# tuning a box to the wave length of a car?

wormy
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Posted: June 03, 2007 at 11:34 PM / IP Logged

I'm not saying its a simple process, but this is the part that I'm most interested in.  I can do tests all day long, but that gives me less of a direction than understanding the concept of the waves.  If I could understand how to predict at least the general reflections of the waves, then I would at least have some direction.

...typically, I just run whatever I randomly pick up off the floor.
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wormy
Copper - Posts: 76
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Posted: June 11, 2007 at 9:26 AM / IP Logged
So, I agree with zhalverson in that this is physics applied incorrectly...to a degree.
This should be one part of the whole formula in finding your resonant frequency.  There is an easier and more straight forward real world method that doesn't require a bunch of formula's.  The formula's are just so cool though!  lol.  The formula you were using was n * v / 2 * L.  The "n" stands for the resonant frequency frequency that you which to obtain.  If you plug one into the "n" variable, then you will obtian the fundamental frequency, two while obtain the 1st overtone, three will obtain the second overtone, and so on.  "v" is the speed of sound.  Different temperatures will equate different velocities for the speed of sound.  "L" is the length you are trying to determine a resonant frequency for.  That equation takes the order of the frequency that you are trying to find, in your case the 1st fundamental frequency, which is 62.5 Hz, and determines the frequency.  Not very useful alone, but if you were to find enough of these throughout your vehicle and matched the wave coming out of your enclosure to that resonant frequency and controlled that wave so that it would follow that path until it reached the point in which it was to be measured, then, theoretically, it should boost your SPL.
The more realistic method is to forget about the physics of it all and just use a meter and sine waves to test the actual SPL at different points.  To do this, play several different, individual sine waves.  Set the meter whereever you need the system to be the loudest.  Find the loudest frequency in that location.  Next, play the sine wave that you determined to be the loudest and move your enclosure into different positions to determine the loudest one for the location you are testing.  Finally, retest for your loudest frequency and change it if another one took its place.  You can use programs, such as WinISD, to build an enclosure that will maximize your vehicle's resonant frequency.
I've known about that method for a while now, but I decided to give your method a try.  I knew that the one resonant frequency alone wouldn't do you any good, but I wondered if I might accurately determine my own using the formulas when I worked enough of the resonant points up.  Maybe when I understand the nature of waves better...lol.  My resonant frequency, for the fundamental, came out to be 101 Hz.  I know my resonant frequency to be 70 Hz.  So, that one resonant frequency alone isn't enough.  You need to calculate alot of different paths.
...typically, I just run whatever I randomly pick up off the floor.
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speakermakers
Copper - Posts: 231
Joined: January 02, 2003
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Posted: June 12, 2007 at 4:28 AM / IP Logged
This is truly one of my favorite topics. There are actually several acoustic phenomenon happening all at the same time. You can break these down into groups prioritized by the expected percentage of effect that each will have on the total transfer function.
1.     Space loading. Space loading is when sound reflects off a solid surface and amplifies its self in the process. This effect increases as frequency drops and is the prominent reason for the large boost in bass in cars. The predicted in car response that most box programs generate for you use this and only this to predict transfer function. This alone will not give you an accurate prediction with in 12db.You can find out more about this here http://www.trueaudio.com/st_spcs1.htm
2.     The second is due to standing waves. There are multiple standing waves in the interior of a vehicle do to the many parallel solid and semi solid reflective surfaces. Check out this video of an experiment that shows how standing waves are frequency dependant and become more complex as frequency rises. http://www.youtube.com/jp.swf?video_id=Zkox6niJ1Wc&eurl=http%3A//en.wikipedia.org/wiki/Standing_wave&iurl=http%3A//img.youtube.com/vi/Zkox6niJ1Wc/2.jpg&t=OEgsToPDskKC1Ua75Pj7BZMCFHMC35jD&autoplay=1. Notice how the effect is always more drastic near the perimeter of the acoustical area. This is why standing waves in a vehicle are so important. Due to the close proximity of your head to a wall at all times in a car the effect is dramatic.
3.     The next is diaphragmatic absorbers. There are many surfaces within a car that will act as an acoustic reflector at certain frequencies and as an absorber at others. This is aggravated by the presence of standing waves at frequencies that over lap these points. One way to battle this is to reduce the gain at the problem frequency in only one channel in a stereo situation. This can actually cause an acoustic gain from the listening position due to the standing wave null physically shifting away from the listening position. This is why Audiocontrol EQs have discrete L and R adjustments.
4.      The next is multiple resonant chambers interacting with each other causing new standing wave formations as well that I like to call “third party resonances”. This is where for example the trunk resonates at one frequency but also resonates at a second frequency when combined with the resonant frequency of the passenger compartment. I know of no accurate way of predicting this in the car environment. In loudspeaker design I deal with this all the time. For example a speaker has a resonant frequency and the enclosure has a resonant frequency and when combined this produces a new resonant frequency. Due to the well documented attributes of enclosure building materials and construction this is not nearly as difficult to predict with in an enclosure. Vented enclosures are a prime example of how this effect can be advantageous. Though this can not be accurately predicted before hand, knowing about it and being able to identify the phenomenon while performing a transfer function test will enable you to take advantage of the phenomenon rather than fall victim to phenomenon.
