New Subwoofer Technology

[bossobass]

Ilkka wrote:

I know that M&C has bass down to 1 Hz, I meant that it might not have very high amplitude material in some small frequency range (for example the wide notch in 3-6 Hz range). But I looked your graphs more closely and you are being correct. There is around 10 dB notch at 20 Hz with the TRWs on. That is very likely a phase issue. What kind of filters (HP or LP) you have there? What are their slopes?

I don't think there is material at 1Hz. There certainly isn't at the level shown in the graph.

I think the spikes at 1Hz, and actually from DC to around 4Hz, are noise from the structure of the house and baffles, or some such resonance phenomenon.

I wonder if there are graphs of scenes where it's known that there is material in single digits that is encoded at high levels, like F'in' Irene, Batman Begins scarecrow scene and WOTW lightning strikes, for comparison?

Bosso

bossobass wrote:

I don't think there is material at 1Hz. There certainly isn't at the level shown in the graph.

I think the spikes at 1Hz, and actually from DC to around 4Hz, are noise from the structure of the house and baffles, or some such resonance phenomenon.

I wonder if there are graphs of scenes where it's known that there is material in single digits that is encoded at high levels, like F'in' Irene, Batman Begins scarecrow scene and WOTW lightning strikes, for comparison?

Bosso

Good question, however, you can still see the 1-10Hz stuff at lower SPLs that don't yet cause any shaking of room objects. The energy appears to be there. But I did have the same concern as you for when the walls were shaking ... some of that room interaction must show up in the spectral plots.

[MACCA350]

bossobass wrote:

I don't think there is material at 1Hz. There certainly isn't at the level shown in the graph.

I think the spikes at 1Hz, and actually from DC to around 4Hz, are noise from the structure of the house and baffles, or some such resonance phenomenon.

I wonder if there are graphs of scenes where it's known that there is material in single digits that is encoded at high levels, like F'in' Irene, Batman Begins scarecrow scene and WOTW lightning strikes, for comparison?

Bosso

This should clear up any doubt about sub sub 10Hz within source material.
Below is a graph produce by Sound Forge showing what is actually recorded in the DD5.1 track (for simplicity I combined all 6 channel to one) of the scene in The Hitch Hikers Guide To The Galaxy where the Earth gets Blown up at the end of chapter 4.


For comparison, below that is the Spectrum Analysis Graph on tzucc's site(I hope you don't mind) showing what the TRW is producing for that scene


Seems to be doing it's job

cheers

[bossobass]

Macca,

Thanks for the info. It looks to support my thoughts. There is a disparity as noted on the graph comparo below 5-6Hz of approximately 10dB from your graph to the in-room graph.

As I said before, I don't think it's room or boundary gain, but rather sympathetic subsonic resonances.

Of course, I could be reading the graphs wrong, and/or guessing wrongly at the cause of the differences I see.

Bosso

[MACCA350]

bossobass wrote:

Macca,

Thanks for the info. It looks to support my thoughts. There is a disparity as noted on the graph comparo below 5-6Hz of approximately 10dB from your graph to the in-room graph.

As I said before, I don't think it's room or boundary gain, but rather sympathetic subsonic resonances.

Of course, I could be reading the graphs wrong, and/or guessing wrongly at the cause of the differences I see.

Bosso

There can be a number of reasons for the differences, the simplest being that my graph is taken at a particular time stamp whearas tzucc's graph is "of a peak hold FFT during the scene"

But the graphs are very similar and shows that there is information that low in the source material and that the TRW is reproducing what is down there. How accurate it is and how well the TRW accomplishes this is something I can't answer.

cheers

[Chrisbee]

This is a fascinating subject. We really need to be reading the response of more subs below the 10Hz mark. There are some tantalising response graphs that seem to continue past 10Hz at much the same level as they do above that point. How useful that output would be is still a matter of conjecture.

I wonder if John M could be persuaded to give us a few more Hz on the REW sweep? (perhaps as a guarded selectable option to protect subs that can't handle it) But then we'd need better microphones than the common RS meter. What's the lower useable limit on the Behringer ECM8000 calibration microphone? It only claims a linear response to 15Hz on the sales blurb.

