What Kind of Speakers Should Be Put in a Van?
Acoustic engineering is far from simple, even in something as architecturally simple as a van. The kind of speakers you opt to use are far less important than the location where you decide to install them and the kind of enclosures you opt to use. Using the highest-quality speakers you can afford is a good start, but no speaker, no matter how good it is, can compensate for bad systems engineering.
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Determine What You're Going For
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Speaker system design depends entirely upon the effect you're going for. Vans are somewhat unique in terms of application -- first off because the large interior volume changes the sound dynamics; secondly, because passengers are spread so widely through the box; and thirdly, because the entire van acts as a giant speaker enclosure. Before you decide on what speakers you'll need, determine how many passengers you intend to carry, what your packaging constraints are like and what kind of decibel and frequency range you're after. In general, you'll want to "target" the speakers on your listeners, so that no one acoustic range overpowers another.
Sound Basics
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Sound happens in a series of pressure waves: the closer those waves are together, the lower the frequency. To get the most out of a sound system, you'll want to mount different-sized speakers so that those waves all collide on your target area, or where the listener will be sitting. Big subwoofers have a longer wavelength, which is why installers typically put them in the trunk, as far away from the driver as possible. Tiny "tweeters" have a much shorter wavelength, and need to be much closer to the listener in order to coincide with the bass wavelength. Without tuning the wavelengths to coincide, one source will drown out the other, or the two wavelengths will cancel out and muddy the sound.
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Frequency and Position
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The "first octave" of sound runs from about 10 Hz to about 80 Hz, but frequencies below 40 Hz are typically inaudible. While technically inaudible, sub-40 Hz ranges are what induce that thumping bass that enthusiasts are after. The 1/4 wavelength (which is what gives you the loudspeaker effect) for a 20 Hz speaker is about 14 feet, so you'll need to put a super-low 20 Hz speaker all the way at the back to target the driver. Speakers with the "standard" bass range of 80 to 250 Hz should sit between 3.5 and 1.25 feet from the listener. Anything smaller than that can sit practically on top of the listener, with smaller tweeters in the headrests. Don't worry about overpowering the bass with close-set tweeters; you can always adjust the individual speaker channel output later.
Controlling Sound
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Sound quality can really fall if you don't use a subwoofer box to control sound output. The 15-inch subwoofers you'll need to maintain a 20 Hz frequency will require a sub box with an interior volume of between 5 and 9 cubic feet. To tune for the 20 Hz range, you'll want to install a port tube in the box; it should measure about 2 inches in diameter and 2.7 inches in length if you're using a 5-cubic-foot box, and it should be 1-inch long if you're using a 9-cubic-foot box. Smaller speakers won't require any specific enclosure, but you might want to consider soundproofing the van's interior, using a soundproofing mat or spray-on soundproofing medium. Spray-on rubber sealant is a fantastic soundproofer, will easily hide behind panels and will keep air pressure contained in the van.
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References
- "Foundations of Engineering Acoustics"; Frank J. Fahy; 2000
- "Fundamentals of Fluid Mechanics"; Bruce R. Munson; 2005
Resources
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