Noise indices are metrics used to quantify and evaluate noise levels in different environments. These indices help in understanding the impact of noise on human health, comfort, and the environment. Here is a detailed look at the most common noise indices:

1. Equivalent Continuous Sound Level (Leq)

Definition:

• Leq is the constant sound level that, over a specified period, contains the same acoustic energy as the actual varying sound levels.

Calculation:

• Leq is calculated using the formula:
•                                                                         
• where $L(t)$ is the instantaneous sound level at time $t$, and $T$ is the total measurement time.

Purpose:

• Leq is used to assess long-term noise exposure and is commonly used in environmental noise assessments and regulations.

2. Day-Night Average Sound Level (Ldn)

Definition:

• Ldn is the 24-hour average sound level with a 10 dB penalty applied to nighttime noise (10 PM to 7 AM) to account for increased sensitivity to noise during these hours.

Calculation:

• Ldn is calculated using the formula:
•                                                              
• where LdLd​ is the daytime Leq (7 AM to 10 PM) and LnLn​ is the nighttime Leq (10 PM to 7 AM).

Purpose:

• Ldn is used for assessing community noise exposure and setting noise standards for residential areas.

3. Community Noise Equivalent Level (CNEL)

Definition:

• CNEL is similar to Ldn but includes an additional 5 dB penalty for evening noise (7 PM to 10 PM) in addition to the 10 dB nighttime penalty.

Calculation:

• CNEL is calculated using the formula:
•                                                           
• where LdLd​ is the daytime Leq (7 AM to 7 PM), LeLe​ is the evening Leq (7 PM to 10 PM), and LnLn​ is the nighttime Leq (10 PM to 7 AM).

Purpose:

• CNEL is used in noise impact assessments, particularly in urban planning and residential zoning.

4. Sound Exposure Level (SEL)

Definition:

• SEL represents the total energy in a noise event, normalized to a 1-second period.

Calculation:

• SEL is calculated using the formula:
•                                                                  
• where $L(t)$ is the instantaneous sound level and $T$ is the duration of the noise event.

Purpose:

• SEL is used to compare different noise events of varying durations and intensities.

5. Percentile Levels (L10, L50, L90)

Definition:

• Percentile levels indicate the sound level exceeded for a certain percentage of the measurement time.
  • L10L10​: Sound level exceeded for 10% of the time, often used to represent peak noise levels.
  • L50L50​: Sound level exceeded for 50% of the time, representing the median noise level.
  • L90L90​: Sound level exceeded for 90% of the time, representing background noise levels.

Purpose:

• Percentile levels provide a statistical description of noise, useful for understanding the distribution and variability of sound levels.

6. Maximum Sound Level (Lmax)

Definition:

• Lmax is the highest instantaneous sound level recorded during a measurement period.

Purpose:

• Lmax is used to assess peak noise events, which can be crucial for evaluating noise impacts in sensitive areas like hospitals and schools.

7. Minimum Sound Level (Lmin)

Definition:

• Lmin is the lowest instantaneous sound level recorded during a measurement period.

Purpose:

• Lmin helps in understanding the quietest periods in a noise measurement, providing insights into the baseline noise levels.

8. Traffic Noise Index (TNI)

Definition:

• TNI is a composite noise index used to assess traffic noise annoyance.

Calculation:

• TNI is calculated using the formula:
•                                                                 

• Purpose:

  • TNI is used to evaluate the impact of traffic noise on communities and set regulatory standards.