Testing stage filler - Sand, Fiberglass, or Nothing - AVS Forum
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post #1 of 176 Old 10-21-2014, 08:45 PM - Thread Starter
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Testing stage filler - Sand, Fiberglass, or Nothing

Stages don't tend to be as variable as risers, it seems, with the majority housing one or more subwoofers and being filled with sand. This is a great example of that style of design (an Erskine design, as shared by BasementBob):



A description of what this means:

Quote:
Originally Posted by BasementBob View Post
- The purpose of the rubber underneath is to decouple the riser from the house slab for soundproofing benefits.
- The purpose of the fiberglass and the green glue is to damp the vibrations resonating through the riser.
- The purpose of the sand is to give the subwoofer enough mass to push off of without moving the riser. The subwoofer-riser/rubber/slab forms a mass-spring-mass system -- i.e. the sand improves the frequency response output of the subwoofer. The 10 times the weight of sand to the weight of the subwoofer, sounds good to me for this purpose. If the riser joists are perfectly flat and continuously touching and possibly glued to a perfectly flat concrete slab (as opposed to rubber, or raised joists to give airflow under them for a bass trap), the need for sand is probably removed as long as the sub is atop a joist.
Of course, nobody is going to suggest putting in hundreds or thousands of pounds of sand if the theater is on the second floor, so an alternative method is to fill the stage with fiberglass. This is considered a lesser solution, but an acceptable one.

I do wonder how much of the sand vs fiberglass recommendation is based on solid evidence and how much of it is just because that's what is done. Something HopefulFred said recently stuck with me (emphasis added by me):

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Originally Posted by HopefulFred View Post
The existing patterns of usage for stages and risers is a matter of tradition and myth. If subs are used on both stage and riser, there shouldn't be a difference in their construction. Here's how I understand the history and tradition, as it is, for these things - I think this will explain what you see in survey of existing home theaters. Stages were probably recommended as sand-filled long before anyone tried to use a riser as a bass trap. This has become a matter of pride and initiation to some degree, but still makes sense. Even in single row theaters, stages are useful. Second rows and the need for a riser are later developments, at least as far as I can tell and in terms of broad usage. Further, the use of multiple subs is a fairly recent trend. And the use of risers as bass traps has only been generally known (though mysterious) for about 10 years. So there's a great deal of inertia in this folk-culture of home theater design. And I say folk-culture since the hows and whys are often not understood, but a great deal of accepted best-practices are passed from one generation of builders to the next, with very few of us being actually capable of the engineering work that would be required to competently do the work from scratch.
I'd like to get some actual numbers on the differences between the various fillings. I'm in the position now where I have not yet built my stage, nor anything else other than my soundproofing shell. So here's what I'm thinking:

Step 1: I run some tests with my subwoofer on the bare slab and record the results
Step 2: I build the stage normally, but don't fill it with anything and only screw down the two layers of OSB top. I re-run the tests.
Step 3: I take the top off of the stage; fill it with fiberglass insulation; and put the top back on. Re-run the tests a third time.
Step 4: Finally, I take the top off; fill it with sand; and put the top back on. Re-run the tests a fourth and final time.

And I do mean final because once the sand is in there, it won't be taken back out!

So here's my questions:

Question 1: Is this a feasible set of experimental steps? Or is there something about this that would make any tests run completely invalid?
Question 2: If the steps are reasonable, then precisely what tests should I run? I wouldn't want to get to the sand stage of the stage and find out that I should have run a different test!

Thoughts?

(As a complete digression, this is my 512th post, which seems very notable to me. Maybe that only sticks out as an important number of CS geeks, though)

UPDATE 2014-11-21

I completed the testing on this, with results found here: Experiment Conclusions (Post 140)

TL;DR: There was no measurable or noticeable difference related to any of the riser filling materials, including sand.
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Last edited by granroth; 11-21-2014 at 07:48 PM.
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post #2 of 176 Old 10-21-2014, 09:02 PM
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I think you would want to measure any stage resonances that are introduced in the theater by comparing the rooms frequency response curve at each step. You need to position the speakers and microphone at exactly the same position and send the identical signal and amplified strength to the speakers

You could put a contact microphone on the face of the riser and see what you get, (I actually think this will produce the most measurable difference) You could also put it on the top but you should try to leave it attached for each iteration.

