Subway speed is not fast, why some areas of noise than high-speed rail is greater?

“Now every ride, in the station and station between the full speed of running when the sound feel to pierce the eardrum” “That shrill sound, really whole i thieves uncomfortable, panic” “very harsh, a lot of people have to cover their ears, call, listen to music is almost impossible” … It’s not uncommon to open a web page and spit comments or posts about the noise of Beijing subway vehicles.

With the rapid development of urban rail transit construction, more and more cities have opened subway lines, Beijing, Shanghai and other first-tier cities, subway has become one of the main means of public travel. But on the other hand, the subway in the convenience of travel, but also to many people have brought trouble, many subway lines in the process of driving noise, there are shaking and other issues, greatly affectthe the comfort of passengers, but also the future direction of urban rail transit, put forward higher requirements.

In addition to the Beijing metro, including Guangzhou Metro Line 5, Shanghai Metro Line 17 and other cities of a number of subway lines, but also the public feedback of varying degrees of noise phenomenon. Under the background of the accelerated construction and rapid development of urban rail transit, “train noise” seems to be a difficult problem.

A few days ago, Changsha Metro Line 5 officially carried passengers, is China’s first full line using permanent magnetic traction system of the subway line. Compared with AC asynchronous traction systems, permanent magnetic synchronous traction systems have significant advantages such as high power density, high efficiency, high power factor and low noise, and light weight, especially in noise reduction, which can greatly improve ride comfort. And this also let the outside world’s eyes, more began to focus on the subway train running smoothness and comfort.

The reporter measured. 

Line 6 is the most noisy Other lines turn ingest noise. 

Is the Beijing subway really “loud” as netizens say? To this end, the journalists made field visits.

In the mall underground garage, the reporter with noise meter detection, environmental noise is only 54dB. Then the reporter boarded the subway Yizhuang line, in the elevated line section, the detector showed that the noise at this time basically remained at about 80db. After passing the old palace station, the subway into the underground driving, at this time the noise in the car increased, especially in the “Little Red Gate – Xiao Village” corner, the noise in the car increased to 86db, in the car can clearly feel the vibration of the wheels and rails.

Subway speed is not fast, why some areas of noise than high-speed rail is greater?

Beijing Metro Line 6 is considered to be the most noise-free and vibrating line in the Beijing subway.

Subsequently, the reporter boarded Metro Line 5 and Metro Line 14, also got similar data – the subway on the straight track, the noise in the car remained at about 80db, but in the train corner stage, the noise will increase. And the faster the acceleration, the larger the turning radius, the greater the noise, often up to about 85db.

Subway speed is not fast, why some areas of noise than high-speed rail is greater?

On a relatively small afternoon, reporters measured noise on Line 6 (East) of the Beijing Metro, with a maximum value of nearly 90db.

And came by netizens nicknamed “similar to the aircraft take-off” of the east section of Line 6, the reporter was also “stunned” by the harsh noise. The journey from Youth Road to Tongzhou Beiguan is a dozen kilometers long, although it has been a straight track, but the noise inside the car is getting louder and louder, and the rails and trains occasionally make a shrill roar.

Directly with the roar is the bump in the car, reporters observed that the time in the afternoon, although the car passengers are not many, but the noise inside the car always remains above 85db, in the Changcamp station near, even at one time close to 90db. Whenever the noise is serious, the bumps in the car are also more serious, standing passengers tightly grasp the armrests, sitting passengers can not sit still, forced to follow the car left and right.

The data show that decibels are the size units of the sound pressure level (symbol:db), and for each doubling of the sound pressure, the sound pressure level increases by 6 decibels. Too high decibels can affect human physical and mental health, and scientific studies have shown that more than 70 decibels interfere with conversation and affect productivity. Long-term living in the noise environment of more than 90 decibels, will seriously affect hearing and cause nervous breakdown, leading to headaches, elevated blood pressure and so on.

Subway speed is not fast, why some areas of noise than high-speed rail is greater?

Sound decibel schematic.

Many passengers also expressed concern, a frequent subway line 6 Mr. Zhang said that line 6 opened six or seven years, but the noise problem has not been solved, very worried about the safety of the train, do not know how to do after decades, after all, the subway is not a bus, maintenance is not easy. Another woman who regularly rides the subway, Ms. Liu, also believes that the Beijing subway is more and more repaired, but the number of miles should match the quality of the ride, excessive noise and bumps, is not conducive to encouraging public transport. Where does the noise come from on the subway?

Poor wheel matching is to blame.

Compared with the high-speed railway moving hundreds of kilometers per hour, the subway is not moving at a fast speed, why has the noise problem become a pain point? Xiao Xinbiao, an associate researcher at the National Key Laboratory of Traction Power at Southwest Jiaotong University, told the Beijing Science and Technology Daily that this is closely related to poor wheel-rail matching, curve passage and fork.

Subway speed is not fast, why some areas of noise than high-speed rail is greater?

The construction site of the subway, which is in the process of laying the tracks.

