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Audiophile listening room helps to define classroom acoustics

Audiophiles and Recording Engineers Generated Advances in Small Room Intelligibility Classroom intelligibility is a specialized application in the field of general intelligibility. The subject of general intelligibility was studied by Art Noxon during the mid 1980's and a number of technical papers were developed and published. The financing and focus of this work in intelligibility was supported by "golden ear" audiophiles in their quest to develop and own high performance "listening rooms". At that time no one imagined the work accomplished in developing and defining high end audio playback rooms would be used some 20 years later to improve the listening experience of students in the classroom.

History

The search for acoustic perfection in high performance small rooms lasted one decade, starting in the early 1980's and coming to an abrupt halt in 1990. This type of investigation was funded by profits in the flourishing hifi industry. A series of three economic setbacks essentially finished off the hifi industry and funding for the research. The first was a national recession in 1990 and the cutback in consumer discretionary spending . By 1992 home computers were introduced and hifi enthusiasts, being among the most sophisticated of electronic equipment buyers were first in line to buy home computers, not more audio equipment. Next came the DVD and Home Theater and any funds set aside for hifi improvements became reallocated for home theaters. Home theater, being a highly visual experience and fairly popular does not yet demand high level acoustic performance. However, the demand and interest for intelligible small rooms has resurfaced today, in the classroom.

Intelligibility Became the High Point in Acoustic Treatment of HiFi Listening Rooms

To become familiar with intelligibility it is very instructive to revisit those old papers on small room intelligibility developed some 15 years ago within the hifi industry. The first paper to study is called "ARTICULATION: Prerequisite to performance". It was written in 1988 and delivered to the Audio Engineering Society convention in June of 89 in New York. This paper represents the overview of the science and applied art of small room intelligibility. It is an excellent place to start because it defines intelligibility starting with the most simple signal, the Morse code, and moves through the range of speech intelligibility and on to the topic of music intelligibility. It is a pragmatic overview and connects the dots that are otherwise found scattered among various papers on intelligibility.

Further study on intelligibility is available in another paper. Here the more practical side of intelligibility is being put to work in developing performance evaluation specification to be met by audio systems that play in small room acoustics. We have "Articulation and the Small Room" written by Art Noxon and presented in November 1988 at the AES Convention in Los Angeles. Here, the basic terms used in intelligibility are defined: Signal to noise ratio, modulation index, modulation transfer function, transmission index speech transmission index and octave masking. Then they are re expressed in terms of an acoustical testing method called the MATT, Musical Articulation Test Tones, a gated tone burst sequence. This test is used in numerous field applications to further explain what does and what does not work in the quest for better intelligibility when applied to small room acoustics.

The third paper was actually an appendix to the 89 paper. However, it is sufficiently separate of a topic that it is now republished under it's own heading. It is a field guide to using gated tone bursts as a diagnostic tool in the evaluation of room acoustics. It is intended to provide the field experimenter with the actual equipment list and settings used to produce modulation transfer functions. This short paper is called "Diagnostic MTF Testing Procedure" and is should be read only after both other papers have been read.

The basic premise for this series of works in intelligibility is that when working with small room acoustics, room resonance becomes a very significant problem. Room modes and their Q or damping factor is a major problem. Acoustic testing of the listening room had been restricted to balloon bursts, slow sine wave sweeps, pink or white noise spectrums and octave noise spectrum levels and octave band RT-60 measurements. But all of these tests missed performance indicators that directly applied to and were valued by listeners in rooms. The tonal type of modulation transfer function however directly addressed and accounted for differences in room acoustic treatments and their perceived value to the listener. It became apparent that steady state measurements and broad band transient measurements were not sufficient to account for intelligibility in small room acoustics.

Now, with opportunities to improve classroom acoustics on the horizon, the fruits of years of study and experimentation in small room acoustics, particularly small room intelligibility seems to have found a new home. It is fittingly appropriate that the pioneering work in high performance listening rooms done by some of the best listeners in the world has now found application in developing classroom acoustics for those whose hearing capabilities have been compromised.

