FLINT: IB Subwoofer Project

[Flint]

Since there were some questions and since I couldn't find the build thread over at S&V, I am quickly putting together a thread on my IB subwoofer.

For years I had wanted to put an Infinite Baffle subwoofer in my HT as back in my audio consulting days I had build a dozen, or so, IB subs for clients and was always absolutely blown away by the results. Not only did they perform significantly better than any of the box subs I could buy or build, they cost less and were easily made invisible in the HT if the user wanted to hide them.

So, after building bass bins for my DIY speakers to give them outstanding performance from about 300Hz to 30Hz, I decided to install an IB sub system in my HT to get a solid, tight, and powerful output with low THD to under 15Hz for both music and movies.

My HT is built in what is supposed to be the master bedroom of my home this room is too big (in my opinion) for a bedroom and it is by far the best room for a HT in the house. It is 14ft wide by 17 ft long and has a 10 ft high ceiling. Since the rest of the upstairs has a 8 ft high ceiling, I could easily mount woofers high in my walls and the volume of air behind them would be the entire attic of my upstairs area. It was perfect for that.

After carefully studying the potential placement options based on the construction of the home and calculating the predicted performance for each potential location based on what would be heard at the primary listening position, I chose to place the subs in the top rear corners of the room. One woofer on the right side and one woofer on the left. They were both the exact same distance from the listener and would create a single, unified signal when wired in parallel so as to perform as a single sub deespite being separated b 14 feet.

I then searched for the best woofer for the job. I decided an 18" woofer was not the best option since it would be difficult to fit them into the wall. But I also only wanted to use two woofers if I could, so I wanted as much output per woofer as possible. Price was also important as I wanted to keep the price below the best vented subs on the market as an experiment to show the effectiveness and value of using IB subs over buying something commercially.

I already knew from research and experience that the best performing IB subs used drivers with properties such as: Low Fs, a Qts between 0.25 and 0.35, a high Xmax, and high power handling. The Qts requirement ruled out nearly all car audio subs, so I looked at HT subwoofer drivers instead. Some people swear by PA woofers, but I wanted a lower Fs and higher Xmax that those offer because I only wanted two woofers in my room. I picked out about 10 woofers based on their specifications and modelled them in software to see which would likely give me the best performance for the money. I modelled their peak acoustic output at their resonant frequency, the relative output at 20Hz, and the excursion needed to generate 100dB SPL at 1 meter at 20Hz. Ultimately I chose the Dayton Audio 15" Titanic MkII from Parts Express for the job.

I then looked into amplifiers and crossovers for my IB sub and fairly quickly chose the QSC RMX2450 amp because it had a defeatable subsonic filter and clean output with more than enough power at loads I would be putting on it. It also had a mono input option and a terminal strip connectivity option which made it very appropriate for me since I wanted to place it in my attic forever. For a crossover I chose the Behringer DCX2496 since it has an amazing capacity for unlimited crossover filter settings and the cricital phase & delay settings. It also provides a fully parametric EQ which is required for IB subs to have a flat response in the room. Most importantly, though, was that I already had a DXC2496 in my rig for my rear speakers which had an extra input and output not being used. So, no additional spending was necessary.

Here's a photo of my drivers and amp when they came in:

 

[Flint]

Since I had already done all my research and virtual design and virtual testing, I just went ahead with an installation rather than experiment with real world stuff. I had total faith in my design desicions.

I started by building baffles to mount the woofers in which would be mounted over large holes I cut in my walls. To make them the most secure as possible, I made the baffles with two layers of MDF and counter sank the woofer into them to help them blend in as much as possible. The baffles were wide enough for me to mount them to the wall simply by using long screws (in this case 5 in lag bolts) driven into the wall studs.

Here's some photos of the baffle construction:

It only took about a two hours to cut the holes in the walls and mount the baffles & woofers:

Since my HT room has gray walls and all the lighting is reflective cans aimed downward, it is difficult to see the woofers even when the lights are on full-blast. As such, they disappear in the room and unless I point them out to people, most visiters never even know they are there.

