If you have great bass when the trunk is open, but it suddenly goes away when the trunk is closed, there is a simple solution - move the sub. When you lose the bass, it is due to reflected frequencies canceling direct frequencies.

In the drawing below, Figure 1 represents low frequency sound waves when the trunk is open. Part of the bass is reflected, but passes out of the trunk opening. What is sent to your ears directly from the subwoofer sounds loud because the wave hits right at ear level in the driver's normal seating position.

In Figure 2, the trunk lid is closed, and this causes a big difference in what you hear in the driver's seat. Now what happens is bass frequencies that are reflected in the vehicle (known as standing waves) do not escape out of the trunk opening, causing cancellation of these frequencies. This happens because the same frequency travels a different pathlength to get to the same point, arriving at different points in the wave. The end result is known as phase cancellation. These points are indicated by a red 'X'.

In Figure 3, the problem is resolved in this case by turning the subwoofer box around. This allows the reflected bass to arrive at the same point in the wave as the direct bass at the driver's position. This is why the bass sounds louder in some cars if you face the woofers toward the rear of the vehicle, as well as why your subs sound louder when you roll your windows down.

Each vehicle totally affects the sound produced within it. Your results will vary depending upon the size of your vehicle, what type of interior you have, how far speakers are placed from your listening position, and what obstacles are between you and the speaker. This is known as 'transfer function', and experimentation will more than likely get you the best results.

If you would prefer to go the technical route (rather than experiment) to find the best position and location for your box, here is what you need to consider:

• The speed of sound (in air) is 1130 feet per second.
• The frequency (in hertz) is the number of cycles per second of a sound wave.
• 1130 (the speed of sound) divided by the frequency equals the wavelength.

For example, the wavelength of 45Hz would be 25.1 feet. You can use this information when placing your box, especially if you are planning on doing SPL competitions and want to target a specific frequency. This will help you determine where the box needs to be in relation to the microphone position.

• One-quarter of a wavelength presents the best perception to the human ear.

Still using 45Hz as the example, the best perception of 45Hz to humans would be at 6.28 feet, 18.84 feet, 31.37 feet, etc. from the speaker that is producing it.

• Cancellation takes place at 1/2 of a wavelength.

With 45Hz, cancellation would take place at 12.55 feet, 37.65 feet, 50.2 feet, etc. from the speaker that is producing it. For an example of this, play a test CD with a frequency sweep on it, and notice how some bass notes seem to almost totally disappear, while others are significantly louder.

• 1130 (the speed of sound) divided by the wavelength equals the frequency.

This would be used if you had a fixed distance to work with, and wanted to find out what frequency would hit the hardest at that distance. For example, if you had 15 feet to work with:
1130 / 15 = 75.33Hz .

By using this information, you can get a better idea of where to place the box depending upon what you want it to do, especially if you are targeting a specific frequency. Just be sure to remember that the vehicle that the system is in plays a big role in what the final results will be.