[bossobass]

I have no doubts as to the claims of output capability of the rotary sub. I do however, question the system capability. I can't find info as to what player/preamp/amplifier is driving the rotary sub, but it looks to be rolling off well above 1Hz, IMO...probably at 5Hz the system is down 3dB by my guess.

In fact, I don't know of any system that's flat to 1Hz.

Tzucc, can you mention the upstream hardware in your TRW system?

Chris, you are absolutely correct and I think it's only a matter of time before we have another 1/2 octave or so of graphing capability from the various measurement software people.

I think that most of the 'flat-to-10Hz-in-room' graphs we've seen to date are the results of sympathetic subsonic structure noise, and those graphs will show output to below 10Hz that isn't coming from the subwoofer.

Still, it's the final frontier and very exciting info, IMHO.

[bobgpsr]

The individual calibration graph on the "Certificate of Traceable Calibration" for my LinearX M31 cal mic shows a smooth rise from +2 dB @50 Hz to +5.5 dB @ 10 Hz. No drop off shown. I wonder how low cal mics really go?

MDF (Microphone Data Format) File
Author=LinearX Systems Inc
Date=Nov 17, 2005  Thr  4:49 pm
Model=M31
Serial=158848
dBspl=   94.000
dBm=  -23.414
Points=   300
Index   Freq(Hz)       dB      Deg
   1      10.000      5.45      0.00
   2      10.281      5.29      0.00
   3      10.570      5.24      0.00
   4      10.868      5.00      0.00
   5      11.173      4.91      0.00
   6      11.488      4.81      0.00
   7      11.811      4.76      0.00
   8      12.143      4.68      0.00
   9      12.485      4.58      0.00
  10      12.836      4.52      0.00
  11      13.197      4.49      0.00
  12      13.568      4.47      0.00
  13      13.950      4.41      0.00
  14      14.342      4.33      0.00
  15      14.745      4.23      0.00
  16      15.160      4.11      0.00
  17      15.587      4.00      0.00
  18      16.025      3.89      0.00
  19      16.476      3.78      0.00
  20      16.939      3.69      0.00
  21      17.416      3.59      0.00
  22      17.906      3.52      0.00
  23      18.409      3.44      0.00
  24      18.927      3.39      0.00
  25      19.459      3.32      0.00
  26      20.007      3.27      0.00
  27      20.569      3.20      0.00
  28      21.148      3.15      0.00
  29      21.743      3.09      0.00
  30      22.354      3.02      0.00
  31      22.983      2.95      0.00
  32      23.630      2.89      0.00
  33      24.294      2.82      0.00
  34      24.978      2.76      0.00
  35      25.680      2.69      0.00
  36      26.403      2.63      0.00
  37      27.145      2.57      0.00
  38      27.909      2.52      0.00
  39      28.694      2.47      0.00
  40      29.501      2.42      0.00
  41      30.331      2.38      0.00
  42      31.184      2.33      0.00
  43      32.061      2.29      0.00
  44      32.963      2.25      0.00
  45      33.890      2.21      0.00
  46      34.843      2.17      0.00
  47      35.823      2.13      0.00
  48      36.831      2.09      0.00
  49      37.867      2.06      0.00
  50      38.932      2.02      0.00
  ...
 275   19993.297     -1.63      0.00
 276   20555.660     -1.80      0.00
 277   21133.842     -2.00      0.00
 278   21728.287     -2.12      0.00
 279   22339.451     -2.18      0.00
 280   22967.807     -2.28      0.00
 281   23613.836     -2.43      0.00
 282   24278.037     -2.68      0.00
 283   24960.920     -2.95      0.00
 284   25663.012     -3.16      0.00
 285   26384.852     -3.34      0.00
 286   27126.994     -3.58      0.00
 287   27890.012     -3.95      0.00
 288   28674.490     -4.36      0.00
 289   29481.035     -4.79      0.00
 290   30310.268     -5.30      0.00
 291   31162.822     -5.84      0.00
 292   32039.357     -6.43      0.00
 293   32940.547     -7.13      0.00
 294   33867.086     -7.95      0.00
 295   34819.688     -8.77      0.00
 296   35799.082     -9.56      0.00
 297   36806.023    -10.51      0.00
 298   37841.289    -11.41      0.00
 299   38905.676    -12.46      0.00
 300   40000.000    -13.51      0.00
End

Bob

[Chrisbee]

bossobass wrote:

I think that most of the 'flat-to-10Hz-in-room' graphs we've seen to date are the results of sympathetic subsonic structure noise, and those graphs will show output to below 10Hz that isn't coming from the subwoofer.

