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Measurement of Highway-Related Noise

8. Building Noise Reduction Measurements in the Vicinity of a Highway

This section describes recommended procedures for the measurement of building noise reduction, i.e., the effectiveness of a building structure in insulating residents from outside noise sources, in this case, highways. In contrast, these procedures may also be used to determine how effectively A structure contains internal noise, especially where the external environment is quieter than the noise environment within the building.(20) The following procedures are in accordance with the American Society of Testing and Materials (ASTM) Standard E966-84.(32)

Two sets of measurements are recommended: (1) exterior measurements of the roadway noise. (Note: If a traffic noise source is not available, a fixed, artificial noise source, such as a loudspeaker, may be used); and (2) interior measurements of the roadway noise within the building itself. The difference between the exterior and interior measured sound levels is the resulting noise reduction performance for that building, or commonly referred to as the "outdoor-indoor noise reduction."

8.1 Site Selection

8.1.1 Site Characteristics

8.1.1.1 Interior Measurements

The interior location should be a completely enclosed Space with, preferably, its largest dimension no greater than twice its smallest. During measurements, all other noise-generating activities in the room should be quieted. In addition, the interior ambient level should be at least 10 db below the lowest-anticipated, vehicle pass-by, maximum A-weighted sound-pressure level (LAFmx).

8.1.1.2 Exterior Measurements

Exterior measurement sites should have the following geometric characteristics:

8.1.2 Microphone Location

8.1.2.1 Interior Measurements

Microphones are placed at 1.5 m (5 ft) above the floor of the interior location and at least 1 m (3 ft) from any walls (See Figure 16). Measurements at several different heights and locations in the room are strongly recommended to achieve statistical precision.

8.1.2.2 Exterior Measurements

There are two potential locations for the placement of the exterior microphone as shown in Figure 16:

Position 1: At least 3 m (10 ft) from the side of the building, at the same distance from the road as the front wall, at a height of 1.5 m (5 ft) AGL. This position must be carefully selected such that the microphone is not shielded from the road by the building, or influenced by noise sources behind the building. This positioning essentially eliminates influences on the measured levels due to reflections. As such, this is the preferred position.

Position 2: Not greater than 2 m (6.6 ft) from the facade, located on the roadway side of the building, at a point opposite the middle of the facade, at a height of 1.5 m (5 ft) AGL. This setup is not recommended if the roadway facade of the building is within 7.5 m (25 ft) of the centerline of the near lane of traffic.

A roadway is shown on the left with a car traveling north in the right lane. Click on image for full description

Figure 16. Microphone positions for building noise reduction measurements.

8.1.3 Artificial Noise Source Position

If a loudspeaker is used, it should be located at a distance from the building facade such that the ratio of the distances from the loudspeaker to the farthest (D1) and nearest (D2) edges of the facade is no greater than two, i.e., D1/D2 ≤ 2. The loudspeaker should be angled at an incidence within the range of 15 and 60 degrees, preferably at an angle of 45 degrees (See Figure 17). This angle, θ, is determined by the perpendicular to the facade midpoint and the line joining the loudspeaker to the midpoint.

A loudspeaker is positioned facing a building façade at an angle. Click on image for full description

Figure 17. Loudspeaker position.

8.2 Noise Descriptors

The equivalent sound level (LAeq) should be used to describe continuous sounds, such as relatively dense highway traffic. The sound exposure level (LAE), or the maximum A-weighted sound level with fast time response characteristics (LAFmx), should be used to describe the sound of single events, such as individual vehicle pass-bys. The day-night average sound level (Ldn) and the community-noise exposure level (Lden) may be used to describe long-term noise environments (typically greater than 24 hours), particularly for land-use planning. Note: Once the LAeq and LAE noise descriptors are established, other descriptors can be computed using the mathematical relationships presented in Section 2. Ultimately, the particular descriptor chosen is of little importance since the objective of these measurements is to obtain a change in sound level.

8.3 Instrumentation (See Section 3)

8.4 Sampling Period

Different sources may require different measurement periods. For multiple-source conditions, a longer sampling period is needed to obtain a representative sample averaged over all conditions. Typical sampling periods are 15 minutes, 1 hr and 24 hr. Measurement repetitions at all receiver positions are required to ensure statistical reliability of measurement results. A minimum of 3 repetitions for like conditions is recommended, with 6 repetitions being preferred. Table 5 in Section 4.4 presents suggested measurement sampling periods based on the temporal nature and the range in sound level fluctuations of the noise source. Guidance on judgment of the temporal nature of the source may be found in ANSI S1.13-1971.(16)

8.5 Measurement Procedure

  1. Prior to initial data collection, at hourly intervals thereafter, and at the end of the measurement day, the entire acoustic instrumentation system should be calibrated. Meteorological conditions (wind speed and direction, temperature, humidity, and cloud cover) should be documented prior to data collection, at a minimum of 15-minute intervals, and whenever substantial changes in conditions are noted.
  2. The electronic noise floor of the acoustic instrumentation system should be established daily by substituting the measurement microphone with a dummy microphone (See Section 3.1.5). The frequency response characteristics of the system should also be determined on a daily basis by measuring and Storing 30 seconds of pink noise from a random-noise generator (See Section 3.1.6)
  3. Ambient levels should be measured and/or recorded by sampling the sound level at each receiver and at the reference microphone with the sound source quieted or removed from the site. A minimum of 10 seconds should be sampled. Note: If the study sound source cannot be quieted or removed, an upper limit to the ambient level using a statistical descriptor, such as L90, may be used. Such upper limit ambient levels should be reported as "assumed." Note: Most sound level meters have the built-in capability to determine this descriptor.
  4. The interior and exterior measurements should then be performed Simultaneously; and the characteristics of the source should be carefully documented (e.g., if actual highway traffic is being used, the volume, speed, and mix should be recorded).

(Note: Appendix B provides example field-data log sheets.)

8.6 Data Analysis

  1. Adjust measured levels for calibration drift (See Section 3.1.4).

  2. Adjust measured levels for ambient (See Section 8.6.1).

  3. Compute the building noise reduction

    (NR) as follows:
    For exterior microphone at Position 1:

    NR = Lexterior - Linterior           (dB)

    For exterior microphone at Position 2:*

    NR = Lexterior - Linterior - 3           (dB)

    For example:

    • Lexterior = 77.0 dB for microphone-position 2
    • Linterior = 65.0 db

    Therefore:
    NR = 77-65-3 = 9 dB

8.6.1 Ambient Adjustments

If measured levels do not exceed ambient levels by 4 dB or more, i.e., they are masked, then those data should be omitted from data analysis.

If measured levels exceed the ambient levels by between 4 and 10 dB, then correct the measured levels for ambient as follows (Note: For source levels which exceed ambient levels by greater than 10 dB, ambient contribution becomes essentially negligible and no correction is necessary):

Ladj = 10 x log10 (10{0.1Lc} - 10{0.1La})           (dB)

where: Ladj is the ambient-adjusted measured level;
Lc is the measured level with source and ambient combined; and
La is the ambient level alone.

For example:

Therefore:

Ladj = 10 x log10(10(0.1 x 55.0)-10(0.1 x 47.0)) = 54.3 dB


Updated: 07/06/2011
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