Automatic Equalization for Live Venue Sound Systems

Damien Dooley, Final Year ECEInitial Presentation, Tuesday 2nd October 2007

Contents

• Background

• Project Description

• Initial Work (Room Acoustic Modeling)

Background

• In live venues it can be quite challenging to get a balanced sound

• Several factors can influence a significant deviation between the desired output and the actual output of the system.

• Factors include, room dimensions, room structure, number of people in the room, furniture, sound source and listener location.

The Project

• This project aims to create a tool that will help automate the process of achieving a balanced sound.

• This will be achieved by developing a DSP system which will acoustically model the environment in which the PA system is setup.

• This system will compensate for the specific properties of the room to ensure that the perfect sound is achieved in each venue.

The system will consist of a microphone which will record the sound at a particular location in the room.

A pre-determined sound sequence will be played through the PA and the DSP system will record the audio signal received.

The system will then estimate the impulse response of the system which will be a combination of the input signal and the room acoustics.

The Project

• Capture audio signals using MATLAB• Develop and implement filters on the audio• Create a simple acoustic model in SIMULINK, (more on this…)• Develop an adaptive filter that can adapt to the acoustic

properties of the room, again thru SIMULINK• Create an active DSP system which will adjust the

discrepancies in the audio in real time (adaptive filtering)

Initial Work

Room Acoustic Modeling

• In order to properly understand how to implement this design, one must understand how sound behaves in various rooms. This is known as room acoustic modeling.

When a sound is generated in a room the listener will first hear the sound via the direct path to the source.

The listener will then hear the reflections in the sound and the magnitude will decrease exponentially after each echo.

Room Acoustic Modeling

• Sabine’s equation for reverberation time

• RT = 0.161V/A• This equation yields the time it takes (in seconds) for the sound

of the room to decay by 60dB. • V = Volume of the room• A = Absorption Window

(A is calculated by taking the total surface area of the room and multiplying it by it’s absorption coefficient.)

Room Acoustic Modeling

Some absorption coefficients are laid out as follows

Material125 250 500 1000 2000 4000

Concrete Block Unpainted 0.36 0.44 0.31 0.29 0.39 0.25Concrete Block Painted 0.1 0.05 0.06 0.07 0.09 0.08Glass, Window 0.35 0.25 0.18 0.12 0.07 0.04Plaster On Lath 0.14 0.1 0.06 0.05 0.04 0.03Plywood Panelling 0.28 0.22 0.17 0.09 0.1 0.11Carpet On Pad 0.08 0.24 0.57 0.69 0.71 0.73Gypsum Board 0.29 0.1 0.05 0.04 0.07 0.09Drapery, Lightweight 0.03 0.04 0.11 0.17 0.24 0.35

Frequency (Hz)

Example

2

5

10

Taking a concrete room painted, with no furniture, pip at 500Hz.

V = 2 X 5 X 10 = 100m3

A = (Surface Area) x (Absorption Coefficient)

A = {2(50) + 2(20) + 2(10)} X {0.06}

A = 9.6

RT = 0.161V/A

RT = 0.161(100)/9.6 = 1.677s

Hence, it takes 1.677 seconds for the pip at 500Hz to drop by 60dB.

Summary

• The project will require a deeper understanding about how sound behaves in different environments.

• It will greatly increase my knowledge of MATLAB and SIMULINK, and the general area of DSP.

Questions?