such as children or people with existing health problems react particularly sensitively.

Even a low continuous sound level can be perceived as disturbing or unpleasant. Imagine sitting in a concert hall, and the perfect musical experience is accompanied by the background noise of the ventilation system. But especially here, where many people are together in a confined space, mechanical ventilation is indispensable.

Volume flow controllers, shut-off dampers, fans and other moving parts can generate noise. Even the airflow within a unit can cause sound emissions when the air flows at high speed through ducts and outlets. Thus, noise is also generated inside sound attenuators.

These noises, however, can be avoided or reduced to the point where they are barely perceptible through good planning and the installation of suitable components.


Acoustics is the study of sound, including its generation, propagation and effect. It is an interdisciplinary science that deals with the study of all waves in gases, liquids and solids. If you want to avoid unwanted sound emissions, you have to understand how sound is created and how it propagates. We also need to define what types of sound are "unwanted" and when sound becomes noise. So, this is where perception and the measurement of sound come into play, in addition to the measured values.

The music concert mentioned above is usually considered pleasant, although it is usually relatively loud. A mosquito flying in the room, however, is relatively quiet, but is perceived as unpleasant. This shows that individual signals - even if they are quiet - can be perceived as disturbing. Therefore, let's take a closer look at the issue of perception.



Have you ever put your finger on the membrane of a loudspeaker? What you feel are the vibrations of a sound source which - explained in a simplified way - are emitted into the air in the form of minimal pressure changes and then propagate in waves. The sound waves cause our eardrums to vibrate, which triggers the process of hearing in the brain. The stronger the change in pressure, i.e. the greater the deflection (amplitude) of our loudspeaker membrane, the louder the sound is perceived. And the faster the membrane vibrates, the higher the tone of the airborne sound (frequency). The unit of measurement of frequency is “hertz,” or Hz for short, measured in oscillations per second. The energy that is acoustically released with the loudspeaker is the sound power. It defines the source strength of a noise source and is independent of the environment. Sound pressure is the effect of the source that a person hears or that can be detected with a measuring device.



In order to be able to represent the enormously large dynamic spectrum of sound, the physical units of pressure [Pa] and power [Watt] are expressed as levels, related to different reference quantities in decibels [dB]. This produces the sound pressure level Lp and the sound power level Lw, both in dB and in dB(A) respectively, yet fundamentally different.

Since the decibel scale is designed for the human ear, it is not surprising that the lower threshold of human hearing is 0 dB. Signals below 0 dB are nevertheless present and can be perceived by some animals, such as dogs or cats, but also by measuring devices. The pain threshold is around 120 dB, but volumes of 80 dB or more over a longer period of time can also cause permanent hearing damage, such as tinnitus.

In human perception, +10 dB corresponds roughly to a perceived doubling of the volume. So, each of the sounds in the following illustration seems about twice as loud as the previous one!


If several noise sources of the same volume act next to each other, this does not mean that the sound pressure level simply adds up. Rather, the sound pressure level increases by the following values:


When determining the total sound level of several noise sources with different sound levels, the sum level increases depending on the difference between the two sound levels.


Most noises are composed of sound components of different frequencies. Just like the already mentioned music orchestra, where many instruments play at the same time and produce a cumulative sound.

If the sound waves occur in a pure sinus curve, this is called a tone. A sound is always a composition of different harmonic tones. Sounds that are perceived as loud noise consist of an unlimited number of individual tones.

If you want to filter out a certain noise component, you would have to let the instruments play individually in our orchestra. Similarly, one can analyse a noise composed of many frequency components, such as that produced by an air handling unit, and determine the frequency components of the individual parts.


aUDible frequencies are divided into eigt sections

This is where our hearing spectrum comes into play, which covers the frequency range from 20 to 16,000 hertz (Hz), children even up to 20,000 Hz. This area is divided into 8 sections, the so-called octave bands. The relevant octave bands are defined as follows according to VDI 2081:

For the acoustic calculation of a system, the specification of the sound power level in the octave band is essential. Our design programmes provide you with product-specific information on all TROX products. 

