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After a few days of playing with all the fun toys I
discovered the bass that goes away when opening the trunk was causing
cancellation when kept in the car (trunk closed), and turning the box backward
made less of a difference when opening the trunk, and moving the box all the way
to the back of the trunk eliminated having an increase in SPL when opening the
trunk totally.
In summary I found:
1. box at front of trunk speakers aiming forward or up
through the rear deck = poor in car bass response, much better with trunk open.
2. box at front of trunk speakers aiming backward = better in car bass response
by far, slightly better with trunk open.
3. box at back of trunk with speakers aiming forward = better in car bass, no
difference with trunk open.
4. box at back of trunk with speakers aiming backward = best in car bass
response and gets less bass with trunk open.
I used sine waves and mapped out the phase relationships
between the incident (direct) sound wave entering the car and the reflected wave
that hits the back of the trunk and reflects forward. Since the reflection is
bounced into the listening area, you can treat them much the same as having two
sources...
I drew some pictures to illustrate what I found.
In this picture, try to imagine the back of the trunk is the vertical
black line at the left of the picture and the little square is the
speaker box. This picture is an illustration of what happens when sound
comes out of the right side (front side) of the little square speaker
box. Sound actually goes forward into the car (incident wave (RED)) and
also backwards to reflect off of the back of the trunk (reflected wave
(YELLOW))... Both the incident wave and reflected wave get to the
listener but they are way way out of phase causing mucho cancellation in
the listening area. If you could open the trunk, the reflected wave
would disappear and NOT reflect back into the car thus no
cancellation...
In this picture, the speaker box has been aimed at the trunk instead of in the
car and it is plain to see the incident and reflected wave are not nearly so
much out of phase as in picture 1! Resulting in much better bass!
This picture it represents the speaker box being moved to the rear of
the trunk with the speakers aiming forward. The waves are a little
closer to being in phase with each other. (were gettin there!)
Finally, in this picture we are aiming the rear mounted speaker box to
the rear so the incident and reflected wave are very close to being in
perfect phase, reinforcing each other quite well.
These pictures are simulating a 60Hz bass
note with the rear of the box mounted approximately 3 feet from the back of the
trunk (reflector) ...
Keep in mind we are only discussing the
incident and rear reflected sound in an effort to try to simplify this, the
reflecting sound waves in a car are much more complex than these drawings
indicate but we must start simple before we work ourselves into the more
complex, hopefully this will be a nice foundation for those of you who wish to
study this phenomenon further.
And for those that have the mindset
that this can't be true because the interior of cars are small in
relation to bass wavelengths, so what? The full wavelength does not have
to completely develop to be OUT OF PHASE or IN PHASE with its own
reflected sound. The pictures above are showing a 60Hz wavelength and
the bounce distance to reflect back out of phase a complete 180 degrees
is just over 4 ft. At higher frequencies the distance is less (120Hz is
2.3ft).
For more information on wavelengths
and how you can predict the interaction of reflections at any frequency
see my wavelength paper.
For actual in car tests check here! (aiming
woofer boxes part 2) |