I have barely scratched the surface of this topic here. There is simply not enough room, but I hope that I have led you to the information that you seek.
There is however a rather simple and scientific solution to a cars low frequency acoustic environment. If you own a computer and a cheap computer microphone you can test your cars transfer function for free and then attempt to either reverse engineer the situation to better understand it or you can take that info at face value and design a sub box taking that information into account. Once you have your vehicles transfer function in hand you can import that info into a program like bass box pro or leap enclosure shop and then see how various box designs with various tuning frequencies will interact with your vehicle.
So what’s the answer to how do you find the right frequency to tune your box to after understanding your vehicles transfer function? This involves becoming familiar with what frequency response curves, cone displacement curves, and group delay curves will get you what you are looking for. I will try to comment on this in future posts but don’t have the time now. But its really not that complex compared to the rest of this.
To test your vehicles transfer function download OscilloMeter 4.14(free) or pay for TrueRTA \$39.95 (better- way more user friendly). Using this software and a regular mic you can send a pink noise (the free version of TrueRTA comes with a good pink noise generator that you can pipe into your system) signal to your amps and use your cheap mic to sample the out put of your sub(s).
First run a long wire from your sub amp to your sub and place your sub on the ground out side of your car. Then place your microphone 2 meters from your sub and place it on the ground pointing towards your sub.
Find a relatively empty and noise free parking lot (no traffic or people) with no buildings or large surfaces within 30 ft. I show up to work early before anyone ells and place the sub box near the corner of my shop (I can’t get 30 ft. from a building). Placing the box at the corner drastically reduces any reflections that the building can send back to my mic. At 30hz any surface within 37.69 ft. of the sub will color your results by at least 6db and that gets even worst as frequency goes down so don’t under estimate the reflections. In this test the ground acts like a reflector and will boost your results by 6 db. That’s the reason why we place the mic 2 meters away. Sound pressure drops 6 db at every doubling of distance so this is the equivalent of placing the sub 1 meter from the mic in the car. When we test in the car we want the mic positioned where we sit. If this is more les than 1 meter we adjust the results by X db using the formula that sound decreases by 6 db for every doubling of distance and our reference distance is 1 meter. The short cut way of doing this is to ignore it. A sub in a car is usually more than 2 meters from the listener and we are more concerned with larger effects on frequency response to the tune of 12 db. This sensitivity information is only for reference and may be quite useful to you in the future. I reiterate don’t under estimate the effects of reflections in your test.
You are not likely to find all of the information required to accurately describe the acoustical environment in a vehicle in a single location. The reason for that is some what complex but in short the educated and experienced engineers that could potentially understand the answers to this either have little interest and experience in the subject as it pertains to cars or simply have bigger fish to fry.
speakermakers
Copper - Posts: 231
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Posted: June 12, 2007 at 4:57 AM / IP Logged
I also have a well tested formula for SPL that goes hand in hand with this testing method. It once got me a 149.9 with 2 12s at the IASCA world finals 301-600 pro (10 years ago). The reality is that whether you are going for sound quality or SPL knowing your transfer function and understanding how and why it is what it is combined with the knowledge of speaker design can gain you miles on everyone else’s inches. Due to the lack of consistency from vehicle to vehicle and the lack of documented reliable acoustical equations the only scientific approach currently is to test and analyze. And I cant stress enough how important the analyzing the tested information is. I can gain 10 db over some one who has but dose not understand their transfer function, and I will have better transient response to boot!
wormy
Copper - Posts: 76
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Posted: June 12, 2007 at 2:21 PM / IP Logged
I LOVE YOU...lol.  Not really, but yeah.  Thanks.  I'll start getting into this as best as possible.  One thing that I am wondering is whether or not its possible to ignore most of the predictions and just force the direction of the waves themselves?  I would like to setup a standing wave right on top of the microphone, but I don't know how to predict the way in which to calculate the change in direction of the waves.  Does it matter whether or not the wave reflects off of a surface at the node, antinode, or a spot in between?  If you find the time, I really would enjoy picking your immense wealth of knowledge on this subject.  ...lol.  I don't suppose its too hard to believe that you never completed high school and yet still managed to become so intelligent.  School only slows some people down!
...typically, I just run whatever I randomly pick up off the floor.
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jazzcustom131
Copper - Posts: 175
Joined: October 10, 2004
Location: United States
Posted: June 12, 2007 at 8:16 PM / IP Logged
Now you've done it Speakermakers...
thank you for ending his nights of sleep.... and mine too.
Haemphyst, Dyohn, Alpine... and all the rest of you guys that know more than I could ever put into a book (though i'm tempted, but it would be a pain to write you guys out royalties, so screw it)
lets pour out some info, yeah? It's alot easier for me and wormy to come to you guys than spend hours scouring the internet for false leads...
course that doesn't mean I won't be occupying the engineering halls of the 5 major colleges here in Colorado Springs...
wormy, i'm gonna beat you senseless for making me use my brain when I get back to the south...
Greed is for amateurs.
Disorder,chaos,anarchy now THAT is fun!!
wormy
Copper - Posts: 76
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Posted: June 12, 2007 at 8:59 PM / IP Logged

No sleep tonight, that's for sure...lol.  Do be expecting several phone calls tonight jazzcustom131!!  Its going to be a long night.  Thank you so much for introducing me to the12volt.  It sure beats spending endless nights at the University Library down here...lol.

