mas Posted December 23, 2006 Author Share Posted December 23, 2006 {Gee, its interesting to see that a bit of the personal attack above has since been edited...} I would suggest more fiber. Let's see, another post decrying less math to balance the plethora of posts decrying any math, despite the inclusion of LOTS of hyperbole that you have missed, just as I suspect this jest and play on words will be as well. (...I guess I have to start pointing this stuff out [*-)] - hyperbolE, hyperbolA...get it? Probably not... Look it up...) "Anybody can do that stuff". Congrats! ...Had I only known that Jethro had arrived to help those with their cypherin'.... Just to save those who already know it all a bit of time...this thread is not for you. You will be very bored here. Nor will we be discussing autotransformers. I know there are many folks here with varying degrees of experience and knowledge. I apologize if I have not acknowledged that some already know everything. Some will be anxiously awaiting your original research. We are not attempting to present any here...but again, it is always entertaining to read of allegations based upon information not in evidence. Neither will I be attempting to present any original cookie recipes nor novel approaches for BBQ sauces. And as my warped sense of humor simply can't resist, I hope we didn't confuse the really erudite folks with the "circle thingies". May I suggest that you take your issues and discuss them with the those who have complained about the few math examples that were included. My tongue in cheek response was for them, not for the more 'erudite' folks such as yourself. But thanks, as you have indeed added allot to our understanding of the topic. BTW, any time you might wish to talk and actually discover what I know, you are glad to PM me. But be careful, we wouldn't want to ruin the the speculation game based upon ignorance. After all, there is more to tuning a room than simply installing polycylinders that simply 'scattering' focused specular reflections rather than actually diffusing them. [] Have a nice holiday. (PS...I suspect you will be in a better humor after you have found a few kids to whom you can tell that there is indeed no Santa Claus. Oh, and for the rest of you kids on the forum who may have just read this..... I was just joshin'...there really IS a Santa! ...Geesh! Why do I have the feeling that I am going to hear from Coytee again and that Chloe is going to be really pissed now!?) Quote Link to comment Share on other sites More sharing options...
Coytee Posted December 23, 2006 Share Posted December 23, 2006 She cornered him for 3 hours last year in our fireplace. Our fireplace is a ventless type. [&] Quote Link to comment Share on other sites More sharing options...
westcott Posted March 20, 2007 Share Posted March 20, 2007 I would suggest more fiber. Let's see, another post decrying less math to balance the plethora of posts decrying any math, despite the inclusion of LOTS of hyperbole that you have missed, just as I suspect this jest and play on words will be as well. (...I guess I have to start pointing this stuff out [*-)] - hyperbolE, hyperbolA...get it? Probably not... Look it up...) "Anybody can do that stuff". Congrats! ...Had I only known that Jethro had arrived to help those with their cypherin'.... Just to save those who already know it all a bit of time...this thread is not for you. You will be very bored here. Nor will we be discussing autotransformers. I know there are many folks here with varying degrees of experience and knowledge. I apologize if I have not acknowledged that some already know everything. Some will be anxiously awaiting your original research. We are not attempting to present any here...but again, it is always entertaining to read of allegations based upon information not in evidence. Neither will I be attempting to present any original cookie recipes nor novel approaches for BBQ sauces. And as my warped sense of humor simply can't resist, I hope we didn't confuse the really erudite folks with the "circle thingies". May I suggest that you take your issues and discuss them with the those who have complained about the few math examples that were included. My tongue in cheek response was for them, not for the more 'erudite' folks such as yourself. But thanks, as you have indeed added allot to our understanding of the topic. BTW, any time you might wish to talk and actually discover what I know, you are glad to PM me. But be careful, we wouldn't want to ruin the the speculation game based upon ignorance. After all, there is more to tuning a room than simply installing polycylinders that simply 'scattering' focused specular reflections rather than actually diffusing them. [] Have a nice holiday. (PS...I suspect you will be in a better humor after you have found a few kids to whom you can tell that there is indeed no Santa Claus. Oh, and for the rest of you kids on the forum who may have just read this..... I was just joshin'...there really IS a Santa! ...Geesh! Why do I have the feeling that I am going to hear from Coytee again and that Chloe is going to be really pissed now!?) I have a question. I know it is probably not answerable but if the control room is manipuling the ITD and the Studio is manipulationg the ITD, how are we supposed to treat a room to address all the various manipulations that have been already implemented? I also want to make sure I got this. Lpowerlevel or sensitivity and directivityQ equals Lpressurelevel? Quote Link to comment Share on other sites More sharing options...
