May 2018

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My house is sold, which means that I'm now house shopping in the province of my soon to be residence. (Alberta, Canada)

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Naturally, being an audiophile, I pay special attention to the basement "rec room" dimensions and living room of any potential new house.

I think to myself, could do a great sound system room in there, and how could I set it up? Is the layout asymmetric or symmetric?  Where are the doors are located? Is there a large opening on one side, and how many cubic meters is the "open concept" ?. Cathedral ceilings are particularly appealing to me, but they aren't typical in my price range. Ideally, a partially finished basement would be the ultimate ticket, as I could finish the room to the exact dimensions I want, and even incorporate an infinite baffle multiple sub-woofer setup.

 

Acoustic treatments can, and do go a long way. However, a good room ratio to start with is much easier to set up. It also requires much less treatment to achieve a great outcome. Yes, the house "sound system potential" plays a significant influence on my house search criteria, along with being close to work, and in a good neighborhood. The back yard is not that important right now.

 

Once upon a time, when visiting a condo unit with my ex, I recall her saying "where will the dog run ?" I laughed, saying that we're buying a house for "us" not for the dog. If I was still with her today, I guess she could nag at me even more now, about buying a house for me and not for my sound system.

AHHH… looking at ourself in the mirror isn’t always gratifying. 

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Now, back to my previous audio room. I started with two 18" infinite baffle sub-woofers in the front, and eventually added two more 18" subwoofers in the back, to get a more uniform response across the room (and much better overall sound too)

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I knew my days in this house were numbered, so I didn't go through the hassle of constructing an infinite baffle in the rear wall and left the two 18"  subs loaded in their own sealed box.

An infinite baffle in the rear wall would had required building a manifold, which would have vented in the adjacent hallway, requiring lots of drywall work. No thanks. Not for the short term. Had that been my "last house" however, then yes…I’d have done it for sure.

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When I purchased two used 18" subwoofers, (my model was out of production), the previous owner built a nice 3/4" reinforced box and painted them in DuraTex. It looked good, but he built the boxes too small at 140 liters net internal volume (once the sub-woofer driver volume had been subtracted)

That resulted in a system Q of ~0.7 which is the bare minimum size to obtain semi-acceptable results. (I prefer Q of 0.5 and lower)

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Infinite and open baffles let a driver move without any pressure difference behind the cone. This is a huge advantage.

In a sealed box, when the cone moves forward, it creates a negative pressure inside the box, and the more the cone moves, the more it has to "fight" against this internal pressure. The smaller the box… the bigger the internal pressure. Internal pressure also creates a "suction effect" that wants to bring the cone back in neutral position. This "spring effect" often induces the cone to bounce back and forth a number of times, before stopping in its resting position. (easy to see this phenomenon on step response measurements)  

 

Obviously, this non-linear effect significantly increases distortion. Every loading (bass reflex, horn, transmission line, bandpass) creates this "cone loading effect" to varying degrees. They all induce this "pressure" effect, versus a "no differential pressure system", like what open and infinite baffles provide.

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Note:

Cone displacement increases: 

• A - as you play louder (keeping same frequency and piston diameter), more SPL always results in added displacement for the cone. 

• B- as you play at lower frequencies. i.e. 20 Hz generates more cone movement than 60 Hz at the same pressure level by the same diameter cone.

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Speaker distortion increase when: 

• A - cone displacement increases (speaker nonlinear: suspension, BL, thermal compression, inductance...)

• B - when you have an asymmetric loading (air pressure applied against the cone like a sealed box. the smaller the box, the worse it is.)

• C- when a speaker plays below its natural resonance frequency (sealed box raises a speaker resonant frequency and increase distortion below the system’s natural resonance)

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Open baffles suffer from "front to back" wave cancellation. In the treble and mid-range, this is a non-issue. But at low frequencies, this effect becomes major, and is directly proportional to the baffle width. To obtain an even response below the "baffle cut-off", one needs to add equalization and "boost" lower frequencies. As per above, this increases cone movement and thus increases distortion quickly. Pick your poison. Only infinite baffles really provide low frequency (i.e. below 200 Hz) advantages.

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Basically, a sealed box induces asymmetric loading, raises FS, requires more power to compensate for air pressure (therefore inducing more thermal compression and more loss).

Nothing good really here. All these items raise distortion of a sealed box system.

Bass reflex add group delays and phase issues, so not much better either.

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But the question now becomes, to what degree is distortion worsened? The answer is, the smaller the box, the worst it becomes. (relative to the speaker electrical parameter and size)

The sealed box main advantages are of smaller size and portability, but certainly not better sound.

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So, here’s the reality of two identical subwoofer drivers in direct comparison. (my very own Mach 5 - 18 inches subwoofers)

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#1 is loaded onto an infinite baffle wall. (no equalization added), natural FS of ~19 Hz and system Q of ~0.3 (Q=QTS of the driver)

#2 is in a small 140 liters  sealed box. (same driver and same amplifier used for the test), system Q of ~0.7, box loading changes the "system FS" to about 36 Hz (simulated only, as the real number may differ, but not a huge change from simulated is expected)

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Because #2 had an induced roll-off at low frequency, this resulted in a difference of frequency response. I added equalization to the sealed box to match the natural frequency response of #1 to ensure, it was an apple to apple comparison.

Of course, the power requirement of #2 is much higher than #1 for the same output level, but we'll come back to this phenomenon another time.

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Measurements below were taken with the microphone at 3" from the subwoofer dust cap. (10 Hz - 60 Hz sweep)

Note the big the distortion difference is between the sealed and the infinite baffle.

Not exactly decimal difference and extremely easy to hear... like two seconds worth of music, maximum.