Xiao Xinbiao explained that the wheel and rail contact on the stepping surface has a micron-level surface uneven, it will be through the wheel rail contact to make the two vibration, vibration energy through the air medium outward seisking, the formation of wheel rail noise. The greater the roughness of the wheel rail, the greater the vibration noise.

“This noise tends to be more pronounced in closed subway tunnels. Because the reverb in the confined tunnel space is obvious and the noise energy cannot be transmitted outwards quickly or decays, more sound energy travels into the carrier. Xiao said.

In addition, if the wheels and rails of the subway train do not match well, it will cause abnormal wave grinding. The so-called “wave grinding” is short for “wave-type wear”, which refers to the wave-like uneven wear ingestion of the top surface of the rail, and is also one of the main forms of steel rail wear. Abnormal wave grinding increases vibration between wheel slings and also increases noise generation.

So why can’t the match between subway trains and tracks be more precise in order to avoid the emergence of abnormal wave grinding? Xiao Xinbiao said that different brands of train wheel rail stomp, different manufacturers of track standards are also different, vehicles and lines there are very complex coupling relationship, from the current technical level, the difficulty of complete coupling is relatively large, the relevant personnel are stepping up research.

Subway noise also has a reason for line design. Subway is different from intercity trains, shuttle in the city will design a lot of curved sections. On the one hand, the curve d’oeuvres are prone to steel rail wave grinding due to the interaction of wheel-rail, and on the other hand, when the train passes through the curved section, it is easy to stimulate the vibration mode in the horizontal direction of the wheel and the rail, which is also relatively easy to produce sound, the acoustic radiation area is also relatively large, like the horn, the formation of significant acoustic radiation.

Subway speed is not fast, why some areas of noise than high-speed rail is greater?

In addition, the fork will also affect the smoothness and comfort of the ride. A fork is a line connection device for a train to transfer from one section to another, and when a subway train passes through a fork, it is easier to generate traffic lateral movement and wheel-rail impact noise because of the line discontinuity in the fork area. The more significant the discontinuity of the fork, the more obvious the passenger’s feeling.

How to reduce subway noise?

“Reduce, divide, suck” three methods of noise reduction. 

Is there a safety hazard because of the heavy noise caused by the subway train due to the wheel rail? In this regard, Xiao Xinbiao said that in addition to the wave grinding will lead to train noise, but also lead to loose vehicle bolts, fasteners broken and damage to the road bed, and even damage to the train frame and body, a long time will indeed form a safety hazard.

However, from the current situation, the safety will not be too big a problem, because the Subway Company every year on a regular and timely operation and maintenance of trains and lines, wheel repair, steel rail grinding, the purpose is to ensure safety and ride comfort.

It is worth mentioning that this kind of maintenance is not infinite, but the use of scientific means and methods. Because the wheels are frequently repaired and the rails are over-polished, it will accelerate its reach the wear and tear limit, at this time it will need to replace the wheel pair or replace the rail, especially the operation line rail change, the cost is very high.

So what can be done to reduce the noise of trains? Xiao Xinbiao said that from the current technical means, can take “minus” “separate” “suck” three methods for comprehensive management.

Subway speed is not fast, why some areas of noise than high-speed rail is greater?

Staff are overhauling and maintaining subway trains.

The so-called “minus” is to reduce the roughness of the wheel rail, the root cause to suppress the wheel rail noise. For example, the abnormal wave grinding section should be regularly monitored and maintained, serious areas should be encrypted monitoring cycle, with grinding or milling, scientific noise rail grinding, to help wheels and tracks better “grinding”, and ultimately reduce the noise of wheel rail vibration, improve passenger comfort.

“Separate” is the use of high vibration-isolated track type, to minimize the impact of wheel rail vibration energy to the surrounding environment along the line, while the use of high sound isolation and airtight body structure, to minimize the wheel rail vibration and noise energy transmission to the car. For example, the Beijing Metro Line 4 train, in the “North Dongmen- Yuanmingyuan” section in order to protect the university laboratory and monuments from the impact of subway train vibration, the use of high vibration line design, so that the wheel rail vibration and generated noise can be maximized isolation, not outward transmission.

Subway speed is not fast, why some areas of noise than high-speed rail is greater?

“Sucking” is the use of step-by-step absorption of consumption, the dispersion of the consumption system’s vibration and noise energy, rather than just at the wheel-rail level of the article. “Our suggestion is that the rails on the line dissipate part, the fasteners dissipate part, the mats dissipate part; on the vehicle, the wheels dissipate part, the suspension element dissipates part, the structure dissipates part; and then to the body to do the corresponding vibration reduction and soundination, so that the noise of the entire train and line system should be a good solution.”

It is understood that in Beijing, for example, in recent years has been in a number of lines to use more noise reduction measures to improve passenger comfort. For example, the installation of noise-cancelling tablets on the wheels of the Metro Line 10 trains, the improvement of the air tightness of vehicles on the Daxing Airport Line, the enhancement of noise insulation and so on.

Journalist/Zhao Tianyu Editing/Liu Zhao.

New Media Editor/Lu Bingxin.

Interview experts:

Xiao Xinbiao (Associate Researcher, National Key Laboratory, Traction Power, Southwest Jiaotong University)