I don't think the effect of structural resonance would be so linear an extension of the frequency response. The curve would also depend a great deal on the structure itself. My listening room is very lightweight being an attic with all wood cladding internally with relatively large areas of thin glass. A concrete cellar should show a completely different infrasonic response to subwoofer excitation. This is something that could be easily investigated using the same model of subwoofer in two rooms of the same size but of totally different construction.

I know from experience that my windows and glazed doors are sympathetically resonant at sub-14Hz. It doesn't matter whether the excitation is coming from my IB or my big SVS cylinder. Neither sub is in the same position in the room. They are about 8 feet apart. I can easily get my roof to heave and creak at 10Hz.

Still, it's the final frontier and very exciting info, IMHO.

Agreed! All subwoofery is still in its infancy compared with the level of development reached by "normal" loudspeakers.

[SteveCallas]

There is a disparity as noted on the graph comparo below 5-6Hz of approximately 10dB from your graph to the in-room graph.

As I said before, I don't think it's room or boundary gain, but rather sympathetic subsonic resonances.

This low, does it really even matter? You'll be hard pressed to find any room that will allow for high accuracy into such low frequencies, and if you do, it will take a whole moo baby moo of a lot of fun out of the experience.

I can't find info as to what player/preamp/amplifier is driving the rotary sub, but it looks to be rolling off well above 1Hz, IMO...probably at 5Hz the system is down 3dB by my guess.

In fact, I don't know of any system that's flat to 1Hz.

Tzucc, can you mention the upstream hardware in your TRW system?

He mentions in his blog that he got lucky his Lexicon passed a pretty solid signal to 1hz using the XLR output. Using the RCA output, he observed a typical electronics rolloff.

Hi gents. Bruce and I think the 3-5Hz dip is some sort of room mode. The 1-3Hz it NOT enhancement from room shaking or whatever... it's just output. I only say this because I see nothing happening or shaking in the room at that freq.

The room seems to resonate with 8-9Hz range... at that freq, the room seems to complain loudly and bitterly I simply will not demo 120dB (let alone 125dB which is easily possible) at 8Hz... if you were there, you'd be cringing too.

The mic that Bruce made for me cost $5 of radio shack parts, and he believes it is reasonably flat to 1Hz. I should have him post his design... it's pretty nifty. I'll put up a snap of it. Put it this way... it picked up a very low SPL 0-1Hz signal during a quiescent time in the room... we were going nuts trying to figure out what it was... then we figured it out. My AprilAire ERV was slowly pumping air through the room!!! Yeah, it's sensitive enough at low frequencies all right.

edit: you can barely feel this air flow by standing up to the ceiling registers, and only by wetting the back of your hand to the airflow. I am still amazed that cheap condenser mic could see that weak flow!

The signal path is:
- HTPC optical out
- Lexicon MC12B optical in
- Lexicon L and R stereo sub to XLR out
- Lake EQ XLR in, 2 channels summed to mono
- Lake EQ XLR out to TRW amp (converted to RCA for now), XLR out to Watchdogs and XS amps
- RCA into Bruce's custom made amp (nothing special, just designed for down to <1Hz, will have XLR and EQ in next revision) at about 200W into 4 ohms or something. The amp has phase, volume, boost and low pass filter controls. Simple and cheap.
- Bruce's amp out to the TRW via 50 ft of speaker cable

It appears that the Lexi is easily flat down to below 1Hz. Thank God, or we'd be nowhere in this project.

Macca, thanks for posting that chart. I assume it's an FFT done right off the DVD bits, which is quite useful to see what the true output frequency spectrum is. Confirms we have a room mode ... which is strange... at these wavelengths, I just don't get how.