I put the amp in the attic near the right woofer above the closet where I keep my HTPC, Tivo, and the amps for the side surround speakers. This made it easy to run a signal cable to the amp since all the cable holes were already in place and that location is also very close to the attic ladder in case I need to get to the amp quickly.

Ultimately, all of the gear was hidden and once setup properly, not touched again.

I used 10AWG cable to connect the amp to the drivers to reduce any ill-effects of cable resistance.

Here's the finished photo from inside the HT:

 

[Flint]

Since the original installation photos were taken I have built and installed new & improved DIY surround speakers. Here's how the IB subs blend in today:

 

[Flint]

Once installed and initially tuned, I made this frequency response measurement of the woofer alone:

And then after tuning to match the main speakers (my older DIY speakers), I made this overall response measrement:

I use the Surround Preamp as the primary crossover between my main speakers and my IB sub. The crossover setting is at 40Hz since I have good bass speakers in my stereo setup. More accurately, the Surround Preamp has the bass management crossover settings set to:

Left = 40Hz
Right = 40Hz
Center = 50Hz
Surrounds (all) = 60Hz

I also have the LFE channel crossover set to 120Hz, the highest setting.

This is the frequency response (at multiple SPLs) today with my new DIY speakers:

Note that there is almost zero dynamic compression above 15Hz clear up to 110dB SPL in my HT!!!


Using TrueRTA, here is the completely unsmoothed output at 110dB SPL:

 

[Orbison]

Very interesting! Any concerns as far as the amp location in regards to attic heat in the summertime?

 

[Flint]

Then, a few years after installing my IB Subwoofer, I purchased a better mic preamp with a flat response to below 5Hz, a better measurement mic, and some professional speaker measurement software and just for fun I made a THD measurement at the listening position in my HT with an average sub SPL of 105dB:

With an effective output above 105dB SPL the THD was well under 3% in the sub's operating range down to the room resonance at 12Hz, and there was measureable output to below 5Hz. Now, we cannot hear sound below 10Hz, nor even feel it at those levels (it is an artifact of a measurement mic that is measured output below 10Hz), but check out what that sub can do from 15Hz to 40Hz.

The peak in the THD at 42Hz is from the crossover.

 

[heeman]

Flint - Nice Job

Here are a few questions:

1. Why mount them high on the wall?

2. Why one on the side wall and one on the back wall?

3. Are the heat concerns with the amp in the attic?

4. Did you isolate the speaker mounts from the wall or are there any resonnant issues?

5. Since there are not ports do the walls/ceiling offer any audio benifit?

6. What was the speaker and amplifier costs?

Thanks!

 

[Flint]

Finally, I made some measurements of the actual dynamic performance of my IB subs, and this is where the IB design truly shines. While the bass output below 20Hz and high SPL and low THD are really cool, the true differentiator from ALL of the box based subs is the dynamic performance.

I created some test tones which were a series of four short bursts of sine waves at various frequencies and captured the output from the IB sub when attempting to reproduce those signals. Here's what the 30Hz burst signal looks like when visualized using the analysis software I had:

As a comparison, this is what the SVS PB10-ISD subwoofer output looked like when attempting to reproduce that 30Hz burst signal at 85dB SPL at 1 meter:

Note how the rise time at the beginning of each tone burst is slow and how there is ringing at the end of the burst only 20dB below the signal.

The same signal reproduced by the SVS 25-31 PC-Plus cylinder sub is much better at the silent parts, but the rise time is slower:

Now look at the IB sub's ability to reproduce that burst signal at 30Hz and 85dB SPL:

Not only is the rise time nearly perfect - every wave of the signal is reproduced with accurate dynamics and the silence between the bursts is nearly perfect - just a small amount of ringing.

I also measured the test tone output at 20Hz and 85dB SPL:

WOW!!!



And, for a another wow! Check out how it handles 15Hz at 85dB SPL:

And finally, to totally blow you away, check out the 12Hz test measurement:

I don't think you will ever find any commercial boxed speaker selling for less than $10K which can match that dynamic performance in any way shape or form.

 

[Flint]

Total time spent installing and calibrating my IB subwoofer, including the baffle building: 12 hours

Total cost of the entire rig (including the cost of the crosover): $1,250

Performance is 10x better than anything you can buy for that price.

