Do you want the best performance from your subwoofer? You'll need to filter out sounds it was never intended to reproduce. You don't want vocals and guitars coming from your subwoofer driver. You don't want your subwoofer bottoming out from playing too low.
We can do these tasks with some simple filters. These filters are likely found on the amplifier.
We're going to explain how to use two basic and inexpensive tools to set your filters like a pro. You'll need a digital multimeter and a tone pack. This is an old school approach. Pros have used this method for 40 years to produce consistent results.
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Step 1: Gather Your Tools
You are going to need two tools. You will need a digital multimeter and a source with tones.
Modern car audio systems have Bluetooth and it is fine to use your mobile phone as a source in that case. Most of the music services have "tone packs" and "set up tracks" available to play. I know they exist on Amazon music and Youtube music, at least.
If you're connecting your amplifier to an older deck with a CD player, you should use a test tone CD. You can make your own tone CD from MP3s or you can order one from Amazon.
Step 2: Determine Frequencies
Determining filtering frequencies is part art and part science. We can't set up "hard and fast" rules with our subwoofer installations. Each installation will be different. Some installations will sound fine with a low pass filter at 100hz. One could push this lower if the installation has large midbass drivers upfront.
There aren't hard and fast rules on low pass filtering for our subwoofer. We need to make adjustments to the filter to let the midbass speakers shine.
Since this is a teaching exercise, we'll make some assumptions. We'll assume the front speaker installation uses 6.5" drivers, which should play down to 80hz. So we want to set the crossover on the subwoofer to 80hz to start.
With the subsonic filter, we have a good "rule of thumb": half an octave below port tuning. If we tuned our ported enclosure to 31hz, one octave below that is roughly 16hz. We can set our subsonic filter to 24hz. This will protect our woofer from unloading at low frequencies.
Step 3: Determine Voltage
To keep the math simple, we're going to use a 100-watt amplifier into a four-ohm load. This will make it easy to compute our target voltage.
First, we have to compute the voltage at 0db. After that, we will need to compute the target voltage at -3db.
Ohms law: Volts = square root of Watts * Resistance
Volts at 0db = square root of 100 * 4
Volts at 0dh = square root of 400
Volts at 0db = 20
To get -3db, we need to compute volts at half the power for the same resistance.
Volts at -3db = square root of 50 * 4
Volts at -3db = square root of 200
Volts at -3db = 14.14
The above assumes you determined 100 watts. There are means of setting your gains with distortion detecting tools. You'll need to compute the voltage after setting the gain. Disconnect the subwoofer from the amplifier's outputs and play a 40hz tone track. Then make a note of the voltage at the amplifier's output. If you measure 20v, then you know it's 100w into 4ohms. If you measure 63v, then it's 1000w into four ohms. Use this measured voltage to determine your -3db target voltage.
Step 4: Set The Filters
First thing's first: make sure you've set the amplifier gains properly.
Let's get to tuning!
- Disconnect the subwoofer leads from the amplifier.
- Set the amplifier's low pass filter to its highest setting
- Set the amplifier's subsonic filter to its lowest setting
- Start playing a test tone whose frequency is equal to the desired low pass filter setting, say 80hz.
- Probe the outputs of the amplifier with your digital multimeter, set to AC Volts
- Turn the amplifier's low pass filter down until the voltage reads your target -3db voltage
- Start playing a test tone whose frequency is equal to the subsonic filter setting, 24hz.
- Probe the outputs of the amplifier with your digital multimeter, set to AC Volts
- Turn the amplifier's subsonic filter until the voltage reads your target -3db voltage
- Reconnect the subwoofer, testing for proper operation.
Final Thoughts on Amplifier Tuning
There are many tools on the market that take some of the mathematical work out of tuning your car stereo. These tools are great if you're a professional who installs several amplifiers a week. These tools are not necessary for the hobbyist.
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