Anyway, I am going to look for SoundForge and run some of that analysis... anyone know which software it is that also gives you those cool color waterfall charts showing freq vs output vs time?

[Sonnie]

The mic that Bruce made for me cost $5 of radio shack parts, and he believes it is reasonably flat to 1Hz. I should have him post his design... it's pretty nifty.

Off topic... uh...errr... well, maybe not. It might be considered new subwoofer "measurement" technology for that price. I've read somewhere that someone could build a very flat mic from inexpensive parts, but lost where I read it.


Seriously though, if you can share this with us, please start us up a new thread in whichever forum would be appropriate (I may copy and sticky it in a couple of forums). One thing I'm curious of though... how can we be sure of how flat it is down to 1Hz? Has there been any calibration against a worthy mic. We can certainly compare it down to 10Hz. I'll build one and compare it myself, but I don't know how accurate I'd be below that.

[Sonnie]

Chrisbee wrote:

I wonder if John M could be persuaded to give us a few more Hz on the REW sweep? (perhaps as a guarded selectable option to protect subs that can't handle it) But then we'd need better microphones than the common RS meter. What's the lower useable limit on the Behringer ECM8000 calibration microphone? It only claims a linear response to 15Hz on the sales blurb.

This is a done deal, the next release sweeps from 0Hz. We are beta testing it now and hopefully John will have it ready soon... lots of new and useful features.

My ECM is calibrated to 10Hz, but most of them are out of the box -2.5db+ from 15Hz on down.

[Chrisbee]

tzucc wrote:

Hi gents. Bruce and I think the 3-5Hz dip is some sort of room mode. The 1-3Hz it NOT enhancement from room shaking or whatever... it's just output. I only say this because I see nothing happening or shaking in the room at that freq.

The room seems to resonate with 8-9Hz range... at that freq, the room seems to complain loudly and bitterly I simply will not demo 120dB (let alone 125dB which is easily possible) at 8Hz... if you were there, you'd be cringing too.

edit: you can barely feel this air flow by standing up to the ceiling registers, and only by wetting the back of your hand to the airflow. I am still amazed that cheap condenser mic could see that weak flow!

Macca, thanks for posting that chart. I assume it's an FFT done right off the DVD bits, which is quite useful to see what the true output frequency spectrum is. Confirms we have a room mode ... which is strange... at these wavelengths, I just don't get how.

Why couldn't your room volume resonate as whole just like any other Helmholtz resonater?

[SteveCallas]

This is a done deal, the next release sweeps from 0Hz. We are beta testing it now and hopefully John will have it ready soon... lots of new and useful features.

My ECM is calibrated to 10Hz, but most of them are out of the box -2.5db+ from 15Hz on down.

Sonnie, will you be extending the RS meter calibration files down to single digits as well then?

[Sonnie]

If I can find someone to calibrate my ECM down to 0Hz for a reasonable price I will attempt to calibrate the RS meter, although I'm not sure if the RS meter mic can handle it, being it is so far off at 10Hz now.

I'm really interested in what Bruce and tzucc have going on with their home made mic with $5 worth of parts that is supposedly flat to 0Hz. I have read previously how the capsules in the very expensive mics are very inexpensive themselves. What I don't know is what makes them expensive. I'm having a hard time believing we can make a mic with $5 worth of parts that is flat down to 0Hz or even 10Hz for that matter, and Behringer, B&K, and all those expensive mic manufacturers can't do the same for hundreds and thousands more money.

[Mark Seaton]

tzucc wrote:

Hi gents. Bruce and I think the 3-5Hz dip is some sort of room mode. The 1-3Hz it NOT enhancement from room shaking or whatever... it's just output. I only say this because I see nothing happening or shaking in the room at that freq.

The room seems to resonate with 8-9Hz range... at that freq, the room seems to complain loudly and bitterly I simply will not demo 120dB (let alone 125dB which is easily possible) at 8Hz... if you were there, you'd be cringing too.

