# Does bass increase or decrease with windows down?



## durwood (Mar 7, 2007)

This is a strange one:

I would like to hear possible reasons this could happen. My buddy and I have discussed this before and asked around before, but we have never heard a good explanation of why or how.

Why does the bass increase in some cars and decrease in others when you roll down or crack the windows?

I'm not talking about major spl, just your basic setup.

I know it sounds strange, but I have seen it first hand.

Theories anyone?


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## Xander (Mar 20, 2007)

Just cases of either cancellation or...gosh I'm tired. What's the word for when a sound wave gets boosted by another sound wave of the same frequency at the same phase? Whatever. The opposite of cancellation. That's my opinion. Every car is different because of the distance between the subs and the windows, so the reflections cause different effects.


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## solacedagony (May 18, 2006)

I'm not sure of the exact reasoning behind this, but remember that the enclosure holding your sub is in ANOTHER enclosure... Your car. When you roll your windows up or down, you're changing the volume of your "car" enclosure.


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## durwood (Mar 7, 2007)

Cancelation/addition of waves is ruled out by the pure fact all you need to do is crack the window open, not remove the surface of the glass all together.

that's pretty close to explanation we heard that fits best. But just curious if it can explained by math/physics. I think it was someone at phoenix gold once suggested it ahd to do with cabin airspace and sizing the enclousre. Something along the lines of taking the ideal enclosure then adding or subtracting 1/3 of the airspace to the voume of the box found. So, if you ahd a large vehicle, and your enclosure was 1cuft, then make it 1.33cuft, if it's smaller, then your box should be 0.66cuft. This was only a suggestion given, but no numbers or rules of thumb or ranges given as far internal cabin airspace.

I think you are close-as like a bandpass enclosure, the only thing that kind of kills it though is that the window can be cracked or rolled all the way down and it doesn't seem to change between the two. Once the seal is broken it's just louder. For most of us it's the opposite, roll the window down, bass decreases.


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## Oliver (Jun 25, 2007)

Here is a pickup truck that went from so-so to allright http://www.the12volt.com/installbay/forum_posts.asp~TID~96179~PN~1


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## durwood (Mar 7, 2007)

Hic said:


> Here is a pickup truck that went from so-so to allright http://www.the12volt.com/installbay/forum_posts.asp~TID~96179~PN~1


no explanantion on that one yet either. he even changed his box and still had the same results. It describes exactly the same issue-if you can call it an issue.

I would actually prefer my bass increase with the windows down.


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## Neil (Dec 9, 2005)

I have heard all sorts of explanations, including that the vehicle behaves as a bandpass enclosure with the windows down. The real truth of the matter is that energy reflects off the window and arrives at your ear delayed and out of phase (at certain frequencies) with other sonic energy that has a more direct approach to your ears. This is simple destructive interference and a form of comb filtering in an acoustical sense. When you roll down the window, this effect is diminished.

As for mathematical calculation, it is predictable, but not something that a novice is likely to jump at.


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## Oliver (Jun 25, 2007)

Durwood,
I lean towards the box in a box theory, lack of space limiting the output, The destructive and constructive string of pearls doesn't seem to apply to this, in my mind.

When people put their subs in smallish boxes they sacrifice low end extension and possibly some output.


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## Xander (Mar 20, 2007)

durwood said:


> Cancelation/addition of waves is ruled out by the pure fact all you need to do is crack the window open, not remove the surface of the glass all together.


Ah, I see. I haven't really experimented that much. It probably has something to do with the pressure of the cabin too. Crack a window, and the air has somewhere to go instead of it being forced to pressurize inside the car. Not sure why the bass would get louder if you opened the window...unless it's less pressure on the cone of the sub so it can reach higher excursions with the same power. But I don't think cracking a window would do all that.


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## Whiterabbit (May 26, 2006)

I dont think in terms of waves bouncing off the window, but rather pressure. regardless of how much pressure is contained with the windows up and released with the windows cracked, there will always be wave interaction with all surfaces inside the cabin.

Sometimes that is constructive, sometimes that is destructive.

Cracking the windows can change the way pressure builds in the cabin. That's good enough for me to explain. constuctive? destructive? I've merely noted in most cases it's constructive when cracked.