You should measure the soundproofing results outside the theater in an area that would be affected by structural vibration.

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post #3 of 176 Old 10-21-2014, 09:27 PM
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Using just insulation in the stage isn't acceptable in my opinion if in an upstairs situation. There are other ways to achieve the goals. This is where engineering ahead of time would become essential. The tests need to be performed outside the room. So, you will need a microphone that has a low noise floor. This is where type I mics become necessary.

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post #4 of 176 Old 10-22-2014, 06:28 AM
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Does it matter if you use 3/4" CDX plywood for the top vs 3/4" T&G OSB?

My Home Theater Build: The Vortex Theater Build
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post #5 of 176 Old 10-22-2014, 05:28 PM - Thread Starter
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Quote:
Originally Posted by BIGmouthinDC View Post
I think you would want to measure any stage resonances that are introduced in the theater by comparing the rooms frequency response curve at each step. You need to position the speakers and microphone at exactly the same position and send the identical signal and amplified strength to the speakers
Something as simple as an RTA snapshot? Or do a full REW style measurement sweep?

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Originally Posted by BIGmouthinDC View Post
You could put a contact microphone on the face of the riser and see what you get, (I actually think this will produce the most measurable difference) You could also put it on the top but you should try to leave it attached for each iteration.
Well that's an interesting idea. I was tending to shy away from direct measurements like that, since they could be pointless. That is, maybe a direct measurement would show a 2x increase in some frequency being transmitted in the material, but if it didn't result in an audible difference externally, then who cares? But that still might be an interesting thing to note and to also check what is being transmitted through the slab.

I've never seen any reference to contact mics being used for analysis like this, though, so I'm not sure what would be a good one to get. Most seem to be geared towards guitarists or similar. I'll do some more research on this angle.
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post #6 of 176 Old 10-22-2014, 05:43 PM - Thread Starter
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Quote:
Originally Posted by SierraMikeBravo View Post
Using just insulation in the stage isn't acceptable in my opinion if in an upstairs situation. There are other ways to achieve the goals. This is where engineering ahead of time would become essential. The tests need to be performed outside the room. So, you will need a microphone that has a low noise floor. This is where type I mics become necessary.
Are you thinking that the tests should only be performed outside the room, or both inside and outside? Do you have specific tests in mind that you think would mean the most?

I wasn't aware of the different microphone levels, so looking that up, I see a reference to type 1 mics having a tolerance of +/- 0.7dB with type 2 mics allowing +/- 1.0dB.

I was planning on getting either a miniDSP UMIK-1 or a Dayton Audio UMM-6. The UMIK-1 has a stated frequency response of 20 Hz - 20kHz +/-1dB, which suggests to me that it's maybe a type 2 mic. The UMM-6 only says 18-20,000 Hz with no +/- figure. I would assume that if the UMIK-1 was type 2, then the UMM-6 would be also, since they are direct competitors.

I'm curious why that 0.3dB difference would matter. Can you explain what a type 1 mic would pick up that a type 2 wouldn't?
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post #7 of 176 Old 10-22-2014, 06:41 PM
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The accuracy isn't really the issue, I think. The measurement Shawn is talking about is measuring the bass bleeding through to other areas. When identifying and parsing elements of a noise floor, high-sensitivity/low noise-floor is the key.

"Our Type 1 meters tend to measure lower levels due the sensitivity of the microphone capsule. This is particularly relevant for environmental noise surveys where low noise levels are being measured." link:https://www.noisemeters.com/help/faq/type-class.asp
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Quote:
Originally Posted by HopefulFred View Post
The accuracy isn't really the issue, I think. The measurement Shawn is talking about is measuring the bass bleeding through to other areas. When identifying and parsing elements of a noise floor, high-sensitivity/low noise-floor is the key.

