What is the primary goal of EN 15461?

The primary goal of EN 15461 is to provide a standardized method for measuring and characterizing the acoustic properties of a railway track. This is done by measuring the Track Decay Rate (TDR) to ensure that tracks used for vehicle pass-by noise tests are comparable and produce repeatable results.

What is Track Decay Rate (TDR)?

Track Decay Rate (TDR) is a measure of how quickly vibration energy dissipates along a rail after an impact. It is measured in decibels per meter (dB/m). A high TDR means vibrations are damped quickly (a quiet track), while a low TDR means vibrations travel far (a noisy track).

Why can't you just measure pass-by noise on any random track?

The track itself radiates a significant amount of noise. Different track constructions have different acoustic properties. Measuring on a random track would make it impossible to compare the noise of different trains fairly, as the track's contribution would be an uncontrolled variable. EN 15461 helps standardize this variable.

Is EN 15461 used for measuring train noise or track noise?

EN 15461 is used to measure and characterize the *track's* dynamic properties, not the train's noise directly. It is a preparatory standard used to qualify a track section as suitable for conducting official train noise measurements according to other standards, like EN ISO 3095.

Europe Standardizes Rail Noise: Fairer Train Tests

EN 15461 standardizes railway track acoustic properties via Track Decay Rate (TDR). This ensures fair, comparable vehicle noise tests, vital for accurate rolling stock assessment and noise reduction.

December 15, 2024 2:02 am

EN 15461: Technical Guide to Track Dynamic Properties for Railway Noise Measurement

What is EN 15461?

EN 15461 is a European Standard that specifies the method for characterizing the dynamic properties of a railway track section intended for pass-by noise measurements. Its primary purpose is to quantify the track’s acoustic behavior, specifically its ability to radiate noise when excited by a passing train. This ensures that noise measurements are comparable and repeatable across different locations by defining a standardized acoustic “background” from the track itself.

The standard focuses on a key parameter known as the Track Decay Rate (TDR). By measuring and classifying the TDR, operators can determine if a track section is suitable for vehicle noise acceptance tests, as prescribed by other standards like EN ISO 3095.

The Importance of Characterizing the Track

When measuring the noise of a passing train (pass-by noise), the total sound pressure level is a combination of noise from the vehicle and noise radiated by the track structure. The track itself acts like a loudspeaker, vibrating in response to the wheel-rail interaction and radiating sound. The characteristics of this “loudspeaker” can vary significantly based on its components.

Comparability: To compare the noise emission of two different trains, the test must be conducted on tracks with similar acoustic properties. Otherwise, a quieter train on a “noisy” track might appear louder than a noisier train on a “quiet” track.
Repeatability: EN 15461 provides a method to ensure that a test track’s properties remain consistent over time, allowing for reliable and repeatable measurements.
Source Isolation: By defining a reference track condition, the standard helps to isolate the noise contribution of the vehicle, which is the primary subject of pass-by noise tests.

The Core Concept: Track Decay Rate (TDR)

The central measurement in EN 15461 is the Track Decay Rate (TDR). TDR is a measure of how quickly vibrations in the rail dissipate or “decay” after an impact. It is expressed in decibels per meter (dB/m) and is measured across different frequency bands.

A high Track Decay Rate indicates that vibrations are quickly dampened by the track components (sleepers, pads, ballast). The rail radiates less sound, making it an acoustically “dead” or “quiet” track.
A low Track Decay Rate means that vibrations travel a long way along the rail before dissipating. The rail vibrates more freely and for a longer duration, radiating more sound and acting as an acoustically “live” or “noisy” track.

Measurement Procedure for TDR

The TDR is determined through a relatively straightforward but precise impact test. The general process involves the following steps:

Excitation: The rail is struck vertically and laterally with a calibrated instrumented hammer at a specific point, typically mid-span between two sleepers. This impact introduces a wide spectrum of vibration energy into the rail.
Measurement: Accelerometers are placed at several defined distances away from the impact point along the rail. These sensors measure the vibration response of the rail as the energy propagates away from the impact.
Data Acquisition: A data acquisition system records the hammer’s impact force and the rail’s acceleration response from each sensor simultaneously.
Analysis: The data is processed to calculate the transfer function between the input force and the output acceleration at each measurement point. From these transfer functions, the rate at which the vibration level decreases with distance is calculated for specific one-third octave frequency bands. This result is the Track Decay Rate (TDR) for each frequency band.

The standard specifies the required frequency range for analysis, typically from 500 Hz to 2000 Hz, which covers the dominant range for rolling noise.

Technical Specifications and Acceptance Criteria

Measurement Equipment

Instrumented Hammer: An impact hammer with an integrated force sensor to measure the input force of the excitation.
Accelerometers: Lightweight sensors to measure the rail’s vibration response without significantly altering its dynamic properties.
Data Acquisition System: A multi-channel system capable of simultaneously sampling the force and acceleration signals at a high frequency.
Analysis Software: Software to perform Fast Fourier Transform (FFT) and calculate transfer functions and decay rates.

Track Condition and Acceptance

For a track section to be considered suitable for official pass-by noise measurements, its measured TDR values must fall within a specific range or “acceptance window” defined in EN ISO 3095. This reference curve ensures that the track is not excessively noisy or quiet, providing a standardized condition for vehicle testing.

Comparison of Track Types by TDR

The table below illustrates the difference between tracks with low and high decay rates, which is the core principle quantified by EN 15461.

Feature	Low TDR Track (Acoustically Live)	High TDR Track (Acoustically Dead)
Vibration Damping	Low damping. Vibrations travel far along the rail.	High damping. Vibrations are quickly absorbed.
Typical TDR Value	Low (e.g., < 1.5 dB/m in key frequencies)	High (e.g., > 3 dB/m in key frequencies)
Noise Radiation	High. The rail acts as an effective radiator of sound.	Low. The rail radiates significantly less sound.
Suitability for Testing	Generally unsuitable for standardized noise tests as it would artificially inflate the measured vehicle noise.	Suitable for standardized noise tests if TDR values are within the acceptance window of EN ISO 3095.
Common Causes / Examples	Stiff rail pads, very cold temperatures, light track components.	Soft rail pads, resilient fasteners, rail dampers, heavy-duty ballasted track.

Relationship with EN ISO 3095

EN 15461 is not a standalone standard; it is a critical supporting document for EN ISO 3095 (Railway applications – Acoustics – Measurement of noise emitted by railbound vehicles). EN ISO 3095 is the primary standard that defines how to conduct pass-by noise tests for type approval and conformity of railway vehicles.

EN ISO 3095 states that the test track must have a known and compliant acoustic characteristic. EN 15461 provides the precise methodology to measure and verify this characteristic. In essence, you use EN 15461 to qualify your track, and then you use EN ISO 3095 to test your train on that qualified track.

Conclusion

EN 15461 is a fundamental standard in railway acoustics, providing the essential framework for ensuring fair and consistent noise emission testing. By quantifying the dynamic properties of the track through the Track Decay Rate, it allows for the separation of track-related noise from vehicle-related noise. This standardization is crucial for manufacturers, operators, and regulators to reliably assess the acoustic performance of rolling stock, drive innovation in noise reduction technologies, and manage the environmental impact of rail transport.