GTi Subwoofer Owners Manual 7/17/98 9:51 AM Page 4
Vehicle Acousticstion sheet for your particular model, wereNormalized Amplitude Response (dB/Hz)JBL SpeakerShop 1.0Enclosure Design3. Now that you have calculated the internal box2. Draw a horizontal line starting from the con-
The acoustics of your vehicle also influencechosen for optimum in-car performance.dBVehicle Transfer FunctionFollow the simple steps below to design anvolume, you should subtract the DRIVER DIS-struction point straight through the chart. Note
bass performance tremendously. FortunatelyYou may find them to be dif, theferent than the6optimum ported box for your GTi driver. PLACEMENT (listed on the specification sheet)that the Lv/Sv axis also appears at the right ofIn Car Response Curve
small interior volume of most cars and trucksrecommendations from formulas used to0while also subtracting approximately 0.1 cubicthe chart to help keep your line parallel all the
�61. Determine the maximum enclosure size
boosts low bass output enough to allow for small-design enclosures for in-home or profes-feet for the port tube (if one is being used). Ifway across the chart.
�12
sional sound-reinforcement applications.available for your installation by measuring er enclosure sizes while still achieving good bassyouOd like, you may calculate the exact dis-
Enclosure-only response and multiplying the height x width x depth (in�18
performance. As a general rule, below 50-80 HzThese OTransfer Function Compensated�Transfer Function placement of the port tube by utilizing the for-3. Select the curved line with the port diameter�24
Compensated Tuning (33Hz)
tuning recommendations result in smallinches) and then dividing by 1728. This will tell(depending on interior size), bass response�30mula listed above (Volume of tubular duct). you would like to use. Locate the point where
you exactly how many cubic feet your enclosure
increases at a rate which approaches 12dB perenclosures with excellent in-car low fre-�36this curved line intersects with your horizontal
4. Using the GTi Enclosure chart (shown on your
quency performance.5 Hz105010050010002000will be.octave as the frequency decreases. This adds aconstruction line and draw a vertical line from
FrequencyGTi driver specification sheet), choose a desired
significant bass reinforcement to the actual output2. To determine the volume of an irregularlythis point down to the bottom of the chart.
This graph shows the result of tuning the enclosure to compensate for the effects box volume on the vertical chart axis, which fits
of the subwoofer. If this rise in bass is notof the vehicle transfer function. Note that the in-car response is smoother and the shaped box, divide the box into sections, calcu-within the size limit you have established. Draw
addressed in the design of your enclosure, bass�3dB point moves down to approximately 22Hz.late the volume of each section individually, thenNOTE: Try to use a port tube no smaller than 1/3 the
a line across from this point until it intersects the
performance may be boosted to the point where itadd the separate volumes to determine the totaldiameter of your speaker. Using a smaller diameter
recommended tuning frequency. Draw a line
is actually too strong for some tastes. For others,box volume. Here are some useful formulas toport can result in Owhistling� or Obreathing� noises.
down to the horizontal axis and read the corre-
there is never too much! The enclosure designhelp you calculate volumes:
sponding tuning frequency. We have drawn an
recommendations which appear on the specifica-4. The required port length is found at the point
example on the chart.
Normalized Amplitude Response (dB/Hz)JBL SpeakerShop 1.0USEFUL FORMULASwhere the vertical line youOve drawn intersects
Normalized Amplitude Response (dB/Hz)JBL SpeakerShop 1.0dB1 cubic foot = 1728 cubic inches5. Use the Port Nomograph on the Driver Specifica-with the scale at the bottom of the chart.
In Car Response with Transfer
dBVehicle Transfer Function6Function Compensated Tuning1 cubic foot = 28.32 liters
tion sheet to determine the port diameter and
601 liter = 0.353 cubic feet = 61 cubic NOTE: The recommended port tube dimensions
length based on the enclosure volume and tuning
0�63
inches = 1000cmshould be considered a starting point. Since it is
In Car Response Curve frequency determined in step 4. Follow these�6�12
Radius = 1/2 diameter steps to determine the port.impossible to predict Oin-car� response exactly, you
�12�18In Car Response with
Standard TuningBox volume = height x width x depth may find that a port length slightly longer or shorter
�18�24
Enclosure-only response with
Port Dimension Calculation
�24Standard OOptimum� Tuning (42Hz)�30Volume of triangular box section = 1/2 base xthan that recommended may produce slightly better
�30�36height x length 1. Draw a line from the chosen enclosure volume toresults. Experiment!
�365 Hz105010050010002000Volume of tubular duct = area of circle x lengththe desired tuning frequency. Continue this line
Frequency
5 Hz105010050010002000pi (1) = 3.1416 until it intersects with the Lv/Sv construction lineLay out plans for your box using the information
Frequency
2(Lv = port length, Sv = port opening surface
This graph shows the effect of vehicle transfer function added to an enclosure Area of circle = pi (1) x radiusdetermined in the above steps and the box con-This is a comparison of in-car response curves with and without transfer function
using Ostandard� tuning. Note the peak at 50Hz, with a �3dB point of compensated tuning.Diameter of circle = 2 x area / pi (1) area). This is called the construction point.struction tips listed on page 6.
approximately 31Hz.
Area of square = height x width
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ACEC
