Resonate absorbers are the most powerful of low-frequency absorption technologies. Pound for pound and square foot per square foot, resonant absorbers can not be matched for low-frequency absorption. They are sometimes called resonance absorbers. We are speaking about real low-frequency absorption which represents all frequencies below 100 Hz. Resonant absorbers are different than other absorbers. They work best in areas of high room sound pressure not high sound velocity areas like porous absorbers that handle middle and high frequencies.
Vibrations & Sound Pressure A resonant absorber is a vibrational system that “runs” on sound pressure. As vibrational science will tell us a resonant absorber is a mass vibrating against a spring. The mass is the cabinet and front wall or diaphragm. The spring is the air inside the cavity of the resonant absorber. If you change the vibrating mass and stiffness of the spring, you can control and tune the resonant absorber to the resonant frequency of choice. The internal mass or cabinet depth determines design frequency. The spring or internal air and cavity are used for achieving the rate of absorption above the unit’s designed for resonant frequency. There are three types of resonant absorbers: Helmholtz and Diaphragmatic and Membrane.
Helmholtz / Membrane A Helm resonator is a box or tube with an opening or slot at its mouth. Air enters the slot which has a calculated width, length, and depth. The slot is attached to a cabinet or cylinder of different widths and depths. A glass coke bottle is a good example of a Helmholtz resonator. It is a resonant absorber or as some would term a resonance absorber. The frequency or resonance is determined by the slot dimensions along with the cabinet or cylinder depth. Helms are frequency specific and narrow frequency band coverage. A membrane absorber works similar to a diaphragmatic. It has a membrane than vibrates in sympathy to sound pressure. This vibrating membrane is attached to a cabinet which has a certain depth and fills material. A diaphragmatic absorber works similar to a membrane with more performance per square foot.
Calculate Resonant frequency of Helmholtz Slot Absorber
Resonant Frequency Formula fo = 2160*sqrt(r/((d*1.2*D)*(r+w))) fo = resonant frequency r = slot width d = slat thickness 1.2 = mouth correction D = cavity depth w = slat width 2160 = c/(2*PI) but rounded c = speed of sound in inch/sec If the gaps vary say 5mm, 10mm, 15mm, 20mm and the wall is angled as shown below, a broad band low mid resonator is created that still keeps the high frequencies alive.
Remember the cavity behind must be airtight! By working out the different slat widths and slat gaps you can create a broadband low mid resonator at specific frequencies.
A lot of activities and businesses have the potential to have negative effects to their environment because of the vibration that they produce. For example, construction (for example during piling), mining and and other vibration-generating activities. This vibration can disturb the comfort and health of people around it, and even can have destructive effects to nearby buildings.
In Indonesia, the vibration limit is regulated through Ministerial Decree of Ministry of Environment No. 49 Year 1996. This regulation was made to ensure healthy environment for human and other living creatures to live in. Consequently, the vibration generated from human activities need to be regulated.
In this regulation, businesses and activities are required to:
Comply to the vibration limit in the decree. This is required for businesses and activities to obtain certain relevant permits to be able to operate.
Use vibration reduction equipment
Report vibration monitoring activities at least once in 3 (three) months to the Governor, Minister, Government agencies that are responsible to control environmental impact, other technical institutions that is responsible for the activities and other organizations that might need the vibration monitoring report.
The vibration limit is separated into few parts which are:
Vibration limits for health and comfort
Mechanical vibration limits based on its destructive effects
Mechanical vibration limits based on building types
Shock limits
The following table and graphs is the vibration limit for health and comfort:
Conversion:
Acceleration = (2πf)2 x displacement
Velocity = 2πf x displacement
The graphic representation of the table above is as follows:
The table below is the vibration limits based on the destructive effects:
As seen above, the peak velocity limit from the vibration is separated into 4 categories which are:
Category A: non-destructive
Category B: Possibly destructive for plastering (crack, or in certain cases the plaster can fell off the wall)
Category C: Possibly destructive for structural components that bear loads
Category D: High risk of destruction of load bearing walls
The following graph is the vibration limit based on destructive effects in a graphical form:
Mechanical vibration limit can also be categorized into the types of buildings. The buildings are categorized into 3 which are:
Buildings for commercial, industrial, and other similar use.
Residential and other buildings with similar design and usage
Structures that are sensitive to vibration and cannot be categorized into category 1 and 2, for example preserved buildings with high cultural value
Below is the vibration limits for the building category above:
The table below is shock limit for buildings:
Category
Building Type
Maximum velocity (mm/s)
1
Old buildings with high historical value
2
2
Buildings with existing defects, cracks can be seen on the walls
5
3
Buildings with good condition, minor cracks on plaster is acceptable
10
4
Buildings with high structural strength (for example industrial building which is made from concrete and steel)
10 – 40
The ministerial decree also describe the measurement and analysis method for vibration as follows:
Instruments:
Vibration transducer (Accelerometer or seismometer)
Vibration measurement device or analysis device (Vibration meter or vibration analyzer)
1/3 octave or narrow band filter
Signal recorder
FFT Analyzer
Measurement procedure:
Vibration measurement related with health and comfort:
Place transducer on the floor or other vibrating surface, and connect it to the measuring device with filtration
Set the measuring instruments to measure displacement. If the measuring instruments do not have that on display, the conversion from acceleration or velocity can be used
Reading and recording is conducted for frequency between 4-63 Hz or with signal recording device
Measurement results with at least 13 data shall be plotted on graph
Vibration measurement for structural health:
The measurement method is similar with the vibration measurement above, however the physical measure that is assessed is the peak velocity.
Evaluation
The 13 data which are plotted on graph shall be compared with the vibration limits. The vibration is considered above the limit if the vibration level exceeds the limit at any frequency.
Definition
The definition used in the regulation of ministry of environment No 49 Year 1996 is as follows:
Building structure is a part of building that is planned, calculated, and functioned to:
Support any kind of load (static load, dynamic load, and temporary load)
Functioned for building’s stability as a whole. For example: frame and bearing wall
Structure’s component is a part of a building structure that contributes to structure’s function. For example: beams, columns, and slab.
Bearing wall is a building structure which is a vertical plane that is functioned to support loads on top of it such as slab or roof.
Non-structure components are parts of building that is not planned or functioned to support load. For example partition walls, door and window frames, etc.
Destructive impact on structure and non-structure:
Destructive impact on structure: Destructive impacts that can endanger building stability (for example destruction of columns that potentially make a building collapses)
Destructive impact on non-structure: Not dangerous to building stability, but can be a danger for building occupants (for example: when a partition wall collapses, it will not make the building collapse, but can injure occupants)
Degree of building destruction:
Light: not dangerous for building stability and can be fixed without reducing building’s strength
Moderate: Destruction that can reduce structural strength. To fix this, added reinforcement must be used.
Severe: Degree of destruction that can endanger the building and potentially makes the building collapses.
