Speaker Boxes in Car Audio Installation

Why do we need them?

The beauty of music doesn’t lie in its loudness or quantity, but the minute details, or the quality. Often, the best part of the music listening experience is enhanced by small parts of compositions, which bring an appreciative smile to an ardent listener’s face. This quality of music can be achieved, by tweaking and improving the music system; among the many tweaks, speaker boxes find a prominent place. They add that extra quality to the sound output, appreciated by the connoisseurs.

A Hi-Fi speaker for home use is always mounted in a box, because it aids in reproducing the best possible sound. If speaker boxes are good enough for home theatres, why can’t they be used for car audio systems? It will increase the listenability of the music- and takes your music listening experience to a different level, which you never thought existed.

Traditionally, speakers in cars are mounted using a technique called ‘simple baffle mounting’, in which the speakers are placed in the side doors or in the hat rack. The sound quality, in these kind of mountings, is often average. Using suitable speaker boxes will lead to better audible experiences, as the power handling capacity increases in a correctly designed box.

If you have pair of "open air" (mounted) speakers in the rear and a subwoofer in your trunk, the rear speakers can get affected by the air pumping from the subwoofer. This will make the speaker cones move a little, and ruins their ability to produce quality sound. This, apart from others, is a good reason to use boxes in sound systems (car or home). Normally, boxes are used for the subwoofer; using them for the rest of the speakers too will result in better sound.

Speaker Boxes – In General

Any speaker box should be built using a stable and air-tight material. The best choice is MDF-board, around ¾th of an inch in thickness. As the boxes grow large, braces should be placed inside them- to avoid resonance. The sizes of the boxes are normally determined by the speaker data, but the type of vehicle and music play an important role too. For example, deep bass demands larger boxes than disco music.

There are different types of speaker boxes. Vented boxes and sealed boxes are the most common and are most widely used.

Vented Boxes

A speaker in a vented box has higher efficiency (about 3 dB; that’s almost double) than the one in a sealed box. Speakers need to be arranged carefully, because the sound from the speaker and the port combine to create a higher sound level in a vented box. The sound output will deteriorate if the sound from the port didn’t come out in the same phase as from the speaker.

Designing a vented box: few pointers

The size of the box should be carefully determined using the speaker data, car type and music type. The size of the car decides the practical size of the box in most of the cases.

A smaller box will have a higher resonant frequency than a larger one; but the size of the box should not be very large; it will cause the speaker to play below its free air resonance (Fs), and reduces the power handling capacity of the system.

Ports in the vented box

The port in a vented box should be installed on the same side of the box as the speaker. The gap between the port opening and the wall behind should be equal to or more than the port diameter. If the port is very long, one can try cutting the port approximately 1 inch from the rear wall, as it fools the speaker to believe that the port is longer than it actually is. This method, though not perfect, works quite agreeably.

The port opening should not be covered with cloth. The area in front of the port should be kept free. Larger ports are required for larger subwoofers to avoid whistling sounds. Ports with conical openings can be used too.

The air from the port pumps out powerfully in a correctly tuned box. If not, the box and port are mismatched. As long as the area and length are correct- the port need not be inside the box.

Here is an indicative list of suitable port diameters for different speaker sizes:

  8": 4 – 8 cm

10": 6 – 10 cm

12": 8 – 15 cm

15": 10 – 15 cm.

There is no rule to restrict the number of ports in a box. One can have as many ports as they wish, as long as the total port area remains the same.

Advantages:

• Cone movement and distortion are reduced after port tuning.
• Port tuning leads to higher sound levels.
• Improvement in the bass output.

Disadvantages

• Restricted cone control below port tuning.
• Possibility of higher frequencies "leaking" through the port.
• Sound from the port and the sound from the speaker cone can be out of phase.
• Trailing sub sound can seep in.

Sealed (Closed) Boxes

Building a sealed or closed speaker box is relatively easy. The size specifications are not strict either; they depend on the speaker data (Fs, Qts, Vas and X-max). Large speakers need larger boxes, two speakers need double the box size, and so on. One thing to keep in mind- is not to use a small box for a big speaker.

Designing a sealed box: few pointers

Sealed boxes are normally used for door-panels or kick-panels; they must be completely air-tight, and acoustic wool should be filled in them up to 75 – 100% of their volume. Most 4", 5. 25" and 6.5" speakers can be used in sealed boxes. Audiophiles listening to classical music, jazz and soft rock will appreciate the effects of putting a subwoofer in a sealed box.

Advantages:

• Can handle high power outputs
• Low frequency reproduction is extended in a sealed box
• Excellent transient response
• Easy to build
• Size specifications aren’t strict

Disadvantages:

• Need more power when compared to vented boxes
• Have lower efficiency than vented boxes

A suitable speaker box is essential to enjoy the music in all its glory; once you are used to it, it will be difficult to imagine how you lived without one in your car.