Combine up to 5 noise sources, compute distance attenuation for point and line sources with atmospheric absorption, and apply A-weighting correction. SPL vs distance chart updates in real time.
What exactly is the "combined noise level"? If I have two machines each making 80 dB of noise, is the total 160 dB?
🎓
Basically, no! Sound pressure level (SPL) is a logarithmic measure of energy. You can't just add the decibels. In practice, you add the sound energies first, then convert back to decibels. For two 80 dB sources, the combined level is about 83 dB. Try it in the simulator above—add two sources with the same level and see how the total changes.
🙋
Wait, really? So why does the formula have a 10*log10? And what's the difference between the "Point" and "Line" source options in the tool?
🎓
Good questions! The 10*log10 comes from the definition of decibels for power or energy quantities. The key difference is in how sound spreads. A point source (like a pump) radiates sound spherically, so energy spreads over an area of $4\pi r^2$. A line source (like a busy road) radiates cylindrically, spreading over an area of $2\pi r L$. That's why the "distance attenuation" term changes from -20 log10(r) to -10 log10(r). Try switching the source type in the simulator and watch how the level drops off more slowly with distance for a line source.
🙋
Okay, that makes sense. But what about the "Atmospheric absorption" slider? When does that matter, and what does the "Peak frequency" have to do with it?
🎓
In practice, air isn't a perfect medium—it absorbs sound energy, especially at high frequencies. The absorption coefficient $\alpha$ (in dB/km) depends heavily on frequency and humidity. For instance, a 4000 Hz tone can be absorbed over 10 dB/km in dry air, while a 125 Hz tone might lose less than 1 dB/km. That's why you set a peak frequency. Move the "Evaluation distance" slider to a large value (like 500 m) and then adjust the "Atmospheric absorption"—you'll see it has a major impact on high-frequency noise over long distances.
Physical Model & Key Equations
The fundamental principle is the logarithmic addition of sound pressure levels from multiple incoherent sources, based on their energy.
Where $L_{sum}$ is the total sound pressure level (dB), and $L_i$ is the level from the i-th source (dB). Each $L_i$ is calculated at the receiver point, accounting for distance and atmospheric loss.
For a single source, the level at a distance $r$ from the reference point ($r_0 = 1$ m) is calculated, with different models for geometric spreading.
Here, $L_0$ is the reference SPL at 1 m (dB). $n$ is the attenuation exponent: $n=2$ for a point source (spherical spreading), $n=1$ for a line source (cylindrical spreading). $\alpha$ is the atmospheric absorption coefficient (dB/km), which is a function of the peak frequency and environmental conditions.
Frequently Asked Questions
A point sound source emits sound from a single point, such as a factory exhaust vent, and attenuates by 6 dB when the distance doubles. A line sound source emits sound continuously along a line, such as a road or production line equipment, and attenuates by 3 dB when the distance doubles. Please select 'Point/Line' in the tool's sound source settings.
It becomes significant when the distance is several hundred meters or more, or when the sound source contains many high-frequency components (e.g., metallic sounds). For typical factory sites (tens of meters), it can often be ignored, but for long-distance predictions, it must be entered. The value of α varies with temperature and humidity.
A-weighting correction applies weighting to each frequency band to approximate the human ear's sensitivity, which is more sensitive to mid-range frequencies (1–4 kHz) and less sensitive to low and high frequencies. It is standardly used in regulatory evaluations and environmental assessments, expressed as dBA.
Once the largest contributing sound source is identified in the bar chart, prioritize noise control measures for that source (e.g., shielding, installing sound-absorbing materials, reducing rotational speed). The tool can simulate the combined result by hypothetically reducing the SPL of that source, allowing you to verify the effectiveness in advance.
Real-World Applications
Environmental Impact Assessments: Before building a new factory or highway, engineers must predict the total noise impact on nearby communities. They model each noise source (compressors, fans, traffic lanes), calculate their contributions at different distances, and sum them to ensure legal limits are met.
Workplace Safety & Industrial Hygiene: In a manufacturing plant, a worker might be exposed to noise from multiple machines—lathes, conveyors, and air compressors. Calculating the combined 8-hour exposure level is critical for determining if hearing protection is required and if it meets OSHA or other regulatory standards.
Concert & Event Sound Planning: Sound engineers need to predict noise levels for audiences and, crucially, for the surrounding neighborhood to avoid noise complaints. They model each speaker array as a point source, account for distance, and sum their contributions to ensure front-row levels are safe and back-of-venue levels are adequate, while keeping spillover noise within limits.
Transportation Noise Modeling: Predicting noise from a railway or a busy road involves modeling it as a line source. Planners assess how noise diminishes with distance and use barriers or landscaping to mitigate it. The difference between point and line source attenuation is key here—road noise persists over much greater distances than a single vehicle's noise would.
Common Misconceptions and Points to Note
When starting with this tool, there are several pitfalls that beginners in CAE simulation often encounter. A major misconception is underestimating the importance of source type selection. For instance, are you inadvertently modeling the noise from a fan several meters long as a point source? If the physical size of the source is not sufficiently small compared to the distance to the prediction point (e.g., a 5m long source for a receiver 10m away), using a point source model will overestimate distance attenuation and calculate a lower noise level than in reality. As a rule of thumb, if the source size is more than about 1/5 of the evaluation distance, you should consider switching to a line or area source model.
Next is the mishandling of the "reference sound pressure L₀". This is the "noise level at a point 1 meter from the source," but it's risky to use catalog values directly without measured data. A difference of several decibels can arise depending on whether the catalog value is for "1 meter from the source surface" or "1 meter from the source center." For large machinery, this difference is not negligible. In practice, always check the metadata of the measurement conditions.
Finally, blind trust in the atmospheric absorption coefficient α. While the tool conveniently lets you set it with a slider, the actual α depends heavily on temperature, humidity, and frequency. The attenuation of high-frequency components differs completely between humid summer air at 80% and dry winter air. Rather than calculating all conditions with default values and considering them absolute, it's crucial to adopt an approach of sensitivity analysis—comparing results from multiple cases with varied parameters, understanding that "high-frequency sounds travel farther in dry winter conditions with low humidity."
Enter sound pressure levels (dB) for up to 5 sources in l0Num fields; use the slider for quick adjustment between 50–120 dB
Set measurement distance r in meters using rSlider; atmospheric absorption coefficient alpha (dB/m) adjusts for humidity and temperature via alphaSlider
Select point or line source geometry; calculator outputs combined SPL, distance-corrected SPL, A-weighting correction, and final weighted level in dBA
Worked Example
Highway noise scenario: three sources at L0=75 dB (heavy truck), 72 dB (medium car), 68 dB (ambient traffic). Measure at r=25 m with alpha=0.008 dB/m (typical humid air). Combined SPL at source: 77.8 dB. After inverse-square attenuation and atmospheric loss: 55.2 dB at 25 m. A-weighting correction at 1 kHz reference: −26.2 dB. Final result: 29.0 dBA. This matches regulatory monitoring for OSHA compliance near roadways.
Practical Notes
Atmospheric absorption alpha varies: dry air 60% RH ≈ 0.003 dB/m; humid air 80% RH ≈ 0.012 dB/m—adjust for seasonal conditions in long-range noise modeling
Line sources (e.g., traffic highways) use 10·log10(r) divergence; point sources use 20·log10(r) for accurate near/far-field predictions beyond 3 wavelengths
A-weighting penalizes low frequencies: 31.5 Hz = −39.4 dB, 250 Hz = −10.9 dB, 8 kHz = +1.1 dB; use for human perception compliance audits per ISO 3891