Transit Stationary Source Leq and Ldn Calculator
This calculator estimates the noise of a stationary transit source at 50 ft from its reference sound exposure level (SELref), the duration of one event and the number of events. It returns the hourly equivalent level Leq(1hr), the daytime level Ld, the nighttime level Ln and the day-night level Ldn, all in dBA, and shows every step of the arithmetic.
Leq(1hr) uses the number of events in the hour of interest. Ld and Ln use the average hourly event rates of the 15 hours from 7 a.m. to 10 p.m. and the 9 hours from 10 p.m. to 7 a.m., and Ldn combines the two periods after adding 10 dB to the nighttime level. The equations are those of Table 4-25 (Eq. 4-38 to 4-41) in the FTA Transit Noise and Vibration Impact Assessment Manual.
The tool is written for transit noise analysts and acoustical consultants. Results update as you type, and each calculation covers one source and one operating condition.
Results at 50 ft
Every step
- The steps appear when every field is valid.
How to use the calculator
- Enter SELref, the reference SEL of the source in dBA. The values of Table 4-24 are listed in the FAQ.
- Enter E, the duration of one event in seconds, and N, the number of events in the hour of interest. The hourly level is Leq(1hr) = SELref + 10 log(N) + 10 log(E/3600) − 35.6 (Eq. 4-38), with log in base 10.
- Enter the number of events between 7 a.m. and 10 p.m. and between 10 p.m. and 7 a.m. as whole numbers. The hourly rates are Nd = day events / 15 and Nn = night events / 9.
- Read Ld, which is Eq. 4-38 with N = Nd (Eq. 4-39), and Ln, which is Eq. 4-38 with N = Nn (Eq. 4-40). A period with 0 events shows none.
- Read Ldn = 10 log(15 × 10Ld/10 + 9 × 10(Ln+10)/10) − 13.8 (Eq. 4-41). Ld and Ln enter unrounded, and a period with 0 events adds 0 to the sum.
- Check the worked lines under Every step to follow each value from the inputs to the result.
FAQ
How are the hourly event rates Nd and Nn found?
Nd is the number of events between 7 a.m. and 10 p.m. divided by 15, and Nn is the number of events between 10 p.m. and 7 a.m. divided by 9. Each is the average number of events per hour in its period, and each replaces N in Eq. 4-38 to give Ld (Eq. 4-39) and Ln (Eq. 4-40).
What happens when the day or night period has no events?
A period with 0 events adds 0 to the sum in Eq. 4-41, and its Ld or Ln is shown as none. If both periods have 0 events there is no Ldn, so the calculator asks for at least one event in either period.
Why are Ld and Ln not rounded before Ldn is computed?
Ldn is computed from the unrounded Ld and Ln so that no rounding error is carried into it. For example, SELref 120 dBA, E 48 s, N 19, 124 day events and 171 night events give Ld 74.8 dBA, Ln 78.4 dBA and Ldn 84.5 dBA (84.4834); Ldn from the rounded Ld and Ln would give 84.4 dBA.
Which reference SEL values does Table 4-24 list?
Table 4-24 (p. 82) lists these reference SELs, in dBA: Auxiliary Equipment 101, Locomotive Idling 109, Rail Transit Idling 106, Buses Idling 111, Track Curve Squeal 136, Car Washes 111, Crossing Signals 109, Substations 99.
Disclaimer. The results are an estimate only and are not a substitute for a qualified engineer. The formulas are Table 4-25 (Eq. 4-38 to 4-41) of Section 4.5 Step 2 Option C of FTA Report 0123, Transit Noise and Vibration Impact Assessment Manual, 2018, p. 83, with Example 4-7 on p. 84.
Results are at 50 ft with no shielding, ground or distance correction, and no impact criterion is applied. Each calculation covers one source and one operating condition. Ferry boat landing and fog horn sources are not covered.