 

[Flint]

Very interesting! Any concerns as far as the amp location in regards to attic heat in the summertime?

It is a commercial amp designed for outdoor, PA, and permament installation use. It has a fan for forced air cooling. I am not concerned at all about heat. It has been running for 6 years in my attic without ever being turned off at all, no problems so far.

 

[Flint]

Flint - Nice Job

Here are a few questions:

1. Why mount them high on the wall?

2. Why one on the side wall and one on the back wall?

3. Are the heat concerns with the amp in the attic?

4. Did you isolate the speaker mounts from the wall or are there any resonnant issues?

5. Since there are not ports do the walls/ceiling offer any audio benifit?

6. What was the speaker and amplifier costs?

Thanks!

1. Why mount them high on the wall?
A: It is in the text already. The walls in my HT at 10 feet high, the walls in the rest of the upstairs are 8 feet high. That means the top part of the walls in my HT are open in the back to the attic, an ideal place for an IB subwoofer. Also, in general subs are best in corners - a top corner is my only choice for an IB sub.

2. Why one on the side wall and one on the back wall?
A: It doesn't matter at all in terms of acoustic performance since they are merely inches away from ether mounting option. I chose those configs because the studs in the corners were located such that I could only do it that way. I considered mounting in the ceiling, but that would have been a much harder task.

3. Are the heat concerns with the amp in the attic?
A: See my previous post in response to Orbison's question.

4. Did you isolate the speaker mounts from the wall or are there any resonnant issues?
A: Speaker mounts? Do you mean the baffles? If so, the baffles are not "isolated" as they are very securely attached to the studs for safety reasons. I did use foam behind the baffles to seal any potential air gaps between the baffle and the wall.

5. Since there are not ports do the walls/ceiling offer any audio benifit?
A: I am not sure what you are asking. Here's my answer to what I think you are getting at: The walls and ceiling are barriers separating the back wave from the front wave, hence the name "Infinite Baffle". The effective design is that the woofer is theoretically mounted to an infinite baffle forever serparating the back wave from the front.

6. What was the speaker and amplifier costs?
A: I listed that above.

 

[Towen7]

My HT is in the corner of the house. That means that I've got only two walls and a ceiling to mount drivers for an IB sub. One wall has attic space behind it but the volume is much smaller than the room and the access door is far from air tight. My biggest concern about using the ceiling is motion of the mounting surfaces. I'm scared to mount drivers (or a manifold) to the ceiling because I don't want my projector to vibrate.

 

[Flint]

I've seen manifolds which are secured not just to he ceiling joists but also to the roof joists. That way the manifold is very unlikely to vibrate in the same way as if it were just on the ceiling.

 

[Towen7]

Yeah... I suppose mounting a manifold with drivers opposing each other would help cancel out some motion as well.

 

[Flint]

Thanks!

The volume of the room you are in matters greatly. My room is pretty small. I came really close to using four 15" drivers.

 

[TKoP]

The conventional wisdom for IB mounting is to not mount them in the rear of the room. By mounting them in the back are you restricted on where you sit for getting the best performance? I mean more than a normal rig would?

I think I asked this question before, but for the life of me can't remember your response.

 

[Flint]

The conventional wisdom for IB mounting is to not mount them in the rear of the room. By mounting them in the back are you restricted on where you sit for getting the best performance? I mean more than a normal rig would?

I think I asked this question before, but for the life of me can't remember your response.

It is purely psychological that a subwoofer with an operating range limited to 40Hz or below should be placed in front of the listener. Most people cannot locate bass speakers with low THD at frequencies below 80Hz, and almost nobody can location acoustic energy below 60Hz.

What does matter is the distance from the speakers in relation to the distance from the subwoofer. In my case it just happened that the distance to my stereo front speakers from my listening position was identical to the distance from the subwoofer drivers to the listening position. However, I was prepared to use the Digital Delay function of my crossover to adjust for any relationship differences between the woofer and the stereo speakers.

For surround sound, the LFE noise and bass redirection is less noticeable in terms of directionality since there are so many sound sources generating competing sounds as the same time. As such, for surround recordings and soundtracks it is much less critical that the sub be time aligned with all the other speakers in the HT.

