"Completely dead" rooms

[damonrpayne]

I'm reminded of something I read, a quote from a guy working on THX certified sound track titles. He claimed the final room should be as dead as possible despite some claiming that some life shoudl be kept in the room, because "Any ambiance we want, we put in the soundtrack". Arrogance, accurate? Discuss.

[mas]

Here we are reminded that there are different acoustic designs for different purposes. And the failure to understand this is the cause of a great deal of misunderstanding and confusion both here and in the industry in general. And I hope that I don't offend anyone, but this is a common source of confusion here as well.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

As this 'guy' is mentioning dealing with "sound track titles" in a mix environment, he is completely correct within a limited application environment! The purpose of the acoustic design of a studio mix booth is to completely remove the room and to focus exclusively on the direct signal, as that is ALL the mix engineer has control over. Hence the use of RFZ (reflection free zone) configurations. But the exclusive use of an RFZ is not appropriate for listening rooms used for accurately reproducing recorded sources. If this seems confusing, it is a definite sign that one needs to do a bit more research. And at the risk (once again) of stepping on toes, this seems to be behind much of the rush to treat rooms generally (rather than surgically) with absorption. ;-)

On the other hand, I will reiterate what I have been trying to state previously with regards to accurate reproduction. This (an anechoic response) is NOT appropriate for listening rooms! If he is refering to a listening room, most** would agree that he is incorrect...but if you say he is from THX, then I am being redundant. ;-) And unfortunately, as I am unable to post attachments on the site, (HELP!!!!!), and subsequent requests for help have gone unheeded, so you will just have to squint a bit harder and imagine the attachments that would make this much clearer. ;-)

What I have been trying to convey are some of the more current thoughts regarding the concept originated by Don Davis that has continued to evolve and which is known as the LEDE (Live End Dead End) configuration. (But to properly understand it, you must move beyond the simplistic description!)

"When the physically and acoustically small room is converted into a physically small, acoustically large room by LEDE means, then this signature is heard from the loudspeakers."

...

"Leo Beranek has written in Music, Acoustics and Architecture, Persons trained in listening for example, bind people, who receive all of their clues about the environment around them through the senses other than the eye can measure the size of the room or judge the distance to the wall behind them by the length of the time interval between the direct sound and the first reflected sound. Beranek goes on to note that this capability is not restricted to the unsighted, but experienced music listenerssense the approximate size of the hallby the length of the initial time delay gap.

"The LEDE technique, by virtue of the distance the direct sound must travel to encounter a first reflection, adjusts the initial time delay gap {to a figure ~2-5 ms longer then the ITD of the untreated room itself - thus establishing an effectively 'anechoic' path to the listener's ear of the direct signal} to the same figure that Beranek judged as desirable in the best concert halls, namely 20 ms. It is no coincidence that the same 20 ms is the optimal delay for the maximum Haas effect in good, diffuse, semi-reverberant spaces."

"William B. Snow, of Bell Labs fame, in 1957 wrote in Application of Acoustical Engineering Principles to Home Music Rooms, The direct sound alone carries the information giving the sense of direction, by allowing the listener to observe the initial transients clearly during the short time interval before the many-directed reflections begin to arrive at his ears.

A 'dead' room does indeed sound 'dead'. A lesson learned quite well (and quickly modified!) in the very frst iteration of the LEDE concept at Wally Heider's. Absorption is used surgically for very specific purposes. It is used to define the ITD/ISD and, in very rare cases, to tame stubborn subsequent focused specular reflections where diffusive means prove insufficient. Other then those surgical applications, diffusion is used liberally to increase the density of the diffuse field occuring later in time after the ITD/ISD, defined by Ld and the Haas kicker.

** To make any absolute statements regarding preferences is a bit absurd. But one is able to make some generalizations regarding the general trend among those who are actively developing and researching such environments. So please understand my statements that may indicate that a particular method or approach is 'correct'.

