Re: Creating the "Horn effect"...
I wouldn't call it a horn effect. A horn is a rather complicated wave guide that is tuned to the Fs of the driver (in most cases). What you're describing is simply cabin gain from the large space in a hatchback. And typically, that works best in both hatchbacks and trunk cars loaded off the back wall.
For a trunk car you have two options... load off the back of the car and utilize the gain from the trunk or fire forward and seal the trunk off from the cabin to prevent standing waves behind the box (in the trunk).
And if you plan to fire through the package tray, thats fine but you'll need to make sure it is very well deadened and sealed up or you'll have a noisy, rattling system.
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