Modeling "Room Gain" in WinISD - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 23 Old 08-14-2019, 01:08 AM - Thread Starter
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Modeling "Room Gain" in WinISD

Several folks have asked how to model the increase in SPL that's occurs in the low frequencies (so-called "room gain" or "cabin gain") in WinISD.

Each room is different and there is a lot more going on in rooms than what is typically referred to as room gain (e.g., reflections, modes, etc.), but having a basic model may be helpful, so here goes...

I'll use the example of a Full Marty, which is roughly a 10 cubic footer tuned to around 16Hz with a UM18 driver.

Full Marty (UM18) raw response with 1100w:



As a ported cab, it is always best to put a protective high pass filter on.

A 2nd order Butterworth (12dB/oct) protective high pass at 16Hz is used. "N=2" means second order.

Full Marty (UM18) with protective high pass:



**Uncheck** the protective high pass filter before proceeding.

The next step is to "force a flat response" so the room gain can be viewed easily. Switch the view at the top to Transfer Function Magnitude and check the box on the Advanced tab to Force Flat Response. The result should look like this:



Then a room gain profile has to be created. A medium size home room will typically have some lift beginning around 30Hz or so, approximately 6dB or so by 20Hz and somewhat more under that. A large room will typically have its gain starting lower in frequency and a smaller room will typically start higher up. [BTW, room gain in cars frequently starts up around 60Hz or higher, which is why small subs can make such enormous bass in cars, but not so much in home-size rooms.]

For a starting point, a Linkwitz Transform with F0=25Hz, Q0=0.707, F1=7Hz, Qp=0.5 is a decent starting point. For small rooms, increase F0 to something like 30Hz and for very large rooms, lower F0 to something like 20Hz.

Again, be sure to **uncheck** the protective high pass filter (and any other filters that may have been created) in order to see the room gain profile only.

A simulated room gain profile for a medium-large room



The final step is to turn the protective high pass filter back on and turn "force flat response" off.

The final in-room response is a combination of the raw response of the sub, the protective high pass filter, and the room gain.

In this case, it is pretty flat to around 14Hz, has an F3 of about 13Hz, and an F6 of about 12Hz.

FINAL RESULT: Full Marty (UM18) with protective high pass and room gain



Note: Be sure to turn **off** the Linkwitz-Transform when checking air speed, cone excursion, amplifier power, etc. because room gain is "free". It is not actual EQ that is being applied to the subwoofer.


Enjoy,

John
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Last edited by LTD02; 08-14-2019 at 01:16 AM.
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post #2 of 23 Old 08-14-2019, 01:19 AM - Thread Starter
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AVS member bossobass estimated room gain of several forum members based on driver response and room response (the difference being the room gain).

I have not attempted to validate his data, but it is generally consistent with what would be expected in home size rooms.



The non-smooth gain of some members' gain profiles is the result of some of that "other stuff" going on in rooms.

...


For folks who are new to it all, actual measured response will probably look more like this because of all the other stuff going on in the bass (reflections, modes, room resonances, and even things like halls and windows acting like ports for the room).

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Last edited by LTD02; 08-14-2019 at 01:31 AM.
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post #3 of 23 Old 08-14-2019, 02:14 AM
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Thanks LTD02!

This knowledge is definitely key, at least for me. When you say "Windows or Hallways acting as ports" does that mean you know what generally causes room nulls in the 30 to 40hz range or is it too complicated to place blame on some one thing like counter floor resonance.