The human ear and its characteristics

The human ear is not equally sensitive to all frequencies - low frequencies are perceived worse than high frequencies. Therefore, it is common in technical acoustics to use rating curves that take into account the perceived loudness.

These rating curves are available for different volume levels and applications. The most commonly used is the A-weighting, which ultimately indicates the dB(A) value, which is well known in building services engineering (BSE) applicatins. The relative level (diagram below) is added to the measured value of the respective spectral value. The logarithmic sum of the frequencies is the A-weighted sum level. It is possible to evaluate the sound pressure and/or the sound power.

The diagram shown represents the relative levels of the A-weighting. Another way of weighting measured sound spectra is NC or NR weighting. NC is commonly used in the US or in countries with high US/ASHRAE influence.



Sound usually travels away from its source in a spherical shape. This happens with a speed of 343.2 m/s (1,236 km/h) in dry air at 20 °C. You certainly know the counting rule for estimating the distance of a thunderstorm: 3 seconds between lightning and thunder correspond to approx. 1 kilometre. Around the flash, the sound spreads out in all directions simultaneously, similar to a wave in the water after a stone is thrown. Outdoors, there is only direct sound propagating from the source.

In BSE, this is relevant, for example, for noise at weather louvres and sound levels in the vicinity, which can vary depending on the distance, the level at the source and also the arrangement of the source (free in the room / in a wall / at the edge ...).

Sound reduction in relation to distance (dB) in the free field:


When sound propagates in a room, direct sound and diffuse sound from reflections from the ceiling, walls and floor overlap. Here, sound reduction is strongly dependent on the size and furnishings of the room. These two factors influence the so-called reverberation time of the room. In addition, the arrangement of the source is decisive, which causes bundling or uniform distribution. In the calculations according to VDI 2081, this is taken into account with a direction factor.



The TROX research and development department in Neukirchen-Vluyn has reverberation chambers and a silencer test duct. Here, the measurements relevant to each unit type are carried out with regard to their sound emissions or sound reduction and improved in extensive tests until the desired properties have been achieved.

To objectively assess the sound power of equipment or the insertion loss of sound attenuators, standards apply that allow comparability. They describe the measuring arrangement, measuring technique and the nature of the reverberation chamber. The following standards are used at TROX in the acoustics laboratory:

DIN EN ISO 5135:2020-12 Acoustics - Determination of sound power levels of noise emitted by air diffusers, volume flow controllers, dampers and shut-off elements by measurements in a reverberation chamber (ISO 5135:2020); German version EN ISO 5135:2020

DIN EN ISO 3741:2011-01 Acoustics - Determination of sound power levels and sound energy levels of noise sources using sound pressure measurements - reverberation chamber method of accuracy class 1 (ISO 3741:2010); German version EN ISO 3741:2010

DIN EN ISO 7235:2010-01 Acoustics - Laboratory measurement of ducted silencers - insertion loss, air-regenerated noise and total pressure drop (ISO 7235:2003); German version EN ISO 7235:2009

VDI 2081 Blatt 1:2022-04 Ventilation and air conditioning - noise generation and reduction

[Source: Beuth Verlag GmbH]

For more complex units or equipment components, several measurements may be required to evaluate specific frequency ranges or characteristic noises.


Building acoustics is a separate discipline in acoustics that deals with the effect of structural conditions on the propagation of sound between the rooms of a building, or between the interior of the room and the outside. Building components such as walls, doors, windows and also crossflow elements are described and defined with regard to their sound insulation in order to fulfil certain requirements.


In a project, the sound insulation data of the individual components can be used to calculate the resulting noise insulation of the entire system consisting of wall, door, window and, if applicable, the air path. The weighted sound reduction index Rw is usually used for this. This describes the ability to insulate the sound between two rooms.

