ROOM CORRECTIONINTRODUCTION TO

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ROOM CORRECTION - INTRODUCTION

It has been known for many years that the room we listen to music in is a significant arbiter of the sound quality we enjoy. A number of different approaches have been tried over the last 40 years to remove the negative influence of the room on the quality of sound reproduction achieved from a given set of electronics. In the early days crude frequency response equalization was performed through application of analog parametric equalizers or graphic equalizers. These devices were typically not designed for high quality home high end audio design reproduction and so gained a negative reputation amongst discerning audiophiles (whether stereo or multichannel) and home theater enthusiasts. Over the last 20 years the increasing availability of cheap and powerful digital signal processing (DSP) hardware has led audio companies to attempt to design and implement ‘digital room correction’ (sometimes abbreviated to DRC and also known as room equalization / EQ) devices.

ROOM CORRECTION - INTRODUCTION

The first company to release a product in this field was SigTech, who were followed by a number of other companies, most notably TacT, who can probably be credited with introducing room correction to the two channel audiophile market. Audyssey, a company that primarily licenses their DSP code to other manufacturers such as Denon, have been responsible for the rapid penetration of room correction into the home theater market. It is probably fair to say that digital room correction is now more embedded in the home theater than it is in audiophile systems, a likely consequence of manufacturers trying to ‘out feature’ each other in the surround sound processor market.

ROOM CORRECTION - INTRODUCTION

Despite these trends, much confusion still exists about what a room correction product does, what problems it can (and cannot) solve and therefore its ‘place’ in a modern high quality sound reproduction system. Part of the challenge of understanding room correction is that it requires a reasonable level of understanding of sound quality, acoustic science, acoustic measurement and psychoacoustics (how humans perceive sound). The majority of the articles I have read online or in print magazines do not cover the fundamentals in enough depth to allow the curious and committed reader a chance to understand room correction on anything more than a cursory level. By the end of this article I hope that you will have learnt enough to judge for yourself what room correction can and cannot do and how best to apply it in the context of a world class music or home theater system.

ROOM CORRECTION - INTRODUCTION

The conclusion I have reached from my study and from its application in my client’s systems is that room correction can be a valuable tool to address certain acoustical problems, particularly those related to room mode resonances. There are many things that room correction cannot, for want of a better word, correct. These acoustical issues can only be addressed through good design, appropriate use of acoustic treatment and calibration (e.g. speaker/listener positioning) techniques.

ROOM CORRECTION - INTRODUCTION

The first task for you on this journey is to:

Learn a vocabulary for describing sound quality through the five sound quality metrics: clarity, focus, envelopment, dynamics and response.

Understand the science behind how a room behaves at different frequencies; above and below what is termed the ‘transition frequency’ at 200Hz.

Relate acoustical measurements to the typical major issues a room introduces and how these impact the five sound quality metrics.

It is important to take the time to study these topics, since the remainder of the discussion on room correction assumes familiarity with the terminology and language used.

HOW A ROOM CORRECTION PRODUCT WORKS

HOW A ROOM CORRECTION PRODUCT WORKS

The process used is, at a high level, the same for nearly all types of room correction products:

Play back a known signal and measure the magnitude versus frequency response of the system (i.e. equipment, speakers AND room).

Compare the measured magnitude versus frequency response to a target magnitude versus frequency response.

Create correction filters to match the measured to the target responses.

Apply the correction filters to the music signal.

HOW A ROOM CORRECTION PRODUCT WORKS

An example will help to illustrate this. At 60Hz the measured response of a nominal system is 78dB. A target has been set of 75dB. The measured response is therefore +3dB louder than target, and a correction filter of -3dB is needed. An additional example is shown in the table below; all figures are in dB:

Frequency Measured Response

Target Response

Difference

Correction Filter

60 78 75 +3 -3

1000 73 75 -2 +2

HOW A ROOM CORRECTION PRODUCT WORKS

An illustration of what occurs in a room correction product; the measured frequency response has been compared to a target and a correction filter calculated.

HOW A ROOM CORRECTION PRODUCT WORKS

Different products may have sub-processes within each of the four major steps or may implement the process differently:

Audyssey digital room correction products, for example, use a swept sine signal during the measurement step. They require that the user takes multiple measurements at different spatial locations within the listening room. Correction filters are generated by finding a solution that will be effective for all locations.

The Rives Audio PARC, which is a product using analog parametric equalizers, requires the user to generate and play a test signal such as pink noise, measure the results on a separate analyzer and calculate the required filters before manually setting up the product. There is no automated filter generation process

HOW A ROOM CORRECTION PRODUCT WORKS

Velodyne subwoofers provide an automated pink noise based measurement and filter generation system using a measurement at a single location.

Some products (e.g. TACT) allow the user to ‘draw’ their own frequency response target. A couple of others (e.g. Audyssey) provide a limited selection of choices. Many however use a simple flat frequency response as the target.

HOW A ROOM CORRECTION PRODUCT WORKS

The only exception to the use of magnitude versus frequency response in the measurement and comparison steps (1 and 2 above) is Meridian. The Meridian digital room correction system follows the same high level process (measure, compare, generate and then apply correction filters) but utilizes the reverberation time versus frequency response of the system. The system analyzes the deviation of the measured reverberation time from a target. Since the Meridian system only operates below 200Hz (i.e. below the transition frequency) it can take advantage of the minimum phase behavior of the room in this region, allowing it to correct reverberation time (or more correctly modal decay time) through magnitude based correction filters.

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