The Octagon-Shaped Room Pt 1 – Site Survey

A Case Study – By Pat Brown

This article by Pat Brown presents a case study of an audio installation of an octagon-shaped room, beginning with a site survey.

Introduction

In this multi-part article I will look at a complete sound system renovation project, from the initial site survey, to validation of the finished results. Along the way I will provide some room models and auralizations used to consider various loudspeaker types and placements. But first, some background…

The Scenario

My first crack at this room was in the early-1980s. I was a struggling sound contractor and this was my introduction to a room specifically designed to provide acoustic support for organ and choir. I hung a couple of loudspeakers near the ceiling that looked aesthetically pleasing. Unfortunately, the resultant speech intelligibility was poor, as was the gain before feedback. It was this experience that prompted me to purchase my first copy of Sound System Engineering by Don and Carolyn Davis (the “yellow book”). Part way through chapter 1 I realized that I had broken all of the rules, and that this failed attempt at sound reinforcement was completely predictable.

The Short-Term Fix

The short term solution was to move the loudspeakers closer to the audience, suspending them on steel pipe. While this dramatically improved the system performance, it was still barely adequate and the congregation never really got used to seeing them. At least I got paid and made payroll that month. I also suggested some acoustical treatment, but this idea was quickly shot down by the music director – a nice person but a purist with regard to traditional music.

Fast-Forward 25-Years

Last year I got a call from an old acquaintance who is now the music director at the church. We played in high school rock bands in the same town and era, and had mutual friends. He asked if I would have a look at the sound system. I agreed on two conditions – that the church hire a local contractor to provide the gear and perform the upgrade, and that I get to use the data as a case study.

Since my stab at this in the ’80s, the church has had two other sound systems. Neither proved adequate, and they were still looking for the final solution. One big change from the first time around is that the musical emphasis has shifted away from organ and choir, and they now hold – you guessed it – a contemporary music service. The idea of room treatment now seemed tenable, as long as the organ sound was impacted as little as possible. I felt that a combination of increased directivity sources and some strategically placed absorption could make a tangible improvement.

The Plan

First, a site survey is necessary to quantify the acoustical environment. My procedure is on the next page. Part 2 will discuss construction of a room model. Part 3 will look at the design process, including the use of auralization for evaluating loudspeakers and placements. Part 4 will wrap things up with an overview of the compromises and some measured end-result RIRs. pb

The Site Survey

The objective of the site survey is to establish a set of “known’s” about the space. I chose a test loudspeaker with a known directivity (a powered 8-inch coax) and placed it at a relevant location (the talker position at the lectern). The brand is unimportant and the data file is unnofficial – made for my own use.

The data collected will be suitable for refining the room model. Once refined, other loudspeakers can be substituted and placed at different positions.

I chose a set of listener positions that will allow the performance of the test loudspeaker to be well-characterized. Using an Ipod source and Sony recorder, I recorded a test track at each listener position. The test track (shown at bottom of page) contains:

• 1. Anechoic speech
  

  EASERA

• 2. A period of silence (room noise evaluation)
• 3. A log-sine sweep (for IR production)
• 4. Some soft piano music (to provide time to move the recorder to the next position).

The RIR is produced by convolving the wet sweep with an inverse of the dry one using the freeware GratisVolver. The octave-band RTs and other metrics can be determined from the RIRs using a measurement program.

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Stereo RIRs for six test positions (one shown at
above right) are available in the SAC Library (Room Survey 20). These were made with a Tannoy
PowerV8 loudspeaker positioned as per the diagram.
The unofficial CLF data file for the
loudspeaker is available here (first device).