Evaluating a Reflection

[dragonfyr]

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Many recent posts have made mention of imaging problems and other anomalies within rooms.

The attachment illustrates the basic windowing framework that defines the relationship of reflections within the time-intensity domains. Note that all reflections that exist within the area defined as Image Shift and Echoes are to be absorbed. Reflections within the rest of the defined areas are not as simple, and a well-behaved decay relationship becomes desirable, as well as addressing the non-linear frequency composition of the component reflections. Within this range specific reflections are treated, not the entire field. And a simple set of instructions to absorb &/or diffuse is not easily stated in 25 words or less, as the particular environments usage will determine the particular treatment. Thus, questions of space utilization (music, conference, lecture, multipurpose, theater, sports, etc.) become a critical determining criterion. Moreover, while the various behaviors are quite well understood, a simple one-size-fits-all treatment may not result in the desired improvement in all but the most grossly misaligned acoustical spaces. Despite the various websites selling such products, there is no one size fits all generic solution. Therefore it is easy to spend a lot of money and end up with a mediocre result.

In the future, I will attempt to provide some actual responses illustrating the reflections within both the frequency domain and the time domain and show how the two domains can augment each other, and how they each fall within this diagram, illustrating very different yet very important information regarding of the same event.

In addition, for those who have expressed concern about the correlation of measured responses with the psycho-acoustical limits of hearing, 1/3 octave smoothing will be utilized as frequency anomalies smaller then 1/3 octave tend to be smoothed out by the ear.

[mikebse2a3]

dragonfyr said:

Not all reflections that exist within the area defined as Image Shift and Echoes are to be absorbed. Reflections within the rest of the defined areas are not as simple, and a well-behaved decay relationship becomes desirable, as well as addressing the non-linear frequency composition of the component reflections. Within this range specific reflections are treated, not the entire field. And a simple set of instructions to absorb &/or diffuse is not easily stated in 25 words or less, as the particular environments usage will determine the particular treatment. Thus, questions of space utilization (music, conference, lecture, multipurpose, theater, sports, etc.) become a critical determining criterion. Moreover, while the various behaviors are quite well understood, a simple one-size-fits-all treatment may not result in the desired improvement in all but the most grossly misaligned acoustical spaces.

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I haven't seen it mentioned often but one option I'm thinking of trying is to "Redirect" the first reflection points(Mirror Image Points)of the left/right side walls in my small listening room instead of absorption or diffusion of these areas. I have found absorption doesn't work well in a small room and diffusers used in small rooms is also complicated by the small spacing created in small rooms.(ie: to early reflections from the diffusors due to the small space).

So I'm asking in your experience would redirecting an interfering reflection point also be an appropriate(Better?) option as opposed to absorption or diffusion so that its energy can be of a benefit to the listener by being delayed and then introduced at a proper point in time?

mike

[dragonfyr]

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On 7/24/2005 3:16:01 PM mikebse2a3 wrote:

I haven't seen it mentioned often but one option I'm thinking of trying is to "Redirect" the first reflection points(Mirror Image Points)of the left/right side walls in my small listening room instead of absorption or diffusion of these areas. I have found absorption doesn't work well in a small room and diffusers used in small rooms is also complicated by the small spacing created in small rooms.(ie: to early reflections from the diffusors due to the small space).

So I'm asking in your experience would redirecting an interfering reflection point also be an appropriate(Better?) option as opposed to absorption or diffusion so that its energy can be of a benefit to the listener by being delayed and then introduced at a proper point in time?

mike

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Please note the correction made to the typo in the original post! NOTE (not "Not" ;-) all reflections in the mentioned Image Shift and Echo zones are to be removed! Wow! The difference that an "e" makes!

Myriad attempts at repeatedly copying and pasting this morning while dealing with the web site problems has taken its toll! Thank you for calling this to my attention!

Regarding the treatment of the various reflections This is the subject that I had hoped to avoid at this point, as it is not as easy as simply saying that you should absorb or diffuse all reflections, as tempting as this might be! And I can just as easily say that you do not necessarily want to ignore them either!!

