Airborne Sound Transmission

Airborne Sound Transmission

Introduction

In the last blog post we took a look at the basics of sound. We covered the basics of what sound is, how it is formed, what a decibel is and also the relationship between frequency and wavelength (phew).

This post we are going to look at how soundwaves move in a simple, open-air environment. We can describe this as airborne sound transmission. When sound travels from a source there are many things that can change it including absorption in the air, changes in temperature and moisture, reflections from other surfaces and more.

There are not many examples of noise sources in such a simplistic environment, but if you can imagine a hovering drone that is quite far away from any reflective surfaces, this would almost look like a single point in the distance. Therefore we can describe this as a point source.

Point Sources

As you can imagine by the term point source, the sound acts as though it is being transmitted from a single small point. Although close up the noise source might be more complex (like the four propellors on the drone), these small sources combine over distance to the point that all of the propellors act as if they are a single source around the drone.

The sound pressure from the point source is transmitted in all directions around the point source equally. This has the effect of the airborne sound emanating from the point in an ever-expanding sphere.

As the energy transmitted into the air as sound is propagating out in all directions, the intensity of the energy is spread out further and further over distance. In laymen’s terms, the sound from the source becomes quieter the further away from the source it gets.

If you have taken a look at acoustics before, this loss of energy intensity over distance can be described using a mathematical term (gasp), often referred to as the inverse square law. This has the effect of meaning that the sound pressure level is decreased by approximately 6dB every time the distance from the source is doubled. For example if the drone is 60dB(A) at 10m distance, it will be 53dB at 20m.

Most standard noise sources will have interaction with the floor and other nearby surfaces. These reflections have the effect of increasing the sound pressure from the source. A point source on the ground would act more like a hemi-spherical noise source (half a sphere).

Line Sources

Some noise sources consist of a long line of point sources (like a busy road) or might actually be a line source (like a gas pipe or flue). If we use a busy main road as an example, we can imagine the noise coming from the road to act like an expanding cylinder shape.

Once we take the effect of the ground into account the line source of a road can be seen to expand in a half cylinder shape. Which allows sound to propagate further than a point source and the sound pressure decreases less over distance.

This has the effect of meaning that the sound pressure level is decreased by approximately 3dB every time the distance from the source is doubled. For example if a road is 70dB(A) at 10m distance, it will be 67dB at 20m.

Noise Barriers

One method used to reduce noise from roads and other noise sources is to install an acoustic barrier at the source or acoustic fencing around a garden. This blocks the direct line of sight between the source and the receiver (person listening).

There are a number of factors that impact the effectiveness of a noise barrier:

The mass of the acoustic barrier
The height of the acoustic barrier

Sound can penetrate through a noise barrier that has a low mass, much like it does in a house with thin partition walls. To avoid this the surface mass density of the barrier should be at least 10kg.m^-2. The barrier should touch the floor and there should be no gaps or holes present.

It should be noted that having a density much higher than this will only provide diminishing returns in performance. This is because sound can travel over the top of the barrier due to the diffraction of the sound waves. As such the acoustic barrier will only provide more attenuation of noise if you increase the height of the barrier.

To calculate the performance of an acoustic barrier we often use the Maekawa method. To do this you must take the distance between the source and receiver into account along with the path difference caused by the height of the acoustic barrier.

You can see the effect that a noise barrier has on higher frequency sound in the video here.

[Model created by Matthew Wright of The Institute of Sound & Vibration Research at the University of Southampton]

HA Acoustics also use SoundPLAN noise modelling software to calculate the effectiveness of noise barriers for our clients. You can find out more about noise barriers here.