There is more to the Signal to Noise ratio than just Noise

Our of these studies only certain aspects of listening room acoustics have been studied. These aspects do not embody "all" there is to know about quality listening. They merely address one aspect of evaluating room acoustics, the measuring of the signal to noise ratio on a tonal basis rather than a 2 octave wide basis as is ordinarily done. The audio playback in small rooms experience of the '80's contributed to the recognition of tonal MTF analysis in determining intelligibility for small rooms. But more, the audiophile and the recording engineer both work in small rooms and they have developed a sensitivity and tradition with respect to not only sound absorption but also to sound reflection.

There are 2 parts to the "signal to noise" view of acoustics. One is the "noise" part and most of the effort by all involved in classroom acoustics is focussed on absorbing or otherwise eliminating the noise. The other part of the "signal to noise" viewpoint is the "signal" part. This is a much less addressed factor. The signal of a sound begins with the direct sound, the one traveling the direct path between the sound source and the ear of the listener. But it also includes all early reflections. The contribution of reflections to listening, how strong they are, where they come from and when they arrive all make up a second quality of sound. The brightness of the sound, it's presence, is a consequence of how many reflections accompany the direct signal. The additive effect of early reflections to the S/N ration is too often not even considered.

The Recording Industry also Contributes to Intelligibility

The last paper presented here does address the "presence effect" of sound signals. It is called "Sound Fusion and the Acoustic Presence Effect" and was presented in 1990 in Los Angeles at the AES Convention. This work is born out of the efforts to improve microphone technique for recording engineers. Here we see that both the direct and the early reflections are important contributors to the strength of the sound "signal" and substantial experience with this type of sound is presented. Curiously, at the end of this paper is found a section that has nothing to do with the recording industry, it has to do with an experiment involving learning disabled children at San Diego University. The value of the Quick Sound Field effect was recognized early on to also apply to those who are learning disabled.

RT-60 is "Necessary but not Sufficient" for Good Intelligibility

The ADA has specified that the noise level in our classrooms should be quiet and the reverb times in the 500, 1k and 2k octave bands be short. The net result is that a substantial quantity of acoustic material has to be added to any accessible classroom, typically 200 square feet of material. There are a lot of factors that restrict what that material might be, and where it can be placed. We have black boards and clocks, windows and doors, light switches, air vents, maps and many more things that were there first. Acoustics is relegated to whatever space is left over. The question remains as to where in this remaining available space should the acoustics be placed.

We have found that it is never satisfactory to simply own a bundle of acoustical material. Even if you were to place the bundle of acoustic material in the right room it still isn't good enough. If you spread out the materials even in the room one effect will be noticed. Rearrange the materials and another effect is noticed. How the room sounds depends on where the materials are placed. Reverb analysis leads to introducing some 200 square feet of acoustic material into a classroom. The RT-60 requirement may be satisfied with any number of material distribution configurations. But which distribution best satisfies the needs of the students? That is the point to which we direct our attention.

More than Reverb Control, Intelligibility means Voicing the Room

We have found that in high performance, high intelligibility spaces, it is critical that the materials introduced into a room to satisfy the RT-60 requirement also have to be placed is specific areas, patterns and configurations in order to develop the inner detail of the listening experience. Absorption manages the RT-60 and knocks down the noise. Reflections enhance the signal and improves the overall S/N ratio. Therein lies the difference between a room that meets spec and a room that really sounds good.

Acoustics is sometimes referred to as being half science and half art. The science of acoustics specifies the material needed to meet the RT-60 spec and the art of acoustics determines where that material is to be located. And that's what we call "voicing a room".


©2002-09 Arthur M. Noxon, PE, MSME, MS
©2002-09 ASC. All Rights Reserved.