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Best Way to Brace a Box


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soundnsecurity 
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Posted: June 22, 2012 at 2:51 PM / IP Logged  
did some digging on the helmholtz effect and found mostly examples about bottles and musical instruments. i understand it a little better(conceptually, not mathematically) though, but i wanted to know how it applies to subwoofer boxes. what got me curious was how they use the bottle as an example. i figured that if you use a car as an example instead of a bottle then the cab of the car would be the same as the inside of the bottle. i dont know if i described that the right way at all but thats how i see it. ive always known that the airspace inside the car mattered in a ported box design but now im thinking that its more important than i thought. what are your thoughts on this matter?
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: June 22, 2012 at 4:06 PM / IP Logged  

soundnsecurity wrote:
did some digging on the helmholtz effect and found mostly examples about bottles and musical instruments. i understand it a little better(conceptually, not mathematically) though, but i wanted to know how it applies to subwoofer boxes. what got me curious was how they use the bottle as an example. i figured that if you use a car as an example instead of a bottle then the cab of the car would be the same as the inside of the bottle. i dont know if i described that the right way at all but thats how i see it. ive always known that the airspace inside the car mattered in a ported box design but now im thinking that its more important than i thought. what are your thoughts on this matter?

You are 100% correct.  The inside of a car does work as a Helmholtz chamber, and a good SPL competitor will tune their system to activate the car's interior to squeeze out every last fractional db they can.  The way a port works is the speaker moves air across the top of the port tube just like those descriptions you found of blowing across a bottle.  This sets up a Helmholtz resonance based on the size of the tube and the area of the enclosure, causing the system to vibrate and make noise at a specific frequency (the "tuning" frequency.)  When properly designed the tuning frequency will add to the system's output and extend the low-end performance.  You don't want to create little unpredictable resonators inside your enclosure, or in your car because of opening a window while driving.  :)

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soundnsecurity 
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Posted: June 22, 2012 at 4:19 PM / IP Logged  
but what i mean is that its the subwoofer that does the "blowing" and the cabin is the bottle, so does the extra output come directly from the port or does it come from cab itself. also if it works that way then wouldnt you tune the port using the cabin volume and not the box volume? see now im probably over thinking it but now im slightly confused...
ive always pictured a ported box working more mechanically, taking the rear wave and physically aligning it in phase with the front wave so that they work together and build higher peak output. but looking at it from this new perspective its starting to seem like the subwoofer and the port are working almost independently to create sound, and then the sound from them are mixed together in the cab to make one single louder sound. i know its way more complicated than that but my whole world is turned upside down now so i dont know what to think anymore Best Way to Brace a Box - Page 2 -- posted image.
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: June 22, 2012 at 6:11 PM / IP Logged  

soundnsecurity wrote:
but what i mean is that its the subwoofer that does the "blowing" and the cabin is the bottle, so does the extra output come directly from the port or does it come from cab itself. also if it works that way then wouldnt you tune the port using the cabin volume and not the box volume? see now im probably over thinking it but now im slightly confused...
ive always pictured a ported box working more mechanically, taking the rear wave and physically aligning it in phase with the front wave so that they work together and build higher peak output. but looking at it from this new perspective its starting to seem like the subwoofer and the port are working almost independently to create sound, and then the sound from them are mixed together in the cab to make one single louder sound. i know its way more complicated than that but my whole world is turned upside down now so i dont know what to think anymore Best Way to Brace a Box - Page 2 -- posted image.

The cabin of the car can work like a Helmholtz chamber, with the subwoofer blowing on the "port" of how the sound wave enters the chamber.  It's actually a relatively unimportant effect: much more important to the overall SPL are the standing waves created inside the vehicle. The subwoofer system will work the same way in a car or out of it.  The port is a separate sound generator from the woofer.  The woofer moves air which "blows" across the port, causing it to resonate based on the size of the enclosure.  The enclosure is the "bottle" and just like if you blow across a soda bottle with different amounts of liquid in it, different sizes of enclosures and ports will resonate at different frequencies.  It has nothing to do with the rear wave of the woofer (except that is what provides the motive force) and it certainly has nothing to do with aligning the waves.  Indeed, the sound created by the port will be out of phase with the sound created by the woofer depending on where the port is located in relation to the woofer, among other things.  Look up an effect called "group delay" if you want to learn more about this.  It'll really mess with your mind for your next install.  Best Way to Brace a Box - Page 2 -- posted image.

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soundnsecurity 
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Posted: June 22, 2012 at 7:24 PM / IP Logged  
that figures, just when i thought i had a good handle on how things worked it turns out that nothing is what it seemed. thanks for your help and patience because im sure ill have more questions once i find the energy to look into group delay. ive heard the term tossed around here from time to time but nobody ever went into any detail about it.
soundnsecurity 
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Posted: June 23, 2012 at 9:57 AM / IP Logged  
any good resources about group delay sir? did a quick search but didnt find anything very substantial for someone who doesnt already know what it is.
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: June 23, 2012 at 10:19 AM / IP Logged  
Written by genius subwoofer designer and good friend Dan Wiggins:  http://www.adireaudio.com/Files/GroupDelay.pdf
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soundnsecurity 
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Posted: June 23, 2012 at 10:55 AM / IP Logged  
that was a good read, havent heard the Adire audio name in a while. i now know a little more about group delay but im still in the dark on how to use it to my advantage when designing a system. it would have been nice to be able to read the response charts. maybe i need to invest in a box design program when i get a little money.
if i have this right group delay is basically the relationship between acoustic phase and output at a given frequency.
i notice that in the vented box that the group delay is the highest just below the tuning frequency, would this be another way to describe how a ported box "unloads" and loses output below the tuning frequency. is this a result of the sub and port being totally out of phase with each other?
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: June 23, 2012 at 1:55 PM / IP Logged  
The reason a vented system has its worst group delay at or around the tuning frequency is because this is where the port output is greater than the woofer's output.  You can't really "use" group delay to your advantage, but if you understand what it does in a system (flabby, booming sound without much definition; high distortion; loss of output due to cancellation) then you can design the system and how the installation will fit a vented system into the vehicle in such a way to minimize the effects.  And, as Dan's paper illustrates, never use a passive radiator system if these effects are a concern.  By playing with port position and how you control it's relative acoustic phase and output level in relation to that of the woofer's output, you can design a system so these things occur at about the same time, maximizing SPL and minimizing the effects of group delay at the listening (or measurement) position.  It's very math intensive... but programs like LSPCad or Praxis can be used to do the math for you.
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soundnsecurity 
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Posted: June 23, 2012 at 10:00 PM / IP Logged  
i guess some experimentation is in order for me now. i might have to rebuild a new box for my current 15 and see what i can do to improve how it sounds in my truck because right now it's the worst part of my system by a long shot.
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