[PrestonTom]

MAS,

There is a simple approach to give a direction to what you are advocating (although perhaps the simplicity is actually simplistic). Although no formal measurement is neccessarily employed (although a much better job would in fact rely on treatment guided by measurement), there is a simple formula: "begin by treating the first reflections points with absorption, and then add some bass trapping"

Yes, of course this is simplistic! However, the absorption will tame the smearing that happens within the first few to several millseconds. Absorption is not practical for the very low frequencies (and most "normal" size rooms will be plagued with room modes), so the second approach of trapping is reasonable.

What is not clear, as a general approach, is if the later reflections will provide enough diffusion fot whatever "presence" or "ambience" is desired. Eliminating reflections for a full 20 msec would not be simple, especially for a room that is already constructed.

Again, this has been a rule of thumb and is not meant to be the last word. It is simply a direction from which to start.

-Tom

[DrWho]

The room is but one part of the canvas on which the sound engineer is painting. So in light of achieving accurate playback, your room should be identical to that of the studio - but that also includes all the source equipment and same speakers as well...I'm not sure too many people would be interested in building rooms and equipment that can change depending on the studio the source material was mixed in.

The way I see it, true accuracy to the live event is never the goal because it's impossible to achieve. Accuracy to what the studio engineer hears isn't the goal either because things usually sound most unpleasant in the studio. So what then is the goal? I don't think there is one.

I'd just use my own ears and decide what flavor I like more - using the technical crap to facilitate the process.

Someone needs to sit down and standardize what constitutes a good listening environment. And then people can just build to the predetermined standard and know they will achieve good sound - and the studios can mix for those environments too. It really shouldn't be the responsibility of the average consumer to purchase crazy fancy measuring equipment nor hire "professionals".

[mas]

I sense the tendency for us to keep going in circles!

The statements above pertain to general acoustical spaces, an acoustical definition that includes rooms of varying sizes but all exhibiting a series of similar specific characterisitcs. In reality, we deal with many sized rooms - the vast majority on this forum, if not all, of which will fall into the "small acoustical space" definition. Hence the tuning of the specific ITD to ~2-5 ms greater than the room's natural ITD.

The primary problem to which I am referring is the over use of absorption, an approach which is predominant here as well as a pronounced lack of sufficient diffusion. With this approach, the resultant lack of a dense diffusive field is not only a definate possibility, it is a near certain probability. And these late arriving higher order specular reflections will very probably exceed the exponential decay rate of a diffuse field and be very audible. This is further aggravated by the common references on the Net to an LEDE exhibiting an 'anechoic' reponse which is wrong in its simplistic misunderstanding of the concept. Hence the rush for absorption!

And if the same trend is followed, more absorption will be used, further exascerbating the problem. Hence alot of time, effort and money to simply move the problem around. But yes! It WILL sound different! And without anything to benchmark it too, most will assume it to be an improvement.

We keep circling the issue here which is that many want a very scientific way to put their finger into the air and determine the optimal way to treat a generic (anything but perfectly rectangular) room without using the proper tools (and I say this as RTA and EQs and SPL meters are NOT the proper tools for this application.)

Additionally, such concepts as LEDE are not random or approximate. They are based upon research and can be quantified and qualified based upon actual measurements that do correlate to a subjective experience.

I can completely understand the desire to be able to enhance their listening room without spending thousands of dollars! Not only that, I completely agree! I personally have no desire to buy magic foam for a thousand dollars! ...Nor do I personally relish the thought of buying 2' x2' diffusive panels for from $200 to $600 each!! I would personally propose making these in a weekend or three. Some sheets of luan (if you are comfortable doing the ripping & edge finishing), or several plantation blinds bought at Home Depot, prefinished in the desired finish, taken apart, and a jig and router make this a simple and fun project.

I can provide as in depth a discussion of the physics as one wants. We can specify how a well behaved room behaves quantified by measurements, complete with tolerances and limits. But these descriptions will not result in a tuned room that is worth the ignorance of the basic tools. If we could, those who have been doing this for a long time now WITH the elaborate tools would be able to save the time, money and hassle of using them. But they don't. There is a very good reason for this.

There are very definate advantages to tools such as TEF, Easura, SMAART, etc., but for $150 to $300, ETF's RPlusD can do what the average person needs. And most are all too willing to spend more than this for interconnects and/or bi-wiring, yet run from the suggestion of learning about real tools that make a real difference. And all one needs for a basic room is the ability to make (and interpret!) basic time, energy, frequency waterfall plots for room modes and impulse responses for absorption, ITD, and diffusive measurements.

