Outdoor Noise Prediction: The Advantages and Limitations of Computer Noise Modelling

30 Aug Outdoor Noise Prediction: The Advantages and Limitations of Computer Noise Modelling

During BKL’s 54 years of acoustical consulting, the method of calculating and predicting outdoor noise propagation has evolved from hand calculations with slide rules to georeferenced 3-D computer models. Noise contour maps can look impressive, but we’re interested in the acoustics!

How does software figure out how loud something is? Is it better than old school methods? Let’s take a quick look under the hood of a 3-D computer noise model and find out.

Software and Calculation Standards

First off, acoustical engineers rarely simulate the actual physics of sound propagation in an outdoor environmental setting. Calculating how air molecules vibrate and interact with each other (which is how sound actually moves) is still not practicable. We salute researchers and scientists around the world who are pushing the boundaries of this aspect of computing.

Instead, BKL uses commercial software that implements industry calculation standards developed at a time when noise propagation calculations were mostly done by hand.

Furthermore, most regulatory agencies require that any computer software used must comply with an internationally recognized calculation standard. One such standard is ISO 9613-2, which has a long descriptive title that we’ll leave for you to Google. In essence, this calculation standard considers how noise propagates from a point source to a point receiver, and considers various factors including these five:

Geometric Divergence

This is the spreading out of sound energy over distance. For example, as you move farther  and farther away from a sound source, less and less of the direct energy will be apparent to you. See the figure below

Atmospheric Absorption

Over a very long distance, the friction with the air molecules themselves absorb some sound energy. One extreme analogy is to imagine a thick sponge getting wet at one end and being substantially drier at the other. Sound in the air somewhat resembles the water in a sponge in that it gets absorbed as it moves through the medium. In reality though, atmospheric absorption is a relatively small effect compared to geometric divergence.

Ground Effect

This is the acoustical influence of the ground between the sound source and receiver. Ground effect is rated as either hard (rock, concrete, ice, etc.) or soft (grass, trees, vegetation, farm land, etc.).

Reflection from Surfaces

You’ve heard echoes in a canyon or on a street full of skyscrapers. This is because sound reflects off hard objects like rocks or buildings and returns to the source. In fact, sound reflection is an important geometric consideration as sounds can bounce off one surface and cause a nuisance to people in a completely different direction.

Screening from Obstacles

This is quite an intuitive phenomenon. A substantial amount of sound energy is blocked by placing an obstacle between the source and receiver. However, sound always leaks around the obstacle. That is why you can still hear traffic behind noise barriers—it’s just a lot quieter!

Benefits of ISO 9613-2 and Computer Modelling

One advantage of this calculation standard is the math is very simple. For each source and receiver, you can perform the calculations for all five factors by hand (and maybe with help from log tables and slide rules). The advantage of modern computer software is that it can scale up in a way that would not have been possible even 10 years ago. So we can, for example, import CAD or GIS data to model accurate geographical locations, calculate hundreds of noise sources and receivers at a time, and consider the acoustical effects of various design layouts or parameters, all with several clicks of a mouse!

At BKL, our work ranges from predicting the noise exposure of an air conditioning unit at a neighbouring house to importing highway upgrade designs through an entire municipality to assess the noise exposure to hundreds of homes. Computer modelling allows us to help designers understand the outdoor noise environment of their designs before they are built. This means they can consider and implement appropriate noise mitigation, if needed, as part of the design phase.

So, is all this computation actually better?

Overall, yes! We are able to do noise predictions that simply were not possible in the past. The merger of 3-D graphics into software packages has allowed us to create noise contour maps to visualize how sound levels change over the terrain. Furthermore, we can incorporate more noise sources and more noise-sensitive spaces into the models.

Accurate Inputs. Quality Predictions

The old saying goes, garbage in is garbage out. It applies to 3-D noise models. The quality of noise predictions depends greatly on the accuracy of the inputs. In particular, one challenge has always been obtaining accurate sound power data from equipment manufacturers. For example, when predicting HVAC-related noise, it is critical that we receive octave band sound power data. Whether it’s from the equipment supplier or a noise measurement, these data need to be added to a model before we can produce meaningful predictions. For road traffic noise predictions, we need information such as traffic volumes, vehicle speeds, and percentage of heavy vehicles, like trucks or buses, on the road.

ISO 9613-2: Not Quite Perfect

However, the ISO calculation standard itself has limitations: It does not consider all situations encountered in the real world. One such limitation is that the standard considers only moderate downwind conditions when determining atmospheric attenuation. This means that calculations do not provide any upwind calculation results. However, moderate downwind conditions are typically more useful when trying to predict a long-term average scenario (over months or years), or when considering a short-term worst-case scenario, as noise is generally louder downwind. Another limitation is the standard provides only estimated accuracies within 1 km. This means that predictions beyond 1 km may differ significantly from measured results.

One reason for these limitations is that, like all calculation models, ISO 9613-2 simplifies the real world to allow for easier computations. For example, the calculation standard assumes a constant set of conditions within the air that never exactly reflect the real world. Also, parameters like ground absorption reflections are treated with discrete values. In reality, the amount of absorption in different ground materials fluctuates, and reflections off real buildings are quite complex acoustically, especially off buildings with many different shapes.

Conclusion

Computer-aided outdoor noise prediction is a powerful tool in an acoustical consultant’s toolbox. At BKL, we use the latest noise modelling software and our advanced instrumentation to give our clients the most accurate predictions possible, so they can consider the effects of outdoor environmental noise, include appropriate mitigating measures in the design, and avoid costly complaints and retrofits.

If you have a project where 3-D noise predictions would be helpful for you, please don’t hesitate to give us a call!

Written by Trevor Cheng