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Noise development and noise during ventilation

Basics and definition of sound

Sound is understood as the wave-like propagating vibration in a gaseous, liquid or solid substance that causes noise. The sound frequency (pitch) denotes the number of vibrations of the sound wave per second, measured in Hertz [Hz]. The human hearing range is between 16 Hz and 20,000 Hz, with the upper frequency range decreasing towards lower frequencies with advancing age. This means that adults tend to perceive high notes more poorly than children. Sound waves with frequencies below 16 Hz, which are inaudible to humans, are called infrasound, while those with a frequency of over 20,000 Hz are called ultrasound. These are used by different animal species for communication or orientation.

The human ear, as a sensory organ for picking up sounds, not only perceives them as soft or loud, but also as pleasant or unpleasant. In addition to the physical laws, the physiological relationships due to the subjective hearing sensation are important when evaluating noise.

Schall - der Frequenzbereich

Sound transmission and types of sound

Sound waves cannot propagate in a vacuum, but need a so-called transmission medium whose moving particles can transmit the wave – for example air, water, rock or metal.

“Structure-borne noise” refers to mechanical vibrations that propagate in solid materials. This cannot be perceived directly by the ear, but is emitted by radiation from walls and other surfaces as airborne sound that the human ear can hear. The surface of the body makes the surrounding air vibrate like the membrane of a loudspeaker. Examples of structure-borne noise are: hammering a nail into the wall, dropping objects or climbing stairs (impact sound).

“Airborne sound” is transmitted directly through pressure oscillations in the air, such as when speaking. The sound propagates in the open spherical in all directions and depends on the distance of the sound source, obstacles on the way and the so-called sound reflection, which leads to the creation of an echo.

From sound pressure to sound pressure level

Sound pressure and sensitivity of hearing

The sound pressure level is perceived by the human ear, which changes depending on the room and the distance from the sound source. A sound wave is a deviation in air pressure from the otherwise prevailing air pressure. This so-called sound pressure [p] is measured in Pascal [Pa].

Our hearing is there to register these rapid fluctuations in air pressure. During a normal conversation, they are around 0.05 Pa (1 / 2,000,000 of atmospheric pressure). Weather changes sometimes cause the air pressure to fluctuate by several thousand pascals. Sound pressure or sound pressure level change depending on the distance, i. H. the further away you are from the sound source, the smaller the pressure fluctuations and the quieter the sound is perceived.

Schallempfindlichkeit des Gehörs und Schalldruckpegel

The hearing can process a sound pressure range from 0.00002 Pa (hearing threshold) to approx. 20 Pa (pain threshold). The human hearing can ignore slow pressure fluctuations, for example due to the difference in height when climbing stairs (several 10 Pa) or when the weather changes. The existing static air pressure has no influence on hearing, because it acts equally on the outside and inside of the eardrum. Pressure equalization can take place during yawning or other jaw movements.

Schalldruck und Geräuschpegel

Sound pressure level or noise level

The volume impression does not only depend on the sound pressure, but also on the perception of the human ear. The ear is very sensitive to weak signals; it is much less sensitive to strong signals.

The sound pressure of a tone is a very small absolute value and the scale is very wide. In order to indicate the strength of the sound, the sound pressure p of a tone is compared with that of a tone that can just be perceived (hearing threshold 1 kHz). The sound pressure level (or sound level) Lp is therefore a relative reference value. The dimensions are given in decibels [dB] on a logarithmic scale. The sound pressure values ​​from 0.00002 Pa to 20 Pa are represented by the decibel values ​​from 0 to 120 dB.

The sound pressure level can be calculated from the sound pressure and the reference sound pressure of 1 kHz using the following formula:

Formel zum Schalldruckpegel

p – effective sound pressure [Pa]
p0 – Reference sound pressure [Pa]

The human ear also works roughly logarithmically, the decibel measure of the sound pressure level thus allows a better representation of the volume impression of an acoustic signal. A person cannot perceive differences of less than a decibel. An increase in the sound level by 10 dB is perceived as a possible doubling of the volume. Doubling the distance to the sound source results in a level reduction of approx. 5 dB.

Soundproofing in ventilation

Wind and weather changes affect the air volume flow that flows through a ventilation system. This can sometimes result in perceptible and annoying noises. Noise reduction can be achieved through manual controls or through automated adaptation using suitable accessories.

