Ethan, Dan, Steve, John, i need some heads-up here....

Hi guys I did some listening test in my CR, you may check my construction thread here : http://www.johnlsayers.com/phpBB2/viewt ... 9955#59955 I would need some cues on what you guys think is happening with this room, and what action should be taken. Thanks

orangenumerik wrote:

I did some listening test in my CR ... I would need some cues on what you guys think is happening with this room, and what action should be taken.

Sorry, I don't have time to plow through six pages of that thread. But I can offer this: Have you optimized the speaker placements, and the listening position to the 38 percent "rule?" Read more about that here: www.realtraps.com/art_room-setup.htm The main null I find in most rooms is related to 1/4 wavelength (and its multiples) distances from the rear wall behind you. For this reason you want to put traps on the rear wall as well as straddling as many corners as possible. --Ethan

Orange, I'm not surprised that so much of your low end is accumulating in the rear corner, since your room is shaped to aim all sound reflections back there. You're definitely going to want to build as big a superchunk as possible in that back corner. If it were my room, I'd probalby go with a huge superchunk in the rear corner, more absorption along the 2 rear walls, slot resonators on the 2 side walls. I'd probably go slot on the front wall as well.

Orange - I'd build something like this in the rear corner cheers john

John, thanks for piping in on this. Interesting about the bass hangers, I know you're a fan of them. Do you think they will be more effective than filling the entire area back there with absorptive material? Well, you must or your wouldn't have suggested it.... I just wonder about the differences between the two approaches.... could you maybe say more about why you prefer that approach?

jwl wrote:

could you maybe say more about why you prefer that approach?

I'm interested too because I know little (okay, nothing) about slat absorbers and I should! --Ethan

well - the main addition was not the hangers per se it's the angled drywall walls. Most control rooms have a bass buildup in the rear of the room, possibly because of the angled walls directing all the signal to the rear of the room. see attached pic. I remember an article way back in the 70s in REP discussing this and the author offered the triangular rear wall solution. It does work - Guruland has a triangular rear wall sot resonator and there is no bass buildup in the rear. Unfortunately it takes up space but in the corner room like this it's space well spent IMO. cheers john

Thanks guys, thanks John Very informative info here. At first i was shocked by John proposition but the more i looked at it, the more i thinks i get the way it works. I must say that i am not a big fan of the hangers principle, but i've seen them so many time in many studios, i may give it a try. four question for John : 1-Does the added "solid wall with drywall" has to be as massive as possible and sealed or just one layer of 5/8 gypsum will do ? 2- The center frequency of the slat resonators on the side walls can only be determined by testing the room with measurement gear ....right ? 3-When you say "heavy insulation" for the rear walls do you mean "Thick" or "high density" (4pcf and up). As we speak i have 3" of RW spaced 3" from the walls 4ft high X 8 ft wide centered at hear height. 4-

Guruland has a triangular rear wall sot resonator and there is no bass buildup in the rear

So if i get this straight, in my case i can't built a triangular rear corner slat resonator because i don't have enough distance between the listening point and any reflective rear surface, so the rear corner has to be absorbent, right? sorry if those questions are just obvious but i am a bit discouraged and i need approval that i am on the right track. :? jwl wrote:

If it were my room, I'd probalby go with a huge superchunk in the rear corner, more absorption along the 2 rear walls, slot resonators on the 2 side walls. I'd probably go slot on the front wall as well.

I plan to have two slot resonators on the front wall over and under the main window, that is for sure. But i still don't know if i will use the front wall corners to soffits the monitors, or as bass traps or even slot resonator... hum Bonus question : When building a bass trap, am i right to think that higher RW densities is prefered. I always tough so but i seem to read just the oposite from time to time. I am using Roxul Safe n"Sound 3" (3pcf) for all my traps. Thanks to all this is great info.

orangenumerik wrote:

four question for John : 1-Does the added "solid wall with drywall" has to be as massive as possible and sealed or just one layer of 5/8 gypsum will do ?

I'd use two layers for extra mass - it is low end we are dealing with here.