Now someone who is selling a turnkey solution may try to tell you it can be done (and FAR TOO many do!!!!!) And some even suggest they can if you send them a sketch (I guess that adds to the psychological feeling of completeness). Heck, save your money and send me a sketch, I will be glad to suggest wall treatments fir free - and you can move the response of the room around. But at some point it you desire to address a real room with any sense of accuracy, you bite the bullet and get the proper tools.

I guess what concerns me most, is that so many will go to such great lengths, and spend more then they need to on treatments rather than to be come familiar with the time based tools that are available that will allow them to identify where they are, what needs to be done, and when to stop!

But there is more to this than simply installing some magic bass traps (based upon ideal gross guestimates of rooms that vary greatly from the ideal), some absorption using the mirror test (yet having no idea of the ITD of the room nor of the Haas kicker - if it even remains), and tacking up some diffusion (actually pretty hard to go wrong here - except that it is done too little and too sparingly). The devil is indeed in the details.

But if you do choose to go this route, use bass traps, and then please, use absorption sparingly for the first order reflections, and then go heavy on the diffusion on the remaining surfaces!!!!!!

Sure there are some simple general guidelines, just as I can tell provide you with the general guidelines to become a 50 home run hitter in the majors without steroids! Unfortunately, they are both equally effective in actually achieving the goal in the real world.

[mas]

Doc,

There are very different acoustic design goals for studio recording rooms, studio mix suites and listening rooms.

Likewise, the design goals and resultant employed techniques for intelligibility in a board room are not the same as for music listening room. They may utilize similar methods and tools, but their implementation is not the same.

Again, we are tending to fall into the trap of treating all rooms the same. And thus far we are still only dealing with "small acousitical spaces." We haven't even considered large acoustical spaces!

I think I understand what you mean, but as I read your statements, I cannot avoid reacting with a bit of playful incredulity:

"I'd just use my own ears and decide what flavor I like more - using the technical crap to facilitate the process."

Literally, then this entire discussion is moot. Just do what you want and what feels good. I reject this radical sense of its being either purely subjective or its purely objective! If they don't correlate, and you are not at least aware and concerned about that, then you are off the path. And I like the concept of 'optimization'.

"It really shouldn't be the responsibility of the average consumer to purchase crazy fancy measuring equipment nor hire "professionals".

Its not! But isn't that what someone is in effect doing when they spend ~$7500 on a pair of Klipsch speakers????? The only difference is that they are ignoring the room in which they will be placed! And the "crazy" equipment and the "Professionals" are simply in another location. Of course they could easily go buy a set of generic wonders from Best Buy, or of course I am wondering more than ever why some like to complain about Bose - who has chosen to cater to the "average consumer"!

But then the room is not mass produced in some plant with standard designs! It is a rather unique element and the technicians must go to it, as , at least the last I checked, it was rather awkward to bring the room to a processing facility.

But I hope that you will be as quick to remind the rest of the forum that these fancy speakers and cables and SPL meters and RTAs and crossovers, as well as the rest of the menagerie of pieces and parts whose total price is more than several hundred dollars can also be considered just a tad bit ridiculous!

This is where the serious science of acoustics diverges from the goals of many here. And to continue to mix the advanced acoustical techniques which have resulted from allot of study and experimentation with the 'whatever feels good school' results in a point of "have fun", as discussion becomes an increasingly divergent exercise that will simply frustrate each group.

And with regards to this, I feel like it is a discussion with the Scarecrow in the Wizard of Oz. Fun, but ultimately one that we must simply decide which group to which we belong and stick to that group. As the discussions of one group are not necessarily productive to the goals of the other. Oh, and lest some feel obliged to go there, the time based measurements do compliment the subjective experience.

I just fail to understand how some can spend justify efforts in expending absurd amounts of money on subtle improvements quickly approaching or exceeding the point of diminishing returns, and on the other hand dismiss real improvements amounting to orders of magnitude.