mas Posted March 20, 2007 Author Share Posted March 20, 2007 I have a question. I know it is probably not answerable but if the control room is manipuling the ITD and the Studio is manipulationg the ITD, how are we supposed to treat a room to address all the various manipulations that have been already implemented? I also want to make sure I got this. Lpowerlevel or sensitivity and directivityQ equals Lpressurelevel? The answer is actually quite simple. You don't. Your job is not to try to re-engineer the source. The purpose of the ITD/ISD is essentially to create a small period of time where your experience is anechoic - you are only experiencing the direct signal without any reflected room effects. The rest of the room treatment is to break up specular reflections and to redistribute the acoustic energy in a well behaved logarithmic decay providing a semblance of 'space' that provides a semblance of a reverberant space (which is not actually present in a small room). The purpose of a mix environment is to focus entirely on this direct signal component, as they have no control over the playback environment. Hence the use of RFZ (reflection free zones) and other techniques useful in a studio mix environment, but not appropriate fro a home listening anvironment. And to digress...a major source of confusion for many is exactly this confusion of studio designs with 'home' designs. Various techniques and aspects are applicable to both, but you do not want to create a home listening environment copying a studio mix environment! And "Lpowerlevel or sensitivity and directivityQ equals Lpressurelevel? " Ah...I am not sure what you are looking for here. Q is a 'measure' (a ration of 'in to out') of the polar dispersion of the speaker - the coverage area. I guess that you are trying to get to an intensity distribution by saying, for example, if a given sound energy is distributed within a high Q distribution, the SPL will be higher as opposed to a given sound energy distributed in a low Q distribution, thus resulting in the same energy being spread over a greater area...But I am not exactly sure what you are trying to do with this. But, to state this from a 'position' that I am more familiar, yes, the smaller the volume of space a given amount of energy is dispersed, the higher the effective intensity. Hence, for example, corner placement is more 'effeicient' than free space placement. Please see the attached graphic that summarizes this. Quote Link to comment Share on other sites More sharing options...
mas Posted March 20, 2007 Author Share Posted March 20, 2007 deleted dupl post Quote Link to comment Share on other sites More sharing options...
mas Posted March 20, 2007 Author Share Posted March 20, 2007 hahaha! NOW ithe system gets religion and says my message in the post cannot be a duplicate when I try to replace the post with a "deleted dupl post" message! [:S] This can only be running on a MS system! Quote Link to comment Share on other sites More sharing options...
mas Posted March 20, 2007 Author Share Posted March 20, 2007 deleted duplicate post Quote Link to comment Share on other sites More sharing options...
mas Posted March 20, 2007 Author Share Posted March 20, 2007 Neat!!!! ...as the Twilight Zone theme plays in the background with a small MS emblem incorporated in it... I submitted the post, received an error, went back and looked - no post, so I then I resubmitted the post. And the result is 4 posts, 2 with the attachment and 2 without! [*-)][^o)][:|][] Quote Link to comment Share on other sites More sharing options...
