Noises in ventilation systems
Understanding causes, avoiding noise
Ventilation systems are an indispensable part of modern building technology—decentralized, centralized, with heat recovery, or classic exhaust air variants. But what happens when the system suddenly becomes too loud or produces disturbing noise? Fans, silencers, fire dampers, and other components in a ventilation system can generate various noises that are perceived differently depending on the operating environment and individual perceptions.
This comprehensive overview provides you with all the important information about noise generation in a ventilation system – from the physical basics to practical solutions for a quieter, noise-free ventilation system.
What is sound?Fundamentals and definition of sound
Sound refers to the wave-like vibration that propagates in a gaseous, liquid, or solid substance and causes noise. The sound frequency (pitch) refers to 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 age. This means that high-pitched sounds are generally less easily perceived by adults than by children. Sound waves with frequencies below 16 Hz that are inaudible to humans are referred to as infrasound, while those with a frequency above 20,000 Hz are referred to as ultrasound. These are used by various animal species for communication or orientation.
What is the difference between sound and noise?
Sound refers to the wave-like vibration in a medium that can be measured objectively. Noise, on the other hand, is the subjective, negative assessment of sound.
As a sensory organ, the human ear perceives sounds not only as loud or soft, but also as pleasant or unpleasant. In addition to the laws of physics, physiological factors also play a role in the subjective perception of sound.
As a sensory organ, the human ear perceives sounds not only as loud or soft, but also as pleasant or unpleasant. In addition to the laws of physics, physiological factors also play a role in the subjective perception of sound.
How is noise generated in the ventilation system?
Sound in ventilation systems is mainly caused by rotating fans, turbulent air flows, and vibrating system components. These mechanical vibrations set the air molecules in motion and generate sound waves.
Sound waves cannot propagate in a vacuum, but require a so-called transmission medium whose moving particles can transmit the wave – primarily through the air in the duct system, but also through solid materials such as duct walls or the building structure.
Sound waves cannot propagate in a vacuum, but require a so-called transmission medium whose moving particles can transmit the wave – primarily through the air in the duct system, but also through solid materials such as duct walls or the building structure.
Sound transmission of different types of sound in ventilation systemsHow does sound transmission work in humans?
The ear can process a sound pressure range from 0.00002 Pa (hearing threshold) to approx. 20 Pa (pain threshold). The human ear can ignore slow pressure fluctuations, such as those caused by climbing stairs (several tens of Pa) or changes in weather. The existing static air pressure has no effect on hearing, as it acts equally on the outer and inner sides of the eardrum. Pressure equalization can occur when yawning or during other jaw movements.
structure-borne noise
“Structure-borne noise” refers to mechanical vibrations that propagate through solid materials. This noise cannot be perceived directly by the ear, but is emitted as airborne noise by walls and other surfaces, which the human ear can hear. The surface of the body causes the surrounding air to vibrate, similar to the membrane of a loudspeaker. Examples of structure-borne sound include hammering a nail into the wall, dropping objects, or walking up stairs (impact sound). Structure-borne sound can spread through the building over long distances and can also be heard as disturbing airborne sound in distant rooms.
Examples of potential structure-borne noise in ventilation systems:
- Fan vibrations transmitted to the building structure via fastenings
- Motor noise transmitted through duct walls
- Vibrations caused by possible imbalances in rotating system components
- Vibrations from compressors or air conditioning units
airborne sound
“Airborne sound” is transmitted directly by pressure vibrations in the air, such as when speaking. Outdoors, sound propagates spherically in all directions and depends on the distance from the sound source, obstacles in the path, and sound reflection, which causes an echo. In ventilation systems, airborne sound mainly propagates through the duct system and can enter the rooms to be ventilated via air outlets. In addition, flanking transmission can occur when sound is transmitted via secondary paths such as ceiling or wall penetrations.
Examples of potential airborne noise in ventilation systems:
- Flow noise caused by turbulent air movements in ducts
- Noise caused by changes in air velocity at deflections or cross-sectional constrictions
- Whistling noises at dampers, grilles, or other fixtures
- Fan noise transmitted directly through the air
What is the sound pressure level or noise level?
