In order to know how to reduce noise with ductless sound traps, we first need a brief introduction to what noise and sound actually are.
Sound is a sensory experience linked to a subjective interpretation of a physical phenomenon - pressure variations in the air around us.
When the pressure variations are perceived by our ear, these variations are called sound waves.
A sound wave is defined by physical parameters, such as wavelength and amplitude.
The experience of sound is linked to our subjective perception of this physical phenomenon and is a personal experience.
In the world of ventilation, sound is an important concept. If there is sound coming from a piece of ventilation equipment, the sound is probably unwanted - and this is the definition of noise (unwanted sound).
This unwanted sound can be greatly reduced with the right use of a duct sound trap.
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- Channel sound trap is also known as:
- Sound trap
- Silencers
- Sound attenuators
- Silencers
- Mufflers
- Tested according to ISO standard
Nordic Steel's duct sound trap is tested according to international standards:
- ISO 7235 "Acoustic measurement procedure for ducted silencers- insertion loss, flow noise and total pressure loss".
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In a ventilation system, there is a continuous process of sound generation and attenuation, caused by obstructions and changes in airflow, which increase the sound level.
Besides sound traps, filters, heat exchangers and dehumidifiers are components that contribute to sound attenuation.
If sound attenuation is used in a duct system, absorption, reflection and transmission are important factors that contribute positively to reducing the sound level in ducts.
Absorption: For internally insulated ducts, absorption is dominant.
The absorption properties are a function of insulation thickness and frequency.
For low-frequency attenuation, thick insulation is required (50-100 mm), for high-frequency sound, 25 mm is effective. (more about absorption sound traps further down in the article)
Reflection: can make a significant contribution in rectangular ducts, especially in bends with a right-angled outer edge and without guide rails.
Transmission attenuation: due to the sound energy "leaking" out of the channel.
Flexible hoses are a good example of effective damping, due to the sound being transmitted out through the wall.
This can be a valuable effect, but more often than not this "leakage" of sound will cause problems in the rooms through which the ducts pass and is therefore an undesirable form of attenuation.
Much of the attenuation in large rectangular ducts can be due to transmission losses.
Measures to reduce transmission losses will be to brace the ducts with steel profiles, preferably in combination with increasing the plate thickness in the ducts.
Please note that a transmission loss out of ducts can be a negative contribution to the surroundings outside the ducts, due to the influence of the noise level outside.
The natural attenuation in the duct system mentioned above is rarely sufficient to take care of the ventilation noise. It is important to install primary sound traps just after the fan, possibly after the ventilation unit.
- Primary silencers should focus on attenuating low-frequency noise in the frequency bands up to 500Hz. Low-frequency noise has high energy that can cause problems for the surroundings.
- It is recommended to use secondary silencers to reduce noise from dampers and flow sound from bends, branches and obstructions in the duct system.
- A sound trap is specialized equipment designed to reduce noise, and should be handled and installed with care - to ensure proper function and efficiency. Improper handling or installation can reduce the capability of the sound trap and affect the noise level in the ventilation system.
That's why it's important to treat sound traps as specialized equipment, following the manufacturer's recommendations, and industry best practices during: transportation, handling and installation.
- Static insertion losses (dB)
- The silencing values are measured as static insertion loss (without air supply).
Loudspeakers are used as a noise source to perform measurements. Difference values for measurements with and without sound trap are presented as static input values, in 1/1 octave frequency band, from 63 to 8000Hz.
- Intrinsic noise (Flow Noise, dB)
- Testing of self-generated noise is carried out with the supply of air (flow noise) with different amounts. Measurement data is presented in 1/1 octave frequency band from 63 to 8000Hz.
- Pressure drop (Total pressure drop, Pa)
- During testing of self-generated noise, the pressure drop across the sound trap is also measured, with different amounts of noise. Differential pressure values are read as total pressure (static + dynamic pressure). The measured values are recorded in Pascal for the relevant air volumes with the associated pressure drop coefficient.
Absorption sound traps have a sound-absorbing insulation (e.g. mineral wool or Dacron) to dampen noise.
The insulation has air pockets that absorb the sound waves and convert the acoustic energy into heat by slowing down the oscillating particles and reducing the noise level.
It is important to have open pores in the insulation for the sound to disappear. The pores must not be filled with dust, moisture or other obstacles.
To avoid fiber entrapment, it is recommended to use fiberglass or polyester cloth (open structure) to prevent the fibers from detaching the insulation.
In some cases, a robust plastic foil is used to prevent moisture. The foil must be as thin as possible and must be laid loosely between the insulation and the perforated sheet.
Always remember to use perforated sheets to protect the insulation!
The length of the sound trap affects the level of attenuation. The longer the sound trap, the higher the level of attenuation created. Larger insulation surface provides better attenuation.
To achieve better low-frequency attenuation, it's a good idea to combine thicker baffles in rectangular baffles, in combination with smaller air gaps.
Some of the acoustic solutions we provide in the field of sound attenuation:
- All types of sound calculations
-Circular sound traps [Type SAC3-50]
-Circular sound traps [Type SAC3-100]
-Rectangular sound traps [Type SAR3]
-Combined sound trap and jet hood [Type SONEX2]
(For calculation of intrinsic noise, please contact us on our website)
Go here for more information about each product:
https://www.nordicsteel.no/acoustics
