How Sound Spreads Is Reflected In Large Open Spaces

Large open cave Tham Lod in Pang Mapha, Thailand (Credit: John Spies / Barcroft Media)

In a room, we typically hear that a voice is louder than in an open area. This is the effect of reflected sounds which comes out of the mouth and is reflected on the walls and finally comes back into the receiver’s ear. Those sound reflections enhance the voice of people so that louder sound and greater clarity is gained indoors.

The sound received indoors can be divided into two parts. If the sound arrives at the receiver directly, it is called a “direct sound”. If the sound is reflected on a surface and finally received by the listener, it is called “reflected sound”, as can be seen in Figure 1.

Figure 1: The sound spreading in an extra-large space.

The further you stand from the sound source, the less you will hear it. Because the voice spreads like a constantly blowing balloon. The bigger the balloon is blown, the thinner it will be. The transmission of the reflected sound also follows this principle. Therefore, the later the reflected sound reaches the listener’s ears, the softer the final sound will be heard.

With the development of construction technology, some extra-large spaces have been built to accommodate more and more complex functions. It can be imaged that if the distance between the talker and the listener is the same, the larger room you stay in, the smaller voice the listener hears. When a space is large to a certain extent, is the reflected sound still important or the sound in extra-large spaces just spread as in the open area?

On-site measurements were carried out in four cases, such as the measurement in Figure 2. The volume of the four cases varies from 7000 to 190,000 m3, representing ordinary to extra-large spaces. Computer simulations were also conducted using image method to obtain more details.

Figure 2 A typical extra-large space – Changchun Station in China

The results show that the first reflection from the floor (FRFF) is significantly larger than other reflections because both the source and receiver are usually closer to the floor which can be used to enhance the existing prediction model.

The received sound energy in a room can be divided into three parts, which are direct sound, first reflection from the floor and other reflected sounds, as shown in Figure 1. They all occupy a significant proportion of sound energy that cannot be ignored in the prediction. The direct sound attenuates exponentially along the source-receiver distance, which is due to the spherical spreading of point sound source. The reflected energy, which is also called the “reverberant energy”, also attenuates near-exponentially, especially when there is a lot of sound absorbing material in the space. As to the FRFF, it is lower than the direct sound in the near-source area and almost the same in the far-source area. This suggests that some furniture or carpeting may be useful for reducing the noise in extra-large spaces.

A modified prediction model of sound pressure level has been developed based on the FRFF and obtains a good prediction accuracy, as shown in Figure 3. This prediction model can be used to predict the sound pressure level in extra-large spaces and it is also important for acoustic design and crowd noise control.

Figure 3 Predicting the difference between the existing classical model and the new modified model.

These findings are described in the article entitled Characteristics and prediction of sound level in extra-large spaces, recently published in the journal Applied Acoustics. This work was led by Jian Kang from University of Sheffield, UK, in collaboration with Chao Wang (Tianjin University, China), Hui Ma (Tianjin University, China) and Yue Wu (Harbin Institute of Technology, China).

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