Introduction to Acoustics & Noise
Acoustics is the branch of physics that deals with the study of all mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. Its importance lies in its ubiquitous presence in our everyday lives, from the sound systems we use to the noise pollution we seek to avoid.
Noise is any sound that is unwanted. Humans can hear all frequencies from 20 Hz to 20,000 Hz. This is the ideal audible range of course. As we grow older, or when we face noise dosage which is beyond the allowable limits as per multiple studies, our eardrums physically get damaged and our hearing gets affected. It is commonly observed nowadays that people beyond the age of 35~40 cannot hear frequencies above 17,000 Hz. The higher frequency range of our hearing is the first to get damaged. When that happens, you start hearing a ringing noise in your ears continuously whenever you are in a very quiet environment. This is called "Tinnitus". Once this happens, as of now, it is irreversible. Observe this the next time you sit inside your car before starting the engine or when you get in your bed before you go to sleep, or when you are in any other quiet environment. There's a very good chance that you already have it to a certain extent. The only thing that you can do about it is to not let it get any worse by controlling the noise dosage that you are subjected to.
Understanding Sound
Sound is a type of energy made by vibrations. All of the sound is created because of Vibrations but not all Vibrations create sound. When any object vibrates, it causes movement in the air particles. These particles bump into the particles close to them, which makes them vibrate too causing them to bump into more air particles. This movement, called sound waves, keeps going until they run out of energy. Any medium, including air, resists this movement and hence the amplitude of the sound waves dies down as it travels more distance. That's why you can't hear the sounds if you move too far away from the source.
Sound waves can be categorized into two main types: airborne and structure-borne. Airborne sound, the most common type, is transmitted through the air, while structure-borne sound is transmitted through a solid structure. For example, if a compressor is mounted on a base frame which is kept on a steel structure, even if you block the air-borne noise by installing an acoustic enclosure over the compressor, the structure-borne noise component of it still persists. The vibrations travel to the base frame and then to the steel structure, and any "unconstrained" or "undamped" part of the connected steel structure may (it most likely will) emit noise in the form of air-borne sound. This is not that big of a problem if you install the vibrating machinery on an RCC concrete foundation since the foundation is better at dampening the vibrations than the steel structure is.
Noise
Noise, in the context of acoustics, is typically defined as unwanted sound. It can be categorized by its character, such as continuous or intermittent, and by its frequency, such as low or high frequency.
There are three main categories of noise emissions:
1. Machinery surface emissions: This noise originates from the mechanical and electrical operation of the equipment and is then emitted by the body of the equipment.
2. Pressurized gas flows through pipes: This type of noise is primarily associated with fluid dynamic noise generated in pipes, valves and other equipment where pressurized fluid flows. This component of noise flows through pipes without getting damped and comes out wherever the pipe is open or where the flow is coming out into the air.
3. Structure-borne noise: This occurs when vibrations from machinery travel through connected structures and is radiated as noise at another part. E.g. Base-frame emits air-borne noise that it receives in the form of structure-borne vibrations from the compressor that is mounted on it. This is especially important in case the enclosure is mounted on the base-frame leaving some part of the base-frame exposed outside the enclosure. That's why foundation-mounted enclosures provide better results than base frame-mounted enclosures.
Understanding these categories is crucial because each type of noise emission requires a different solution.
Industrial Acoustics
Industrial acoustics focuses on the control of noise in an industrial setting, where machinery and processes can generate significant noise levels. It is crucial for maintaining a safe and productive work environment. Typically up to 85 dBA of Sound Pressure Level exposure for 8 Hours every day is acceptable according to international safety standards.
Noise Measurement and Sound Pressure Level (SPL)
Understanding noise is incomplete without acknowledging its measurement. Noise is typically measured in terms of the Sound Pressure Level (SPL), which is sensed by a microphone and recorded in decibels (dB).
Sound pressure is a field quantity that measures the fluctuation in air pressure caused by a sound wave. A microphone, functioning much like our ears, senses these fluctuations and converts them into an electrical signal that can be analyzed and interpreted.