2- The center frequency of the slat resonators on the side walls can only be determined by testing the room with measurement gear ....right ?

Not really - you are aiming for the low mid frequencies that the insulation doesn't absorb , i.e 100 - 350hz.

3-When you say "heavy insulation" for the rear walls do you mean "Thick" or "high density" (4pcf and up). As we speak i have 3" of RW spaced 3" from the walls 4ft high X 8 ft wide centered at hear height.

yes - that's the pcf but I'd do the whole wall, niot just an 8' x 4' area. 4-

Guruland has a triangular rear wall sot resonator and there is no bass buildup in the rear So if i get this straight, in my case i can't built a triangular rear corner slat resonator because i don't have enough distance between the listening point and any reflective rear surface, so the rear corner has to be absorbent, right?

correct.

sorry if those questions are just obvious but i am a bit discouraged and i need approval that i am on the right track. :?

cheer up mate - It's not brain surgery ;)

jwl wrote:

If it were my room, I'd probalby go with a huge superchunk in the rear corner, more absorption along the 2 rear walls, slot resonators on the 2 side walls. I'd probably go slot on the front wall as well.

I plan to have two slot resonators on the front wall over and under the main window, that is for sure. But i still don't know if i will use the front wall corners to soffits the monitors, or as bass traps or even slot resonator... hum Bonus question : When building a bass trap, am i right to think that higher RW densities is prefered. I always tough so but i seem to read just the oposite from time to time. I am using for all my traps. Thanks to all this is great info.

Roxul Safe n"Sound 3" (3pcf) is fine. BTW The further forward you can build that rear wall unit the better, work out where the rear wall works best and built it out to there. cheers john

Interesting, John. Thanks for taking the time to explain the angled wall. Very good idea. I'm still curious about the bass hanger vs. absorptive mass/superchunks approach. It seems to me that the greater mass of absorptive material from a superchunks approach would provide more absorption that the bass hanger approach, and therefore would flatten out the low end more effectively. But then I still don't quite understand the theory of the bass hanger approach. What if you built the small angled wall as you suggest above, then fill the spaces behind the false wall and around the angled wall with absorption? Or perhaps asked a bit more succinctly, what advantage does the bass hanger approach have over a superchunks (or fill-the-space-with-absorption) approach?

These are all good questions jwl, i'd like to know too. BTW thanks to John for answering all my questions so fast, it's really appreciated.

good question - the superchunk is a new method for me so I'm making assumptions here. The hanger traps have never been officially explained as far as I know - they were an idea put forward by Tom Hidley in his studio designs. The only explanation for how they work was demonstrated to me one day when I was down by the river - :D there were oyster pots hanging in the water by the river and a boat went past creating a wave - as the wave passed through the oyster farm all the hanging oyster pots moved together and the wave was absorbed and didn't make it to the shore. I realise this is not a scientific explanation but it's the only one I have - perhaps Eric could offer a better idea. The advantage I see in hangers over superchunk would be simply less materials. Filling the whole corner using the superchunk method would use up a lot of insulation, and I mean a lot!! hangers on the other hand would probably use less than a quarter of the amount. Also corners are not the only place you want to absorb low end. How about in a ceiling? are you going to fill a whole ceiling using the superchunk method? I think not. cheers john

