Box shape - golden ratios and cubes - Realm of Excursion



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Old 06-21-2006   #1 (permalink)
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Default Box shape - golden ratios and cubes

Some info for any of you building your first box looking for a starting point on the shape to make it. I've heard all of this for many years, but rather than put it all in my own words, I'll cite some references. The summary is, a 3:4:5 (mathematically same as the one listed below) ratio is *said* to create less standing waves while a 1:1:1 (cube) is said to be the worst.

Quote:
The "Golden Ratio" should be used whenever possible unless space is restricting which is a normal occurrence when dealing with an automobile. This ratio is 1 x .7937 x 1.2599. For subwoofer cabinets, dimensions with this ratio will sound less boomy and have less panel vibration.
http://www.geocities.com/the_technoweenie/#golden

http://www.ajdesigner.com/speaker/bcv.php

Quote:
There are some ratios which are typically used to determine an enclosure shape that will help to minimize standing waves in the familiar rectangular Speaker box. Never Build a cube shaped box, which is the absolute worst enemy of a smooth frequency response curve.
http://www.carstereo.com/help/Articles.cfm?id=5

*what I'm about to say is questionable because I can't find any other evidence for it. .. just my personal observations.*

Under perfect conditions, a subwoofer should be nearly centered in it's excursion in and out. If your sub seems to lift out of the box like the eyeball of a sexually aroused pomeranian (or retract into it), you have problems with standing waves and either need to totally rebuild your box or experiment with some angled board to break things up a bit. One box I built had 4 subs in 2 sealed chambers, 2 per side. The subs had non-uniform movement within the same chamber (at various frequencies, one would have more excursion than the other and it would switch off). I can discount manufacturing differences because the other chamber was behaving the same way in unison. One guy I used to look to for advice said my subs were "fighting each other" because they were in the same chamber. I find this ridiculous because subs in a symetrical box on the same plane don't behave this way. The only real explanation was soundwaves in the box, which is going to happen no matter how many drivers you have in the same box. Although, I can see that standing waves from one sub might affect the other in non-symetrical conditions, but there's not really any way you can predict this unless you have your own personal supercomputer. The best you can hope for is a good guess based on experience.
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Old 06-21-2006   #2 (permalink)
 
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I got lost reading the title to this thread but I noticed you said the subs were fighting eachother or some odd thing... Couldn't you remove the inner or outer two subs from the box and reverse them to make up for the fight?
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Old 06-21-2006   #3 (permalink)
 
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wtf are you talking about colossus? lol

but ive also heard that watching two subs next to each other can play tricks on your eyes, like you sometimes think that one sub excurds before the other one or something like that.
random fact too, ive heard for port wars, competitors dedicated to that Build cube boxes or boxes which produce the most standing waves b/c thats supposed to somehow improve their scores, but im not sure how port wars work really.
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Old 06-21-2006   #4 (permalink)
 
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I said I got lost!!!! oh well.. i'm in over my head and it's too late to delete my old post now..
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Old 06-21-2006   #5 (permalink)
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This was definitely not a trick of the eye. I'll just talk about 1/2 of the box, 1 chamber. So, I have subs in a basicly rectangular box with the back angled about 20 degrees. One sub is on the top of the box and one is on the front of the box. I wish I could take a video (can't because it's not installed anymore), because it's very clear that the subs don't have the same extension from 0. Most people probably have boxes with the subs on the same side of the box, so they'll probably never see this.

For instance, I would put money on it that this guy ( http://i30.photobucket.com/albums/c3...newbox0003.jpg ) is going to notice the same thing with the way his box is set up. It's probably not a bad thing, in most cases.

If you want to understand this better, download all the test tones from 5 to 200 or so. Play them on your system. Ideally, your subwoofer should pull in as much as it pulls out. The middle point of your excursion and (incursion?) should always be the at-rest position of your cone. I bet that at some frequencies the woofer will pull in from 0 (resting position) and at some it will push out. My thinking is that you will get best efficiencies at the frequencies at which your woofer is nearest this 0 position and not either pulled in by the trough of a standing wave or pushed out by the crest of one. I'm not totally convinced there is a resonant frequency of a car. Has anyone on here ever built multiple boxes for their car and found that even though you have changed boxes that your resonant frequency is the same? If there is a resonant frequency for a car, matching it with a box that doesn't have standing wave problems at that frequency would probably give you some major db boost.

You might ask why I don't try this myself. Well, a couple months ago my steering broke and my Tahoe went crashing off into a house. So, now I have a Jeep with a miniscule area in the back that I haven't motivated myself to work on. Until I found this site, I really didn't give a care about SPL or tweaking anything. I just put in SQ systems for my own listening.
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Old 06-21-2006   #6 (permalink)
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Oh, and to simplify this golden ratio thing.... examples would be:

a box 3" x 4" x 5" ok.. so you can't fit a sub there.. so let's multiply by 5 for the same ratio
a box 15" x 20" x 25"
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Old 06-21-2006   #7 (permalink)
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Alright, I found an example. Watch the first 15s of this video over and over and over and over and pay attention to the frequency of the note and how it dives off.. and how the woofer seems to float out of the box at certain frequencies. Under a free air condition, I bet this woofer would remain centered on it's resting/off position. Probably his highest SPL tones are somewhere in the upper bass range at a point where the woofer is oscillating at near center and not trying to rip the surround off the basket trying to escape. (No, I'm not saying anything bad about this box. I've seen the same behavior in most everything I've built as well)

https://www.realmofexcursion.com/vide.../solox18.5.wmv

Again, I've never been in comp audio so I'm welcoming people who have to comment on these observations. I'm guessing people who win comps do so at a frequency at which the sub is closest to it's median travel range. OR, maybe competition boxes are so big that they are close to freeair and you don't see this kind of thing.
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Old 06-21-2006   #8 (permalink)
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Default Re: Box shape - golden ratios and cubes