On the other hand, maybe some should be telling the folks who are adept at designing and optimizing passive crossovers that they worry far too much about component values, and that they use far too fancy test gear! Besides, as I personally like the color blue, we should use the blue caps, and as I like the sound of the word 'resistors', so we need a few more of each! And those inductors! Heck, just their name sounds illegal! And regarding speaker design, I tire of terms such as throat dimension, path length, excursion limits, Thiel-Small, compression, and words like "tractrix"! Can't they just stick a speaker in a box!? After all, in high school I did that and they sounded OK... ;-) ;-)

"Someone needs to sit down and standardize what constitutes a good listening environment. And then people can just build to the predetermined standard and know they will achieve good sound - and the studios can mix for those environments too."

And given that far too many engineers are having great difficulty simply mixing the DIRECT signal, I will thank then to forget trying to tell me what 'approved' listening environment I am supposed to use! I will assume control over that! And considering the myriad types of 'legitimate' listening spaces there are, who will determine this? And are we to omit speakers (monpole/dipole. plnaer, electrostatic, horns, and the myriad types of loading configurations, full range, 2 way, 3-way, more-way), amplifiers and analog and digital, tube and solid state, etc.? Must the rooms be dedicated, multi-function, strictly music, home theater, or mixed use, cinder block, drywall, ICF, tile, carpet, hardwood flooring, windowed or 'dark', timber framed or log, etc.from this 'standard' as well? After all, we already have the saleman's tool called THX! I am rather shocked that you, of all the folks here, would make this statement.

Not to mention...I assume that you are pursuing your degree simply for your own altruistic pleasure, as, by your reasoning, I would wonder why anyone should expect to pay you the wages you will expect! Why should the average consumer purchase products from a company who has purchased "crazy fancy measuring equipment" and hired superfluous "professionals" such as yourself? Besides, anyone can mix (can't they???)...just go to the average show for ample evidence! ;-) ;-) ;-) ;-P

Sure, if you like, you are welcome to guestimate to your heart's content.

But there are those who are concerned with a more accurate prediction and correlation of the subjective experience with objective tools. No, we don't know it all, and that is what makes acoustics so exciting compared to many other disciplines where there are very few practical unknowns. But the discipline has advanced at an incredible pace over the past 35 years, and most of this has been made possible precisely by the time domain models these 'fancy tools" make available.

Addendum:

For future reference and clarity, may I suggest that we dilineate each thread on the subject of acoustics as either being one being a purely subjective approach, and as such, feelings and emotion determine one's goals, actions and judgements; or the other thread category as being one based upon systematic research, study and the necessity of utilizing the appropriate tools pertinent for the job. Each person can determine where they reside. But in any case, it is foolish to try to arbitrarily mix the two.

PS Doc, I hope you received this with the grin intended! As I quite enjoy your posts! ;-) ;-) ;-)

[PrestonTom]

MAS,

I appreciate your frustration over this. Yes, people do spend a great deal of money on items that are of secondary importance relative to the "speaker/room" part of the equation. Getting the room imnproved will yield much -greater improvements compared to cables, tweaking crossovers and exotic amps.

I freely admitted that my approach was simplistic. But I also appreciate that most folks will not have the resources or or knowledge to do the proper measurements. The classic frequency domain measures are difficult enough to perform and interpret. Although it can be done.... The time-domain measures you are advocating are more difficult to perform and certainly more difficult to interpret. IMO this is a simple and unavoidble fact.

I agree that it is very easy to quickly have too much absorption in a room. Although, we might disagree on whether this is the worst of the sins. Again, I am being pragmatic in my approach.

Let's take a simplistic approach as a start. Once again I am labelling this as simplistic and it is not meant as the ultimate solution. I think most folks can understand and get some intuition on what a waterfall display represents (frequency response measured over a number of sequential time slices). I also think there is an intuition about a reflection with a short delay equates to a "smear" in the perceived frequency response, while a reflection that results from a longer delay is percieved as a temporal effect (reverberation, echo, ambinece etc). The former being something that should be avoided and the later reflection being more percptually benign.