DrWho Posted March 20, 2007 Share Posted March 20, 2007 I'm sure mas can answer this better than I, but I'll give it a shot... Ultimately, if you're asking the question, then I don't think you quite understand how the audible effects of the ITD? You don't want the ITD to be infinitely long because that's what an anechoic chamber sounds like. You don't want it infinitely short either though unless you dig the sound of comb-filtering. The way I understand it, you want the ITD just long enough to clear the Haas window so that any reflections are not percieved as influencing the output of the direct sound...basically, achieving the effect of no percieved comb-filtering (since that always sounds bad). Once the ITD is established, it's up to the end user to determine what kind of semi-reverberant field they prefer. In fact, it's up to the end user to determine how long they want the ITD to be too. It's all just personal preference really - the art of acoustics so to speak? I'm sure there's plenty of research going on to try and describe generally ideal scenarios, but no room is ever going to be close to the ideal. The purpose of the science then becomes one of understanding the systems at play so that the end user can pick a set of least compromises for the dollar. Right now, my understanding is that you want the ITD to be right at the edge of the Haas window....so around 20-40ms depending on the frequency range. Beyond that, you want a strong Haas kicker (strong first reflection), say about 6-10dB down from the direct sound and then as closely as possible approximating an exponential decay with the ensuing reflections. The most important criteria being that spikes in the semi-reverberant field need to be non-existant since they distract your attention on the direct sound. You'll rarely be able to achieve a long enough decay to be "ideal" so it's usually a question of how long can you make it... As far as differences between the studio and the home....in the studio, you want to hear exactly what is on the recording and then you use your mixing experience to predict what it will sound like in the environment of your target audience. This often means mixing only the direct signal and trying to ignore the acoustics of your control room. You need to be able to differentiate between ambience and spatial cues that need to be in the record from ambience and spatial cues of your own room, since your control room acoustics rarely exist in the target audience scenario. The home listener then takes this recording mixed for the direct sound, and puts it into their listening room where they still get to hear the initial direct sound....followed by whatever semi-reverberant field they have determined they feel sounds most natural. Pretty much any amount of reverberant field sounds the same (provided it's dense enough), so there's really nothing the end-user can do to distroy the intended sound....they just get to pick the flavor they want. But to bring it back to the ITD....pretty much any amount of percieved comb-filtering is going to destroy the image. At least I'm yet to hear a situation where it sounds good. Anyways, that's just my take on the issue...if I'm completely wrong, then perhaps it'll spur some conversation to guide us in the right direction. Quote Link to comment Share on other sites More sharing options...
DrWho Posted March 20, 2007 Share Posted March 20, 2007 ahhhhh, overlapping posts! lol You're outta control Mark....it's prob cuz you're not using a windows machine on the forum [] Quote Link to comment Share on other sites More sharing options...
westcott Posted March 20, 2007 Share Posted March 20, 2007 thus resulting in the same energy being spread over a greater area...But I am not exactly sure what you are trying to do with this. But, to state this from a 'position' that I am more familiar, yes, the smaller the volume of space a given amount of energy is dispersed, the higher the effective intensity. Hence, for example, corner placement is more 'effeicient' than free space placement. Please see the attached graphic that summarizes this. THAT pretty much clarifies what I was asking. I will keep reading to better understand how a room should be treated to avoid a studio like listenting room. I do not have the luxury of randomly placing solutions so I want to make sure that I do what is best before making the pitch to the wife in our shared space. I have a large "small" room and I will have to be very judicious in my approach. So, if you were going to pay a professional to come in and make in room measurements, who would you suggest,what questions would a prudent customer ask, and what would one expect to pay (per cubic foot?). Oops. I should have PMed you. I will next time, I promise. Quote Link to comment Share on other sites More sharing options...
mas Posted April 4, 2007 Author Share Posted April 4, 2007 This is just another overview. I will not be going onto the actualspecifics of particular diffusor or absober design in the piece.Besides, it is not the time to be doing so anyway if we do not haveactual measurements identifying the required characteristics. Thatcomes later. The Masonite/hardboard polycylinders are more properly labeled'scatterers', in that they reflect sound in lower intensity specularwaves - meaning that the 'smaller' reflections have less intensity thanthe incident waveform, but are still specular (focused) in nature. Theywere the early precursors to more effective diffusion methods whichwere ushered in with the research performed by Manfred Schroeder. PeterD'Antonio was the first to transform Schroeder's ideas into a productin the the form of his QRD (quadratic residue diffusers) - which alongwith many advances since, more accurately and completely diffuse thereflected sound. But back to the absorption/diffusion conundrum.All materials are partially both. All materials have an acousticimpedance. And just like in electronics (where some think they can makeall sorts of definitive statements as opposed to that oh so confusingand imprecise acoustic realm), materials will both absorb and reflectsound waves in a frequency dependent manner. Those which tend tobe more reflective at the frequencies of interest are generallyreferred to as reflectors or possibly diffusers(depending upon