COMMON SENSE
ALWAYS DISCONNECT THE NEGATIVE TERMINAL OF THE BATTERY WHEN
WORKING ON YOUR SYSTEM. (This is not a rule, it is a LAW and should
only be broken by God's and anyone who thinks that dying is an acceptable outcome!!!!)
SPL
The secret
to achieving high Sound Pressure Levels (SPL) is to let the laws of physics work for you,
rather than trying to fight them. All vehicles have a resonant frequency, and it is at
this freq. that the most transfer function will occur. Transfer function is when the audio
system couples with the environment of the vehicle to boost a specific section of the
acoustic spectrum, this occurs in the bass region.
If we know this freq. then we can design a set-up for max output at this
freq. for maximum SPL for a given amount of power. So our first step in the pursuit of high
SPL is to work out this freq.
The cheapest way is using a "radio shack" type handheld SPL
meter, a bass tuning disc with tones at fixed freq. and a sub in a sealed
box (and a
suitable amp). The box for the sub can be fairly rough although it will still need to be
sealed well, as its just used to test with. The type and size is not critical although a
10"-12" is better.
Now the test sub system is placed in the car and we are ready to
begin. Play the bass freq tracks, and set the volume so the meter is reading around 80dB
and then step thru tracks noting dB readout. You should notice that at and around a
specific freq. you will get an increase in dB and when you go either side of this freq the
level drops back to original 80 dB or so, this is often between 45-75Hz
It is this freq. that is your vehicles resonant freq. and this is
now the freq. that the system should be designed for. When deciding which subs to go for,
try to get subs whose resonant freq. is exactly the same as the cars. If you get subs that
match, then things will work better, but getting close as possible is the goal. The
sub enclosures should be designed to provide maximum output around this freq. as well.
If Sound Quality (SQ) is not an issue, then the box should also be
limited in operating range for maximum efficiency at the resonant freq and bandpass or
ported boxes can provide the best efficiency and flexibility in design.
Placement and direction of subs will also effect SPL and often
experimentation here can yield significant changes in results. This is why SPL
competitors often have raw uncarpeted boxes, as they constantly experiment,
trying to get the extra dB to win.
SPL boxes are best designed by people with plenty of practice using CAD
programs for box design and so contacting your local specialist store, to whip up the
final design may be the best for most or even contacting the manufacturer of the subs, as
they often are only too happy to do this for their customers, and have the appropriate
experience. Now its just a matter of building and installing it all (he
he:).
I would also
suggest using an adjustable "infra-sonic" or "sub-sonic" filter as
this is essential for preventing damage (especially for port/bandpass
boxes), and it will also help maximise SPL by
preventing sub-sonic power robbing bass content reaching the amp, therefore allowing the
amp to put its power into the region its needed for maximum SPL.
This is just the beginning when chasing SPL and doesn't take into account
many things such as measuring and optimising impedance curves, vehicle
bracing and reinforcement, and numerous other factors.
Achieving high SPL is a science, and the method suggested is a starting
point only, as often the chase never ends.
Good Luck and remember to always protect your hearing by using quality
ear protection.
ALWAYS PRACTICE SAFE SOUND
And don't crank your system at Intersections as its RUDE behaviour, and
will give us all a bad name!