Diffusion is a means of redirecting the reflections in such a manner as their acoustic energy is spread over a greater area (more appropriately, volume!)

Simplistically speaking, you do want to address many of the reflections, but in order to benefit from Wally Heiders experience with the first implementation of the LEDE concept (which was overly damped), you want to surgically address various components of the reflections. And this is further complicated by the fact that each reflection consists of a wide frequency spectrum as well as various reflections of various intensities emanating from the various sources (drivers, diffractive sources, etc.). Thus, depending upon the anticipated use of the room*, not only is it prudent to address the intensity/gain of the reflection, but it is also appropriate to address aspects of the frequency composition. And to this end, simply using non-specific absorptive or diffusive products without concern for their very specific frequency sensitive characteristics will result in results that may very well be as detrimental as the original problem.

This is why I have straddled the proverbial fence to the chagrin of some by advocating both the need for treatment of the early reflections when this issue is ignored or dismissed, while also dissenting when simple generic absorption has been suggested or employed! What has heretofore been referred to as magic or an art is in fact understanding the spectral balance that comprises the reflected signals, and to surgically address the aspects that affect the quality of the total sound energy field.

And it is to this end that the proper tools become invaluable. Simply knowing that there are reflections, and understanding their impact upon the total system has not proved to be sufficient in and of itself. This has been a primary focus in the learning curve that has pre-occupied many people since the discovery of the ability to quantify and qualify the components of this domain. And it is precisely this aspect that has resulted in great strides being made in the quality of rooms being designed.

Of particular note, is the work of Dr. Peter DiAntonio of RPG Systems. His use of quadratics and fractal geometry has furthered the ability to treat very specifically some of the variable mixtures of frequency isolations, absorption and diffusion in very specific ways. And while I do not expect the average person to delve into, nor to enjoy, the mathematical modeling employed, it is truly amazing. (I had hoped those days were over after the period of slave labor politely and euphemistically called grad school!)

Additionally, I should add an additional comment here. It is easy, too easy in fact, to become focused on a particular domain, when in fact both provide invaluable insight into the real world phenomena! And both complement each other. Small path lengths are perceived in the frequency domain, and the perceived affect measured in the frequency domain. Whereas when the signal paths are large, we perceive the anomalies in the time domain. (But note that the component signals can only be measured as such in the time domain!)

As we can see from the polar and comb filtering responses, when the physical dimensions are less then ¼ wavelength, multiple sources combine in a well-behaved manner and the frequency domain is pre-eminent. But when the physical dimension is greater then ~4-5 wavelengths, detrimental effects manifest themselves in the time domain. Comb filtering may be present, but they are closely spaced (within the 1/3 octave range where our hearing smoothes them) and thereby not audible.

So, it is the region > ¼ wavelength and < ~4-5 wavelengths, where physical dimensions are comparable to wavelengths, that the dilemma lies. It is in this area of transition between small and large that the real problem area lies.

And on this note I will end for now! I hope to obtain some plots that will illustrate how the two domains overlap and how multiple sources, or a source and reflections (in effect multiple sources) can appear in some rather unusual ways. And how they deceptively appear when we look at them in measurements. Also, I hope in the near future to post an article that Pat Brown has been working on regarding some alternative methods of obtaining and convolving impulse responses with some evolving freeware tools base on some new evolving standards!

(* Hereafter, I will generally try to assume the room is for a mixed use, music and home theater environment emphasizing vocal intelligibility. But please note that there are different criteria and subsequently different tunings for different uses!)

[Nklipsch]

Ah yes I remember that image stating which reflections and which action to be taken. What is it from? An online pdf I think

Mike.e

[dragonfyr]

It is a 'classic' diagram of the various zones within an ETC which has been used for at least 17 years (I have copies dating from 1988!). This particular scanned copy is from my SynAudCon seminar notes from a presentation by Dale Shirk. Pat Brown has also used this same image in many presentations as well. You should be able to source it from many places.