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post #4 of 23 Old 08-14-2019, 03:00 AM - Thread Starter
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Quote:
Originally Posted by kagtha View Post
Thanks LTD02!
This knowledge is definitely key, at least for me. When you say "Windows or Hallways acting as ports" does that mean you know what generally causes room nulls in the 30 to 40hz range or is it too complicated to place blame on some one thing like counter floor resonance.
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i suspect that for most folks, peaks and dips are caused by room modes and reflections, but there can be other stuff going on too. one time we were trying to figure out what was going on with one member's response and when we modeled a hallway attached and his room as a one giant ported cab, it seemed to correlate with a frequency response aberration that seemed to have no other explanation. might have been the 12hz dip in ricci's response. i forget now though.
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post #5 of 23 Old 08-14-2019, 04:48 AM
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Thank you for this! This will for sure be another "Sticky" GO-TO.
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post #6 of 23 Old 08-14-2019, 05:08 AM
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WOOT! Im modeling better! Thank you so much! We the community appreciate all your contributions. Now all I have to learn is how to model HS/LS filter haha
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post #7 of 23 Old 08-14-2019, 06:10 AM
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I've been measuring the transfer function of my better-half's car, trying to get something to play nice and flat down to 20ish hz. Here is one of the $30 JBL's from Best Buy, playing flattish down to 15 hz. Gonna re-design for a quicker rolloff around 20 with a smaller box.

Red = close mic (kinda close)
Blue = mic on center of dash



EDIT: I forgot that this was actually with a BASH300 sub amp, which has a HPF around 18hz... so the graph would have actually been 3dB higher at 18 and still rising at 15 I'd say. I'm going to have to go quite a bit smaller on the box.
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post #8 of 23 Old 08-14-2019, 06:12 AM
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Unfortunately... my house on the other hand, with it's very long rooms, doesn't start getting gain until 10-15hz. And I don't even care about bass that low lol
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post #9 of 23 Old 08-14-2019, 12:50 PM
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Quote:
Originally Posted by LTD02 View Post
it might have been the 12hz dip in ricci's response. i forget now though.
Nah...Mine at the old suspended floor place was due to the floor resonance. Floor jumped like crazy at 12-13Hz acting like a giant out of phase bass shaker and I always had a notch in SPL at that frequency band. That's not a coincidence. I've seen a ton of measurements from different rooms and suspended floors usually have a notch in response somewhere in the 10-20Hz octave. No such issue in the basement at my current place. Another measurement artifact I can tell a lot from is if there is a big peak in the 40-60Hz region. It usually indicates sitting near the center of the room and can even give you a guess at the room dimensions in some cases.
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post #10 of 23 Old 08-14-2019, 01:04 PM
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Quote:
Originally Posted by Ricci View Post
Nah...Mine at the old suspended floor place was due to the floor resonance. Floor jumped like crazy at 12-13Hz acting like a giant out of phase bass shaker and I always had a notch in SPL at that frequency band. That's not a coincidence. I've seen a ton of measurements from different rooms and suspended floors usually have a notch in response somewhere in the 10-20Hz octave. No such issue in the basement at my current place. Another measurement artifact I can tell a lot from is if there is a big peak in the 40-60Hz region. It usually indicates sitting near the center of the room and can even give you a guess at the room dimensions in some cases.

Imagine the hump in my un-EQed response from sitting EXACTLY in the center of my room, combined with a high inductance RE sub lol
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post #11 of 23 Old 08-14-2019, 01:10 PM
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Imagine the hump in my un-EQed response from sitting EXACTLY in the center of my room, combined with a high inductance RE sub lol
Been there. Exactly in that situation. Believe it or not.
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post #12 of 23 Old 08-14-2019, 01:12 PM
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Been there. Exactly in that situation. Believe it or not.

lol

What subs are you using for low frequencies these days?
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post #13 of 23 Old 08-14-2019, 04:08 PM - Thread Starter
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Quote:
Originally Posted by Ricci View Post
Nah...Mine at the old suspended floor place was due to the floor resonance. Floor jumped like crazy at 12-13Hz acting like a giant out of phase bass shaker and I always had a notch in SPL at that frequency band. That's not a coincidence. I've seen a ton of measurements from different rooms and suspended floors usually have a notch in response somewhere in the 10-20Hz octave. No such issue in the basement at my current place. Another measurement artifact I can tell a lot from is if there is a big peak in the 40-60Hz region. It usually indicates sitting near the center of the room and can even give you a guess at the room dimensions in some cases.
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post #14 of 23 Old 08-15-2019, 07:31 AM
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A medium size home room will typically have some lift beginning around 30Hz or so, approximately 6dB or so by 20Hz and somewhat more under that. A large room will typically have its gain starting lower in frequency and a smaller room will typically start higher up.
What is the approximate size of a small, medium and large room?
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post #15 of 23 Old 08-15-2019, 09:50 AM
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^curious about that too. Also guessing you model the same way for a sealed driver but just verifying.