The perceived volume depends not only on the sound pressure, but also on the sensitivity of the human ear. The ear is very sensitive to weak signals, but significantly less sensitive to strong signals.
The sound pressure of a tone is a very small absolute value. To indicate the strength of the sound, the sound pressure p of a tone is compared with that of a tone that is just perceptible (hearing threshold 1 kHz). The sound pressure level (or sound level) Lp is therefore a relative reference value. The measurement is given in decibels [dB] on a logarithmic scale. Sound pressure values from 0.00002 Pa to 20 Pa are represented by 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:
p – effective sound pressure [Pa]
p0 – reference sound pressure [Pa]
Human hearing also works in a roughly logarithmic manner—the decibel measurement of sound pressure level thus allows for a better representation of the volume impression of an acoustic signal. Humans cannot perceive differences of less than one decibel. An increase in sound level of 10 dB is perceived as a doubling of volume. Doubling the distance from the sound source results in a reduction in sound level of approximately 5 dB.
Sound insulation for ventilation What sound insulation measures are available for ventilation systems?
Wind and weather changes affect the air volume flow through a ventilation system. This can sometimes cause noticeable and disturbing noises. Strong winds in particular can increase the noise generated by ventilation systems, both central and decentralized residential ventilation systems can increase. Noise can be reduced by manually adjusting the ventilation level or by automated adjustment using suitable sound insulation accessories.
Noise reduction in ventilation systems
When comparing ventilation systems, fans are the main source of noise in any ventilation system. In central and decentralized ventilation systems, silencers can significantly reduce noise levels. Fire dampers in the system, such as those used in ventilation systems in hotels and micro-apartments, can also contribute to noise generation due to their design. The exhaust air not only transports air pollutants, but also sound through the entire ventilation system.
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As a general rule, the higher the air volume flow, the louder the noise that can be generated by a decentralized ventilation system. Reducing the air volume flow therefore reduces the flow noise. There are two basic ways to achieve this reduction in air volume flow:
- Active adjustment by the user
- Passive use of accessories to reduce flow noise and air volume flow
Our inVENTer accessories are ideal for passive noise reduction:
- Sound insulation inserts reduce airflow noise directly in the ventilation unit
- Sound protectors dampen noise at critical points in the duct system
- Special wall-mounted housings minimize the transmission of structure-borne noise to the building structure
- Wind protection grilles prevent wind-induced turbulence at the outer end
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“Here in northern Germany, we have always had the problem that strong north winds caused the fans in our ventilation systems to overdrive. You could even hear it. With the inVENTer PAX, we finally have a solution for all affected residential units in the coastal region.” – Olaf Elbinger
Measurement and analysis of soundStandard 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 expressed in watts and basically describes the power of the sound source. It can be determined by measuring the sound pressure at several points in a closed room. The sound power of the sound source can be calculated from the sound pressure levels at a specific distance from the sound source, or, given the sound power, the sound pressure level at a specific distance can be calculated.
Calculation of sound power:
I – Sound intensity
A – Reference surface
Sound power [P] is a quantity that is independent of distance and space and is suitable as a starting point for all acoustic calculations. Sound power level [Lw] is the logarithmic representation of sound power and is the quantity commonly used in practice.
P – Sound power [watts]
P0 – Reference sound power [watts]
Calculating with sound pressure levels
Sound pressure level values cannot simply be added together. Only the sound power (or sound intensity) of two sound sources, which is proportional to the square of the sound pressure, can be added together. To double the sound power, the sound pressure only needs to be increased by a factor of √2. It is then possible to convert this into the sound pressure level.
The following relationships apply:
Doubling the sound power results in an increase in sound level of 3 dB: – 20∙log(√2) = 3.01
A tenfold increase in sound power results in an increase in sound level of: 10 dB: – 20∙log(√10) = 10
If two sound sources, each with Lw = 0 dB, are added together, the result is – 0 dB + 0 dB = 3 dB.
