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Old 09-28-2005, 10:24 AM   #21
DevilDriver
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assuming that the cabin is perfectly sealed (which i know, is a hard assumption to make) you WOULD experience exactly 12dB/octave. and if iirc, sub/enclosure location does matter, but the direction you face the subs does not or at least that's what vance dickason says.
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Old 09-28-2005, 11:12 AM   #22
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Theoretically speaking, yes, but if you've ever measured the transfer fuction of numerous cars you would see otherwise. There is alway a bump in the graph.

It's been a long time since I read the LSDC. Was Vance talking about anechoic response or the automotive enviroment? To be honest, I usually get the approx response I need, build the box, put it in the car, and tweek from there. So I really havn't moved then remeasured to verify. But I know moving the sub around in the home, a much less complicated enviroment, causes a drastic effect on sub response. I can see how it could be less emphazied in car due to the wavelenths and small space, but I can't see how it would not have any effect. Was he just saying the the delta in distance from various trunk locations isn't audible?
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Old 09-28-2005, 11:36 AM   #23
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Quote:
Originally Posted by MIAaron
Theoretically speaking, yes, but if you've ever measured the transfer fuction of numerous cars you would see otherwise. There is alway a bump in the graph.

It's been a long time since I read the LSDC. Was Vance talking about anechoic response or the automotive enviroment? To be honest, I usually get the approx response I need, build the box, put it in the car, and tweek from there. So I really havn't moved then remeasured to verify. But I know moving the sub around in the home, a much less complicated enviroment, causes a drastic effect on sub response. I can see how it could be less emphazied in car due to the wavelenths and small space, but I can't see how it would not have any effect. Was he just saying the the delta in distance from various trunk locations isn't audible?

nope, he was referring to installed in a vehicle. a lot of the major audio guys i have spoken with have said the same thing. however, for clarification, i'm gonna start a thread on CAF just to confirm or i could be wrong, i have been before. only once though... heh
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Old 09-28-2005, 01:56 PM   #24
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here is a good post on it:
http://www.caraudioforum.com/vbb3/sh...1&postcount=28

the whole thread is here:
http://www.caraudioforum.com/vbb3/sh...d.php?t=226884
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Old 09-28-2005, 02:51 PM   #25
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haha, that's a funny post. That's a fancy way to say that in his experience a 12db/oct slope is a good general estimate. As it has been pointed out, you do need to measure though.

As the rest of the thread shows, you always have to measure, and position does count, but shouldn't have a major difference in a ga. The only variance is the difference in max wavelenths from the sub to the front of the car, so about a ft or two. It'll shift the bump a little, but it won't make a huge difference.

When I read your post before I thought you said distance doesn't matter, but orientaion does. I just noticed your post said distance matters, but orientation does not. In home it doesn't really matter, but in car orientation makes a huge difference in the sound. They both matter, with position making much less of a difference. In home the drivers suspension will determine more about the enclosure orientation than the sound will.
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Old 09-28-2005, 03:29 PM   #26
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i agree that it matters, but does it change the cabin gain/transfer function? not that i'm aware of.
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Old 09-29-2005, 07:14 AM   #27
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Sorry, but I'm going to let you think about that one. Examine the situation/variables and explain the reasoning behind your answer.
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Old 10-01-2005, 05:57 PM   #28
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If the actuallity followed the theory perfectly, it would not. But what we often find IRL, is that the cars length/airspace is such that it matches certain sub-100 Hz wavelengths, and hits others at various divisions (1/2, 1/3, etc), causing phase cancelation in a very real way. Firing the subs backwards minimizes this effect. So I guess the right answer would be "No, driver position has no effect on actual pressure gain in the cabin - but itwill sound like it does!", .
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Old 10-03-2005, 07:51 AM   #29
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Just one more example that shows how little we truly understand about audio.

As far as my question, or punkoramas question actually, I was going for an answer with a little problem solving. If nothing else has changed except the orientation, and there is a change in output, the transfer fuction is the only thing that could have changed as the speaker is still performing the same. Whether it is due to reflections, cancelations, etc it is still an inherent property of the vehicle.
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Old 11-20-2005, 06:03 PM   #30
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a brief thing i put together...
Q: How do i pick the right enclosure?
A: well, which enclosure is right can change depending on the situations. first we will run through the various enclosure types and generalities associated with them.

