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

[John Sayers]

unfortunately there are no measurements taken before/after.

would be interesting.

cheers
john

[Eric_Desart]

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

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?).

[John Sayers]

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.

[Eric_Desart]

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.

[gullfo]

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".

[Eric_Desart]

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. Hence: Paradox or Magic?

[gullfo]

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

[gullfo]

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?

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):

[Eric_Desart]

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.

[John Sayers]

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

[Eric_Desart]

Hi John,

Thanks

I think only real live measurements, comparisons and investigations can give enough answers. But that means time and money.

I checked that study I referred here again triggered by your reply, to go into some more detail.

To know how such traps ought to be build, they visited some Londen Studios including "Nomis" (a Tom Hidley design) and "Britania Row" (during refurbishment work conducted by Philip Newell) in order to check the details of the employed trapping system. (I believe I can safely assume they did this with knowledge and approval of the designers)
Confidential access*** to construction plans of a major studio, to be build around that time, from "Tom Hidley Designs, Switzerland" were defining in the geometry and disposition of these traps in the scale model. Hence there was direct personal contact between "Tom Hidley Designs" and the guy making this extensive study.

I checked the sub-studies and numerous measurements they performed to make sure the properties of the material used in the 1:10 model matched the scaled real live situation. (can assure you this is heavy stuff).

Hence this whole thing is really based on scaling Tom Hidley's principles and ideas.

Yet the outcome was how I entered before in this thread (questioning this vibrational thing).

• I now even start assuming that the fact that this study is nowhere to be found or referred, while Tom Hidley himself (or whoever in his company) was sideways involved, even providing confidential data on that studio in design, must have some reason.
  He MUST know that study, and NOT being happy with it.
  The quality of this study really is an Univ level end work .....

With everything we collected already about this, I think the only thing one can say is:
Real live applications prove they work, and so and so we got this kind of results, but it's not really clear how they work, hence we're still far from whatever mathematical modeling or quantification, with exception of some empirical analogies.

• As per your info it's center core vibration related.
• As per me (belly) I assume some 1/4 to 1/2 wave resonance is somehow involved (not in panel). This should need the core. But this are just assumptions, in order to explain something I can't explain.
• As per Delta acoustics some anechoic wedge behavior is simulated (not even referring to that center core).
  Delta acoustics suggests that no real controlled lab measurements where executed by, or known from the original designer.
• Jeff Cooper states in his book that "sometimes" a core is used (meaning: giving it some undefined not necessary function) and that they are meant "To obstruct the sound waves" (whatever that means), and "diffraction".
  In his book he refers to a mathematical to be defined critical distance and configuration in function of a project, without having any related math to offer or refer to.
  And for now I DO question that cutoff frequency calculation unless I see what that means in some curve(s).
  . What's e.g. the cutoff of all these broadband wall traps from the traditional suppliers?
  . A cutoff suggests a sudden steep drop related to some phenomenon, above which it must be close to max.
• And as per this study I referred (after rechecking triggered by your response, almost certain well known by Tom Hidley, possibly even discussed) they call the observed effects still a paradox needing more investigation, even explicit investigating and questioning this vibrational stuff.

With this in mind I should be careful to divert too much from the main applications as these ceiling traps and large wall bass traps, rather than hoping the principle will work in every corner where undersized models are pushed somewhere hoping they perform their magic stuff.

*** Additional footnote in study: "Private communication from Tom Hidley Designs, Switserland"

[gullfo]

thanks for this Eric. its very helpful to consider the relative effectiveness of different types of trap configurations. It is indeed a mystery as to how this type of trap became popular throughout the world if it had no substantiated effectiveness compared to X type of trap (still want to know what is the most effective type of trapping). it points to the need for careful consideration as to when to use a given type of trap based on space vs. effectiveness and what you're trying to solve...

I do remember (having nearly a photographic memory ) everything about our visit to Galaxy and our conversations. Eric is a kind soul and a bonafide genius (in case anyone didn't know that already...). Respect.

[Eric_Desart]

Many thanks Glenn,

My question remains about the use of L (lenght , Height).
Otherwise I've no idea what to do with that cutoff approach.
Can you check my question please?

still want to know what is the most effective type of trapping

Isn't anyone ???

[gullfo]

my understanding is the length of hanger + hanging distance = about 1/7th of the frequency. so if you have hangers that are 2' long + hang 0.5' for a total of 2.5' your lowest frequency affected (presumably the "cut off frequency") is 64.5hz. presumably smaller hangers are a higher cut off, and longer hangers are a lower cut off.

width should be proportional to the length. you may have some other ideas on how these dimensions play a role.

[John Sayers]

It is indeed a mystery as to how this type of trap became popular throughout the world if it had no substantiated effectiveness compared to X type of trap

The Hidley design came about because of the high levels being created in the Rock studios at the time. Sure the big bands of earlier times were loud but they preferred a reverberant room and still had dynamic range.

By the 70s the loud rock bands had guitarists with stacks of Marshall Quad boxes, similarly with bass amp stacks. The extremely high spl levels in the studio couldn't be controlled by the standard acoustic treatment of the day (absorbers mounted on wall modules) so Tom and Westlake Audio invented the hanger trap which appeared to work. Up to 1/3 of the studio floor space and most of the ceiling space was taken over by trapping. One area of the studio was devoted to the bass amp - you fired the amp directly into what was called a "bass trap" which was made from hangers. Initially they were open faced but later on Tom started to place them behind walls in an attempt to brighten up the top end of the very DEAD control rooms and studios. It was also the period where he started quoting flat down to 20hz.

For example the live room he designed for Paradise Studios in Sydney in 1979 had angled 3/4" mirrored glass walls floor to ceiling (14') but there was a port at the floor/wall intercept that allowed the sound into the heavy trapping (2' - 3') behind the glass. Similarly the ceiling was false with hangers above. (btw - I actually watched the construction and had access to the plans)

I recorded a few albums at Paradise and for me (and many other engineers) the live room was TOO live so we used to set the drums up next to the open doors of the live room and use it as a reverb chamber whilst retaining a close miked sound.