posted
Hello i was reading up on the concept of having the backwave and frontwave meet at the same time and it makes the wave that much louder correct? How exaclty is this acheived? Does it depend on the port length? or the speed of the driver? or what hz you are tuned to? Some help would be much appreciated i'm trying to gather as much info as i can before i start competing. Thanks
-------------------- Right now am bumping in my '89 Thunderbird Super Coupe. It has a huge 120a stock alternator, can't beat that, 5% tint and a stalker.
Currently have two boxes : Triple twelve alum12q box running to a one ohm load for sq DD3515c 4cu. after displacement tuned to 32.25hz for sq
New meter means new boxes In the works: 4 - 6.5" dd midbass tuned for sq. Also designing one for a 2 - 3515 setup cause the second one is on it's way Posts: 33 | From: Philadelphia | Registered: Nov 2002
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posted
Put some quick thought to it.. The backwave leaves the driver at exactly 180 degrees out of phase to the front wave, right? So if you sum the two together at some point, you will have complete cancellation.
The ideal is to have a complete phase change of the back wave over its pathlength vs. the frontwave, so the two arrive at a particular point and are constructive.
Remeber, constructive interference can only be up to a doubling of incident energy, which is a very small change.. Cancellation can be anywhere from zero to complete, which is a very much larger impact on the incident energy at the microphone... Good stuff
ShadowStar
-------------------- You can't build a reputation for what you're GOING to do.. But you can build one for TALKING about it!
It's all about knowledge, love and respect. Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
thanks. What steps would i need to take besides making a tunnel box to get the backwave to meet the front wave if i have the port and sub on the same side of the car? Can this wave theory really only work with wall type setups or will i be able to do it in a plain ported box?
-------------------- Right now am bumping in my '89 Thunderbird Super Coupe. It has a huge 120a stock alternator, can't beat that, 5% tint and a stalker.
Currently have two boxes : Triple twelve alum12q box running to a one ohm load for sq DD3515c 4cu. after displacement tuned to 32.25hz for sq
New meter means new boxes In the works: 4 - 6.5" dd midbass tuned for sq. Also designing one for a 2 - 3515 setup cause the second one is on it's way Posts: 33 | From: Philadelphia | Registered: Nov 2002
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quote: "Remeber, constructive interference can only be up to a doubling of incident energy, which is a very small change.. Cancellation can be anywhere from zero to complete, which is a very much larger impact on the incident energy at the microphone... "
what he said
so if i read you right then its easier to lose db than to gain it so dont do this unless your reely good?
Posts: 22 | From: here to there with bass! | Registered: Dec 2002
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a doubling of energy equates to a 3dB change. it's just as easy to gain dB as it is to lose them...it just depends on the phasing of the two waves when they hit the mic
there is constructive interference and destructive interference, sooo...interference isn't necessarily a bad thing in this case.
all he's saying is that cancellation can either be zero (minimum) or complete (maximum which occurs when the two waves are 180 degrees out of phase).
draw up a sine wave on a piece of paper...now draw one 180 degrees out of phase. Notice that when one peaks the other is at it's minimum? The total energy is therefor zero.
now...visualize the wave that's out of phase as slowly being moved forward so it's closer and closer in phase with the original wave. Notice how the difference between the amplitudes of the two waves at any point becomes less and less? eventually it will be equal and there will be no abnormal effect on the energy levels. It will be like having a single wave. if the phase changes from this point the one wave will be out of phase and cause destructive interference.
i have heard of people moving their wall forward and backward as well as moving their entire port in either direction trying to get these waves to line up better...never tried myself. prob will mess with it next season
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
DiRtY SoUtH DEAF SqUaD
Elemental Designs
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--------------------------------- Loud...it's when you can see the ground move, it's when you have to force yourself to breath, it's when the rest of the world ceases to exist. Posts: 4136 | From: Lafayette, LA | Registered: Sep 1999
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posted
I think you are referring to a Transmission Line box...in which the "port" is a measured length of a certain freq. This isn't accurate by any means, but let's say a 20Hz tone is I dunno...20ft long...well the "port" would be 20ft long or w/e so it comes out of the port in phase with the woofer wave.
-------------------- 1994 Ford F-250 7.3L Powerstroke Diesel Super Cab Long Bed 33" Mud Terrains Yeah, it's big, and yeah, it's got balls. Posts: 876 | From: Decatur, IN | Registered: Apr 2002
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posted
Yeah i meant transmission line box at the time i couldn't remember the name but thanks for all the posts keep it going the more the better.