Adding two sound sources, each with 65 dB, results in: – 65 dB + 65 dB = 68 dB
Doubling the sound pressure results in an increase of 6 dB: – 20∙log(2) = 6.02
Doubling of sound pressure at an output of Lp = 40 dB: – 40 dB + 40 dB = 46 dB
Measurement method standard sound level difference
The standard sound level difference is a measurement used to characterize the airborne sound insulation of a component and is frequency-dependent. It measures the component’s ability to insulate sound between two rooms. The value indicates the sound pressure difference between the source room and the receiving room (measured in dB).
The test setup for measuring the standard sound level difference: a noise of 100 dB is generated in the transmitter room while the fan is switched off. The noise is transmitted to the receiving room through the ventilation system. There, microphones are used to measure how many dB the noise arrives through the ventilation system.
Calculation equation:
Lp1 – mean sound pressure level in the sending room [dB]
Lp2 – mean sound pressure level in the receiving room [dB]
A0 – reference absorption area [m2]
A – equivalent absorption area in the reception room [m2]
T – reverberation time [s]
V – room volume (receiving room) [m3]
Sound power measurement procedure
The total sound energy emitted by a sound source per second is the sound power [P]. It cannot be measured directly, but only determined using specific measurement methods. A sound source has a constant sound power that does not change when it radiates (emits) into a different room environment.
The test setup for measuring sound power is as follows: The noise source is off, the fan is on. The transmitter room is quiet, but the fan creates noise in the receiver room. Microphones are used to measure how much of the ventilation noise reaches the receiver room.
The test setup for measuring sound power is as follows: The noise source is off, the fan is on. The transmitter room is quiet, but the fan creates noise in the receiver room. Microphones are used to measure how much of the ventilation noise reaches the receiver room.
Calculation equation:
- Lw – Sound power of the sound source under investigation [dB]
- Lp – Average sound pressure level – External noise correction [dB]
- T – Reverberation time of the reverberation chamber [s]
- T0 – Reference time 1 s
- V – Reverberation chamber volume [m3]
- V0 – Reference volume 1 m3
- λ – Wavelength [m]
- S – Total surface area of the reverberation chamber
- B – Barometric pressure [bar]
Frequently asked questions Noise and sound in ventilation systems
Why is my ventilation system suddenly noisy?
A ventilation system can become noisy for various reasons. Common causes include dirty fans or blocked exhaust air ducts. Therefore, maintenance and cleaning of a decentralized ventilation system is essential. Strong winds and hurricane-force gusts can also cause noise.
How is the sound power of a ventilation system measured?
The sound power is determined using special measurement methods in which the ventilation system is tested in a controlled environment. Fans are switched on while the transmitter room is silent. The system then generates measurable noise in the receiver room.
What does standard sound level difference mean in ventilation?
The standard sound level difference shows how well a ventilation system attenuates sound between two rooms. During measurement, the system is switched off and a 100 dB noise is generated in the transmitter room. The exhaust air ducts then transmit part of the sound to the receiver room. Silencers have a significant influence on this value.
How do structure-borne noise and airborne noise occur during ventilation?
Fans generate both structure-borne noise and airborne noise. Structure-borne noise is caused by vibrations in the ventilation system and is transmitted to walls and ceilings. Airborne noise is transmitted directly by the exhaust air and supply air. Since structure-borne noise spreads particularly well via solid connections, correct installation is crucial in residential ventilation systems in order to minimize this transmission.
What role does frequency play in ventilation noise?
Noise generation in a ventilation system occurs in different frequency ranges. Fans often produce low-frequency noise, while exhaust air flows can generate higher frequencies. The human hearing range of 16 Hz to 20,000 Hz is decisive for perception. Frequencies that are perceived as particularly loud should be taken into account when planning the system.
Which accessories help with noisy ventilation systems?
Depending on the cause of the noise, various accessories can help: sound insulation inserts for direct noise reduction on the fan or sound protectors for the duct system.