1. sealed - sealed enclosures are very easy to make and design. they are usually smaller than both ported and bandpass enclosures. a sealed enclosure generally has good extension (can play lower and higher notes well) and usually has lower group delay (delay between when signal is played and when sound is heard).

2. ported - ported enclosures are more difficult than sealed but easier than bandpass. a ported enclosure is "tuned" to a certain frequency. this frequency is determined by the cross sectional area and length of your port in comparison to the net volume of the enclosure. being able to control the tuning frequency of your enclosure is very helpful in controlling the response curve you would prefer. in general, tuning higher will create a larger peak in frequency response (good for spl competitions) but your response curve will not be very smooth. in addition, any notes played below the tuning frequency will cause larger than normal excursion of your subwoofer and can easily damage it. tuning lower allows for a smoother frequency response curve and can allow for incredible extension (you can get really really low) depending on how low you tune it. again, any material below the tuning frequency of your enclosure can cause your sub to be damaged, so make sure you set your subsonic filter correctly. a ported enclosure is more efficient than a sealed enclosure and, as such, requires less power to reach a given output. the main disadvantage of ported enclosures is that they require a much larger enclosure. the lower extension you would like, the larger the enclosure has to be. it is an unfortunate trade off in the car audio world where we don't have much space to work with. a ported enclosure also has a usually larger group delay.

3. bandpass - bandpass enclosures are the most difficult of the three main styles. the reason for the term "bandpass" is that the range of material that can be played well is generally a small bandwidth, but as such, the output is greatly increased in these areas. the bandpass enclosure has less than stellar high and low extension. a bandpass enclosure is also more detailed and a miscalculation can cost your frequency response dearly. bandpass can easily have the largest group delay.

now that you know the three main types of enclosures, you need to look at which one will work for your situation. if you are really lacking space (ie. reg cab), then a sealed enclosure would most likely be best for you. if you are intending to compete in spl competitions, consider a ported enclosure or a bandpass if you really know what you're doing. if you have room and want a very smooth response curve, consider going with a large ported enclosure, tuned very low, perhaps in the 20hz range. please try to refrain from buying a prefab enclosure unless you know that it was designed for your sub or will meet your subs requirements.

most manufacturers recommend a specific enclosure and/or size. for example, if you look HERE at Incriminator Audio's 10" Li, they recommend a sealed enclosure that is 0.8 cubic feet to 1.4 cubic feet and a ported enclosure that is 1.4 to 2 cubic feet, has 14-20 inches of port area, and is tuned to 28-35hz. if you were to model any one of those in a program like WinISD, you would see that any of those listed enclosures would net you a fairly consistent response curve.

if you can't decide whether you should have a ported enclosure or a sealed enclosure, there are a couple of parameters you can use to help decide what enclosure to use. included with every sub's owners manuals should be a list of what's called Theile-Small Parameters. the two main things we will consider are EBP and Qts.

EBP stands for "Efficiency Bandwidth Product" and is determined by this formula: EBP=Fs/Qes. if your EBP is 50 or less, your sub is best suited for sealed enclosures. over 90, it would be best suited for a ported enclosure. anywhere in between, it's your choice.

Qts is a Theile-Small Parameter that takes into account both Qms and Qes. Qms is a measurement involving suspension and Qes involves motor force. by considering both of these factors (unlike EBP which only considers motor force), picking enclosures based on the Qts is a fairly accurate way to determine what type of enclosure to use. because of the difference in the factors considered for EBP and the factors considered Qts, your end result can be slightly different depending on which route you go. however, they should be fairly similar. if the Qts of your sub is lower than 0.4, it should be ported. if it's above 0.5, it should be sealed. again, anywhere in between can go either way.

there are a few last things to consider when picking your enclosure.the t/s parameter called Xmax refers to the maximum linear excursion of your sub (the "throw" of your sub). if you have a sub with high Xmax (18 mm or higher), it may be a good idea to consider a sealed enclosure where the spl relies more on excursion, whereas in a ported enclosure, you can get away with less excursion because it relies more on motor force. with that said, a lot of Xmax is always nice.