thereflected wave behavior) while those that tend to be more absorptive atthe frequencies of interest tend to be referred to as absorbers. Ofcourse there have been developed products that exhibit bothcharacteristics in a desirable way and hence their being referred to asabfusors, diffsorbers, etc. With measurements the pattern andbehavior of sound at a given spot is relatively easy to identify and toascertain the effect of applied treatments. But stated in words, a roomhas a finite amount of energy upon stimulation. It acts much like acapacitor storing energy and releasing it over time. The tendency is toattempt to solve the acoustical problems by the use of absorption,which at first seems a pretty reasonable solution. But in a smallacoustical space this quickly proves to be a problem, resulting in anacoustically 'dead' room. And if all you want to listen to is thedirect sound, you are in heaven...a very 'dead' sounding heaven. Most people on the other hand prefer a sense of space, and this space is provided by a semi-diffuse sound field. Sothe way that this is achieved while also maximizing the intelligibilityand imaging of the direct signal is to provide surgically appliedabsorption at the first order reflection points, providing for what isreferred to as the ISD or ITD, an initial signal delay gap. During thisperiod where perception is dominated by the psychoacoustics of HenryPrecedence and Haas Effects, we establish an anechoic response, wherebyfor a short period you experience only the direct sound, without thereflections which are detrimental to intelligibility. But we want to absorb onlythat energy that arrives within that time window that we cannot diffuseeffectively to remain in the room system to be used to 'construct' and'shape' a well behaved semi-diffuse field that will provide the senseof space - the sense of being in a large acoustic space. And since thisremaining sound energy in a small acoustic space consists of specular(focused) reflections which are detrimental to perception in this form,our goal is to create a well behaved diffuse sound field that decays inintensity in a predictable manner with respect to time. This is donevia diffusion. Oh, and all of this is done in addition toaddressing the fundamental issue of room modes - LF standing waves.These wavelengths are sufficiently long that they establish resonantpatterns based upon the room geometry, just like a pipe organ relies ontuned pipes to establish resonances that correspond to the frequenciesof the various LF notes. And while in an organ, their goal is to createand to utilize this resonance, in a room, we are trying to accuratelysupport the reproduction of a source signal rather than create music.And just as amplifier distortion can be a valuable aid to a performerin creating a particular sound or effect, such distortion is an enemyin the reproduction of the recorded signal; and in a similar manner, LFstanding waves that reinforce certain LF modal frequencies aredetrimental to the reproduction of source in an acoustic environment. Andthese are addressed in much the same way as the higher order soundwaves are addressed as mentioned above. We first desire to distributethe resonant peaks in order to minimize their summation, which servesto amplify the problem frequencies. And as this is determined primarilyby the room dimensions, our next line of 'defense' is to then attemptto address them via room treatment. And the treatment of roommodes is done via the two methods mentioned above as well. The mostcommonly mentioned treatment is via 'bass traps' - LF absorption. Andthere are many types and configurations. Most may only befamiliar with one or two popular commercial products, but the realityis that these are simply some of the simpler designs available. MostLF absorption takes advantage of the Helmholtz configuration, of whichthere are many styles. It must be remembered that the LF standing wavesexhibit ALLOT of energy. And in order to decrease the signal strengthof such resonances, it requires 'big' tools capable of addressing andacting as an effective sink for such energies. And just like electricalsinks, the acoustical impedance of the sink must match the impedance ofthe acoustical source very closely in order to increase theeffectiveness of the device in order to effectively reduce the'offending' resonance. Considering the large amount of energy andthe generally moderate to high Q of the offending resonance, typicallya tuned Helmholtz resonator is used. And while the various types arebeyond the scope of this entry, suffice it to say that many options areavailable for a given application. These can take the form of wallsegments, applied panels, tubes, 'jars', columns, porous or slottedsections...in other words, just about any shape that one has theknowledge to design. And they can be designed to have a very high Q ora lower Q with a wider bandstop. Of course, with the wider effectivefrequency band, the efficiency of the absorption decreases. Butabsorption is not the only tool. Diffusion can be effective aid as well,although the wavelengths of the frequencies involved require that theeffective size (depth more so than length and width) of the diffusersbe correspondingly large as well. And given the size of the space, andwhether or not you are dealing with a limiting WAF, these may or maynot be an appropriate alternative. The primary measurements used for this are cumulative spectral plots (waterfalls) for the LF standing waves, and although they are not necessary, they provide a wonderful visualization of the relative behavior of frequency and the waveforms persistence with regards to time. In the higher frequencies, ETC (Envelope Time Curves) are of particular usefulness. Again, many other measurements can provide additional detail, but these are the most generally useful. Quote Link to comment Share on other sites More sharing options...
gaspr Posted April 5, 2007 Share Posted April 5, 2007 Good stuff. Glad to see this thread back on top here. Quote Link to comment Share on other sites More sharing options...
gaspr Posted April 5, 2007 Share Posted April 5, 2007 Mas...Could you please tell us more about how we should go about measuring a room and interpreting the data. Quote Link to comment Share on other sites More sharing options...
