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Active Noise Cancellation (ANC) technology - NoiseTrap® Active Application
As there are many misconceptions and questions about active noise control, this post aims to provide a brief explanation of the basics behind the technology, as well as illustrating some successful applications of ANC systems including Sonobex's own NoiseTrap® Active product
The World Health Organization (WHO) considers excessive noise as one of the most important factors harmful to public health. Environmental noise contributes to health issues such as noise induced hearing impairment, sleeping disturbance, fatigue, increased risk of developing negative cardiovascular and metabolic effects.
In order to mitigate the risk of high level noise exposure, there are many different techniques that can be applied, ranging from passive to active noise canceling systems, which are recently gaining more traction.
Basics of Active Noise Canceling
In simple terms, active noise canceling (ANC) works on the principle of generating an anti-wave to the original sound wave of the noise. The anti-wave has to be identical in sound level (amplitude) to the original noise source, but played back with a delay (180 <sup>0</sup> phase shift) related to half of the wavelength of the frequency - tone to be canceled. This effectively inverts the original sound wave. When both waves meet in space, they theoretically should destructively interfere with each other and as a result cancel each other decreasing overall sound pressure level of the noise. It is like adding -1 + 1 = 0.
Active noise canceling systems need a sensor to recognize the noise to be canceled (microphone or accelerometer), an actuator to create the anti-wave that can counteract the original noise (usually a speaker) and a control unit, currently most likely a digital signal processor (DSP) that computes the anti-wave and plays it back with the right amplitude at the right time. There are of course more components that tie the whole system together, but that is not the point of this post to list them all.
In reality, perfect noise cancellation is an extremely challenging task. Sound waves as any other waves are subject to the laws of physics, which means they are reflected or diffracted by the obstacles and boundaries present in the environment making the overall sound field quite complex to control. This in turn, can in the worst case lead to such effects as unwanted constructive interference, which reinforces sound and makes the noise even louder in certain points in space. Due to these complexities it is important to understand the physics ruling sound waves radiation, influence of the environment and limitations of noise canceling systems, as it is only really possible to engineer a highly effective ANC system when careful consideration and thought is put into it.
General rule-of-thumb limitations of active noise canceling systems:\
Frequency range of effective ANC.
This range is related to the wavelength of the sound wave.
Generally speaking low to mid frequencies can be canceled effectively, so approximately a range between 50 Hz to 500 Hz, although it depends on application. Headphones or long ducting systems with small cross section area can deal with frequencies that are in low kilohertz range.
Open or semi enclosed spaces (3D environment).
In open space or inside rooms / enclosures, the complexity of how sound waves behave and interfere with each other makes it very challenging to control them in any meaningful way in order to achieve global attenuation of noise. The higher the frequency, the shorter the wavelength and the interaction between waves becomes very complex. This introduces issues with control of such a sound field in even fairly small spaces or enclosures. There is a possibility to create small targeted zones of attenuation, eg around ears of the listeners, but the position of the ears in space must remain fairly static and the frequency range is also limited to lower frequencies.
In order to have any chance to control a complex sound field in 3D space, there is a need for multichannel systems. These use multiple speakers and microphones as well as complex algorithms and high power DSP units that can handle all the signals and calculations. The high complexity, trouble with installation and costs involved in development of such system tailored to specific applications can be prohibitive and not practical in real life situations.
Type of noise - broadband, impulsive, tonal.
Random noise that changes very fast or impulsive noise can be very challenging to be attenuated. The signals in an active system must travel from the acoustical domain into electrical domain and back into acoustical domain where the original noise meets its processed inverted version. This process involves many operations, especially in the electrical domain, that take time and sometimes there is not enough of it for the system to generate the anti-wave fast enough to counteract the disturbance. In feed-forward systems this issue is called causality and for the system to be able to cancel out noise, the time for the noise wave to travel from the input sensing microphone to noise canceling speaker must be greater than the total sum of all delays involved on the electrical side, called electrical delay.
Tonal periodic noise is most forgiving as only an adjustment of the amplitude and phase of the signal over one cycle of sine wave is required, which removes the causality problem.
Successful applications of ANC
Now that we know what active noise canceling is supposed to achieve and what are its main limitations, let's go through a couple of applications where it has been proven to be effective.
Active Noise Canceling technology found its home inside a headphone earcup.
The number of headphone manufacturers having in their product range at least one model with Active Noise Cancellation technology is increasing everyday.
Headphones are one of the successful applications of ANC systems. This has to do with the fact that sound field to be controlled is very small, in the range of single centimeters (just an earcup and a tiny ear canal), enclosed and it is in contact with ear without any possibility for additional disturbing sounds to interfere along the path into the eardrum.