Back to the waterfall measure. One could shoot for the intial time slice (I'll let you choose the length of the time window) to be relatively flat. We can quibble about whether a 3 dB drop per octave at the higher fequencies is perferred (I do not prefer it however). Now we are in position to tweak crossovers, use a bit of equalization (sparingly) and attenuate the first reflections with a bit of absorption. So much for the first time slice.

The later time slices: what to do? One approach, and I am trying to keep this intuitive, is that the time for decay (choose wahtever measure you like, a 60 dB down point for instance) should be not too long and not too short (sounds like Goldilocks). One target might be 400-600 msec (please don't focus on the particular number - there are several considerations)) and it should be approximately similar across frequency. Now both of us are satisfied. Too much absorption will kill the delay times at the mid and high frequencies (while absorption wil have little effect at the lows). This is something you are concerned about and it is something that can be inadvertantly done with a casual use of foam & panels etc. But your suggestion of diffusion helps with this since the mid and high frequencies will bounce (diffuse) while the low frequencies will not. This is my reason for the bass trapping (and if you can measure it adequately, the traping should, of course, be targeted for the the offending frequencies (wavelengths)).

I guess where we might disagree regards the sin of too much absorption. If push comes to shove I would rather have too much absorption. My reason follows from a more basic assumption. When I put on a recording of symphonic music, chamber music, girl with a guitar, live jazz in a club, I have a faith in the original recording. The faith is that the ambience, and acoustics of the original performance was done properly. I should be hearing the concert hall acoustics and I would want to minimize the acoustics of my living room. The reflections of the my living room have short delays (leading to frequency smearing). I am interested in the acoustics of the hall not in the acoustics of my small living room. Back to where I started: A simplistic approach is to minimize the first reflections (minimize the smearing) and trap the low frequencies. The trapping will help hide the room modes (uneven frequency response in the first time slice) and reduce the decay time at the low frequencies (which will typically be much longer than those at the mid & high fequencies).

I beleive we are in agreement that measurement (including time domain measures) should be the driver on all of this. I am less optimistic about the practical feasibilty of using some of these measures and interpretting their impact on perception. My approach is simple by design, but it is meant to be a "user-friendly" guide.

Good Luck,

-Tom

[jacksonbart]

just by a good set of headphones and take the room out of it.

[PrestonTom]

I should probably modify my previous comments about the need to first tackle the absorption of the first reflections. I just finished reading a review article by Cox and D'Antonio (RPG diffusers). I am now understanding why diffusers can be used more effectively (MAS's point). Since diffusing the reflection attenuates the reflection, standing waves and comb filtering are "decreased". It is a nice strategy. I have not been thinking deeply enough about this...

Mea culpa,

-Tom

[damonrpayne]

Do you have a link to this article?

[PrestonTom]

Do you have a link to this article?

Sorry, what I have is hardcopy in Acoustics Today. It is supplement for Journal of the Acoustical Society of America. They will have these at a University library.

-Tom

[mikebse2a3]

I should probably modify my previous comments about the need to first tackle the absorption of the first reflections. I just finished reading a review article by Cox and D'Antonio (RPG diffusers). I am now understanding why diffusers can be used more effectively (MAS's point). Since diffusing the reflection attenuates the reflection, standing waves and comb filtering are "decreased". It is a nice strategy. I have not been thinking deeply enough about this...

Mea culpa,

-Tom

http://www.rpginc.com/products/skyline/index.htm

Here is a link to the RPG Skyline Diffuser for anyone interested In seeing a Diffuser Design that suppresses the Specular Reflection while offering much needed diffusion. I like using these over absorption at some of the mirror image points in a room. Even diffusers placed in the wrong location can actually make things worse especially in small rooms were the rooms dimensions cause the diffusers to be located to close (especially in the context of Time) to the listener. Each room is so unique that if you are the least bit interested in good sound and are willing to learn and make efforts to advance your understanding why your room/system sounds like it does then Computer Programs like ETF are a real bargain.