-------------------- Right now am bumping in my '89 Thunderbird Super Coupe. It has a huge 120a stock alternator, can't beat that, 5% tint and a stalker.
Currently have two boxes : Triple twelve alum12q box running to a one ohm load for sq DD3515c 4cu. after displacement tuned to 32.25hz for sq
New meter means new boxes In the works: 4 - 6.5" dd midbass tuned for sq. Also designing one for a 2 - 3515 setup cause the second one is on it's way Posts: 33 | From: Philadelphia | Registered: Nov 2002
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posted
yeah, a transmission line enclosure is tuned something like that. but even with regular ported boxes there are things you can do to help reduce any phase cancellations
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
DiRtY SoUtH DEAF SqUaD
Elemental Designs
To cheat is to admit defeat
--------------------------------- Loud...it's when you can see the ground move, it's when you have to force yourself to breath, it's when the rest of the world ceases to exist. Posts: 4136 | From: Lafayette, LA | Registered: Sep 1999
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Don't get caught in the WEB Gimmie some bass baby keep your XXX and others, MTX for me Is that me in the top 10 now?? Do you smell what the BASS is cooking... Posts: 6799 | From: cincinnati, oh, USA | Registered: Jun 1999
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posted
actually spydermann, he's got a good question but yeah, you are right they do get closer to reinforcing each other
as a material gets denser, at least this is what i was taught in my transmission media class with regards to light and fiber optics, the wavelengths get smaller but the velocity also decreases. Since the velocity of the wave decreases and the wavelengths are smaller, the cycles per second (Hz) remains the same...
velocity = 3*10^8 / n
3*10^8 is speed of light in a vacuum in meters/second
n is the refractive index of the material the waves are passing thru (1 being a vacuum, n equal to or greater than 1 only)
the wavelength in the new material is:
X = Xfreespace / n
X is the new wavelength Xfreespace is the wavelength in free space (vacuum) n is the refractive index
and frequency = v / X (velocity of light in the medium divided by the wavelength of the signal in the medium)
since both the wavelength and the velocity in the new medium are inversely proportional (right term? hehe ... i mean that both are divided by n) the cycles per second will remain the same.
rather than work thru those equations with nutty numbers, i'll just use some simple ones and work most of the way thru it so you can see what i'm saying:
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
DiRtY SoUtH DEAF SqUaD
Elemental Designs
To cheat is to admit defeat
--------------------------------- Loud...it's when you can see the ground move, it's when you have to force yourself to breath, it's when the rest of the world ceases to exist. Posts: 4136 | From: Lafayette, LA | Registered: Sep 1999
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posted
wait a mnute were dealing with peaking spl at the peak frequency inside of a car which doesnt change its shape or size or volume so if the velocity increases wouldnt that mean that the frequency would change - i mean the point of focus at the mic would be the same but the wavelengths would change length so wouldnt that change the peak frequency
if the velocity increases then the wavelengths would go further in the same amount of time,or grow so that makes it seem to me to mean that the frequency that you would be triing to focus would be lower because the lower frequency now looks like the higher one did before the pressure made the wavelengths grow?
Posts: 22 | From: here to there with bass! | Registered: Dec 2002
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posted
He is right I think, smaller wavelength and less velocity = same frequency. I have not taken any classes on this as I am only 16 but it seems like it could be, but I have never heard of frequencies changing wavelength, always beaing a constant.
-------------------- 1994 Ford F-250 7.3L Powerstroke Diesel Super Cab Long Bed 33" Mud Terrains Yeah, it's big, and yeah, it's got balls. Posts: 876 | From: Decatur, IN | Registered: Apr 2002
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posted
two3four...yes, as the SPL in the cabin increases, the density of the air increases. BUT...remember, since we're dealing with a gas and the waves have positive and negative amplitudes, the density is never constant.
But yeah, generally the resonant frequency of the vehicle does change slightly at higher SPL's because of panel vibrations and such. I was just answering your question in a general fashion...at higher densities the wavelengths do get shorter, but the velocity of the wavelength decreases also which means that the cycles per second remains constant. Kind of a weird concept i know...i had to look at it for a little while too before i figured it out.
asauer...frequency and wavelength are two connected ideas, but you have to realize the differences.
first...what is frequency? frequency is cycles per second (i.e., # of times the waveform is repeated fully).
second, what is wavelength? Wavelength is the length between the same two points of a wave, as long as the TIME between these two points remains constant, the frequency will remain constant.
so if it's say 20 feet between two points and it takes 1/50 of a second for the wave to make a complete cycle, it's 50Hz (cycles/second).
