Many of the organizations that sanction sound-off contests also approve SPL or Sound Pressure Level competitions. These contests can be a lot of fun for both the participants and spectators. A judge places a microphone into a car and a meter measures how much Sound Pressure is present when a CD or tape is playing. The measurement is compiled in decimals and different types of weighing scales to determine the dB level. Each organization also has different rules dealing with where the microphone is placed in the car. What is so attractive to spectators is the booming sound and the number that results from that big bang. Everyone knows that the largest number wins and so it is easy to pick a winner.

No doubt, many lowrider enthusiasts are attracted to SPL competitions. And, no doubt, many of you would like to participate in the contests. You can do it and win, but you need to prepare your car and your sound system specifically to achieve a high SPL score. There are plenty of things that you need to do to put yourself in position to win-from preparing the car and choosing components that lend themselves best to achieving high SPL to figuring out the little nuisances of your car and you system to tweak out that extra dB that will result in you going home with the prize.

Once again, we talk to Isaac Goren, the owner of Sounds Good Stereo in Woodland hills California, and Tom Orozco, an installer at Sounds Good. Goren has been constructing SPL systems for years and he also created an SPL competition called SoundQuake. Orozco has been involved in the auto sound industry for 10 years and has spent the last five years heavily involved in SPL contests. He has judged SPL competitions for CMAA, USAC and dB Racing, and he has been a contestant in dB Drag Racing, IASCA, USAC, SoundQuake, CMAA and NACA SPL contests.

THE PRELIMINARIES

If you intend to become a participant in the wonderful world of SPL contests the first thing that you need to do is get a hold of the rules of each of the organizations that sanction these events. As previously mentioned, each group has different rules. You need to know what those rules are so that you can construct, tailor and tweak you SPL system within those rules.

The next concern is the car. If you are really into this and want to choose the best car the would help achieve the highest SPL, Orozco advised that you choose a small car. He explained that the SPL system is using its cone area to compress air. The smaller the area, the more compression that takes place and the higher the SPL score. For those of you who are not fanatics about this and wish to use the family car, thetas fine. There are things you can do to make the car right.

Next, there are modifications that you need to perform on your car. Goren noted that SPL requires a lot of current. Moreover, SPL requires a lot of voltage. The more current and voltage that you can get into the amplifier(s), the more power is put out regardless of the brand of amplifier(s) that you use. That means that you will need a second, third or fourth battery. One battery remains as the source of energy for the car. The other batteries provide juice for the SPL system. Goren cautioned that you isolate the car's battery from the SPL battery or batteries. This will avoid the car's battery for draining and you will be able to start your car and drive away after the contest.

Of course, the batteries will need to be charged and it is the car's alternator that provides the charge. The factory alternator, however, will probably be incapable of charging so many batteries under this condition. So, you will need to upgrade your alternator or add a second alternator. You should also upgrade the power cable. The larger the cable, the better the flow of current.

Before you install any piece of your system you will need to line your car with some kind of deadening material. Gut your car so that you can apply the material to the floor, firewall, trunk and headliner. You also want to use this material to get rid of rattles or vibrations.

According to Goren, this will make the car more solid. In addition, Orozco has seen people add two sheets of medium-density fiberboard to the door panels to close in the sides of the car by an inch and a half. These same people also cover the roof with three or four sheets of medium-density fiberboard. The idea is that this compresses the volume of the car so it is smaller. This means that there is less volume in which to compress the air and that means a high SPL score.

THE COMPONENTS

The radio/CD player or cassette will serve as the source of the sound. You will want a radio that supplies a high-voltage output to the amplifiers, said Goren. If you already have a radio and it does not offer a good amount of voltage, you can add a pre-amp line driver which can boost the signal coming form the radio to seven, eight, nine or even 10 volts. The higher the voltage coming from the radio to the amplifier(s), the better. He added that you will be required to control your sound system from outside the car. So you must be able to work the radio with a wireless remote control. It should be noted ere that SPL will damage your ears. So certain precautions need to be taken.

The next concerns are the amplifiers and subwoofers. According to Goren, the more amplifiers and subwoofers you use the better. You should choose amplifiers that are stable down to 1 ohm. You will be running you system at low impedance and you want to be certain that the amplifier(s) can take the abuse. To build in a little security for the amplifiers, Goren suggested that your system be set up so that the amplifiers sense a 2-ohm load. He noted that when you push for SPL, the impedance will go down below 2-ohms.