One good example for someone is: Have you ever noticed how your sound system/speakers can sound so Real and Life Like Outside of the Listening Room say in another part of the house (of course there will be no stereo effect but the realness and yes clarity and even a good sinse of tonal balance of the sound can be perceived as in real life when you have heard good live music from outside of the room that it is being performed in.) yet when you enter the listening room the sound is irritating and the tonal balance is all screwed up and clarity/details have in a large part disappeared. Thats a very good indication that uncontroled reflections and room modes are killing your sound and the Best Equipment and Speakers will be masked by them until they have been given the proper attention. Its very eye/ear opening as you begin to get the room under control how much the Sound from Inside the Room begins to match and outperform what you hear from Outside of the Room.

mike tn[]

[damonrpayne]

Do you know of anyone who sells those online? Audio Advisor lists a 24"x24" panel as being $169.95. Ouch!

[mikebse2a3]

Do you know of anyone who sells those online? Audio Advisor lists a 24"x24" panel as being $169.95. Ouch!

I've never seen any on line and yes they are expensive but I often see people spending $$$$ on trading around equipment to only gain for the most part just a slightly different sound but not anywhere near the difference/improvement that good room treatments placed properly can offer.

Like anything even room treatment can be overdone or used wrongly and knowing how to use it and when to stop is were programs like ETF can help in identifying what your real problems are. Very small rooms(say less than 1500 cubic feet) acoustically speaking are really hard to deal with and your chances for great sound gets much better as the room volume/dimensions increase. Your personell taste plays a part in what you want to do also. For example using diffusers such as the skyline versus say absorption at the first reflection points will effect the perceived image size. The Absorption will often create a smaller more point source type of image versus in my experience the Diffuser which will give the image a more life size effect. Also the absorption treatment can often lead to a very dead and unnatural sound and in the long run an increase in listener fatigue that I've not experienced with using diffusion as long as the diffusers aren't placed in such a way that they send sound toward you to early.

mike tn[]

[WMcD]

This is all very interesting and many thanks to mas for his input.

The use of absorbers versus diffusers reminds me a bit of the technology of stealth aircraft. The goal is to not create a direct reflection of radar.

This is accomplished by some abosorption in the surface of the aircraft, though this appartently not the most effective.

Rather hard, curved surfaces diffuse. Like my polycylidrical argument. (I thought I'd sneak that in.)

In other cases, hard surfaces are broken up in to systimatically arranged ragged surfaces, like RPG.

Of course in rooms, we're not shooting the signal off into open space. Rather, an early direct reflection is sent toward other surfaces where there is so much delay that it becomes part of the reverberant field. We don't have to attenuate, just manipulate delay.

Gil

[damonrpayne]

I have RPlusD and am waiting on a USB sound card for my laptop to come in so I can play around with it.

Does diffusion help the decay/RT60 stuff? I am such a nerd and so obsessive that I can see myself in the basement till the wee hours with RPlusD, sorta like staying up too late playing video games []

[mikebse2a3]

Of course in rooms, we're not shooting the signal off into open space. Rather, an early direct reflection is sent toward other surfaces where there is so much delay that it becomes part of the reverberant field. We don't have to attenuate, just manipulate delay.

Gil

-------------------------------------------------

Yes Gil

Thats part of what I was talking about when trying to use diffusers in Physically/Acoustically small rooms is that you have to be carefull that those redirected reflection still don't arrive to early in Time with to high an Amplitude or that can just lead to different problems and not get you to your goal. You really need the assistance of test equipment that can show what is happening in Time as well as Amplitude and Direction.

mike tn[]

[mikebse2a3]

I have RPlusD and am waiting on a USB sound card for my laptop to come in so I can play around with it.

Does diffusion help the decay/RT60 stuff? I am such a nerd and so obsessive that I can see myself in the basement till the wee hours with RPlusD, sorta like staying up too late playing video games []

Diffusion and Absorption will both effect the way sound decays in the room. For example if you take the same quantity of absorption and placed it all on one wall in a room the way the sound decays over time will be changed versus if you take the same treatment and spread it over all the room surfaces. If you don't have enough diffusion in a room then the sound will vary dramatically as you move around in the room and it's decay will vary at different frequencies due to lack of a diffuse soundfield being developed.

Yes expect to spend alot of time doing this but if you like this sort of thing you are going to have alot of fun. Just walk away when you get tired and don't be surprised if you walk away with more questions than you had when you began.

mike tn[]

[mas]

Does diffusion help the decay/RT60 stuff?