@silverfox@ Posted April 6, 2007 Share Posted April 6, 2007 from gaspr post this would be most benifical to those who would liketohave some type of real life situation to get a clearer view ,there isa large amount of info to sift through, & focusing on aparticular project would definately benifit some one like me . excelent means of bringing this topic into focus. Quote Link to comment Share on other sites More sharing options...
DrWho Posted April 7, 2007 Share Posted April 7, 2007 If I might jump in, it's actually really easy to measure the room - provided you have all the right equipment. If you have a laptop handy, you could probably put together a decent measuring rig for around $250. If I'm understanding correctly, all of the analysis of the fancier toys out there revolves around obtaining the impulse response of the room and then working some numbers to see whatever it is you want to see (like the frequency response, waterfalls, energy time curves, etc...). But I too think it would be beneficial to look at some measurements of a real room and then get walked through the treatment process, seeing how things change and improve the sound in the room rather than imagining it. While it won't be completely exhaustive, I think it would make more real a lot of the concepts at play here. On a related note, I've finally finished putting together a portable pre-configured measurement system, which I was intending to ship around to those unable to obtain their own (too expensive, too difficult, whatever). There would be a small fee associated with it intended to cover the cost of wear and tear and to fund the purchase of more advanced software (so not trying to make a profit by any means). I would encourage everyone to build their own measurement rigs, but just thought I'd offer an alternative since most people probably will never need to measure more than once. I've only just begun to dabble with the measurements and it's absolutely fascinating how everything correlates to what you hear and even correlates with all the cool info in this thread. And if it hasn't been said already, thanks Mark for all the time you're putting into this (despite how slow you're moving) [][] Quote Link to comment Share on other sites More sharing options...
gaspr Posted April 7, 2007 Share Posted April 7, 2007 Hi Doc. I really find it truly amazing how much info canbe convolved from one little swept sine wave chirp....boggles themind. Good to hear about your roving measurement kit idea. Hope some people a little closer to you can take you up on this. Thanks for weighing in here...keep us up to date on your measurement ventures. Quote Link to comment Share on other sites More sharing options...
mas Posted April 28, 2007 Author Share Posted April 28, 2007 {Please Note: Associated graphics in next frame!} (In partial response to theSpeaker Placement thread in the Architectural forum begun on 4/26/07 http://forums.klipsch.com/forums/1/906285/ShowThread.aspx Only two walls of the room havebeen sheet rocked, still have two walls left and the ceiling. No flooring hasbeen laid (still concrete). So I should be taking measurements in the room eventhough the construction phase of the room is not complete? First, you will please pardon me,when you mentioned the state of your room, I mistakenly thought that you weresaying that because it was small and only partially had sheetrock that perhapsmeasurements didnt matter or were not useful! Oops! Pardon my misunderstanding... A few thoughts and general comments some of which do not specifically apply in your specific situation There still seems to be ageneral/common confusion among some folks between sound transmission through walls (and other flanking pathvectors) with the behavior of the sound fields and component reflections within the room and the role ofmeasurements. For the most part, they are two separate and distinct topics,despite the fact that they both involve walls. Aside from the potential forbuilding some traps, etc. into wall surfaces, the easy way to think of this isthat transmission from one area into another has to do with wall mass andconstruction techniques, while the rooms sonic character is a function solelyof its surface (albeit conditioned by the previous qualification). Techniques for minimizing soundtransmission are actually rather well understood, despite our penchant forreinventing the wheel. I think what causes the most hassle for most is thatsuch considerations are entered after a portion or a majority of the project iscomplete, rather than planning from inception which BTW is MUCH easier folks!;-) And as such they are not the focus of this post. Instead lets agree to focus on thein room response, characterized by the geometry and by the room surface. Once the skin of the room isinstalled (as this is not a modeled room, meaning its behavior and responsecharacteristics were not first modeled and anticipated in software) we can takepreliminary measurements. The effect