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Low frequency sound / noise

What are low frequency sound / noise and possibilities for attenuation?

What is low frequency sound / noise and what can be done to reduce the nuisance from this?

Sound is energy waves that travel in all directions, through air, water and solids. The waves travel in oscillations below and over an axis, and the number of oscillations it makes per second is measured in Herz. (Hz).
We humans can hear from 20 up to 20,000 Hz (if we are lucky).

The lower frequencies, and with high energy, often seem uncomfortable. They can be reversed / experienced as vibration that is noticeable and even make you sick when you are hit by sufficiently low-frequency noise. If you get below 20 Hz - (micro noise) that few people hear, it can be quite critical for some.

A little higher up on the scale you hear the song, but if there is little energy it is neither unpleasant nor harmful. If you send an e-mail, the machine often says "swoosh" - this is very low frequency but neither unpleasant nor dangerous.

On a daily basis, we encounter some typical sources of annoying low-frequency noise.

Step noise: blows and thumps from steps, objects thumping, furniture being pushed. Modern thin click laminates on the floor produce more footstep noise than solid thicker parquet poles.

Step noise has become a major problem in today's buildings. However, it is possible to evaluate certain materials, as well as floor combinations, by subjecting them to special impact tests with small steel hammers in a comparable system and trying their hand at the frequencies and obtaining an average of values. This value is called the IIC (Impact Insulation Class). The higher the better the noise insulation.

Many people use what is called "floating floor". This is not an active absorbent form, but a term for an assembly where you have a plate layer in the floor that is not glued / screwed to the substrate and does not touch the walls. This achieves a small reduction in noise transport. It is best to use an active noise-absorbing material such as Texfon.

Engines running. In homes, there can be a number of motors, large and small, that make low-frequency noise that is easily propagated in the trunk and structure of the house. There are washing machine, heat pumps, fans, pumps, air conditioning, generators, etc.

Music covers a wide range of frequencies. Some instruments go more in low-frequency registers than others. Speakers with amplified bass normally provide significant vibration (subwoofers, etc.) Large speakers for cinema screenings as well.

Machinery in industry as well as Engines in boats and cars is a very field.

As mentioned above, low-frequency noise often transitions to what we call vibration.

Vibration, like other noise, goes in all directions from surface to surface where they can most easily radiate as noise. For example, often it is not the direct noise from an engine that is most annoying, but the reflected noise from the panel or structure where the engine is attached. If you are really unlucky, the frequency of the noise will "collide" with the same frequency in the substance the noise source is attached to (the natural frequency) and what we call resonance occurs, which can be experienced as an amplified unpleasant vibration.

Using vibration isolators (our Sylomer and Heron) can reduce vibration from one point to another and reduce noise.

The type of insulator can be so many. The purpose is to achieve the least possible transfer of energy between two surfaces under dynamic load.

We can also not beat all the vibration in hartkorn, there is a big difference in the needs. A machine that makes a noise above 250 Hz is relatively quick to pick up on a firm but softly resilient surface. But then we go down the scale - a motor of 1800 rpm (30 Hz) requires something quite different. And this increases downwards with logarimic values, so if you want to isolate a noise of 10 Hz, you will need 25 times more insulation than noise of 50Hz.

Types of insulation - what do we have in the toolbox?

For larger tasks, there are mechanical dampers such as special steel springs. For more common tasks, strong rubber mats or blocks are good. But they have the downside that by continuously working between pressure and expansion, they will be able to get tired over time. Plastic mats are also good here, especially for specially designed tasks, but perhaps not quite as good as the softness of the rubber.

TR (Transmissibility) is a measure of a material's ability to insulate. It can be expressed in eg dB. In short, it can be said that it measures the vibration absorbance divided by the result of the vibration in the system. That is, its lower transferability, its better job does it.

Impact sound. One of the most common ailments is the low frequency noise of footsteps. Here we have an exceptionally efficient 3.2 mm cloth, TEXFON which gives a great noise reduction.

For machines, one must try to find the right amount of vibration insulation that matches the weight of the machine. We have suppliers of air suspension, leaf suspension, etc. which is a special product. These can design larger tasks. Here we have our Heron vibro mats.

For heavier machines, you will know the weight of the machine, rotation, noise frequency, desired degree of reduction, space available under each suspension point, etc., so they can calculate the need and find a solution. Heron Vibromatter will help a lot.

Platforms with absorbents underneath are often used in industrial halls, or perhaps for some instruments in rehearsal rooms, such as percussion.

In residential buildings, so-called floating floors can be built that rest on absorbent and resilient materials that minimize contact and reduce the transfer of noise energy from low-frequency noise to the surfaces below.

Low-frequency sound in rooms is thrown around in such a way that a great deal of the energy operates in the 4 corners of the room. Here it may be worthwhile to place so-called "Bass Traps" - triangular cushions made of acoustic foam that absorb well in this area.

In high-ceilinged large rooms with a lot of low-frequency noise, you can hang from the ceiling a number of plates - so-called "Baffles", which can also provide a sharp reduction.

Furthermore, there are other forms of reduction / absorbance of low-frequency noise / vibration.

CLD (Constrained Layer Damping) mats consist of lightweight panel boards that can either be glued to metal surfaces or placed under noise sources. The plate changes the frequency of the vibration, in that one of the 3 laminates in the plates, namely the intermediate viscoelastic, will change the frequency character significantly. The plates are 1.3 mm thick, elastic and can be used in a variety of contexts.

Vinyl-based absorbent mats, made for significant reduction of vibration.

Paint (SoundPaint) is a special paint that is applied thickly on surfaces that emit strong vibration. Used in engine compartments, i.a. on ships on bulkheads and roofs.

Special elastomeric bands and plates that can absorb significant amounts of low-frequency noise. They are elastic and very useful under rails, girders and quick separations between surfaces in ceilings, walls and floors.