"Our Type 1 meters tend to measure lower levels due the sensitivity of the microphone capsule. This is particularly relevant for environmental noise surveys where low noise levels are being measured." link:https://www.noisemeters.com/help/faq/type-class.asp
Correct. Remember, target noise floor in the room is 20 dB. Type II Mics generally have a higher noise floor than 20 dB. Outside the theater, it should be no more than +3dB over ambient. Plus, using a higher sensitivity mic, you can test for leaks in the shell outside the room. You measure in and outside the room. The equipment you mentioned above usually is not sensitive enough. The signal of the room will be lost in the background noise of the mic (SNR). Further, you should be using a battery powered device for measurements this low since AC lines can introduce noise into the measurement. Type I mics are often pricey. Here is the exact one I own. We use this for testing THX theaters. I don't break it out too often because the mic is sensitive, it is very susceptible to fluctuations in temperature and humidity over time, which requires recalibration.

http://www.audiocontrol.com/t37/6229...icrophone.html

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post #9 of 176 Old 10-22-2014, 10:59 PM - Thread Starter
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It looks like the typical noise floor of the UMIK-1 and UMM-6 is about 30dB which is indeed definitely higher than the target of 20dB. Still, I'm not getting why that's a limiting factor in this case. I plan on running the sub at around 100dB, far far above any noise floor or ambient noise. My assumption is that if the difference in stage filler is meaningful, then the difference in dB will be likewise very obvious.

Put in reverse, if the ONLY way to tell the difference between the stage fillers is to require a fantastically sensitive microphone, then it seems like that confirms that the material doesn't matter in real world use.

What am I missing?
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post #10 of 176 Old 10-23-2014, 12:24 AM
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Quote:
Originally Posted by granroth View Post
It looks like the typical noise floor of the UMIK-1 and UMM-6 is about 30dB which is indeed definitely higher than the target of 20dB. Still, I'm not getting why that's a limiting factor in this case. I plan on running the sub at around 100dB, far far above any noise floor or ambient noise. My assumption is that if the difference in stage filler is meaningful, then the difference in dB will be likewise very obvious.

Put in reverse, if the ONLY way to tell the difference between the stage fillers is to require a fantastically sensitive microphone, then it seems like that confirms that the material doesn't matter in real world use.

What am I missing?
Run it and see what happens. All you can do. Still not sure why you are testing to see if fiberglass will be of benefit. Sand doesn't absorb. It just weighs a lot. It takes a massive amount of energy to move it (ie vibrate). Transference is diminished as a result. I don't feel using a foot of loose batt will do much to deaden the energy especially the lowest frequencies. Also consider placing speakers on top of the stage. Not just the sub and also place multiple subs as well. Make sure you post the frequency response plot of the sub as well. My initial thoughts are It's more that the mass of the stage would yield greater reductions than the fiberglass. Anyway, await your results.

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post #11 of 176 Old 10-23-2014, 06:44 AM
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Some time ago when I was reading about constrained dampening layers (Green Glue) I stumbled via Google on this Master Thesis that talked about constrained dampening layers applied to small aircraft landing gear. Anyway on page 26 of the thesis he lays out the instrumentation he used to measure vibration.

http://scholar.lib.vt.edu/theses/ava...imore_Rev1.pdf
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post #12 of 176 Old 10-23-2014, 11:03 AM
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Quote:
Originally Posted by SierraMikeBravo View Post
Correct. Remember, target noise floor in the room is 20 dB. Type II Mics generally have a higher noise floor than 20 dB. Outside the theater, it should be no more than +3dB over ambient. Plus, using a higher sensitivity mic, you can test for leaks in the shell outside the room. You measure in and outside the room. The equipment you mentioned above usually is not sensitive enough. The signal of the room will be lost in the background noise of the mic (SNR). Further, you should be using a battery powered device for measurements this low since AC lines can introduce noise into the measurement. Type I mics are often pricey. Here is the exact one I own. We use this for testing THX theaters. I don't break it out too often because the mic is sensitive, it is very susceptible to fluctuations in temperature and humidity over time, which requires recalibration.

http://www.audiocontrol.com/t37/6229...icrophone.html
Ok Shawn - I have to ask, did you also get the Apple iOS interface device for $500?
http://www.audiocontrol.com/720362/p...Interface.html