Also, since a well designed, installed, and tuned IB sub has such low THD it is much more difficult to audibly place its location in the room. People think that since it is so easy to locate their cheap sealed subs in a room that must be the same for all subs. In fact, small and most sealed subs have high enough THD that a signal at 30Hz will be reproduced with the addition of lots of audible harmonics at frequencies high enough to more easily locate the speaker via purely audible cues. Since a a really good IB sub has almost no audible THD it is nearly impossible to audibly locate the sub when it is operating (depending on how low you set the crossover).

 

[Alien]

So Flint, when you watch a movie, how good does it sound? How would you describe it? Realistic? If you don't mind me asking, what amps, crossovers, processor and room acoustics are you using? Do you have it posted somewhere? I really wish I had that room . I think I'd save up and get one of those new 75" Mitsubishi Laser TVs with no lamp replacement.

Do you think you're going to replace your tweeter after testing all those new tweeters?

To paraphrase what I've read about what other people say; it's realistic enough to not be able to tell between it and really being where ever the movie takes you.

 

[Flint]

So Flint, when you watch a movie, how good does it sound? How would you describe it? Realistic? If you don't mind me asking, what amps, crossovers, processor and room acoustics are you using? Do you have it posted somewhere? I really wish I had that room . I think I'd save up and get one of those new 75" Mitsubishi Laser TVs with no lamp replacement.

Do you think you're going to replace your tweeter after testing all those new tweeters?

Movies? I design everything to perform as close to absolutely perfect for the most critical stereo audio reproduction - to recreate a real acoustic experience as if it were happening in my room. Due to the level of perfection I reach with that, movies are a breeze because it is extremely rare for movie directors to choose to make things realistic - instead they tend to use exaggerated sounds to represent real life as it is more thilling and "better" than reality. With movies which are realistic, like Heat and Ronin, the sound is stunning. For movies which are comic extreme exaggerations of reality, like LOTR and Ironman, my system has way more headroom than you can ever imagine and it is really hard to push it too far without hurting your ears first.

How would I describe it? Well, realistic when called for, over the top when called for. Basically, it come extremely close to perfectly capturing what is on the original recording in an acoustic environment - you hear what the director or producer recorded.

What am I using? Well, that is in my HT build threads and DIY speaker threads over at S&V. I may, someday, replicate them here, but that is a ton of work.

For me, TV is important, but audio is top priority. Sound is a more primal instinct which our brains can fool us into thinking we hear (the first rocket launch movies used toilets flushing as the sound for the rocket engine because to our brains it was more realistic sounding than what the field microphones recorded at the actual rocket launch). However, it takes brain power for us to interpret non-real sounds as real sounds, and that brain power can reduce the comfort, ease, and pleasure of enjoying an event. Potentially all that brain energy being wasted with interpretting sounds as real could lead to listening fatigue and headaches. More often it lead to an unconcious loss of interest in watching movies or listening to music which you can never really explain, though most people just assume that they really weren't as interested in that hobby as when they started out.

That explanation of how we hear and interpret sound, and how it can irritate us, is what I attempt to perfect by removing the need for my brain to interpret sounds. In real life a bass drum is not so deep and loud as it is in most modern recordings. High pitched sounds are not piercingly bright and loud. But most people buy and tune their speakers to have way too much loudness in the bass and treble which seems exciting (exciting because it is NOT realistic), then they get irritated by it without realizing the cause of the irritation.

I go for accurate sound, measureably accurate.

 

[Flint]

it sometimes takes a few weeks of experiece, but the relaxation, natural pleasure, and real emotional engagement with the music that comes with a well tuned subwoofer is usually vastly prefered to the gut-wrenching, balls-busting, hard-pounding unnatural experience of poorly setup subs. Once you are listening to the music instead of the speakers you can really get lost in a different world.

Of course, that isn't for everyone. But for me, a music fanatic, it is the only way to go.

A well designed and setup subwoofer should never call itself out. It should simply add depth to the music, provide the foundation, that added sense of reality, and when the music calls for a big boom or a deep, deep rumble, it will provide it with ease. One should rarely notice the sub as a sub. One should just hear music.