In my case since my family room is open to most of the house I’m guessing all that factors in and I’m in a REALLY BIG room lol


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post #16 of 23 Old 08-16-2019, 01:05 AM
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Quote:
Originally Posted by Ricci View Post
Nah...Mine at the old suspended floor place was due to the floor resonance. Floor jumped like crazy at 12-13Hz acting like a giant out of phase bass shaker and I always had a notch in SPL at that frequency band. That's not a coincidence. I've seen a ton of measurements from different rooms and suspended floors usually have a notch in response somewhere in the 10-20Hz octave. No such issue in the basement at my current place. Another measurement artifact I can tell a lot from is if there is a big peak in the 40-60Hz region. It usually indicates sitting near the center of the room and can even give you a guess at the room dimensions in some cases.
Geez! (Smacks forehead.) 40-50 hz null issue seems so obvious now.

This is minutes after spending a couple vacation days performing a million different VBSS measurements! (closing doors, slanting them, moving them by inches in a pre determined area, changing tunes, adding carpet, methodically adding and removing sets of PEQ settings, etc)

I was flabbergasted when I had one PEQ setting remaining and said what the h3ll... I'll pump up that null to see if I can move up the edges a decibel or two. AND BOOM! The 40 hz null disappeared.

Wtfudge!? Time to put in for some more days off. :/



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post #17 of 23 Old 08-16-2019, 06:06 AM
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Quote:
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Geez! (Smacks forehead.) 40-50 hz null issue seems so obvious now.


He said peak, not a null.
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post #18 of 23 Old 08-16-2019, 06:24 AM
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That is a great point you have there.

I am delirious. I need to go back and start at the basics. I think MTG90 has an amazing sub rew guide somewhere here.


I had NO IDEA that bass shakers actually create SPL. I mounted 2 to 3/4 MDF on the bottom of our sectional. They create an 85 dB avg from 15 to 180 hz AND a 90 dB avg from 25 to 125 hz.

2 more bass shakers arrive today, time for a 2 day nap.




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post #19 of 23 Old 08-16-2019, 10:41 AM
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Cool thread! I get 20 dB of gain at 10hz and 30 at 5hz in my former room. I get with one sealed re xxx18 about 122 dB from 3-40hz and 118 above. Porting in my room usuallu causes a big peak from 7-15hz as seen in many of my sweeps. This is why sealed always sounds great and still with tons of spl since I have two, I want two more!

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Quote:
Originally Posted by MKtheater View Post
Cool thread! I get 20 dB of gain at 10hz and 30 at 5hz in my former room. I get with one sealed re xxx18 about 122 dB from 3-40hz and 118 above. Porting in my room usuallu causes a big peak from 7-15hz as seen in many of my sweeps. This is why sealed always sounds great and still with tons of spl since I have two, I want two more!

Must be nice! I'd get about 102dB with my long ass room haha
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post #21 of 23 Old 08-16-2019, 02:01 PM
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I know, I used the basement purposely for it. Since my flood I am hoping I can achieve the same thing once again.

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post #22 of 23 Old 08-16-2019, 06:20 PM - Thread Starter
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Quote:
Originally Posted by MKIII View Post
What is the approximate size of a small, medium and large room?
there are many factors that make the answer to that question unspecific. for a very rough ballpark, when the long dimension of the room is about 1/2 a wavelength, room gain will begin building. a medium-large room might be 12'Wx20'Dx10'H and have gain building in the low to mid 20's s in the OP.

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post #23 of 23 Old 08-17-2019, 06:15 AM
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No need to make guesses, you can easily specify the start of room gain. It is just below your lowest room mode, which can be calculated using various tools.

In a concrete room you can enter "1" for every value in the linkwitz transformer. (except f0 of course...) The room gain is nearly 12db/oct with completely reflecting walls.

Once there is e.g. drywall/lots of windows/etc the gain will be less prominent.
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