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A state of the art, Made for iPod, microphone preamp. With 48v phantom power for a microphone, a balanced line input, very low noise, and precision A/D conversion, this is the perfect measurement oriented preamp. Add to this an internal Li-ion battery, balanced line output, plus a Toslink output and you have a product with limitless uses.
Though it's kinda dated with a 30pin interface not the newer one.
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post #13 of 176 Old 10-23-2014, 11:39 AM
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Ok Shawn - I have to ask, did you also get the Apple iOS interface device for $500?
http://www.audiocontrol.com/720362/p...Interface.html


Though it's kinda dated with a 30pin interface not the newer one.
Nope. I have the following:


Equipment used:

Video Calibration
CalMan Professional 3.7/4.6
X-Rite i1 Pro Spectrophotometer
Konica-Minolta CS200
Quantum Data 780 THX Version Handheld HDMI Analyzer 1.4a compliant
Accupel HDG-4000
ControlCal Software for Pioneer and Panasonic displays
Avia Pro
DVE (DVD, HD-DVD and Blu-ray)
AVS Blu-ray disc
S&M Blu-ray and HD-DVD test discs
THX DVD Test Disc
THX Blu-ray 3D Test Disc

Design
TurboCAD Professional v.18 Platinum
MATLAB v.7 for acoustic software design
AV Pro Software

Audio Calibration
Sencore SP495 EX
AudioControl CM 145 Type I Mic
AudioControl CM 20 Type II Mic (4)
Sencore MX399
Sencore DAG5161
SoundPro Link
RS-95 Acoustic Software
QSC Signal and Venue Manager
Audyssey v.3.5
Numerous other software packages and test discs


I guess I should mention that the gentleman who created the AudioTools/Sencore boxes is the same person running Studio Six Digital and is the creator of the device you mentioned. Basically, he installed everything in the high end Sencore boxes into the iPad and iPhone. Decent stuff without having to get into the super high end professional gear. Further, the biggest advantage the Sencore boxes and iPad app has is that they are very portable. All of my equipment is except one. This comes in really handy. I can take an item out of the box and take accurate readings in 10 seconds with everything I have including video. If you need to take a quick reading of something, this saves enormous time. Thus, it is all battery powered. Until it gives up the ghost, I'm happy. I went through many years of iterations of equipment to arrive at what I have now. Just a lot of trial and error.
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post #14 of 176 Old 10-23-2014, 03:03 PM - Thread Starter
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Quote:
Originally Posted by BIGmouthinDC View Post
Some time ago when I was reading about constrained dampening layers (Green Glue) I stumbled via Google on this Master Thesis that talked about constrained dampening layers applied to small aircraft landing gear. Anyway on page 26 of the thesis he lays out the instrumentation he used to measure vibration.

http://scholar.lib.vt.edu/theses/ava...imore_Rev1.pdf
Hah! Okay, that's well beyond the test gear that I'd ever have. I did use some LabView controllers back in the day, but never at home.

Also... how could a paper like that ever get released with so many misspellings? You'd think somebody would have run that through a spell-checker at some point.
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post #15 of 176 Old 10-23-2014, 03:17 PM - Thread Starter
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Originally Posted by SierraMikeBravo View Post
Run it and see what happens. All you can do. Still not sure why you are testing to see if fiberglass will be of benefit. Sand doesn't absorb. It just weighs a lot. It takes a massive amount of energy to move it (ie vibrate). Transference is diminished as a result. I don't feel using a foot of loose batt will do much to deaden the energy especially the lowest frequencies. Also consider placing speakers on top of the stage. Not just the sub and also place multiple subs as well. Make sure you post the frequency response plot of the sub as well. My initial thoughts are It's more that the mass of the stage would yield greater reductions than the fiberglass. Anyway, await your results.
I know that you poo-poo using fiberglass, but it is extremely common. Looking through the AVS history, I'm seeing that until maybe 2006, most people were talking about stuffing stages with fiberglass in order to keep the stage from being a resonant chamber. By 2008, though, the meme had totally shifted into using sand as a vibration damping agent. This is a very representative thread post-shift:

acoustic properties of sand

Note that Winer is still recommending fiberglass at this point in time, but the overall tone is definitely pro-sand.