of embellishments such as carpet can be predictedrelatively easily. This can be done in a very cursory manner by calculating thethickness of the pad and carpet calculated at a 45 degree incident angle. Andthen by allowing this distance to correlate to ¼ wavelength, determining thefrequency to which this correlates. (Note: This will vary from the actual, as the two desperate densities andacoustic impedances of the carpet and the pad will not act as a singleboundary. And the pad will also act as a reflective layerthus reinforcing themuch more complex real world of non-linear surface behavior.) Thus a ¾ inch carpet and pad wouldcorrelate to a 45 degree incident thickness of 1.31 inches which correlates toa ¼ wavelength of 10,308 Hz. Well above the frequencies where we will beworried about dense specular reflections. Thus you can see that most carpet andSpiderman beach towel tapestries, etc. will have little positive effect on theacoustics. To amplify and qualify this, assuming that the carpet is ¼ inch andthe pad is ½ inch, the ¼ inch carpet would correlate to a frequency of 38,200Hz. And the 1.2 inch pad to frequency of 19.097 Hz. So it should become prettyapparent that we are dealing with frequencies that are not only extremelyeasily controlled and damped, but we are also not dealing with frequencies thatreally need control. All of this is to show that some ofthe more commonly accepted treatments will have effects, but they are not theones that we really want to focus upon. They will tend to damp frequencies thatwe really do not need damped and ignore the lower frequencies containing muchmore energy that constitute more problematic specular (focused) reflections. But lets take a step back beforewe get caught up in lots of minutia and lets look at the major divisions ofapproaching a small acoustic space: Generally speaking, lets summarizethe various groups of acoustic distortion: Below 300Hz: Problem 1: Room Modal Response Solutions: Room Dimensions, SpeakerPlacement, Tuned Absorption (Corner Bass traps, Helmholtz resonators) Problem 2: Speaker BoundaryInterference; Solutions: Speaker/Listenerposition, Tuned Absorption Above 300 Hz:Problem 1: Specular reflections (1st order early reflections)Solutions:Surgically placedAbsorption, Diffusion (Create an effectively anechoic Initial Signal Delay Gap) Problem 2: Specular reflections (later arriving reflections) Solutions: Diffusion, Selectively placedAbsorption (rare) (Create a well behaved diffuse semi-reverberant sound field)Derivative Problem 3: Comb filtering/PolarAnomalies (Superposition of direct and reflected signals) Solutions: Signal alignment, Selectively placedAbsorption, Diffusion, Solets start by looking at room modes and standing waves The room calculator programs forstanding waves can give us a few ideas, allowing for their assumption of theroom as a perfect space consisting of 6 parallel surfaces with no doorways,irregularities or dimensional variances. Not exactly any room that you or I canget into or out of! Especially as they do not allow for doorways! But its agood place to start! From this, the average room willhave doorways, alcoves, open passageways connecting adjoining rooms, windows,perhaps with inset frames, and a myriad other surface and structuralirregularities. The net effect of this will be to modify the modal structure ofthe room. The reason for this is prettysimple, and a basic understanding of tuned spaces is helpful here. The useful information illustratedabove is in the location of the nodes and antinodes, the peaks and the nulls ofthe standing pressure wave. Remember, a standing wave is a wave that basicallyreflects back and forth with the high and low pressure zones reinforcing itselfat particular resonance frequencies. Beginning with a fundamental frequency andrepeating at harmonic intervals (multiples of the fundamental frequency). Please note. In a closed end of a resonantspace as illustrated in the diagrams, there is a node, a high pressure zone. Ifthe room is open, the open end ends in an antinode or a low pressure zone. Ifthe room is closed on both ends, you have high pressure zones at both ends,with an antinode in the center (in the most basic form), or a series ofdistributed nodes and antinodes. If a room is not a perfectrectangular space and exhibits the common asymmetrical anomalies mentionedbriefly above, these well behaved patterns are modified. And in the morecomplex examples, alcoves, adjoining rooms, be they joined by large openings orsimply doorways act variously as multiple resonant spaces, summing in verycomplex ways. This phenomena is referred to as acoupled space, and its behavior is anything but simple. Oh, some may besimple, but most become very complex very quickly! The significance of thisfact being not so much that they are complex, but to reinforce theunderstanding that predicting them from simple ideal calculators is almost impossible.And thus the easiest way to address them is by the actual measurement