I confirm John here. I never found a real conclusive explenation and have been discussing this at the Catholic University Leuven. I do have measured them and compared with other alternatives. There is one real study I know about (a book of 127 pages which was a dissertation submitted for the degree of Master of Science from the University of Southampton Faculty of Engineering and Applied Science Institute of Sound and Vibration Research Dated 1990 by L. E. B. Soares It has costed me (period 1989-1991) A LOT of effort to 1) discover the very existance of such study and 2) to get me a hard copy of that (AFAIK non-published) thing. An extract from the opening summary: ....... The results present no significant evidence of low-frequency absorption by a vibrating process. The mechanisme responsible for the effects observed in real studio control rooms is therefore still a paradox, and further research is needed. This is an extensive study, directly related to the hangers as described here, where one searched for the supporting theory and tested in a special for this purpose build scale model at 1:10. I have done numerous measurements in real life and in the lab on hanging baffels, with and without core, as applied in more industrial applications. These measurements do not confirm these low frequent qualities. Possibly, but here I'm assuming, has this to do with the smaller distance between the hangers in studios, causing a damped 1/4 wave resonator effect when close to a reflecting wall and a 1/2 wave resonator effect when rather distant from a wall (+ whatever other and in-between phenomena). I also have a baffel study from the Cath. Univ. Leuven where they investigated positioning from these thing, edge effect, screening etc. At one point in time someone here quoted a book where the author ice-cold told these things could be calculated including the low frequent cutoff, but without explaining how, nor refering to any source whatsoever. I really should be interested in more physical or mathematical data. When hearing/reading that here I could only smile and don't trust such an author anymore. I've seen a lot about this stuff and analogies with baffels. I still don't know anyone (serious) who told he had the final answers. It's like aspirine. One knows it works, but it's not very clear how or why. But the explenation I often read here about that vibrating core, is found to be not or little defining as per the referred study. Measurements I personally executed in the lab with baffels with and without core also didn't show any significant influence in the low frequent absorption. Hence the answer must be found elsewhere. I'm sure this core is important but related to their mutual and array positioning. Note that that guy got his Masters degree with as end conclusion that the effects as observed in control rooms is still a paradox. It's because I didn't understand either what I was looking at when I saw these hangers at first that I wanted to test them, just because I didn't trust things I couldn't explain/understand enough or wasn't sure about. But that still does NOT mean that I fully understand why they do what they do.

Possibly, but here I'm assuming, has this to do with the smaller distance between the hangers in studios, causing a damped 1/4 wave resonator effect when close to a reflecting wall and a 1/2 wave resonator effect when rather distant from a wall (+ whatever other and in-between phenomena).

Eric - when I hang hangers in the ceiling I put the first one at the halfway point between two parallel walls. I then divide that and keep dividing until the hangers are around 8" - 12" apart. In other words at the nodes. I also vary it with hangers in the opposite direction by 90 degrees for the other walls.. here's the verbtime of that room cheers john

Thanks John, Do you have before/after measurements? (with exclusion of other possible altered, more or less significant influencing elements). In numbers? If so, combined with some data of the room itself one could calculate a rough estimate of the absorption per piece or m2 ceiling surface.

unfortunately there are no measurements taken before/after. would be interesting. cheers john

Just enter this document in this thread to keep this hanger stuff together: DELTA acoustics: 50 Years of Sound Control Room Design. Revised Aug 1, 2006

av126205.pdf wrote:

In order to obtain as much sound absorption in as large a frequency range as possible Hidley created his famous “bass traps” consisting of elements of mineral wool (batts) hanging vertically side by side at a height of maybe 2-3 metres. The effect can be compared with the effect of the mineral wool wedges of an anechoic chamber. The wedges create an impedance matching between the air and the rigid boundaries of the room, so there will be no reflections from the boundaries back to the room Because the length of the bass trap (or of the wedge) along the sound direction the effect can be extended to rather low frequencies, e.g. 50 Hz. A peculiar detail is the name “bass trap” as the absorber is not especially effective at low frequencies. It is in fact a broadband absorber, just like the wedge. The bass traps were never documented by laboratory measurements, maybe because of the difficulties of doing laboratory measurements at such low frequencies.

Since the document gives no details, but is more a summary of 50 years Control Room design, I've no idea in how far this technical explanation is correct or complete (see previous post with reference to end work in which these things were studied, not resulting in clear answers, calling the observed effects still a paradox). After rereading this quoted description, it surprises me that the document does not refer to the center core in these hangers. Is that an error (oversight) or were the original Hidley hangers indeed without core (just mineral wool batts?).

