tuning a box to the wave length of a car?

The anti-node is where SPL will be at its height within a standing wave form. But there is a big “but” in car audio due to the reflective surfaces at such close proximity. Car audio is the quantum Physics of the acoustic world. Many rules and formulas have to be amended or simply do not apply, and many new rules come into existence. As a reference you may want to skim over http://zone.ni.com/devzone/cda/tut/p/id/264. This is an explanation of acoustic test environments and will give you a good background on the effects of reflections though not specifically in cars. The first thing that you must know in order to understand sound waves within the interior of a car is that 2nd dimensional analogies often don’t apply (string and air column for example). Sound waves are symmetrical, 3 dimensional objects (spheres). These spheres of energy at low frequencies are several times the physical space in which they occupy. Therefore, they must compress and fold in order to stay confined within that space. The pressure generated by this action is what causes the huge increase in SPL below about 100hz (11.3 feet). At this point the sound sphere will not fit within the interior of the vehicle due to dimensions of the interior on multiple sides. This is why vehicles of different sizes sound different. The boost starts at a different frequency and the over all gain throughout the low frequency range will also be affected. The effects of a standing wave will influence the slope of this rise in bass response due to the formation of the fundamental, and harmonics as well as other byproducts of the wave. A standing wave is a significant gain and loss factor that colors the overall frequency response of a vehicle’s interior though it is not usually the most prominent factor. The reflections and boundary effects of all low frequencies below the point in which the waves will not physically fit within the confines of the vehicle before compressing are the most prominent factors. Due to the fact that the waves that you are trying to direct are actually consuming the entire listening space and then some it is not possible to avoid crossing paths or even lengthen the distance between crossing paths. Although you can minimize the distance between crossing paths by creating barriers that interfere with the formation of standing waves at low frequencies and create standing wave conditions for higher frequencies. This can be very helpful if a standing wave is creating a gain or a loss at the listening position. One good example of this in practice is a boat that I built two 12” band pass sub boxes for. The boxes where placed in the engine compartment with the ports firing into a short wide storage compartment directly in front of the engines. To allow the sound to pass through this compartment with minimal influence from the compartment I cut two 10” holes directly in front of each port on the opposite side. You would think that the sound would pass straight through but due to the large physical size of these waves they reflected and caused a standing wave that ran across the width of the compartment. The standing wave and the boundary effects with in this compartment created strong cancellation zones directly in front of the 10” holes that I cut to let the sound through, and this basically killed the bass response. To remedy this I placed a simple barrier in the compartment that broke it up into uneven portions. This made any standing waves occur at a much higher frequency (outside the listening range). The reason that the portions where made uneven was to avoid the two chambers from resonating at an equal frequency creating a harmonic resonance. The result was an astounding average 13.5 db rise across a 35hz bandwidth (35-70hz). This is the difference between absolute success and failure. The reasons that most people try to avoid standing waves in car audio is due to a misunderstanding of what they are. In the event that a standing wave creates a node at the listening position it is sometimes absolutely necessary to kill it. Getting to know all of the effects of reflecting waves within your ride will help you achieve whatever you are looking for. However, as for SPL I would not depend on predicting it due to the unpredictable nature of multiple resonant and diaphragmatic walls that form the reflectors in your vehicle. Understanding standing waves, taking precautions to avoid unwanted waves and recognizing the existence of a standing wave after testing is the key. From your diagram of your testing area it looks like you might get some reflections off of the left side. I suggest testing your sub in a well sealed enclosure of a known size (actual size doesn’t matter). Then model the enclosure by software and get a good idea of what slope the sub will roll of at on the low end. Calculate the frequency that is of equal size of the distance to any reflector in your test area. Then compare your software predictions to the ground plane test. You will be able to easily spot any effects that reflectors in your test area might produce. The reason that it is important to actually conduct this test is so that you can limit any doubt about the accuracies of your software predictions and manufacturing tolerances of your sub (25% is not out of the norm). The effects of a reflecting surface will cause a sudden bump in the response. This is why I suggest a sealed enclosure. The low end roll off should be smooth regardless of the slope or frequency at which it occurs. Also the only way that I know of augmenting the resonant frequency of a car involves drastically reducing the boundary gain. Not what you want to do For SPL. If you are willing to settle for very loud (even abnormally loud) but not ultimate SPL you can design your box to perform at a lower frequency range. You will not get the gain of the resonant frequency but you will get a gain that increases as frequency drops. Personally, I like this effect because it gives you a real kick in the butt without destroying the quality of your bass. Generally speaking and heavily depending on the design of the driver.

