Bass is louder with trunk open?

[Da64u 10+ year member]

So I've searched many threads on this but none with the same setup. Those seemed to be pointing to a ventilation issue where the sub was in a sealed trunk and the sub wasn't breathing. .

When the trunk is open not only does it sound louder, but the subs are definitely moving more and working harder.

My setup:

Chevy Cavalier

2 Kicker 15"'s in a sealed box.

They are firing to the front. but firing to the rear doesn't sound any different.

The rear seats are removed, and the trunk is fully accessible from the car.

The sub box is located where the rear seats were, so sitting right behind the 2 front seats instead of the trunk.

Any ideas why the bass sounds twice as loud with the trunk open, when the subs are in the cabin?

Thanks.

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[IonRL205 10+ year member]

Because the cabin is not sealed off from the trunk

Without the car sealed up, the sound will travel inside the trunk as well as the cab. When you open your trunk, you are turning your car into a giant 4th order bandpass and your trunk basically turns into a giant port. (4th order bandpass is a sealed box firing into a ported box).

If you sealed off the trunk from the cabin, the sound waves would only have the cabin to travel through, instead of going inside the trunk. Just setting your box in the back seat area without sealing the cabin from the trunk will cause cancellation.

It's the same concept as to why a lot of people's systems get louder when the windows are rolled down; your open windows act like a port. If you want your issue fixed, you will have to seal off the trunk from the cabin.

 

[basshead7890 10+ year member]

Either i'm misunderstanding what he's trying to do or he's misunderstanding what he's trying to do and when/why it should be done. Judging from the posts he's made it sounded to me like he wanted to completely seal off the trunk from the cabin to "negate that sound wave issue in cars with trunks" (cancellation). But he never made any mention of firing his sub(s) into the cabin, furthermore he was asking "what dimensions did you make the sheet of MDF", until he chimes in to clear the air I can only assume his plan is to simply put up a wall directly behind his back seat to seal his trunk from the cabin. The only way that would work well is if he is:
A.) Going to run his sub(s) in IB alignment

B.) Going to build a 4th order and port it through the rear deck or ski pass opening.

or

C.) Going to run his sub(s) and port forward with that wall being used as the baffle of the box.

If he's not planning on doing either of the above, sealing off the trunk is a bad idea.

The whole purpose to sealing off the trunk is to eliminate the issue of cancellation due to the front and rear sound waves being out of phase with each other. How do you know if you have cancellation? Easy. Sit in your car and listen to a song you're familiar with, while you're listening to the song have a buddy open the trunk, if the bass gets louder then you indeed are having cancellation issues. How can the bass get louder with the trunk open? Makes no sense huh?

In this picture the vertical line is the back of the trunk and the black box is the subwoofer box which is facing towards the cabin. When the sub hits, the sound goes foward into the cabin (red wave) but also reflects off the back of the trunk to become the reflected wave. When those two waves are way out of alignment as shown in the picture there will be major cancellation. Simply opening the trunk will cause that reflected wave to "disappear" and not reflect back in the trunk and not cause cancellation in the process.

In this picture the sub is aimed at the trunk and the result is that the waves aren't nearly as out of phase as the first picture, resulting in much better bass response but still not as good as it can be.

In this picture the sub box is moved so it's closer to the back of the trunk with the sub firing towards the trunk resulting in the waves nearly being in phase with each other resulting in the best bass response.

Firing the subs/port into the cabin with the baffle of the box sealed off from the trunk results in a wave patter much the same as the last picture, directs all the sound/air into the cabin area and nearly eliminates that horrendous trunk rattle on high powered systems too.

..