This really isn't "modal" behavior, but probably a combination of structural absorption (mass loaded walls on RC channel) in combination with various attached or coupled air pathways such as the HVAC ducting and the multiple chambers created with the rear "room" for the sub and the pathway to the garage. We don't think of these things as resonances for most woofers since they are out of the operating range, but such VLF resonances are not uncommon in HVAC systems that have not given consideration to it. Many workspaces deemed "uncomfortable" are so due to high-Q resonances that tend to be just below the threshold of audibility (as a tone/sound). There are also plenty of documented cases of alleged "hauntings" which were traced back to structural and/or acoustic VLF occilations.

The mic that Bruce made for me cost $5 of radio shack parts, and he believes it is reasonably flat to 1Hz. I should have him post his design... it's pretty nifty. I'll put up a snap of it. Put it this way... it picked up a very low SPL 0-1Hz signal during a quiescent time in the room... we were going nuts trying to figure out what it was... then we figured it out. My AprilAire ERV was slowly pumping air through the room!!! Yeah, it's sensitive enough at low frequencies all right.

Years back in the DIY speaker building communities (anyone else from the old BASS-List still around?) there were a few published projects using the Panasonic mic capsules. You used to be able to purchase assembled kits with cal-files that were referenced to a calibrated B&K. I believe they used to sell for about $40. I have one of the old assembled kits in my basement somewhere that I should dig out some time. Note that this same capsule is what is commonly used in many of the cheaper mics like the Behringer ECM and the SuperLux measurement mic. I think people are mistaken to expect that a generic correction curve can be used for some of the cheaper mics, especially something like an RS-meter. They meet their spec of consistency in measuring pink noise, but frequency response is not a target tolerance. Fortunately the bigger variations I've seen are at higher frequencies, not low. Bruce should be able to give some better insight as to what the typical variation is capsule to capsule and the tolerance inherent to the pre-amp circuitry.

In doing some prodding at the VLF extension issue on my own, what jumps out as the path of least resistance is in using a digital input signal all the way to the amplifier. The pro market has already made this step, so we'll see how much of that ever makes it to the HT world. All that's needed is a digital audio signal output that can be connected to common pro-audio DSPs that almost all handle digital I/O these days. One can only hope that with Dolby now owning Lake Technologies someone from opposite sides of the house might realise that they should be talking!

[Mark Seaton]

Sonnie wrote:

If I can find someone to calibrate my ECM down to 0Hz for a reasonable price I will attempt to calibrate the RS meter, although I'm not sure if the RS meter mic can handle it, being it is so far off at 10Hz now.

I'm really interested in what Bruce and tzucc have going on with their home made mic with $5 worth of parts that is supposedly flat to 0Hz. I have read previously how the capsules in the very expensive mics are very inexpensive themselves. What I don't know is what makes them expensive. I'm having a hard time believing we can make a mic with $5 worth of parts that is flat down to 0Hz or even 10Hz for that matter, and Behringer, B&K, and all those expensive mic manufacturers can't do the same for hundreds and thousands more money.

There are a few major cost contributors here, but the Behringer and SuperLux mics are good examples, and in large quantity, you'd see that the pricing suggests this level of internal parts quality. I would wager a guess that a huge % of the cost is the metal housing of the microhpone. There is also other circuitry inside, and remember, they're going to charge whatever people are willing to pay. Why sell a mic for $35 when you are killing the competition at $49?

The other factor is calibration and consistency of the microphone. I don't claim to be an expert on microphones, but basically you have two subsets of microphones. Those with polymer based diaphrams and those with various metal diaphrams. The much more expensive microphones from B&K, Earthworks, etc., all use metal based diaphrams. The most significant difference is the environmental stability of the microphone. Metal diaphram microphones shift by only hundreths of a dB per deg C of change. Polymer/plastic based diaphrams can shift both temporarily and permanently with significant environmental changes. Both have their place, and there is an order of magnitude in cost between them, but do realize the <$100 mics are not the same as the top quality microphones, even if they may have no added benefit for your particular application.

I do feel we are sorely lacking a reasonble cost, well documented measurement microhpone, and I have personally been harrassing Mark Schifter and those working on their RDES unit to integrate a mic preamp into the unit as you already have to connect via USB to program the unit. Tough to say if or when that might come to be yet, but it is being looked at.