Having an amplifier that is stable down to 1 ohm means that it won't crash. Goren noted that when you put as much power into an amplifier as you do for SPL, there's a good chance that the amplifier will get very hot and go into clipping. Goren cautioned that even the terminals may melt. So you need to choose an amplifier(s) that can withstand the abuse. You may want to use amplifiers with built-in continuously variable crossovers as well.

As for the subwoofers, Goren pointed out that when a subwoofer creates the low frequencies for SPL, it will move a lot and go into excursion. When it moves it creates heat for too long some subwoofers will blow. He cautioned that you cant test many of the specifications that you see on the box that the subwoofer comes in. For example, the box may show that the subwoofer inside can handle 300 watts. But you don't know how the manufacturer came to that conclusion. In order to avoid being mislead, Goren suggested that you buy subwoofers from a recognized manufacturer.

Orozco added that you have a choice of three sizes of subwoofers. The 10-inch subwoofers will hit the higher frequencies, but wont produce SPL numbers. He guessed that the you will need about 30 to 40 10-inch subwoofers to hit 160 dB. The 12-inch subwoofers is the all-around sub, said Orozco. But it is usually not used for SPL because its cone area is too small to achieve the low frequencies that you want to get. The 15-inch subwoofer moves a lot more pressure, but it takes up a lot of space. He recommended that you use the 15s. And the more, the better.

The subwoofers should be loaded into a wall, which is a sealed enclosure made of 3/4-inch medium-density fiberboard. You want to make certain that the wall is strong; it will be taking a lot of abuse. A flat wall will result in a lot of cancellation, said Orozco. To avoid this and include the most amount of subwoofers as you can, you should design the wall in a V or W shape. The subwoofers should be pointing toward the front of the car. Goren added that you can include another series of subwoofers in a ported box behind the wall. The port can funnel the sound around the sides of the wall.

Goren added that you should include a control on the dash, pre-amp or radio that will permit you to adjust the subwoofers from the driver's seat. The control can work the subwoofers' amplifier gain, voltage or volume. This way, when you go into competition, you can adjust things to get the best results for SPL. But when you are driving home from the contest and you want some good sound quality coming from your system, you can adjust thing to a minimize SPL. If sound quality is very important to you, you may want to rig your system so that you actually dedicate subwoofers and amplifiers strictly to SPL. Include a switch on the dash that will allow you to switch off these dedicated components when you want sound quality.

Finally, Orozco suggested that you include an equalizer. Why will become more apparent later. There should also be capacitors in the system. The capacitors will store energy and release it quickly when the subwoofers boom out its SPL for five to 30 seconds.

TWEAKING THE SYSTEM

Once your system is installed, there are things you can do to tweak it so that it will achieve its best SPL. Orozco explained that you can figure out what your car is tuned to. You can do this with a tone generator. Hook up the tone generator to an amplifier or pre-amp using an RCA cable. You should also use a real time analyzer (RTA) to monitor the volume of the frequencies playing throughout the system. Mount the microphone of the RTA inside the car and generate a tone using the tone generator through the system.

The RTA readout will show a spike for a particular frequency. That is what your car it tuned to. You can then go to you equalizer and boost that frequency as much as you can. This will help you achieve better SPL. You can also use this information if you intend to include a second vented subwoofer enclosure behind the wall. Once you know what your car is tuned to, you can tune the vent to the same frequency.

Competitors have been known to also play with such things as the car's windows. Some have found that their systems perform better when the windows are closed or partially rolled down. They have determined how far the windows need to be rolled down to achieve the best SPL score. Use an SPL meter to test out your system to find out what you can do to get that added advantage. Remember, you need to wait an hour or so after each test that the amplifiers and subwoofers can cool off.

Again, you need to be aware that SPL will damage your ears. There are not ifs, ands, or buts about it! So when you test out your system, wear headphones. You can acquire headphones from a gun shop. Sound pressure levels in excess of 110 dB have been proven to cause permanent hearing loss. Please use common sense in determining listening levels and wearing appropriate ear protection.