Could this be acting as a bandpass?

[Immacomputer 10+ year member]

Can you explain what that is?

When you put a sub into a box, you are applying a transfer function. A transfer function is used to describe the frequency relationship between an input signal and an output signal. When you add an enclosure to a speaker, you change the frequency response which means you have applied a transfer function to it.

Your car looks like it acts much more like an expanding transmission line/horn rather than a bandpass. Most vehicles' transfer functions are based around either a transmission line setup or a horn like setup.

If your car is about 9 feet from the trunk to the windshield, I would bet that you have a peak around 40hz with the windows down and low frequency extension getting into the mid 20s but slightly lower than the 40hz range. I bet with the windows up, that 40hz area peak gets reduced and you're left with a much flatter sound with a little more gain in the lower 20s compared to the windows down. With the sunroof open, you would be mass loading the line/horn and you probably get a peak somewhere in the mid to high 20s.

Like stated previously, your vehicle's acoustics play a large role in the final sound. I have found that modeling a car's interior like an expanding transmission line gives a pretty good idea of the transfer function. I was able to get a very close to life model of my sub/enclosure/vehicle response by applying the theoretical plotted graph of my sub/enclosure and combining it into a model of a large, expanding transmission line. I found that the theoretical plot gave me a peak around 25hz (mine is actually at 22hz) and a dip around 33-35hz (mine dips by 1 dB around 28-30hz) with it being flat up until 70hz (mine is flat to about 75hz). I thought it was pretty accurate with the loose dimensions I gave it as they weren't extremely accurate.

 

[Pioneer~Saturn 5,000+ posts]

http://photos-b.ak.facebook.com/photos-ak-sctm/v203/137/0/829895047/n829895047_2231169_5608.jpg

there's a pretty cool one. Not really what I was talking about...but it gets the idea (kinda)...

 

[Rashaddd 5,000+ posts]

When you put a sub into a box, you are applying a transfer function. A transfer function is used to describe the frequency relationship between an input signal and an output signal. When you add an enclosure to a speaker, you change the frequency response which means you have applied a transfer function to it.
Your car looks like it acts much more like an expanding transmission line/horn rather than a bandpass. Most vehicles' transfer functions are based around either a transmission line setup or a horn like setup.

If your car is about 9 feet from the trunk to the windshield, I would bet that you have a peak around 40hz with the windows down and low frequency extension getting into the mid 20s but slightly lower than the 40hz range. I bet with the windows up, that 40hz area peak gets reduced and you're left with a much flatter sound with a little more gain in the lower 20s compared to the windows down. With the sunroof open, you would be mass loading the line/horn and you probably get a peak somewhere in the mid to high 20s.

Like stated previously, your vehicle's acoustics play a large role in the final sound. I have found that modeling a car's interior like an expanding transmission line gives a pretty good idea of the transfer function. I was able to get a very close to life model of my sub/enclosure/vehicle response by applying the theoretical plotted graph of my sub/enclosure and combining it into a model of a large, expanding transmission line. I found that the theoretical plot gave me a peak around 25hz (mine is actually at 22hz) and a dip around 33-35hz (mine dips by 1 dB around 28-30hz) with it being flat up until 70hz (mine is flat to about 75hz). I thought it was pretty accurate with the loose dimensions I gave it as they weren't extremely accurate.

Hmm...interesting. From the front seats, loudest by far is front two windows down, or just the opposite window down, hard to tell which is louder between those. From the back seats, it gets louder with the rear windows down, and then even louder again by lowering the front two windows also. The sunroof has always been hard for me to tell exactly what I get out of it...other than not having it flap so **** much when I open it, but it usually seems quieter with the sunroof open than with the front windows down atleast in the 35-60 range, I get air movement out of the sunroof like a mother****er though at 15-25hz though, so you very well may be right.

I've always been good at visualizing stuff you can't see like this...but for some reason I have a hard time visualizing sound pressure transfer in a car and the effects of opening doors/windows/etc changing the environment

Oh, explain this, if I have one front window down, then the opposite door flexes and flaps in and out like hell, much more than any other door is. Also, does the fact that my backglass flexes alot more when I open the sunroof fit in with your theories? Can you explain these two things to me? I'm just curious and you seem to know what you're talking about

Oh and one more thing lol, I notice I have a HUGE, and I mean HUGE difference between having your head right outside the window (like hair trick close) vs. fully inside the window (like where the driver's head normally would sit). Like when people stick their head right outside my window (if they've never heard my system before) its like oh cool nice bass...and then when they get inside its more like "OH FCUK!" I mean we're literally talking like the difference between me giving 50 watts a piece to these subs and 800 watts a piece to these subs, is that normal? Because it isn't in my experience with other people's systems, at least not to that extent. Also, if its abnormal, what does it mean?

 

[Ferendon 10+ year member]

Yes, it could be acting like one.