...typically, I just run whatever I randomly pick up off the floor.
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speakermakers
Copper - Posts: 231
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Posted: June 13, 2007 at 12:18 AM / IP Logged
Near the windshield at the far lower corner will always be the second loudest location due to it always being a point of reflection of all low frequencies. Even the effects of a standing wave will be at its greatest here. This is common to all vehicles and that’s why the mic is placed there during a competition. It provides a level playing field. All gains including the effects of standing waves will always be at a high level there. The highest SPL location though is directly in front of the port or for sealed boxes directly in front of the sub. I have often entertained the idea of designing a Band pass box for SPL competition that has a long enough port that it could occupy where the passenger would normally sit and would terminate at the corner where the mic is placed. This would yield an extremely hot spot for SPL. I know that you are searching for the physical location of a standing wave but that is not where the SPL will be at its height, because there is no reflection at that point. Think about creating a splash in the middle of a bath tub. You will end up with a clearly visible standing wave in the center but the height of the wave diminishes rapidly as it soon cancels its self out. Now try creating a splash near one end of the tub you will immediately notice that the waves created are larger, cleaner and amplified by the reflections. Near the reflective walls there is a reduction of energy due to the distance from the point of origin but this is more than compensated for by the increase of energy created by the reflection. This happens at every bass frequency (though at different levels per frequency) in a car because of the relatively small distance compared to the relatively large size of the sound wave. Standing waves will always occur at an equal distance between two parallel reflecting walls. The distance between the walls determines at what frequency this occurs, but varying frequency does not shift this collision point towards one wall. So predicting the existence and frequency of a standing wave will not allow you to shift its position. The position is always center. But shifting the point of origin (the sub) towards one reflective wall will increase the force and consequently the gain of the reflections at the boundaries. This is why it is so important to always locate the sub cone and port as near as possible to a reflecting wall. Although you want to avoid situations where the cone is close to the reflecting wall and the enclosure creates a barrier that in turn causes the formation of a new acoustic environment. This will produce yet another variable that is much more likely to harm than to cause a gain. Having said that I am currently working on developing a usable formula that will allow me to build a sub box within an acoustic reflector that will emit a focused pass band that compliments the acoustical environment creating a second lift in a lower frequency range. I have not finished this yet but I can tell you that it is not straight forward and accidentally creating this environment is not advised. The effect works using frequency multipliers and therefore will not respond to traditional adjustments in a predictable manner.
I have to say though that all of the information that I have is based on my own tests that are limited to my education and the material that I have read. I am very interested to find out what one of your professors think about some of this stuff.
I believe that there is no practical way or need to direct the waves due to the fact that the gain is prominent and predictable in the areas where both your head and the mic will reside. Having said that I do have several methods that allow you to kill unwanted standing waves. Though most of these methods are only required and useful when you are attempting to pass sound through a space without coloring it before it reaches another destination. I use these type of techniques largely in marine applications.
Thanks for the props guys that’s what makes me write this stuff. Its nice to know that I am not the only one who ponders this stuff.
wormy
Copper - Posts: 76
Joined: August 03, 2006
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Posted: June 13, 2007 at 9:00 AM / IP Logged

I was trying to direct the wave and measure its reflection towards another wall, hoping that a standing wave would setup along the entire wave from the enclosure to the window, so that I could achieve lower resonant frequencies.  I'm trying to extend the travel distance so that I can achieve these lower frequency standing waves.  I'm sorry, I'll be reading both your post and looking over those sites again, just as soon as I find the time, which should be later this afternoon.  My girlfriend has a job interview here shortly, so I'm reading this on the fly.  I do need two things answered with at least a yes and/or a no, please.

1st

Say we are outside.  I setup a few feet away from a wall at a 45 degree angle to the wall.  There is another wall a few feet away from the first wall and it is set at a 45 degree angle.                                       \    <--wall

wall--> |
/    <--enclosure
...lol.  Don't laugh too hard.  I tried...lol.  I'm just figuring that I might could lower the resonant frequency in a tight space, such as a vehicle, by making the standing wave longer.  I'm figuring that a standing wave isn't reproduced unless it returns directly to the source along the same path in which it traveled.

2nd

Where is the point on a standing wave that generates the greatest SPL.  Is it on a node, antinode, or some point in between.   I think you already told me not to worry about this point.  I'm still curious though.

I appreciate the help.

...typically, I just run whatever I randomly pick up off the floor.
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wormy
Copper - Posts: 76
Joined: August 03, 2006
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Posted: June 13, 2007 at 9:08 AM / IP Logged

Why are so many people trying so hard to avoid standing waves?

...typically, I just run whatever I randomly pick up off the floor.
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