Now...say it's 40 feet between the points but the wave is moving fast enough that it still only takes 1/50 of a second to make a complete cycle. that means it's still cycling 50 times per second aka 50Hz
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
DiRtY SoUtH DEAF SqUaD
Elemental Designs
To cheat is to admit defeat
--------------------------------- Loud...it's when you can see the ground move, it's when you have to force yourself to breath, it's when the rest of the world ceases to exist. Posts: 4136 | From: Lafayette, LA | Registered: Sep 1999
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posted
ala, I don't have the EE background you do, I was just trying to put it in laymans terms
-------------------- spydermann
Don't get caught in the WEB Gimmie some bass baby keep your XXX and others, MTX for me Is that me in the top 10 now?? Do you smell what the BASS is cooking... Posts: 6799 | From: cincinnati, oh, USA | Registered: Jun 1999
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quote: alaska-"at higher densities the wavelengths do get shorter, but the velocity of the wavelength decreases also which means that the cycles per second remains constant"
thats not what i would guess,i thought that as the density increases the wavelength gets longer since the acoustic conductivness is better the energy or wave can cover more ground faster so it would seem like it should ellonggate or grow
but i was talking about the focal point of the peak spl frequency changing and a different frequency coming into focus at higher spl levels at the mic where you would be triing to focus the acoustic energy
[ 12-18-2002, 01:22 AM: Message edited by: two3four ]
Posts: 22 | From: here to there with bass! | Registered: Dec 2002
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posted
I predict that in the not so distant future, we will be designing our entire systems from the ground up.The box, to the port, to the drivers area, moving mass, coil length...everything..... The mass of the sub is soooooooo important!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
If you want to make a real high peak transmission line box, take a length lets say 16' take one fourth of it and make tube that is 4' long and then take three fourths of it and make tube that is 12' long and put speaker in middle, make the output of the two tubes next to each other you will line the peaks of the waves together.
posted
thanks alot guys i'll start working on some new stuff for my boxes then it looks like
-------------------- Right now am bumping in my '89 Thunderbird Super Coupe. It has a huge 120a stock alternator, can't beat that, 5% tint and a stalker.
Currently have two boxes : Triple twelve alum12q box running to a one ohm load for sq DD3515c 4cu. after displacement tuned to 32.25hz for sq
New meter means new boxes In the works: 4 - 6.5" dd midbass tuned for sq. Also designing one for a 2 - 3515 setup cause the second one is on it's way Posts: 33 | From: Philadelphia | Registered: Nov 2002
| IP: Logged |
quote: alaska-"at higher densities the wavelengths do get shorter, but the velocity of the wavelength decreases also which means that the cycles per second remains constant"
thats not what i would guess,i thought that as the density increases the wavelength gets longer since the acoustic conductivness is better the energy or wave can cover more ground faster so it would seem like it should ellonggate or grow
but i was talking about the focal point of the peak spl frequency changing and a different frequency coming into focus at higher spl levels at the mic where you would be triing to focus the acoustic energy
As materials get stiffer or denser, sound moves faster.. IE, sound moves faster in water than air, faster in metal than wood, faster in wood than water, etc..
Because velocity = freq. * wavelength, when your wave velocity goes up your wavelength will go up for the same emitted frequency, so wavelength of a 40hz wave in water is much higher than a 40hz wave in air.
To think about it, how close together are the air molecules that pass the wave along? Now, in metal, how close are the molecules that pass the wave along.. You can see how the wave energy could pass from one point to another point much faster if it doesn't have any empty space.
alaska- you can't use light as an example, because light is self propagating.. Because of this, it travels slower in denser mediums vs mechanical waves which travel faster in denser mediums.
ShadowStar
-------------------- You can't build a reputation for what you're GOING to do.. But you can build one for TALKING about it!
It's all about knowledge, love and respect. Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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quote: shadowstar-"Because velocity = freq. * wavelength, when your wave velocity goes up your wavelength will go up for the same emitted frequency, so wavelength of a 40hz wave in water is much higher than a 40hz wave in air. "
ok so if the velocity does increase as the air gets more dense-or does it reely get more dense,alaska says it isnt constant so is it reely a denser material then usual,but if it does get more dense and the wave travells farther in the same amount of time-it would mean that the peak frequency at 150db would be lower-before i said higher but after i thought about it it would have to be a higher frequency that would focus at the mic at 170db than at 150 db if the wavelength grows-or not?
Posts: 22 | From: here to there with bass! | Registered: Dec 2002
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