Okay, so if I'm going to test this, then I need to be sure I'm testing the right thing. That is, what will be the expected results that will differ dependent on the filler material? You've mentioned "mass" several times, and Dennis also goes into a little detail on that:

Quote:
Originally Posted by Dennis Erskine View Post
To properly anchor multiple subs and speakers, you need to couple the speakers to something with considerably more mass than that of the speakers. Once you do so, kinetic energy is transferred to that mass. If that mass happens to be your house, you'll find a wall somewhere in your house that is now a speaker.

The sand provides mass (the stage is NOT a bass trap). The sand provides excellent mechanical damping. Sand would not be used in the seating platform riser.
This suggests to me that the expected variable would be in SPL for the frequency that the sub is working at. That is, if we're trying to stop the vibrations, then the vibrations in question must be measured as sound. Is that true? Would just a RTA snapshot done in a few select locations capture that well enough?

For the record, I only have one sub right now and no available speakers, so my test will have to be just the single 12" sub.
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post #16 of 176 Old 10-23-2014, 04:07 PM
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I know that you poo-poo using fiberglass, but it is extremely common

Bad judgment and the Flu are extremely common as well. If you want to use fiberglass, go for it. It's your room, not mine, and I don't have to listen to the final result.


What's the purpose of fiberglass in the stage? To avoid cavity resonance. It would not be a bass trap because you don't have it set up to be a pressure absorber.


What's the purpose of sand in the stage? Two reasons. One, you have the equal but opposite reaction issue and you therefore want the subwoofer *bonded* to something of significantly more mass than the subwoofer in order for it to perform at its best. Of course, if you're using an undersized, cheap, under performing subwoofer, what the hay. Two, you need to damp the vibrational energy to prevent it from being transferred throughout the structure of the home. The subwoofer should have all of its energy directed to moving air in the room, not moving the structure of your house.


Quote:
That is, if we're trying to stop the vibrations, then the vibrations in question must be measured as sound. Is that true?
Sound is vibrational energy. Not all vibrational energy is sound and therefore your presumption is false.


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post #17 of 176 Old 10-23-2014, 07:28 PM - Thread Starter
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Quote:
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Bad judgment and the Flu are extremely common as well. If you want to use fiberglass, go for it. It's your room, not mine, and I don't have to listen to the final result.
Hah! Well, I am absolutely going to have sand in my stage, since that's going to be the final step of my testing and there's absolutely no way I'm going to remove it. Ever.

I'm wanting to do this tests, though, to have quantifiable information on why you'd want to do that. Not just "it's a good idea" but "it's a good idea because these numbers show the notable difference"

And that's what I'm really trying to figure out now -- HOW do I actually test all this where I could get useful data?


Quote:
Sound is vibrational energy. Not all vibrational energy is sound and therefore your presumption is false.
My presumptions are often false, so no surprise there! But I want to explore all of this some more.

First, if the vibrational energy in the solid substances surrounding the riser are not detected as sound, then do they even matter? It's a "tree crashing in the forest with nobody around deal" -- if the vibrations are such that I can't even detect them, then should that be something that we're trying to prevent since nobody will even know they are there?

Quote:
What's the purpose of sand in the stage? Two reasons. One, you have the equal but opposite reaction issue and you therefore want the subwoofer *bonded* to something of significantly more mass than the subwoofer in order for it to perform at its best. Of course, if you're using an undersized, cheap, under performing subwoofer, what the hay.
Okay, to see if I'm understanding this -- the common problem with a subwoofer cabinet flexing is that that energy is "wasted" and thus not being used to push the sound waves. Therefore, you'll have reduced output which translates to reduced SPL. A subwoofer that does not move as much, will transmit more of its energy to the sound waves and thus be "louder". That suggests, then, that I could test that using an SPL meter or an RTA graph? That is, if I have the exact same output level on the sub and the exact same mic location, then I should expect to register a loss in DB when the sub moves from the floor to the empty stage, and then a subsequent rise in DB when the stage is filled with sand? Correct?