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## durwood (Mar 7, 2007)

I still have a hard time believing that it is reflections when the simple fact of just cracking the window just an inch is the same as rolling it compleltly down. With it cracked, you still have plenty of surface for reflections. Unless by cracking it you interupt or increase some type of waveguide.

The more I think about it I do beleive it could be destructive interference, but I would like to duplicate it.

It's not like the bass is gone with the windows up in the situation I'm referring to, there is plenty of low end output. It's just like a slight bump or rise in output in the low end.


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## forty5cal1911 (Sep 11, 2006)

I have noticed this phenomenon several times over the years. My question is has anybody noticed this with an IB installation?


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## durwood (Mar 7, 2007)

Xander said:


> Ah, I see. I haven't really experimented that much. It probably has something to do with the pressure of the cabin too. Crack a window, and the air has somewhere to go instead of it being forced to pressurize inside the car. Not sure why the bass would get louder if you opened the window...unless it's less pressure on the cone of the sub so it can reach higher excursions with the same power. But I don't think cracking a window would do all that.





Whiterabbit said:


> I dont think in terms of waves bouncing off the window, but rather pressure. regardless of how much pressure is contained with the windows up and released with the windows cracked, there will always be wave interaction with all surfaces inside the cabin.
> 
> Sometimes that is constructive, sometimes that is destructive.
> 
> Cracking the windows can change the way pressure builds in the cabin. That's good enough for me to explain. constuctive? destructive? I've merely noted in most cases it's constructive when cracked.


kind of more along the lines I was thinking. So in theory we should be able to model it if this were the case and come up with a rule of thumb. At least I would love to think that, although it's not going to be easy.

You essentially have a box, which is inside a larger chamber that leaks into an even larger chamber that is contained until you crack the windows and vent into a huge chamber of free-air. There would be one less chamber if it's all open like a truck/suv/hatchback.


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## Neil (Dec 9, 2005)

Whiterabbit said:


> I dont think in terms of waves bouncing off the window, but rather pressure. regardless of how much pressure is contained with the windows up and released with the windows cracked, there will always be wave interaction with all surfaces inside the cabin.
> 
> Sometimes that is constructive, sometimes that is destructive.
> 
> Cracking the windows can change the way pressure builds in the cabin. That's good enough for me to explain. constuctive? destructive? I've merely noted in most cases it's constructive when cracked.


They are waves of pressure. Sound is simply a wave of pressure, created by compression and rarefaction of the air in your environment.



durwood said:


> I still have a hard time believing that it is reflections when the simple fact of just cracking the window just an inch is the same as rolling it compleltly down. With it cracked, you still have plenty of surface for reflections. Unless by cracking it you interupt or increase some type of waveguide.
> 
> The more I think about it I do beleive it could be destructive interference, but I would like to duplicate it.
> 
> It's not like the bass is gone with the windows up in the situation I'm referring to, there is plenty of low end output. It's just like a slight bump or rise in output in the low end.


Well, I've never noticed that there was no difference between an inch and all the way down, but even small changes in the vehicle can shift a null up or down in frequency, or in rare instances, remove it entirely.


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## durwood (Mar 7, 2007)

So what are the right variables to make something like this happen? Spin your head around that one for a while.


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## Oliver (Jun 25, 2007)

An enclosure that you can make bigger and smaller , does that sum up our variables, and see if we get low end extension with a larger enclosure.

Make sure it is air tight, then maybe a air pressure guage to see if pressure builds when the speakers go backwards and forwards, finally a pressure relief valve, to dump the built-up pressure.


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## Oliver (Jun 25, 2007)

What did I leave out?


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## durwood (Mar 7, 2007)

I'd like to know what the right ratio of cabin airspace to trunk airspace to box airspace and sub output would create such a result, if that is the case.


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## Whiterabbit (May 26, 2006)

a result at what frequency? 

and is that frequency in the intended response?


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## durwood (Mar 7, 2007)

Whiterabbit said:


> a result at what frequency?
> 
> and is that frequency in the intended response?


how about anything between 20-40Hz? A nice wide pass band. Or split it down the middle at 30Hz


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## Oliver (Jun 25, 2007)

Any sized vehicle is what the pressure buildup and then subsequent release of pressure would work in.

This is what it has been noticed in.


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## Oliver (Jun 25, 2007)

If reflections are what you want to test for, maybe curtains and towels would work,[ help to absorb ].