Eric - I was fortunate to observe many Hidley rooms in construction from the early Record Plant etc in LA in the early 70s through to studios built in the 90s. I also spend an afternoon listening to tapes in the Westlake Demo Room in LA and in the Record plant's main room in 74. The hangers are made with a central core of soft fiberboard. We can buy it in Australia under the brand name "Canite" It's the stuff people use for notice boards as the soft fiberboard allows for drawing pins, you can snap it as it breaks easily. Standard pink fiberglass was glued to each side. When the hangers are hung correctly moving one will result in all the others moving as well - you can see the action move through the hangers as they all move in sequence. When I was in Singapore in 92 I visited a Hidley complex under construction. Studio A and C were up and running and Studio B was still in construction. There would have been up to 4 feet depth of hangers all around the room behind angled solid walls - hence the name Trap, the low end gets in but is trapped. This is also why I object the the term "Trap" being used for a sheet of 703 wrapped in cloth. ;) The ceilings of these room were also filled with hangers - horizontal hangers of around 3" x 1" hanging in groups of around 6 - 8 with each group turned 90 degrees on the other. The article failed to mention a couple of things. Firstly Hidely invented "The Compression Ceiling" which was a solid reflective surface above the console higher at the front of the room coming down to just above the engineer. This was to increase bass response. The other factor was stone walls either side of the speakers for diffusion. Their purpose was to scatter the high frequencies across the console. Because the consoles were very wide when adjusting the snare on channel 1 you could be 4 - 5 feet off the center line of the room and outside the splay of the horns. The horns were focused a couple of feet behind the engineer for the same reason, to spread the sound across the length of the console. He also didn't mention that the LEDE rooms were all ripped out as in reality, they didn't work.

John, Thanks for sharing your clarification, correction and additions. Such a paper is nice, but it's good that it is checked by people with related background.

also, Jeff Cooper's Building a Recording Studio book has some basic calculations on hanging traps as well as expected response curve. he calls it a "broadband trap with adjustable low frequency cut-off".

gullfo wrote:

also, Jeff Cooper's Building a Recording Studio book has some basic calculations on hanging traps as well as expected response curve. he calls it a "broadband trap with adjustable low frequency cut-off".

Glenn, Look at this thread: http://www.johnlsayers.com/phpBB2/viewtopic.php?t=6008 Can you please elaborate on that? That he calls it that way without whatever reference to whatever, doesn't sound convincing to me. Or is the content of that thread I linked here in relation to that book wrong? This was the book I remembered (functionally) without recalling the Author's name. By you entering the name I could retrace the thread. Also note that I wrote before that I have done lots of measurements on baffles (with and without core), and still the logic isn't clear. For me the discrepancy is that baffles are known by lots of people (and lots of measurements) of having poor low frequent absorption, while practice in control rooms seems to point otherwise (therefore the paradox comment from that Master Thesis study I entered before). I never saw random incidence measurements with something I could call a low frequency cut-off. Its like telling the low frequency cut-off of a 6" thick fiberglass board (maybe when these real live hangers are measured at straight incidence in an impedance tube, but I doubt anyone ever did). Can you scan the relevant parts/pages of that book? PS: It's handy that this stuff comes back together .... In fact Ethan once did a measurement at IBM with a comparable setup of MiniTraps (:) I have a very good functional memory, but hopelessly forget what I did 15 minutes ago, I know what's in books, but have a hard time recalling names), and these results also showed this relative poor low frequent absorption (which is why they were never published on Ethan's or RealTraps' sites). And my personal tests in an Univ lab show little difference between with and without center core (which in its turn is also confirmed by splitter silencer design principles and tests). Still it does something in Control Rooms as practice seems to proof. :mrgreen: Hence: Paradox or Magic?

I'll pull the relevant info from the book and my communications with Jeff outlining the math he has used in designs.

Eric_Desart wrote:

Also note that I wrote before that I have done lots of measurements on baffles (with and without core), and still the logic isn't clear. For me the discrepancy is that baffles are known by lots of people (and lots of measurements) of having poor low frequent absorption, while practice in control rooms seems to point otherwise (therefore the paradox comment from that Master Thesis study I entered before). I never saw random incidence measurements with something I could call a low frequency cut-off. Its like telling the low frequency cut-off of a 6" thick fiberglass board (maybe when these real live hangers are measured at straight incidence in an impedance tube, but I doubt anyone ever did).