The zone of attenuation is minimized, making it possible to be controlled.
The effectiveness is still usually limited to the lower frequency range under 1kHz and the remaining noise spectrum is handled passively by earcup material and foam padding.
The small size of powerful DSP chips, quality MEMS microphones, as well as very favorable environment for the sound to be controlled enabled ANC technology to be integrated into headphones to become a mass market product.
2. Car cabin
Car manufacturers have managed to implement active noise canceling systems in their ranges of high-end models to treat tonal components of engine noise or road noise to a certain extent. This has become possible, as cars are already equipped with sophisticated infotainment systems. These consist of high quality multichannel audio systems with DSP units for sound management, several (sometimes as many as 8 to 14) surround speakers and arrays of microphones around the cabin to provide hands free telephony but that can be also used as sensors for ANC system . As there is a multitude of sensors and actuators already in place, the volume of the cabin is fairly small and the position of passenger's heads is known <em>a priori</em>there is a chance to effectively cancel out tonal low frequency components generated by the engine and some low frequency road noise that originates as a vibration and can be transmitted through the chassis into the car cabin. Clean signals can be taken directly from the ECU (engine order noise) or accelerometers mounted on the car chassis as well as inside engine bay to provide reference signal for the system. Some of the latest systems try to improve upon the operational frequency range by creating small silent zones around the ears of passengers through incorporating additional speakers into car seat headrests. Unfortunately wind noise that is broadband in nature and cannot be easily predicted with enough speed is very troublesome for an ANC system to be dealt with.
Nevertheless the complexity and cost of active noise canceling systems in such applications can be justified as it is adding functionality to the existing audio system and improving the ride comfort, even if the results are somewhat limited.
3. HVAC duct sections
Long circular or square HVAC duct sections can be combined with an ANC system to provide low frequency tonal attenuation and some low frequency band limited attenuation depending on the cross section size of the duct.
Practical industrial applications put the limit of operational frequency below 500 Hz as the duct size can't be too small. The rule is: the smaller the cross section, the higher the operational frequency. A duct is a known well-defined static environment where the sound wave at low frequencies is traveling as a 1D planar wave the wave fronts are uniform across the whole cross section of the duct, which makes it manageable to be controlled. In order to enforce planar behavior the duct section cannot be too short.
Performance of such systems is usually very good, although it is worth noting that even under such favorable environment conditions, HVAC applications where there is high velocity airflow create turbulent flows that can limit performance. Additionally due to resonances and higher order modes generated inside the duct some frequencies might be more difficult to be attenuated than the others, limiting to a small degree the overall performance and increasing the complexity of system.
Sonobex NoiseTrap® Active - Designed for power transformer ventilated enclosures
There are many places around the world where the population density is relatively high and that have become very industrialized. In such surroundings, the demand for energy is already high and will only keep increasing to enable factories, offices, shops as well as households to keep running. To deliver energy to all these facilities we need power transformer substations. These are key components of the electrical power grid allowing for voltage conversion, transmission and distribution of electricity to the receivers. Power transformers installed in the substations generate heat and produce a distinctive low frequency hum that can travel over long distances without much attenuation. Acoustic enclosures are a good way of attenuating the noise, but they can contribute to heat retention inside the enclosure,
To help cool down the assets there is a need for ventilation in the noise enclosure, but every opening becomes an acoustic weak spot where the noise can leak through. There are noise mitigation techniques such as passive silencers installed in the openings to deal with low frequency noise radiated from a power transformer, but they are usually very long (in the range of meters) and they decrease the amount of airflow through the narrow channels of silencer. Additionally as the main mechanism of attenuation in silencers is due to energy dissipation inside a porous material such as mineral wool, it does contribute to thermal insulation preventing warm air from escaping the inside of a noise enclosure.
As low frequency noise can travel far and require excessive means to be attenuated Sonobex has been on a path to develop an Active Noise Canceling system for ventilation openings inside Power Transformer enclosures.
The NoiseTrap® Active product is designed to attenuate the periodic low frequency tonal noise of power transformer hum radiating through ventilation openings in a noise enclosure. There is no forced airflow creating turbulent flow and limiting the performance. The depth of solution is under 500 mm, which is unheard of to be possible with other solutions. Speaker and microphone placement can be optimized for best performance. All of the parameters of the ANC system, such as size, number of microphones and speakers, is the same in every application and thus can be standardized. The high performance and very large open area can provide both the required noise attenuation and natural ventilation into a power transformer enclosure.