In addition to the Rw value of the components, area ratios play a major role in the calculation of the entire system. It is easy to imagine that a large window, for example, has a greater impact than a small window. If the area definition for windows and doors is quite simple, this is not the case for crossflow elements. Here, the manufacturer is free to choose whether, for example, only the open area to the room is to be used, or the entire unit area. Depending on the surface used, the associated Rw value varies. This means that the Rw value is rarely comparable 1:1, as different reference areas are often used. Calculated in the resulting value of the total system, the varying reference areas have no influence, as these are included in the calculations.

Depending on the reference area (dark blue), different insulation values result for the ventilation element - type TROX CFE .       Nevertheless, the insulation value of the wall as a whole remains identical.



In addition to the weighted sound reduction index Rw, there is also the alternative specification of the weighted standard sound level difference Dn,e,w.

The weighted standard sound level difference describes the ability of a component to insulate or attenuate sound - in our example, the CFE crossflow element. Therefore, the standard sound level difference is mainly used for small components (area < 1 m²), whereby the actual area "S" of the component is replaced by a reference area A0 = 10 m².

In contrast to the weighted sound reduction index, the values of the weighted standard sound level difference can be directly compared with each other.

As can be seen clearly, the reference areas S1 (total unit area R=22.7 dB) or S2 (only unit opening R=13.5 dB) lead to different results for the sound insulation value of the CFE overflow element when inserted into the formula. In contrast, the use of a uniform reference value A0 enables comparable values.

  • L1 Energetically averaged sound pressure levels in the transmission room [dB]
  • L2 Energetically averaged sound pressure levels in the receiving room [dB]
  • S Area of the tested component [m2]
  • A equivalent sound absorption area in the receiving room [m2]
  • A0 Reference absorption area 10 m2

However, comparability is provided by the "standard sound level difference" (Dn,e,w). Here, a fixed value of A0 = 10 m2 is defined as the reference area, which is then placed in relation to the total absorption area of the room. The larger the value, the better the damping behaviour of the component.

The standard sound level difference Dn,e describes the capacity of a building component with an area of less than 1 m2 - in our example a CFE crossflow element - to insulate or attenuate sound.

Accordingly, the standard sound level difference is mainly used for small components. Here, the actual area of the component S is replaced by a reference area of A0 = 10 m2



Are you looking for something very specific? Here you will find an overview of all TROX products with sound attenuation properties.

Silencers at a glance



Depending on the area of application and the type of noise source, there are different products that effectively reduce the sound power level.

For example, there are Splitters, which are used in particular to attenuate low frequencies, or those that are particularly good at reducing high frequencies. Different techniques are used for this purpose.

With the absorption silencer, part of the sound energy is converted into heat energy. Porous materials such as mineral wool are suitable for absorption. The effect of sound absorption is enhanced by multiple reflection within the material. An absorption silencer mainly attenuates medium and high frequencies.

A resonance silencer, on the other hand, converts sound energy into kinetic energy. In this process, resonance plates that have no fixed connection to the frame are made to vibrate by the effect of sound. The energy for the oscillation is taken from the sound power. In addition, sound absorbing material behind the resonance plate acts like a damping element. A resonance silencer mainly dampens low frequencies.


If several attenuating elements are integrated directly into the duct next to each other, this is referred to as Splitter sound attenuators. They are often used in central air handling units such as the TROX X-CUBE and attenuate the sound virtually directly at the source. In the process, the air is passed between the splitters as efficiently as possible. The sound absorbing material and/or the resonance plates contained therein extract the energy from the sound.Circular silencers are used in round air ducts. They are also available in curved form for air ducts around or in corners when other options are not available due to limited space.

Secondary silencers are mainly used to reduce air-regenerated noise in control units. They are usually mounted directly behind the control unit in the air duct.

Crossflow elements, such as the TROX CFE, have integrated silencers on an absorption basis. They serve to acoustically separate two rooms. In the process, the air is guided either Z-shaped or T-shaped inside the unit from one room to the other.



TROX uses non-combustible mineral wool with RAL quality mark, which is non-hazardous to health due to its high biosolubility, according to TRGS 905 and EU Directive 97/69/EC. The waste produced during the production of splitters is completely collected and returned to the supplier for recycling (Rockcycle®). These processing and recycling procedures contribute sustainably to relieving the burden on the environment.


