JBL GTiAutomotive - Manual (Page 5)

Enclosure Design
Follow the simple steps below to design an
optimum ported box for your GTi driver.
1. Determine the maximum enclosure size
available for your installation by measuring
and multiplying the height x width x depth (in
inches) and then dividing by 1728. This will tell
you exactly how many cubic feet your enclosure
will be.
2. To determine the volume of an irregularly
shaped box, divide the box into sections, calcu-
late the volume of each section individually, then
add the separate volumes to determine the total
box volume. Here are some useful formulas to
help you calculate volumes:
USEFUL FORMULAS
1 cubic foot = 1728 cubic inches
1 cubic foot = 28.32 liters
1 liter = 0.353 cubic feet = 61 cubic
inches = 1000cm 3
Radius = 1/2 diameter
Box volume = height x width x depth
Volume of triangular box section = 1/2 base x
height x length
Volume of tubular duct = area of circle x length
pi (
π
) = 3.1416
Area of circle = pi (
π
) x radius 2
Diameter of circle = 2 x area / pi (
π
)
Area of square = height x width
3. Now that you have calculated the internal box
volume, you should subtract the DRIVER DIS-
PLACEMENT (listed on the specification sheet)
while also subtracting approximately 0.1 cubic
feet for the port tube (if one is being used). If
you’d like, you may calculate the exact dis-
placement of the port tube by utilizing the for-
mula listed above (Volume of tubular duct).
4. Using the GTi Enclosure chart (shown on your
GTi driver specification sheet), choose a desired
box volume on the vertical chart axis, which fits
within the size limit you have established. Draw
a line across from this point until it intersects the
recommended tuning frequency. Draw a line
down to the horizontal axis and read the corre-
sponding tuning frequency. We have drawn an
example on the chart.
5. Use the Port Nomograph on the Driver Specifica-
tion sheet to determine the port diameter and
length based on the enclosure volume and tuning
frequency determined in step 4. Follow these
steps to determine the port.
Port Dimension Calculation
1. Draw a line from the chosen enclosure volume to
the desired tuning frequency. Continue this line
until it intersects with the Lv/Sv construction line
(Lv = port length, Sv = port opening surface
area). This is called the construction point.
2. Draw a horizontal line starting from the con-
struction point straight through the chart. Note
that the Lv/Sv axis also appears at the right of
the chart to help keep your line parallel all the
way across the chart.
3. Select the curved line with the port diameter
you would like to use. Locate the point where
this curved line intersects with your horizontal
construction line and draw a vertical line from
this point down to the bottom of the chart.
NOTE: Try to use a port tube no smaller than 1/3 the
diameter of your speaker. Using a smaller diameter
port can result in “whistling” or “breathing” noises.
4. The required port length is found at the point
where the vertical line you’ve drawn intersects
with the scale at the bottom of the chart.
NOTE: The recommended port tube dimensions
should be considered a starting point. Since it is
impossible to predict “in-car” response exactly, you
may find that a port length slightly longer or shorter
than that recommended may produce slightly better
results. Experiment!
Lay out plans for your box using the information
determined in the above steps and the box con-
struction tips listed on page 6.
5
Vehicle Acoustics
The acoustics of your vehicle also influence
bass performance tremendously. Fortunately, the
small interior volume of most cars and trucks
boosts low bass output enough to allow for small-
er enclosure sizes while still achieving good bass
performance. As a general rule, below 50-80 Hz
(depending on interior size), bass response
increases at a rate which approaches 12dB per
octave as the frequency decreases. This adds a
significant bass reinforcement to the actual output
of the subwoofer. If this rise in bass is not
addressed in the design of your enclosure, bass
performance may be boosted to the point where it
is actually too strong for some tastes. For others,
there is never too much! The enclosure design
recommendations which appear on the specifica-
tion sheet for your particular model, were
chosen for optimum in-car performance.
You may find them to be different than the
recommendations from formulas used to
design enclosures for in-home or profes-
sional sound-reinforcement applications.
These “Transfer Function Compensated”
tuning recommendations result in small
enclosures with excellent in-car low fre-
quency performance.
4
5 Hz
–36
–30
–24
–18
–12
–6
0
6
dB
10 100
Frequency
Normalized Amplitude Response (dB/Hz) JBL SpeakerShop 1.0
1000 2000 500 50
Enclosure-only response
Transfer Function
Compensated Tuning (33Hz)
Vehicle Transfer Function
In Car Response Curve
This graph shows the result of tuning the enclosure to compensate for the effects
of the vehicle transfer function. Note that the in-car response is smoother and the
–3dB point moves down to approximately 22Hz.
5 Hz
–36
–30
–24
–18
–12
–6
0
6
dB
10 100
Frequency
Normalized Amplitude Response (dB/Hz) JBL SpeakerShop 1.0
1000 2000 500 50
Enclosure-only response with
Standard “Optimum” Tuning (42Hz)
Vehicle Transfer Function
This graph shows the effect of vehicle transfer function added to an enclosure
using “standard” tuning. Note the peak at 50Hz, with a –3dB point of
approximately 31Hz.
In Car Response Curve
5 Hz
–36
–30
–24
–18
–12
–6
0
6
dB
10 100
Frequency
Normalized Amplitude Response (dB/Hz) JBL SpeakerShop 1.0
1000 2000 500 50
In Car Response with Transfer
Function Compensated Tuning
In Car Response with
Standard Tuning
This is a comparison of in-car response curves with and without transfer function
compensated tuning.
GTi Subwoofer Owners Manual 7/17/98 9:51 AM Page 4