of them.Then the focus is not so much oh how they occur, as it is on simply what isoccurring! Oh, and while we are here, I willalso mention that the modal response is not one dimensional. We are dealingwith a 3Space space, and as such, we have 3 primary modes. Room modes consistof three different types of resonances; these are known as axial,tangential & oblique modes. Axial modes consist of waves resonatingonly along one dimension: the length, width or height of the room, Tangentialmodes involve two dimensions, the length & width, length & height, orwidth & height. Oblique modes involve all three dimensions in each mode ofresonance. Normally the axial modes have the most strength while the obliquemodes have the lowest strength. The concept of dimensioning a roomis to attempt to more evenly distribute the frequency centers of the variousmodes in an attempt to minimize their summation and hence greater peaks andlower nulls, resulting in a more evenly distributed series of peaks ands nullsexhibiting lower peak and higher null SPL levels. A very simple example of an axialmode would be that formed along the length an example room: L=25ft, W=16ft,H=8ft. The first axial mode along the room length would be:SpeedOfSound/(2*RoomLength) = 1130 ft/sec/(25.0 ft*2) = 22.15 Hz. Additionalmodes would exist at integral multiples of 22.15 Hz : 44.3 Hz, 66.45 Hz, etc.The width dimension gives a first axial mode at 1130/(16*2) = 35.31 Hz, 70.62Hz, etc. The height dimension gives a first axial mode at 1130/(8*2) = 70.62Hz, 141.25 Hz, etc. You will quickly note that just forthis axial mode, that the frequency centered around 70 Hz will be a verypopular frequency for many of us! And common multiples of any dimension willtend to create additional nodes at common frequencies that will sum! And justas in a Dickens novel, these coincidences portend future issues that willrequire addressing. What all of this means is two things.One, there will be zones in the room where the low frequencies will tend to bemuch more pronounced than at others. And two, the bass will tend to overdrivethe small space and render it boomy and not very distinct and tight. Andthis quality is the primary problem encountered in small acoustic spaces.Higher order reflections, etc tend to get more attention, but room treatmentMUST begin with the fundamental treatment of room modes and Lf energy. So, for most rooms that exhibitclosed ends, there will be a node at the rear corners. This is furtherreinforced by the corner placement of speakers. Additional nodes may be spacesat intervals in the room, usually ½ and 1/3 the distance of each lateral wall.And for the additional modes, at the intersections of each of the lateralsurfaces walls, floors and ceiling in all of the corners. We can address additional room boundary concerns which affect alternative speaker placement at a later date, but personally I try to avoid them if at all possible either via corner placement or via acoustically isolated soffet (in the wall) installation... Generally speaking, corner trapsconsisting of any of a variety of tuned absorptive traps are effective. Theseare most commonly of a tuned tube or pipe, or a modified version thereof, or inthe form of panels. The fundamental physics model which is employed is that ofthe Helmholtz resonator. This is a tuned enclosure that resonates at aparticular frequency. A very common example that most have experienced is thatof blowing across the top of a coke bottle or a jug. Likewise the use of a tuned pipe to create asound in a traditional pipe organ. The varying volume of the enclosed spacewill result in a particular resonant frequency. A trait that is most helpfulwhen tuning a jug for use in the weekend jug band festivals that are fastapproaching! And if we refer back to ourknowledge of impedance, a properly terminated source with a load of identicalimpedance will result in the maximum power transfer, and all of the incidentenergy will be absorbed and none will be reflected and returned back into thesystem. And since a room exhibits an acoustic impedance, and each surfacelikewise exhibits an acoustical impedance, they will absorb and reflect incidentenergy. And since nothing is ever simple, each surface exhibits a compleximpedance, resulting in a non-linear absorption of some frequencies, and areflection of other frequencies. So each wall surface will absorb or reflectenergy in varying ways, and with varying frequency spectrums, in effect EQingthe reflected sound. (And this is a critical consideration, as many willerroneously believe that an absorber simply absorbs! NOT TRUE!) The Helmholtz absorber is aprecisely tuned enclosure, meaning that it tend to be a high Q device,efficiently absorbing a very narrow band of frequencies and tending to ignoreothers (although lower Q, broader absorptive band designs are possible, theyare correspondingly less efficient over the frequency band). Again, many variations in thedesign of Helmholtz enclosures