Room modes are caused by perfect constructive interference between a sound wave traveling between two boundaries. This article provides a high level overview of room modes and their impact on sound quality. Two examples of how to use subwoofers to reduce the impact of these modes on sound quality are explained – placing the subwoofer in a pressure null and destructively driving a mode with two subwoofers.

Deep nulls in the frequency response in the bass region can be caused by phase cancellation between the direct sound wave from a sound producing device and the indirect sound wave that has reflected from a nearby boundary such as the floor, ceiling or walls. This phenomenon is called speaker boundary interference or SBIR for short. This article introduces SBIR and explains how these deep nulls can be ameliorated through use of a subwoofer.

Today I’m going to look at room correction products specifically designed to be used with subwoofers only. Included in this article is examination of products by Audyssey, Velodyne, DSPeaker and SVS.

In the next couple of articles I’m going to create a comprehensive directory of the room correction products available on the market and assess them against the functional criteria that have been explained in the last two articles (here and here). Today I’m covering Meridian (the 861 and G68 surround processors), TacT (including the 2.2XP) and Lyngdorf. Future articles will examine Audyssey, Velodyne and DSPeaker.

Last week I talked about the importance of letting any end user choose the frequency range to which room correction is applied and allowing them to specify a target frequency response. In this weeks article I cover the last two functional criteria for a room correction product. Whilst these are less important than the first two, they are still important, and just make good sense! This week I cover the importance of not performing needless analog to digital conversions and providing measurement and filter generation capabilities.

My research has led me the conclusion that there are a number of basic functional elements that ALL room correction devices should possess. These conclusions derive from a room acoustics viewpoint rather than a sound quality perspective. It is interesting that there are many devices on the market that do not meet these two basic criteria…

According to Stereophile magazine, the top (‘A’ ranked) room correction or room EQ devices are the: Audyssey Sound Equalizer; Meridian 861 with MRC room correction; Rives Audio sub-PARC / PARC; SVSound AS-EQ1; Velodyne SMS-1 and the Z-Systems RDP-1 Reference. This article explores the pros and cons of each device from a purely functional perspective…here is an excerpt from the section on the SVSound AS-EQ1: “Designed for use with either single or dual subwoofers, the AS-EQ1 is based on a implementation of Audyssey’s DSP room EQ code. Plug in the included measurement microphone and install the custom Windows based software onto your computer and you are ready to go! Because it uses Audyssey code many locations in the room are measured (up to 32) before the correction filters are generated. This makes it particularly suitable for use in a multi-seat home theater where the bass response and therefore degree of correction required will vary significantly from seat to seat”

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.