I'm very curious at how much mass is "enough". If we assume a 5x13 stage with 2x8s or 2x10s and dual layers of 3/4" OSB, then we're looking between 500 and 600 lbs empty. A typical 12" commercial sub appear to weight around or under 50 lbs. So we see a 10x ratio of mass of sub to stage in this case. Not enough?

Finally, when you say "bonded", do you mean physically attaching the sub to the stage? Or just the weight of the sub pressing down on the stage?

Quote:
Two, you need to damp the vibrational energy to prevent it from being transferred throughout the structure of the home. The subwoofer should have all of its energy directed to moving air in the room, not moving the structure of your house.
So this is the case where the true test should likely be done outside of the theater. Is this a simple SPL test as well? That is, if I have a sub going at 100dB at 35Hz, then go through the house running an RTA test, then some notable fraction of the measured SPL at that frequency will be coming through the slab. I would expect to see a notable drop when the sub is placed on a sand-filled stage vs directly on the slab.


Just to be clear, I am not in any way suggesting that you are wrong about any of this. I tend to treat what you say about theater building as the "word of god". But... I'm the type of guy that can't be TOLD something but needs to be SHOWN it. I haven't ever seen any quantifiable numbers that SHOW the differences between stage types and so I want to gather those. That requires that I run the right tests in the right ways or I'm just wasting my time.
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post #18 of 176 Old 10-23-2014, 08:10 PM
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Originally Posted by granroth View Post
Hah! Well, I am absolutely going to have sand in my stage, since that's going to be the final step of my testing and there's absolutely no way I'm going to remove it. Ever.

I'm wanting to do this tests, though, to have quantifiable information on why you'd want to do that. Not just "it's a good idea" but "it's a good idea because these numbers show the notable difference"

And that's what I'm really trying to figure out now -- HOW do I actually test all this where I could get useful data?




My presumptions are often false, so no surprise there! But I want to explore all of this some more.

First, if the vibrational energy in the solid substances surrounding the riser are not detected as sound, then do they even matter? It's a "tree crashing in the forest with nobody around deal" -- if the vibrations are such that I can't even detect them, then should that be something that we're trying to prevent since nobody will even know they are there?



Okay, to see if I'm understanding this -- the common problem with a subwoofer cabinet flexing is that that energy is "wasted" and thus not being used to push the sound waves. Therefore, you'll have reduced output which translates to reduced SPL. A subwoofer that does not move as much, will transmit more of its energy to the sound waves and thus be "louder". That suggests, then, that I could test that using an SPL meter or an RTA graph? That is, if I have the exact same output level on the sub and the exact same mic location, then I should expect to register a loss in DB when the sub moves from the floor to the empty stage, and then a subsequent rise in DB when the stage is filled with sand? Correct?

I'm very curious at how much mass is "enough". If we assume a 5x13 stage with 2x8s or 2x10s and dual layers of 3/4" OSB, then we're looking between 500 and 600 lbs empty. A typical 12" commercial sub appear to weight around or under 50 lbs. So we see a 10x ratio of mass of sub to stage in this case. Not enough?

Finally, when you say "bonded", do you mean physically attaching the sub to the stage? Or just the weight of the sub pressing down on the stage?



So this is the case where the true test should likely be done outside of the theater. Is this a simple SPL test as well? That is, if I have a sub going at 100dB at 35Hz, then go through the house running an RTA test, then some notable fraction of the measured SPL at that frequency will be coming through the slab. I would expect to see a notable drop when the sub is placed on a sand-filled stage vs directly on the slab.


Just to be clear, I am not in any way suggesting that you are wrong about any of this. I tend to treat what you say about theater building as the "word of god". But... I'm the type of guy that can't be TOLD something but needs to be SHOWN it. I haven't ever seen any quantifiable numbers that SHOW the differences between stage types and so I want to gather those. That requires that I run the right tests in the right ways or I'm just wasting my time.
Wow, I guess I'm chopped liver. I said pretty much the exact same thing. This is really a simple physics problem that can be easily thought through. You really don't need anything quantifiable, but if you need it... In essence, the insulation does nothing of any real consequence. All it is effectively doing is keeping the stage from becoming a resonance chamber. Nothing more. Further, you should consider using a range of frequencies...say around 20 Hz to 50 Hz. Test each frequency in that range. You've got some days of testing ahead of you. Go for it!!!