are possible, from tuned pipes, topanels exhibiting various resonant/flexure, porous, or slotted designs.Thedesign of each of these is beyond the scope of this diatribe. But they are veryprecise, and able to be tuned to quite a variety of frequencies. Often the mostcritical aspect is not the design, but of obtaining materials with knownqualities (density, porosity, mass, etc) allowing the construction of a trapthat correlates closely with the design! So, to make a very generalrecommendation In a room that is generally closed, LF traps in all of thecorners consisting of a combination of corner and lateral flanking tuned LFabsorptive traps is almost always advantageous. The trick is to know whatfrequencies are problematic and then responding by designing and building trapssufficient to accurately and efficiently trap them. Additionally, roomfurnishing can have an impact, both good and bad on the room response as well.So as things become more complex and more variables are introduced, its niceto know what is actually happening. Thus measurements knowing exactly what you are dealing with comes in much more handy then a calculator that provides a very pretty graphand a guess. Oh, and since many are usingHeritage speakers, this means LF traps placed above the corner speakers in the front of the room as well! Oh, and while we are here, andsince there is so much more to any of these topics, at the risk of introducing as many or more problems as the techniqueactually solves in the complete overview of the topic, I might note that variablesubwoofer placement offers a mixture of both good and bad options! The good isthat dependent upon its placements about the room, the subwoofer will tend toshift the primacy of the modes from axial to tangential to oblique. With thisshift, different modes will be reinforced differently, thus offering thepotential to reinforce different modes at differing fundamental and subsequentharmonic frequencies and reinforcing various nodes and antinodes and theirplacement according to the dynamics of the particular modes. Additionally, additional units can be placedin positions so that certain modal peaks and nulls are cancelled via 180 degreeout of phase superposition. So, varying the position of the subwoofer canpotentially be used to your advantage. This is the focus of the oft citedHarmon whitepaper on subwoofer placement. But there is a cost! Despite the mantra that LFs are omnidirectional, thenotion that LFs are non-localizable is optimistic at best, and simply incorrect at worst. Oh sure, at sufficient distancesthis becomes more valid; witness the attempt to localize a approaching lowflying helicopter. But in a small acoustical space, where the Henry precedenceeffect is still valid, the difference in arrival times at each ear still allowsquite a bit of localizable cues, despite the frequencies at hand. Additionally,the lack of signal alignment in the time domain also causes radical signal offsets (significant andperverse group delay anomalies) and crossover anomalies manifested viasuperposition, ALL of which are audible effects that ideally should be avoided.But, I mention this as you do have a variety of variables at your disposal, butit must be remembered that these have negative ramifications as well aspositive ones. And generally speaking, aligningsignals in the time domain minimizes frequency response and intelligibility anomaliesand should be your first goal. Afterall, once signals are aligned within the time domain in a minimum phaserelationship, many well behaved options, including EQ, are available foradjusting the character of the direct signal. There is much more than can beaddressed, but I prefer to avoid going into too much arcane detail if it is notproductive. So lets start with this and see where the needs are. Oh, and let me make a request. Atthis point we are still talking about the basic concept. And my goal is to tryto present an opportunity for folks to understand the basic issue at hand. I amnot going to go off on lots of tangents here chasing lots of how do I solvethi or that at this point. After we get the basic concept down,there will then be plenty of opportunity to pursue specific solutions. Andhopefully by then, most will have a basic idea of how to begin to go aboutdoing this. Besides, I will have my hand fulltrying to avoid getting off on tangents myself. Trust me, I dont need any helpfinding tangents to chase! So help me stay focused and I will try to helpyouwith lots of help from some very able folks here! Quote Link to comment Share on other sites More sharing options...
mas Posted April 28, 2007 Author Share Posted April 28, 2007 Tuned pipe resonance and Mode diagrams Quote Link to comment Share on other sites More sharing options...
Roc Rinaldi Posted June 6, 2007 Share Posted June 6, 2007 Tuned pipe resonance and Mode diagrams Huh? Quote Link to comment Share on other sites More sharing options...
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