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## saMxp (Jun 22, 2007)

Punk0Rama said:


> They are waves of pressure. Sound is simply a wave of pressure, created by compression and rarefaction of the air in your environment.


Right, but these waves are upwards of 20-50 ft in wavelength. Could you really nullify them by changing a reflective surface by an inch?


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## Whiterabbit (May 26, 2006)

that depends on how much marketing your company is based on 

try setting a glass bowl of marbles on the dash to see if that helps bass reflections. Make sure they are special audio grade marbles


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## durwood (Mar 7, 2007)

I've seen some wooden blocks will those work? How about a disco ball hanging from the rearview?


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## Abmolech (Nov 2, 2006)

To get destructive interference you require several conditions.

First is a reflective surface and/ or speaker. 
This reflective surface/speaker needs to reflect the wave back into another wave (angle of incidence)
The need to be the same frequency
This occurs at a distance that has to be within quarter of a wave length.

For example inside a sub enclosure, you may have suitable reflective surfaces, at a suitable angle incidence, the frequency are the same, however it is missing one vital aspect to cause destructive interference. The distance between the reflective surfaces are NOT on quarter length apart. (unless your enclosure is huge)

So with the windows we have a reflective surface, at an angle of incidence to another surface (the opposite window) therefore we can calculate the first destructive frequency be measuring that distance and dividing by four.

The highest sub woofer frequency is 80 Hz, therefore the distance has to be a MINIMUM of 3.6 feet between reflective surfaces. If we measure the distance between the windows we can affirm the frequency at which this first null or summing will occur.

As durwood is pointing out, the main cause of this is likely to be acoustic loading.

http://www.trueaudio.com/st_spcs1.htm

By reducing the loading, you decrease the SPL.

One my favourite subjects is the use of baffles as a wave guide in a car. The car body in this case is a wave guide.

If we took the time to use proper baffles we could/should gain significant advantage in SPL, and therefore decrease distortion. Candidates are the door panel inner, the kicks area and the windscreen to dash. All these provide significant gains if were to utilise them.

The steradians loading is the reason why people love listening to bass in a car.


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## zukiaudio (Jan 31, 2007)

here is a thread which discusses it.

http://www.diymobileaudio.com/forum/showthread.php?t=17326&highlight=chuyler1

the place that built the sound works regal.

found that their demo suv sounded better with the windows down.

so they rolled the window down in very measurable increments. and when they felt it sounded best.

measured it.

then cut a hole in the floor of the suv. that exact same size.

that way. no matter how it was tested.

it performed well.

been along time since i read anything about the soundworks demo cars.


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## Xander (Mar 20, 2007)

zukiaudio said:


> so they rolled the window down in very measurable increments. and when they felt it sounded best.
> 
> measured it.
> 
> ...


Cool, I should do that to my car. Time to get out the jigsaw!


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## durwood (Mar 7, 2007)

zukiaudio said:


> here is a thread which discusses it.
> 
> http://www.diymobileaudio.com/forum/showthread.php?t=17326&highlight=chuyler1
> 
> ...


I'll have to look around then but I wondered if they knew the physics/math as to why it was happening and not just how they accounted for it.



Abmolech said:


> To get destructive interference you require several conditions.
> 
> First is a reflective surface and/ or speaker.
> This reflective surface/speaker needs to reflect the wave back into another wave (angle of incidence)
> ...


My friends car already has the sub loaded in the far corner of his trunk and has it low passed <40Hz, might even be as low as 31.5Hz.


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## Oliver (Jun 25, 2007)

In large open spaces, I think that corner loading works, I believe if you hang a speaker from a high enough height you can get a reading similar to anechoic chambers or no reflections.

You can test this if you have an SPL meter though, move your box around and record your decibels as you do.

That link showed an increase of so many dBs for each surface, 1 wall , 2 wall , floor , ceiling ,

I don't think I have seen where this has been shown in a thread or post, I have heard corner loading discussed and sworn too


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## Abmolech (Nov 2, 2006)

> In large open spaces, I think that corner loading works, I believe if you hang a speaker from a high enough height you can get a reading similar to anechoic chambers or no reflections.


If your interested in doing this, the best is to use your ground (land) as a baffle (wave guide) Simply dig a suitable hole and place an enclosure in it with the driver facing out, using the ground for a wave guide.
This should give you a 2 steradians load. You can therefore measure to your hearts content.


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