[bossobass]

Here is a comparo between higher priced mics and the panny electret cap:

http://www.purebits.com/miccomp.html

This is the kit I have from ACOPacific. The 1/2" cap is only a relatively small portion of the cost. The stainless case contains the preamp, there is a power supply, choice of mic caps, etc. IIRC, the total kit $ was around 1500.00:

http://www.acopacific.com/micsystems.html

Point is you need a fairly precision supply and preamp with the Panny cap, which SC provides from the higher priced mics, but doesn't mention in the comparo.

Still, I'm sure one could use the cap and add a decent Mic pre/PS and fashion a case and purchase accessories (cables, clamps, windscreen, etc., for a good bit less.

Also note, that even though I know SC graphs to 1Hz, their test is only to 10Hz.

Bosso

[bossobass]

I'm still sticking with my original thoughts regarding the graph of M&C. Here is the spectrograph of the entire scene. I just think there are better scenes to make the point clearer as far as in-room response to 1Hz that could also be compared to this one.

Much appreciated if there are such comparo measurements...

Bosso

[Sonnie]

Hey Bosso... is this merely heatshrink around the cap and wires? Amazing results!

Bosso, good sleuthing. But I am not sure how accurate I would take that software reading... I'd have to think carefully about the FFT windowing algorithm and how it affects and is able to capture short peaks. Not saying you are wrong, just not convinced yet either way. Of course the FFT windowing on my end, the HTPC doing the measurements off the mic has the same question.

I think we'd need a signal processing expert to weigh in here... I am real rusty on that stuff nowadays, though I used to do it for a living.

On the mic, I have asked Bruce to post a schematic of his mic, along with a parts list. I should step back from saying the mic is flat to 0Hz... I really have NO idea that this is true. What I should have said, is that the mic has surprising sensitivity down to 0Hz. I think to properly and accurately calibrate the mic would have required equipment well beyond what I have or am willing to pay for.

I will ask Bruce about this... he has spent alot more time at his shop with this cheapie mic configuration and can speak more to it's correction curve.

About the RS digital meter, I found it's useless under 18Hz... when I did the measurements you'll find on my blog posting of last nite, I didn't include the RS meter readings cause no matter what f and SPL that the SpectralLabs showed, the RS meter seemed to vary randomly between 75-79Hz.
This contradicts Bruce's findings which he does us an old RS analog meter and he has come up with his own callibration 'file' for it down to 5Hz. I think he said at 5Hz the RS meter reading was -19dB too low.

p.s. on my mic... you gotta believe that it's sensitivity goes down as frequency goes down... so if the SpectraLabs shows a big rise in SPL levels during M&C cannon shots, one has to believe the air pressure change is more than there at the mic.
Also interesting is that the cheapie built in sound chip/circuit doesn't AC couple the mic input... they just let it all in!

[Mark Seaton]

tzucc wrote:

On the mic, I have asked Bruce to post a schematic of his mic, along with a parts list. I should step back from saying the mic is flat to 0Hz... I really have NO idea that this is true. What I should have said, is that the mic has surprising sensitivity down to 0Hz. I think to properly and accurately calibrate the mic would have required equipment well beyond what I have or am willing to pay for.

I will ask Bruce about this... he has spent alot more time at his shop with this cheapie mic configuration and can speak more to it's correction curve.

I recall Bruce mentioning that the mic was corrected via a calibration file that gave him flat response to about 1Hz. I'm sure it is useful below there, but may not be calibrated. In your own system the mic is unlikely to live outside the basement room, and is likely to stay quite consistent over time.

[bossobass]

Sonnie wrote:

This is a done deal, the next release sweeps from 0Hz. We are beta testing it now and hopefully John will have it ready soon... lots of new and useful features.

Hi Sonnie,

I missed this post the first go around.

You're Beta testing the new TrueRTA version that graphs to 0Hz? I sure hope you'll update us here, if you don't mind. Is there a link to a thread where this is being discussed?

Appreciate the heads up

Bosso

[Sonnie]

Actually we are beta testing the new Room EQ Wizard which does indeed graph to 0Hz. The next release will be nothing short of awesome.

Unfortunately there's not a public thread for discussion right now, but hopefully the full release will be available soon... I just don't know an exact date. John is working on it as best he can and doesn't have a date yet. We'll most likely have another beta version or two before full release.