task4

Description

The goal of this task is to investigate several acoustic devices such as barriers, scattering and sound absorbing surfaces. For that purpose we used a 1:8 scaled model.

The other goals of this lab are to obtain some knowledge about:

• The effective frequency response of the reflection/transmission from the investigated devices
• The typical directivity for those devices, and
• How to do simple scale modeling experiments.

The devices were put on a ping-pong table (without net) in the main experiment hall in order ot create a hard surface. This is not what the AES standard describes to do but it still enables us to get valuable results.

Due to the properties of all the equipment used the frequency range corresponds to 100-2000 Hz. Since the model is scaled at 1:8, the "real" frequency range is 800 - 16000 Hz.

This lab is divided into three parts. In the first one the reflection properties of the table are investigated. The second one is for us to "draw" a directivity graph for the scattered sound. In the last one measurements have been done to calculate the insertion loss due to the barrier put on the table.

Those different steps are well described in each session.

I. Acoustic devices: Unlimited knowledge

- Reflection characteristics of the table top

- Directivity of Scatterers and Absorbers

- Barrier Insertion Loss

- Auralization

II. summary

By comparing the level obtained by the direct path and the reflected path, we can deduce the reflection characteristics of the table, which is our measurement platform. In order to achieve these characteristics we have to take into account several parameters as the frequency scaling and the compensation of the difference of path distance.

The second task was meant to give us the directivity of scatterers and absorbers. For that we placed small scattering and absorbing patches randomly on the table and measured again, the direct and the reflected path. Similarly to the first task we deduced the difference between direct and reflected path and deduced the directivity of those patches.

Then we were to find out the insertion loss of a barrier which we placed between the loudspeaker and the microphone. Making two measurements gave us the difference between the two cases. Comparing these results to a wall insertion loss given in the ATA book, we get a good match.

Finally we auralized the results. We convolved speech/guitar sound samples to the frequency responses we got from the insertion loss measurements. We can clearly hear the influence the wall has on the sound and the speech (that corresponds to the values given in the ATA book). The barrier, however shows a much smaller influence on what we hear. What seems incoherent is the fact that the convolved sound samples have clearly lost their low frequency components which is contrary to what the figures of the insertion losses show.