The formula for calculating the SPL from Acoustic Pressure (p) is:
SPL (dB) = 20 x Log (P / Pref)
where where P is the measured acoustic pressure in pascals and Pref is the reference sound pressure, typically 20 micro pascals in air – the quietest sound that the average human ear can hear. The use of this reference level is based on the standard threshold of human hearing. That is why It is chosen as reference pressure because it represents a level that is very quiet but still audible to people with good hearing, thus giving a wide scale over which human hearing operates.
Decibels, on the other hand, provide a relative measure of sound pressure level on a logarithmic scale. This scale is used because of the incredibly wide range of sound pressures to which human ears can respond.
The 'A' and 'C' weighting scales further refine this measurement. They are filters that simulate the response of the human ear at different sound levels and frequencies. 'A' weighting is generally used when considering noise impacts on people, as it provides a good approximation of our ears' sensitivity at moderate sound levels. 'C' weighting, however, includes much more low-frequency sound and is more applicable to situations where there are high sound levels.
Industrial Noise Control Solutions and Their Application
Given the varied sources of noise in an industrial setting, it is essential to match the noise control solution to the specific source. Here's a tabular overview of common noise sources, recommended noise control mitigation strategies, and ARK's product recommendations:
Sr. | Machine | Source of Noise | Approx. SPL (dBA) | Mitigation | ARK's Product recommendation |
|---|---|---|---|---|---|
1 | Pumps | Body emission | 90~95 dBA | Removable Acoustic Jacket for 6~10 dBA Noise reduction | ARK NoiseLAG & ARK NoiseLAG+ |
2 | Pumps | Flow through Pipes | 90~100 dBA | Insulation over the Pips, Avoid bends. | ARK NoiseLAG or Conventional Insulation |
3 | Compressors | Body Emission | 95~105 dBA | Removable Acoustic Jacket or Acoustic Enclosure | ARK NoiseLAG+ (Up to 10 dB Noise Reduction) or Acoustic Enclosure (12+ dB Noise Reduction) |
4 | Compressors | Inlet or Outlet Opening | 120~130 dBA | Inlet or Outlet Silencer (In-Line) | ARK Absorptive Silencers (20+ & 30+ dB Insersion Loss Rating) |
5 | Compressors | Blow off Line | 120~130 dBA | Blow Off silencer | ARK Outdoor Blow-Off silencer (30+ dB Insersion Loss Rating) |
6 | Large Steam Turbines | Surface Emission + Piping | 90~95 dBA | Open roof Acoustic Enclosure | ARK 60S Acoustic Panel Open Roof Enclosure (10+ dB Noise Reduction) |
7 | Mechanical Presses | Stamping Noise | 105+ dBA | Closed roof Acoustic Enclosure | ARK 100S or 100S+ Panel Closed roof Enclosures (30+ dB Noise Reduction) |
8 | Boiler Start-up Vent or Steam Blowing | Pressurised steam being released into air | 140+ dBA | Steam silencers with controlled release | ARK Start-up Vent Silencer and ARK Steam Blowing silencer (40+ and 50+ dB Noise Reduction) |
9 | Power Generation Diesel Engines | Body Emission | 110+ dBA | Acoustic Enclosures | ARK 100S+ Acoustic Panel with 30+ dB Noise Reduction |
10 | Power Generation Diesel Engines | Exhaust Pipe Noise | 120+ dBA | Engine Exhaust Silencer | ARK Engine Exhaust Silencer with Spark arrestor (30+ and 40+ dBA Noise Reduction) |
Conclusion
Understanding the basics of acoustics, the different types of noise, and the solutions available for industrial noise control is key to creating a safe, efficient, and productive industrial environment. At ARK Noise Control - India, we are committed to sharing this knowledge and helping everyone to better understand the science of noise control.
Stay tuned for more informative and educational blog posts about industrial acoustics!
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