Quote:
Originally Posted by Gebrochen
Some info for any of you building your first box looking for a starting point on the shape to make it. I've heard all of this for many years, but rather than put it all in my own words, I'll cite some references. The summary is, a 3:4:5 (mathematically same as the one listed below) ratio is *said* to create less standing waves while a 1:1:1 (cube) is said to be the worst.

Quote:
The "Golden Ratio" should be used whenever possible unless space is restricting which is a normal occurrence when dealing with an automobile. This ratio is 1 x .7937 x 1.2599. For subwoofer cabinets, dimensions with this ratio will sound less boomy and have less panel vibration.
http://www.geocities.com/the_technoweenie/#golden

http://www.ajdesigner.com/speaker/bcv.php

Quote:
There are some ratios which are typically used to determine an enclosure shape that will help to minimize standing waves in the familiar rectangular speaker box. Never build a cube shaped box, which is the absolute worst enemy of a smooth frequency response curve.
http://www.carstereo.com/help/Articles.cfm?id=5

*what I'm about to say is questionable because I can't find any other evidence for it. .. just my personal observations.*

Under perfect conditions, a subwoofer should be nearly centered in it's excursion in and out. If your sub seems to lift out of the box like the eyeball of a sexually aroused pomeranian (or retract into it), you have problems with standing waves and either need to totally rebuild your box or experiment with some angled board to break things up a bit. One box I built had 4 subs in 2 sealed chambers, 2 per side. The subs had non-uniform movement within the same chamber (at various frequencies, one would have more excursion than the other and it would switch off). I can discount manufacturing differences because the other chamber was behaving the same way in unison. One guy I used to look to for advice said my subs were "fighting each other" because they were in the same chamber. I find this ridiculous because subs in a symetrical box on the same plane don't behave this way. The only real explanation was soundwaves in the box, which is going to happen no matter how many drivers you have in the same box. Although, I can see that standing waves from one sub might affect the other in non-symetrical conditions, but there's not really any way you can predict this unless you have your own personal supercomputer. The best you can hope for is a good guess based on experience.
nice find!
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Old 06-21-2006   #9 (permalink)
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Quote:
Originally Posted by COLOSSUS
I got lost reading the title to this thread but I noticed you said the subs were fighting eachother or some odd thing... Couldn't you remove the inner or outer two subs from the box and reverse them to make up for the fight?
I don't think "fighting each other" was a good way to describe what was going on. If I had to translate what my friend really meant, I think he was saying "Yo dude, your subs are going crazy whack and I really don't know why so I'll blame it on your undivided enclosure". The only time I can see "fighting each other" in an enclusure would be if you had different amps driving each sub or different subs, in which case you may be ok if you've thought about it well enough or you might get some really crazy stuff going on.
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Old 06-22-2006   #10 (permalink)
 
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im still trying to figure out what exactly you mean as its a little complicated for me lol but i recommend posting this at termpro.com as there are a lot more serious competitors on that forum and may be able to comment on this more. that forums a lot slower though so youll have to give em time to respond.
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Old 06-22-2006   #11 (permalink)
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I might try the termpro site later. I don't really *need* to know this, I was just wondering if anyone else had noticed it.

This guy seems to have observed the same thing, but no one really answers him: http://srforums.prosoundweb.com/index.php/m/81462/0

This seems to debunk my theory, still searching though:
From http://www.fastfoursforumscarclub.co...tudios/faq.asp
Quote:
Standing waves
Standing waves cause violent response fluctuations inside the enclosure but for a standing wave to exist the distance between parallel boundaries (the enclosure walls) must be half the wavelength of the frequency at which the standing wave exists. Considering that sub-bass waves vary from 17.19 meters (@ 20 Hz) to 3.44 meters (@ 100 Hz) the generation of a standing wave is going to be impossible in an enclosure designed to fit in your average sedan or hatchback. People often install sound deadening and diffuser panels unnecessarily inside their enclosures to combat these standing wave issues (this is also the reason why some people are reluctant to employ certain square enclosure shapes). As you can see above though; in reality, due to the small dimensions of most car audio subwoofer enclosures, there is little chance of generating standing waves in the enclosure.
I'm 50/50 on whether I think that's true. It sounds reasonable that the bass waves are too long to create this, but it also seems like you could still get standing waves at 1/2, 1/4, 1/8 etc of the wavelength because the crests would match at these frequencies as well.
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