I've actually never seen data on hangers anywhere so I'm not sure if empirical data exists beyond what I'm reporting here. if you have some data on their performance (presumably done in controlled circumstances in accordance with the usual standards for this type of analysis) I'd like to see a copy (I promise not to sell it) :-) otoh, i see a lot of use of "acoustic baffles" hung from ceilings in large spaces to control bass and reverberation, or even "whispering gallery" effects. but besides the normal coefficients of absorption, not much information on how to optimize there placement. whats the difference between the baffles and the hangers? do baffles in large spaces work but hangers in smaller places do not? what is the most effecient low bass absorber? high efficiency for the smallest space? a membrane panel? a lined cavity? a simple layer of dense fiberglass over a cavity?

Jeff Cooper wrote:

From our experience, broadband traps have an effective low frequency cutoff equal to 1/7th of the wavelength absorbed. To compute this, measure the total panel length (i.e. L+S where L is the panel size and S is the hanger size per the diagram in the book) and use the formula on p.5 for wavelength. For example, a ceiling hung trap constructed of free-hanging fiberglass-covered panels, each measuring 24" long, on 6" hangers (total length = 30" or 2.5 feet) would be effective above 2.5' x 7 = 17.5' Effective cutoff wavelength 2.5 x 7 = 17.5 ft. (per formula on p.5) Frequency = V/l = 1130/17.5 = 64.5 Hz As long as the entire cavity was lined with absorptive materials and the trap blankets were at least 12" on center, this trap would be effective at all frequencies above 64.5 Hz. To decrease the cutoff frequency and make a basstrap effective down to 40 Hz, the panel length would need to be approximately 4 ft. long (including hangers). Very often, this type of broadband trap is constructed with varying sized panels, giving a more contoured absorption curve. An exact formula, relating panel spacing, size, hanger depth, etc. is empirical and not readily available. Regards, Jeff Cooper

quoted from Jeff Cooper's book on Building a Recording Studio (1976):

Glenn, Thanks for this info, while it isn't much in fact (as data/physics/math I mean, not your work you've done for us). But I let my feeling going over it. Just a question? What is meant with length here? If I have a hanger say 4' x 2' by 6" thick And I hang them on the ceiling 4' long and the height from top to bottom 2' Then for me 4' = L 2' = H 6" = T How does he use that here (he refers to 1 dimension, which one, .... the height?)? Does that in- or exclude the suspension height? Does he nowhere has any measurement with and without from which one could extract the absorption of a certain setup? How does such a cutoff shows in an absorption curve? Why does that curve stops at 125 Hz? if we speak about bass-traps? This still sounds vague to me. Concerning my data. If I once find the time I will digitize that data. Note that I did acoustics long before the net was common stuff. Lots of what I have are hard copies in several forms. Not all data I own (I have it as acoustician, but doesn't mean I can distribute it). And as you also understand this data is an asset to me. Hence I'm not sure what, when and if. In fact you ask a question, what is best ..... I can't be 100% sure (calls for too much, and expensive investigation in a difficult to investigate frequency range). What I do know (and you know since I told you during your visit) that I compared hangers as applied by Eastlake Audio (with a center core as described by John), which is established by Tom Hidley (if I'm correct here), with thick wool at a cavity, within the same space contraints, and I don't use hangers. With everything I know from the net it still feels that I'm the only one who ever compared this stuff. But that does not mean that hangers haven't proved their usefulness and value. What I do feel without being able to prove it, that in some cases here hangers are used in an unlucky corner without space to hang some serious stuff, that my belly tells me, "now you're overdoing it guys". To put this in context: I was recently really honored by a visit from Glenn (:) damned nice gentleman) and we made an extensive visit to Galaxy.

which is established by Tom Hidley (if I'm correct here),

The original company was Westlake Audio, established in LA. Westlake was an equipment supplier. Tom Hidley eventually left and moved to Europe where he established Eastlake Audio. One explanation that was given to me back in the 70s was that the central core of the hangers has it's own resonant frequency and it will resonate in sympathy when it's note is sounded - the insulation then absorbs the resonance. cheers john