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post #19 of 176 Old 10-23-2014, 10:57 PM - Thread Starter
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Originally Posted by SierraMikeBravo View Post
Wow, I guess I'm chopped liver. I said pretty much the exact same thing. This is really a simple physics problem that can be easily thought through. You really don't need anything quantifiable, but if you need it... In essence, the insulation does nothing of any real consequence. All it is effectively doing is keeping the stage from becoming a resonance chamber. Nothing more. Further, you should consider using a range of frequencies...say around 20 Hz to 50 Hz. Test each frequency in that range. You've got some days of testing ahead of you. Go for it!!!
Shawn, you'll notice that I never said you were wrong about any point either!

Seriously, I'm just trying to get some basic questions answered to make sure I'm not completely wasting my time doing the testing. If I conclude that the tests that I should run are an RTA test for multiple frequencies in multiple (but repeatable!) locations, then I have two primary questions remaining (in addition to the other little questions that are still outstanding in the above posts):

1. You mentioned that a type 1 mic would be necessary. I still want to know why!
2. What kind of mass ratio is needed to be effective?

Here's a concern of mine. Say I go through these steps and when I crunch the numbers, I see absolutely no real difference at all. Then somebody comes back and says "well obviously -- your mic wasn't sensitive enough and here's why" or "you needed a much bigger subwoofer, since your ratio of 50lbs to 500lbs is sufficient on its own"... or something else like that. I want to cover my bases up front to make sure that this is a worthwhile endeavour.

Keep in mind that I'm not going to directly benefit from the tests, since I'm going to end up with the canonical recommendation of a sand-filled stage no matter what. My hope is that this will be useful for future stage builders. Hence it needs to produce applicable results.

So here's one more question for all y'all experts:

What kind of results are you expecting? I don't need exact numbers, but general ballparks would be very handy to benchmark against.
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post #20 of 176 Old 10-23-2014, 11:09 PM
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This is the last I'm going to say about this:

When a tree falls in a forest there is no sound if nothing is around to hear it (by the definition of sound). That does not mean the rules of physics have fallen apart and energy transfer does not occur in the absence of some 'within ear shot'.


1. SPL measurements with cheap equipment won't tell you much.
2. You need to measure frequency and phase as well.
3. Vibratory energy in solids (and some liquids for that matter) don't have SPL and are not sound (changes in pressure detected by animal hearing mechanisms).
4. There is specialized equipment which can, and does, measure vibration in solids and the speed of propagation.








In a real example, a room was built. Sound from that room was not audible in any adjacent room at levels less than 60dBB. However, two floors higher in the residence and in a far opposite corner, low frequency sounds were clearly audible. Vibration was getting into the structure and a particular wall in the upper better had the same resonance frequency(ies) as a wall in the sound room. (An artifact of making walls consistently 16" O.C. with 2x4/2x6 and all 8', or 9' or 10' high throughout a residence.) The subwoofer(s) were bonded to a common floor and vibrations from the sub were carried throughout the structure.

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post #21 of 176 Old 10-24-2014, 05:14 AM
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To add to what Dennis has said and to answer your question. On the type 1 mic, you need to have a baseline reading before you do the test. I assume you'll turn everything off to try to get as low as possible. What if your house in that moment is below 30 dB? With a noise floor of the mic you chose, you would never know. Further, the stated specs are on average. Therefore, your mic could be worse or better. How will know unless you have actual readouts from your mic compared to a standard. You would have to get it tested if it wasn't already. Now, on your other question. I cannot really tell you a ratio. Doesn't exist. It would have to be empirically derived. There are way too many variables. Mass of the wood, subwoofer capabilities, number of subs, number of speakers on said platform, how the isolation of the room was done, structure of the house, configuration of the house, materials of the house, and on and on. You'd have to quantify these somehow, and someone would have to be able to have repeatable results. The method we use is empirically derived since we get consistent results time and again without having a target number. Just results. Do we exceed the minimums required? Yes, by the results, but what is minimally required is yet to be determined due to all the variables, and frankly we just don't have to time to determine a minimum of a criteria we have already met by exceeding the minimums. So, to me that would be a more worthwhile pursuit. Just how much sand do you nee? But it will be difficult for others to repeat since every space is different. Hope this helps!!
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post #22 of 176 Old 10-24-2014, 06:21 AM
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Irrespective of what the scientists say it would be interesting to see what a layman can measure relative to the effectiveness of sand. What ever you do may make it easier for the next guy who wants to strap wings to a bicycle. Granroth you obviously have caught the attention of a couple of big guns in theater design, imagine the interest of all us poor soles who have carried sand into the basement bag by bag. Do it.