To prevent abrasion of the mineral wool from entering the airflow, the absorption material is covered by a very fine glass fibre fabric, which provides effective protection up to max. 20 m/s airflow velocity. The surfaces can be wiped and cleaned with a damp cloth (if accessible). The corresponding products and components are marked VDI6022 compliant. (see products)


Also important in terms of sustainability is the energy efficiency of the silencers. Thus, during development, attention is paid not only to minimised flow losses through profiled frames, but also to particularly high casing tightness. Because by raising the leakage class, additional primary air can be saved.




Silencers can also be used in areas with potentially explosive atmospheres. For example, in ATEX-certified central units of the X-CUBES you will find correspondingly suitable splitters. The ATEX manufacturer's declarations are available on the website for all suitable products. ATEX certified silencers may be used in EX-areas of zone 1, 2 and zone 21, 22 (outside) according to directive 1999/92/EC. "Outside" means that the sound attenuator may be used in the specified areas, but no explosive atmosphere may be passed through the sound attenuator (inside).

Kitchen extract air

Especially for kitchen extract air, high hygiene standards must be maintained. The discharged air must be cleaned via aerosol separators so that suitable extract air conditions are established. For planning as well as installation and maintenance during operation, it is recommended to observe VDI 2052.

Splitters in the standard version, without perforated plate or expanded metal, are particularly suitable for sound attenuation. This is because the glass-fibre laminated surfaces are resistant to grease and acid and very easy to clean. When choosing the material for the silencer, it is best to take the duct system as a guide.

Clean room

Silencers are also frequently used in cleanroom environments. Because in the pharmaceutical industry or in the production of sensitive electronic components, maximum concentration is required. Clean room technology in accordance with VDI2083 can be easily implemented with TROX standard silencers. The following points should be considered:

• The maximum airflow velocity in the silencer should not be exceeded.

• The silencer should not be damaged.

• Before commissioning, a flushing operation of at least two hours should be carried out.

Production halls

Large and open halls often have poor acoustic properties, as the ceiling and wall surfaces reflect sound over long distances, resulting in unpleasant reverberation effects. This not only makes communication more difficult but also has a negative impact on the working atmosphere.

Suspended splitters under ceilings or on walls contribute significantly to reducing reverberation times and can be easily installed. To achieve optimal results, it is recommended to have project-specific calculations carried out by a specialised building acoustician.

production {"X-Frame-Options"=>"SAMEORIGIN", "X-XSS-Protection"=>"1; mode=block", "X-Content-Type-Options"=>"nosniff", "X-Download-Options"=>"noopen", "X-Permitted-Cross-Domain-Policies"=>"none", "Referrer-Policy"=>"strict-origin-when-cross-origin", "Content-Security-Policy"=>"frame-ancestors 'self'", "Strict-Transport-Security"=>"max-age=31536000; includeSubDomains", "Content-Type"=>"text/html; charset=utf-8"}

Share page

Recommend this page

Recommend this page by sending a link by mail.

Fields marked with an (*) are required fields.

Share page

Thank you for your recommendation!

Your recommendation has been sent and should arrive shortly.


We are here for you

Please specify your message and type of request
Tel.: +49 (0)2845 / 202-0 | Fax: +49 (0)2845/202-265

Fields marked with an (*) are required fields.


Thank you for your message!

Your message is send and will be processed shortly.
Our department for Service-Requests will contact you asap.
For general question regarding products or services you can also call:
Tel.: +49 (0)2845 / 202-0 | Fax: +49 (0)2845/202-265


We are here for you

Please specify your message and type of request
Tel.: +49 (0)2845 / 202-0 | Fax: +49 (0)2845/202-265

Attachment (max. 10MB)

Fields marked with an (*) are required fields.


Thank you for your message!

Your message is send and will be processed shortly.
Our department for Service-Requests will contact you asap.
For general question regarding products or services you can also call:
Tel.: +49 (0)2845 / 202-0 | Fax: +49 (0)2845/202-265