Ground Absorption Noise Correction Calculator

1) Ground Absorption Noise Correction Overview

Outdoor sound interacts with the surface between a source and a receiver. Porous ground absorbs energy and can reduce reflected sound, while hard ground can reinforce it. This calculator estimates the ground-effect correction, reported as A_gr in dB, across standard octave bands.

2) Key Inputs and Units

Enter source height h_s, receiver height h_r, and horizontal path distance d_p. Heights can be in m, cm, mm, ft, or in, and distance can be in m, km, ft, or in. Consistent geometry is essential because small height changes can shift interference behavior.

3) Ground Factors with Typical Values

Ground is represented by factors from 0 to 1: G=0 for hard surfaces (concrete, water, packed pavement), G=1 for porous surfaces (grass, cultivated soil, loose earth), and G≈0.5 for mixed terrain. You can specify G_s, G_m, and G_r for the source, middle, and receiver regions.

4) Octave Bands Considered

The tool computes corrections at 63, 125, 250, 500, 1000, 2000, 4000, and 8000 Hz. Mid bands (125–1000 Hz) are usually most sensitive to terrain and geometry, while higher bands often show steadier behavior controlled by how porous the ground is.

5) The Combined Correction Term

For each band, the total correction is A_gr = A_s + A_r + A_m. A_s uses (h_s, d_p, G_s), A_r uses (h_r, d_p, G_r), and A_m accounts for the middle region using G_m and a geometry weighting q. More negative A_gr generally means more attenuation.

6) Geometry Weighting q and Distance Threshold

A near-field threshold is calculated as 30 × (h_s + h_r). If d_p is below this threshold, q is set to 0, reducing the middle-region influence. For longer paths, q increases toward 1 using q = 1 − threshold/d_p, strengthening the middle ground contribution.

7) Practical Ranges and Quick Checks

With hard ground (G near 0), A_gr often stays near 0 to about −1.5 dB depending on band. With porous ground (G near 1), mid-band corrections can become several dB more negative. If results look extreme, verify units, distance, and that G values are within 0–1.

8) Worked Example and Reporting

Example: h_s=1.5 m, h_r=4 m, d_p=200 m, with grass-like terrain (G_s=0.8, G_m=0.9, G_r=0.8). The threshold is 165 m, so q≈1−165/200≈0.18. Export the full band table for modeling, and record the terrain description, G values, heights, and distance in your report.

FAQs

1) What does a negative Agr mean?

Negative A_gr indicates attenuation from ground interaction. In many propagation workflows, you add A_gr to other terms. More negative values typically reduce predicted receiver level.

2) How do I pick G for mixed terrain?

Use an intermediate value such as 0.3–0.7 based on the fraction of porous versus hard surface along the path. If conditions vary, set G_s, G_m, and G_r separately.

3) Why are there three ground factors?

Ground can differ near the source, along the middle path, and near the receiver. Separate factors let you model combinations like a paved source area, grassy mid-path, and hard receiver platform.

4) What is q and when is it zero?

q is a distance-based weighting for the middle-region term. If d_p ≤ 30×(h_s+h_r), q is set to 0, reducing the middle contribution on short paths.

5) Which octave band should I use?

Choose the band that matches your source spectrum or reporting standard. Mid bands like 500 Hz or 1000 Hz are common for comparisons, but equipment noise may peak in other bands.

6) Does this replace a full noise model?

No. It estimates the ground-effect correction only. Full assessments may also need source directivity, atmospheric absorption, barriers, reflections, and meteorological assumptions.

7) Why do small height changes affect results?

Ground interference depends on geometry. Changing h_s or h_r alters the balance between direct and reflected paths, especially in 125–1000 Hz bands, so modest input adjustments can shift A_gr.