also consider "Hoffman's Iron Law". Hoffman's Iron Law stipulates that the efficieny of your subwoofer is directly proportional to the enclosure volume and the cube of it's cutoff frequency. this is a very valuable law when you consider the implications. let's say your cutoff frequency is 40h and you would like to reduce this by a factor of two to 20hz. in order to remain as efficient, the enclosure volume would have to be: 2^3=8 times larger!. you can of course keep a smaller enclosure, but in order to keep the same level of output, you would need to increase power, meaning more/larger amplifiers and more money. of course, as you increase power, the power compression becomes intense and you end up with non-linearities (distortion) due to heating of the voice coil. if the cutoff frequency stays the same but the volume of the enclosure gets larger, then the efficiency increases. there is a limit to this law. once you have reached 10 times the Vas (another t/s parameter), the enclosure becomes infinite baffle. group delay is an opposite effect. a smaller enclosure will have less group delay than a larger enclosure. of course, small difference in group delay are not often noticed.

one thing i have not mentioned is the specifics of the port itself. when designing the port, it is important to try and keep port velocity respectable. if you are encountering port noise, sand down or flare the port. it's a good idea to consider keeping the total port area less than the sd of your sub. sd is another theile/small parameter that refers to the surface area of your sub. the reason you want to have more port area is that the less compression (which is caused by less port area), the more efficient the port itself becomes. of course, you reach a point of diminished returns where too much port area becomes a problem. hence why i recommended keeping it less than but close to the sd of your sub.
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Old 11-20-2005, 06:09 PM   #31
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..brief? good info tho
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Old 11-21-2005, 08:50 AM   #32
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Good info. The only info that I would consider really off is the Infinite baffle=10x Vas claim. 10x Vas, 4x Vas, >1x Vas are all thrown around for how much airspace is needed for an infinite baffle setup. All infinite baffle means is that the response of the driver is not effected by any enclosure effects. So simply, it is the point where the box Q is equal to the qtc of the driver. With a box calculator, it will be very easy to figure out how much volume you need simply by playing with the volume of the enclosure. Also, if you know you'll be using an infinite baffle setup, like a car door or rear deck, you will know if the speaker will give you the sound you are after by it's Qtc.
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Old 11-21-2005, 03:39 PM   #33
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Quote:
Originally Posted by MIAaron
Good info. The only info that I would consider really off is the Infinite baffle=10x Vas claim. 10x Vas, 4x Vas, >1x Vas are all thrown around for how much airspace is needed for an infinite baffle setup. All infinite baffle means is that the response of the driver is not effected by any enclosure effects. So simply, it is the point where the box Q is equal to the qtc of the driver. With a box calculator, it will be very easy to figure out how much volume you need simply by playing with the volume of the enclosure. Also, if you know you'll be using an infinite baffle setup, like a car door or rear deck, you will know if the speaker will give you the sound you are after by it's Qtc.
iirc, 4-5x vas is the point of diminished returns, but the definition of IB by t/s is 10x Vas. i could be wrong, but i'm fairly sure that's what it is.
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Old 11-21-2005, 04:26 PM   #34
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The point of diminished returns is subjective. When the box volume is larger than the required volume for the woofer to see an infinite space...which is where the Qts of the enclosure is equal to the Qtc of driver, any additional volume will make absolutly no difference. I understand where you have gotten the info from, but it is far from fact. The 10x Vas quote is a very vague generalization, which is often giving a lot of overkill. Sure 10x Vas will work, but what if someone knows the volume they are working with, for example our trunk that is 14.3cuft, and wants to know if a speaker will work? Given the 10x Vas myth they might not think a certain speaker would work, when in fact it very well could. The point is, the exact volume is easily quantifiable, so why deal with generalizations?
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Old 02-17-2006, 01:25 PM   #35
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just thought i'd throw in a lil side comment about the stand up saws at lowes or home depot. while not having used the saw at home depot... i can say that the saw at lowes will make a perfectly straight cut, providing the person doing the cut really cares and takes the time to make the cut straight. being a vendor... well used to be now i'm in college so i can't travel like i used to be able to . being a vendor, i've done cuts myself for aisles and bays in the store. if you go slow it will cut straight. if you clamp it down and go slow it will cut even straighter. but most lowes or home depot employees will rip through it fast and thus is where you get your 5 inches off error or whatever. just thought i'd toss that bit of info out there. so in any case if you go to home depot or lowes.. make sure you get a guy that cares. which is usually hard to do... but some places in different towns do care... depends on the person. good luck with that.
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