^^

 

[Immacomputer 10+ year member]

Hmm...interesting. From the front seats, loudest by far is front two windows down, or just the opposite window down, hard to tell which is louder between those. From the back seats, it gets louder with the rear windows down, and then even louder again by lowering the front two windows also. The sunroof has always been hard for me to tell exactly what I get out of it...other than not having it flap so **** much when I open it, but it usually seems quieter with the sunroof open than with the front windows down atleast in the 35-60 range, I get air movement out of the sunroof like a mother****er though at 15-25hz though, so you very well may be right.
I've always been good at visualizing stuff you can't see like this...but for some reason I have a hard time visualizing sound pressure transfer in a car and the effects of opening doors/windows/etc changing the environment

Oh, explain this, if I have one front window down, then the opposite door flexes and flaps in and out like hell, much more than any other door is. Also, does the fact that my backglass flexes alot more when I open the sunroof fit in with your theories? Can you explain these two things to me? I'm just curious and you seem to know what you're talking about

Oh and one more thing lol, I notice I have a HUGE, and I mean HUGE difference between having your head right outside the window vs. fully inside the window (like where the driver's head normally would sit). Like when people stick their head right outside my window (if they've never heard my system before) they're like oh thats pretty nice...and then when they get inside its more like "OH FCUK!"

In a transmission line, there is a closed end and open end. At the closed end, there is a high pressure build up and at the open end, there is a high velocity section. When you open one side window, you create the open end of the line there and you mass load it by using a smaller opening than what the actual line cross sectional area is. This creates a high pressure area near the high velocity area which should make the seat with the raised window slightly louder than the seat with the lowered window. That would also explain why the door flexes on the closed window side as it is a high pressure area while the window down is a high velocity section of the line. With the sunroof open, you're matching the natural resonance of the glass with the resonance of the line and so the high pressure area will cause the glass to set in motion. I notice the same exact behavior in my car as well. With the sunroof open, I get a ton of flex from the hatch but the second the sunroof is closed too far, the hatch stops flexing even though the output inside the car stays the same. It's really all in matching resonance points between your mass loading and vehicle.

Again, that huge difference from outside the window to inside is based on the high velocity area vs the high pressure area. If they're standing outside the window, they can probably feel their hair move and air movement but the sound will be much louder in the high pressure zone that you're creating by opening the windows.

Martin J. King has written some great papers on transmission line and horn theory and they really help take the "mystery" out of vehicle transfer functions. It's not perfect as objects in the vehicle and vehicle materials will play a large role in damping which will play a large role in the final response of the system. But, it's a lot more accurate than assuming no change by the cabin.

 

[Rashaddd 5,000+ posts]

In a transmission line, there is a closed end and open end. At the closed end, there is a high pressure build up and at the open end, there is a high velocity section. When you open one side window, you create the open end of the line there and you mass load it by using a smaller opening than what the actual line cross sectional area is. This creates a high pressure area near the high velocity area which should make the seat with the raised window slightly louder than the seat with the lowered window. That would also explain why the door flexes on the closed window side as it is a high pressure area while the window down is a high velocity section of the line. With the sunroof open, you're matching the natural resonance of the glass with the resonance of the line and so the high pressure area will cause the glass to set in motion. I notice the same exact behavior in my car as well. With the sunroof open, I get a ton of flex from the hatch but the second the sunroof is closed too far, the hatch stops flexing even though the output inside the car stays the same. It's really all in matching resonance points between your mass loading and vehicle.
Again, that huge difference from outside the window to inside is based on the high velocity area vs the high pressure area. If they're standing outside the window, they can probably feel their hair move and air movement but the sound will be much louder in the high pressure zone that you're creating by opening the windows.

Martin J. King has written some great papers on transmission line and horn theory and they really help take the "mystery" out of vehicle transfer functions. It's not perfect as objects in the vehicle and vehicle materials will play a large role in damping which will play a large role in the final response of the system. But, it's a lot more accurate than assuming no change by the cabin.

Its all starting to make a lot more sense to me (or at least I think i'm gettin it), thanks alot for the responses man, seriously, appreciate the insight //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[Immacomputer 10+ year member]

Its all starting to make a lot more sense to me (or at least I think i'm gettin it), thanks alot for the responses man, seriously, appreciate the insight //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

In all honesty, it's all starting to make sense to me too haha! I spent a lot of time this summer playing around with a meter in my car playing test tones under all sorts of different conditions. I first thought of a car acting like a ported enclosure but all the models I did just didn't give me numbers that I was seeing in my car. I figured it was based on transmission line theory so I read deeper into it.

A couple of weeks ago, I found that I could modify Martin King's mathematical model for transmission lines and use them to simulate a car's interior and even account for the damping of the panels and seats and whatnot. It's pretty neat stuff.

I'm going to play around a little more with it and see if I can design an SPL based enclosure using it and a back loaded horn. That won't be any time soon though as I am back in school and away from my workshop.

 

[Buck 5,000+ posts]

possibly