I imagine that there is a certain level of trepidation that you might not measure any difference and lend some credence to the "you don't need sand" segment. But then if you can't measure anything except with the absolute best in microphones can we actually hear the difference on a typical day?

I've stood on stages with and without sand and stomped my feet and my ears have told me it makes a difference. So, I'll be pouring sand until proven otherwise.
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post #23 of 176 Old 10-24-2014, 06:29 AM
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Agree with Big, I encourage you to acknowledge the limitations and caveats and proceed with as much data as you can get.

Before you do anything, there should be a careful consideration and listing of the parameters, the equipment, the protocols, and all their limitations. Shawn and Dennis have started much of that, but there's still a lot technically unclear. Obviously you're a smart guy, so this may sound a little condescending - but please don't take it that way: this should be treated as much like a science experiment as possible I you want to be able to generalize the results.
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post #24 of 176 Old 10-24-2014, 07:14 AM
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Oh, forgot to mention this. Most microphones do poorly in the low end. Another reason to use a type 1 is to get the best possible, consistent results in the low end that you can if you want to get serious.
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post #25 of 176 Old 10-24-2014, 08:43 AM
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I've seen some dudes put the mic in a plastic bag for close up sub measurements. Any validity to that?

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post #26 of 176 Old 10-24-2014, 10:58 AM
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No validity to plastic I've ever heard of. However, yes to placing it close if you are trying a near field response plot of the sub.
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post #27 of 176 Old 10-24-2014, 01:05 PM
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Quote:
Originally Posted by SierraMikeBravo View Post
Now, on your other question. I cannot really tell you a ratio. Doesn't exist. It would have to be empirically derived. There are way too many variables. Mass of the wood, subwoofer capabilities, number of subs, number of speakers on said platform, how the isolation of the room was done, structure of the house, configuration of the house, materials of the house, and on and on. You'd have to quantify these somehow, and someone would have to be able to have repeatable results. The method we use is empirically derived since we get consistent results time and again without having a target number. Just results.
What are the results?

 

 

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post #28 of 176 Old 10-24-2014, 03:07 PM
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Greatly reduced or eliminated vibration transference
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post #29 of 176 Old 10-24-2014, 07:51 PM - Thread Starter
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Okay, I ordered a Dayton Audio UMM-6 microphone from Cross-Spectrum Labs. I was originally leaning towards the miniDSP UMIK-1, since that can also work with AudioTools on an iPad (with an adapter), but Cross-Spectrum was backordered on them. I figured that it was more important to get the mic professionally calibrated than to go elsewhere to get the UMIK-1 for a feature I may not actually use. The UMM-6 arrives in a couple weeks, likely.

I also got a cheap piezo pickup from Amazon, just for fun. I'm going to stick it on the stage and the floor and the walls and just see what kind of results I get. I have zero expectations of anything useful, but it was only $8 so no big loss. I'll just make a cigar box guitar or something with it later, if need be.

I'm currently playing around with REW to familiarize myself with how it works. I'm using a cheap-o Radio Shack 33-2055 SPL meter as the microphone until my real mic arrives. I'm planning doing a run-through of a potential series of tests this weekend and posting the results, with the recognition that the numbers are meaningless but rather to use it as an attempt to validate procedure.
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post #30 of 176 Old 10-25-2014, 04:38 AM
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