Wow, so my picture definately didn't turn out at all like I had hoped...lol....typically, I just run whatever I randomly pick up off the floor.

1995 Ford Ranger Supercab

MECA member

Team CSS

I appreciate the info!  Now I need to start looking it over again and again and pull back out alot of my old sources and see if I can't figure out exactly what I want to do.  I'll start with all of your advice and go from there.  Thanks again!...typically, I just run whatever I randomly pick up off the floor.

1995 Ford Ranger Supercab

MECA member

Team CSS

jmelton86 wrote: Vehicles' panels, as any part of the vehicle really, has its own fs. This is due to the size, the way it's held in place, where its held in place, along with innumerable(sp?) other factors that affect the particular piece. Now, the vehicle is actually louder at one frequency than others -just where the panels hit 'common ground' with each other. This is the actual fs of the vehicle. What I want to know is would building a ported box tuned to the vehicle fs be the best way of getting a comp-worthy vehicle? Also, since to have the front and rear waves hit the mic at the same time is optimum for comps, how would one going about this? -what this thread was started for, I think.

I once had an Alpine head unit that allowed you to enter the distance in inches of each speaker to a listening point that you defined in order to time them all.

I'll try to come up with a model number.

'94 Ford Explorer / Kenwood KVT-815DVD / RF Power T1682C 6x8 (all doors) / RF Power T10001 / 12" Kicker L5 (x4) / Optima Yellow Top Battery

Time alignment can be a great way of dealing with or creating standing wave issues, how ever this only shifts one speakers phase relative to others in a system. This will not allow you to change a standing wave due to reflections from the cars interior. This is not the same thing as changing the subs physical location inside of the vehicle. The reflections and all of the effects of those reflections remain the same. Although adding delay to a single sub in a multi sub system though can make or break standing waves and other phenomenon that exists do to interaction between the subs. Decks with time alignment on board is a great thing. But beware! This feature is nearly useless if the deck dose not let you manually adjust this feature increment by increment. Many do not. Also beware that many decks like my Eclipse 8455 do not have time alignment on the sub channels. I use a stand alone processor for this and still love my deck due to its extreme value in other areas.

Hmmm...things I want to say to jmelton86...lol.

Time Correction.

Not what we're talking about.  Think with me in simple terms everyone, just to make sure you understand where I am coming from.  ONE speaker.  We need to keep this simple.  Time correction at no point will affect the wave coming off of the from of the speaker as the velocity of the front wave will be nearly the same as that of the rear.  The only reason why I say nearly is because the temparature from one side of the vehicle to the other may be different enough to change the velocity of the waves to such a degree as might allow the front wave to reach the point of destination before the second wave.  Time correction only affects the difference in time between a signal being sent to speaker A as opposed to speaker B.  Meaning, it is only a time delay for the signal containing the information of the music between the different channels.  The left channel might get the signal 0.01 seconds before the right channel.

Control of the strongest wave.

Definately the topic at hand.  Feel free to continue discussing this topic further so that I might pick up new things to research.

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

1995 Ford Ranger Supercab

MECA member

Team CSS

My ability to put together sentences is slowly degressing...lol....typically, I just run whatever I randomly pick up off the floor.

1995 Ford Ranger Supercab

MECA member

Team CSS