Noise reduction in ventilation systems

Basically, the higher the air flow, the louder the noises that can be caused by a decentralized ventilation system. When the air flow is reduced, the flow noises decrease.

To achieve this reduction in the air flow, there are basically two options:

inVENTer PAX for high sound insulation

inVENTer PAX sound insulation ventilators are barely audible and offer a solution for increased noise protection requirements, for example in buildings exposed to the wind or buildings with thin walls.

“We have always had the problem here in northern Germany that the strong north wind caused the fans of our ventilation systems to overdrive. You could also hear that acoustically. With the new inVENTer PAX, we finally have a solution for all affected residential units on the coastal region. ” – Olaf Elbinger, managing director of Brüggemann Energiekonzepte GmbH

Schallvergleich und dezentrale Lüftung

Measurement method for standard sound level difference and sound power

Relationship between sound power and sound power level

The radiated sound power [P] of a noise source per unit of time is given in watts and generally describes the power of the sound source. It can be determined by measuring the sound pressure at several points in a closed room. From the sound pressure level at a certain distance from the sound source, its sound power or, given a given sound power, the sound pressure level at a certain distance can be calculated.

Calculation of the sound power:

Berechnen von der Schallleistung

I – Sound intensity
A – Reference area

The sound power [P] is a variable that is independent of distance and room and is suitable as a starting point for all sound engineering calculations. The sound power level [Lw] is the logarithmic representation of the sound power and, in practice, the common quantity.

Formel zum Berechnen der Schallleistungspegel

P – Sound power [Watt]
P0 – Reference sound power [Watt]

Calculating with sound pressure levels

Sound pressure level values ​​must not simply be added up. Only the sound power (or sound intensity) of two sound sources, which is proportional to the square of the sound pressure, is added. To double the sound power, the sound pressure only needs to be increased by a factor of √2. A conversion to the sound pressure level is then possible.

The following relationships apply:

The doubling of the sound power leads to an increase in the sound level of 3 dB:

The tenfold increase in the sound power leads to an increase in the sound level of 10 dB:

If two sound sources with Lw = 0 dB each are added, this results

Leise Lüften mit Schallschutz

Adding two sound sources with 65 dB each results in

Doubling the sound pressure results in an increase of 6 dB:

Doubling of the sound pressure with an output of Lp = 40 dB:

Measurement method standard sound level difference

The standard sound level difference is a measured variable for characterizing the airborne sound insulation of a component and is frequency-dependent. Here, the ability of the component to provide sound insulation between two rooms is measured. The value indicates the sound pressure difference between the generator room and the receiving room (measured in dB).

The experimental setup for measuring the standard sound level difference is as follows: A noise of 100 dB is generated in the transmission room while the fan is off. The noise is transmitted to the reception room through the switched off ventilation. There, the microphones are used to measure how many dB the noise reaches the receiving room through the ventilation.

Messverfahren der Normschallpegeldifferenz
Applied standards: DIN EN ISO 140-10 / DIN EN ISO 3362

Calculation equation:

Formel zur Berechnung der Normschallpegeldifferenz
  • Lp1 – Average sound pressure level in the transmission room [dB]
  • Lp2 – Average sound pressure level in the reception room [dB]
  • A-Reference absorption area [m2]
  • A – equivalent absorption area in the reception room [m2]
  • T – Reverberation time[s]
  • V – Room volume (Reception room)  [m3]

Measurement method sound power

The total sound energy that is radiated from a sound source per second is the sound power [P]. It cannot be measured directly, but can only be determined using certain measuring methods. A sound source has a constant sound power that does not change when it radiates (emits) into another room environment.

The experimental setup for measuring the sound power is carried out as follows: The noise source is off, the fan is on. It’s quiet in the sending room, but the fan is making a noise in the receiving room. The microphones measure how much of the ventilation noise arrives in the reception room.

Messverfahren der Schallleistung
Applied standard: DIN EN 23741

Calculation equation:


Formel zur Berechnung der Schallleistung
  • L– Sound power of the examined sound source [dB]
  • L– Average sound pressure level – background noise correction [dB]
  • T – Nachhallzeit des Hallraums [s]
  • T– Reference time 1s
  • V – Reverberation volume [m3]
  • V– Reference volume 1 m3
  • λ – wavelength [m]
  • S – Total surface of the reverberation room
  • B – barometric pressure [bar]

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