I can identify the base cause for much of our disagreements and misunderstanding! It can be summed up in two very misused words: reverberant field.

We need to go back and review...And this is one topic regarding which Everest is largely to blame! While he mentions the issue, he is both ignorant and negligent in both his understandng of, and his treatment of the topic!

There are five general classes of sound fields: free fields, diffuse (reverberant) fields, semi-reverberant fields, pressure fields and ambient noise fields.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

In a reverberant sound field "the time average of the mean square sound pressure is everywhere the same. The flow of energy is all directions is equally probable, which requires an enclosed space with essentially no acoustic absorption. The reverberant sound level is labeled Lr".

In a semi-reverberant sound field, sound energy is both reflected and absorbed. Energy flows in more than one direction. Much of the energy is truly from a diffused field; however, there are components of the field that have a definable direction of propagation from the noise source. The semi-reverberant field is the one encountered in most of the architectural acoustic environments. The early reflections (< 50 ms after Ld) are labeled Lre.

Lets look at a very important qualification. To quote a section for Sound System Engineering, Davis (p.211):

To quote Ted Shultz (formerly of Bolt, Beranek, and Newman):

In a large room, if one has a sound source, one can determine the total amount of absorption in a room by measuring the average pressure throughout the room. This total absorption can them be used to calculate the reverberation time from the Sabine formula. This method fails badly in a small room, however, where a large part of the spectrum of interest lies in a frequency range where the resonant modes of the room do not overlap but may be isolatedIn this case the microphone, instead of responding to a random sound field (as required by the validity of the theory on which these methods depend), will delineate a transfer function of the roomIt does not provide a valid measurement of the reverberation of the room.

What is often overlooked in the attempted measurement of RT60 in small rooms is that the definition of RT60 has two parts. The first part is unfortunately commonly overlooked.

1.) RT60 is the measurement of the decay time of a well-mixed reverberant sound field well beyond Dc.

2.) RT60 is the time in seconds for the reverberant sound field to decay 60 dB after the sound source is shut off.

In small rooms there is no Dc, no well-mixed sound field, and, hence, no reverberation. There is merely a series of early reflected energy. Consequently the measurement of RT60 becomes meaningless in such environments.

The control of the early reflections becomes most meaningful because there is no reverberation to mask them."

Thus a "reverberant field has a very definate meaning! And a 'small acoustical space does not have a "reverberant field". Forget RT60s in a small acoustical space!!!

So, fundamental to our discussion of acoustical spaces are two issues:

1.) What is the level of the reverberant field?

2.) How uniform is the reverberant sound fields level (gain) distribution in the space?

But as we are dealing with a small acoustical space, these questions must be reframed accordingly:

1.) What is the level of the Semi-reverberant field?

Please note: While habit may result in referring to a small acoustical spaces reverberant field, to do so is to render the term as slang since, by definition, it is a semi-reverberant field. It is critical to understand that this term in a small acoustical space has a radically different meaning than its proper use in a large acoustical space! In this semi-reverberant space, the early specular reflections Lre follow a pattern characteristic very near to direct signals which follow the inverse-square law, allowing only for small reflective losses. In a truly reverberant space, the reverberant field level remains constant provided energy continues to be input at a reasonably regular rate.

2.) How uniform is the semi-reverberant sound fields density and level distribution in the space?

And here is the rub In a small acoustical space the semi-reverberant field is characterized by relatively strong, focused and audible specular reflections, Lre. We lack the existence of a well-mixed random sound field! Thus the specular reflections must be diffused in order to become a denser (more uniformly scattered) diffuse field consisting of lower energy randomized reflections that decay in intensity at an exponential rate with regards to time.

[WMcD]

I'm with you on this.

I was trying to inject some Gestalt. There are a lot similar concepts out there which people do understand at a subliminal level. With some contemplation, they will all click together, to form a whole, with perfect understanding.

Gee, I've got this FM multipath issue. The buildings in the Chicago's downtown look like RPG diffusers, injecting random phase reflections. The FM detector in the Mac is confused and doesn't lock up.

Smile,

Gil