Re: how do figure out the rise of my box
Almost completely plagiarized from our friends at Digital Designs:
How to Calculate Amp Power and Impedance
The Volt-meter/Multi-meter test leads should go directly to the channel (+ and -) to test.If you have a mono-block amp with two pair of outputs and both are being used, just pick a – and pick a + to poke your test leads into. Set your Volt-meter/Multi-meter to AC Voltage and turn on the Peak Voltage feature. If your meter has a Range Selection feature, set the range to display only one digit to the right of the decimal point. We are dealing with Volts rather than mVolts when testing amplifier output.
The Ammeter/Clamp-meter should be clamped around only the + speaker wire coming from the amplifier outputs. Set the clamp meter to AC Amperage. Turn on the Peak Hold feature. You do not use the test leads of your Ammeter/Clamp-meter in this process.
When you test your system for output power and impedance at one specific frequency, you must test using that single tone. I use the note I burp at. If you want to know your impedance curve and power curve, you must test using a list of tones from 70Hz down to 20Hz in 5Hz increments; i.e. 70Hz, 65Hz, 60Hz, etc. This gives you a great understanding of what power the amp is making and what impedance the amplifier is seeing at that specific frequency.
The easiest way to test is with a pal; one guy to operate the source unit and the other to verify that both readings were taken and que when to start and stop the test. If you want power and impedance numbers for a daily driving system, we suggest warming up the subs and amp to get readings congruent with how you actually play the system, SPL competitors might take readings with coils/amps at room temperature.
Now put both meters in place and set them correctly (as pictured and outlined previously), burp or roll up the volume at the head unit that is just under clipping indication on the amplifier (as relayed to you by your friend). The Voltage and Amperage numbers are simply multiplied together to get your Wattage. (P=VI or Power equals Voltage times Amperage).
To find out what impedance your amplifier is seeing, simply divide the Voltage by the Amperage. (R=V/I or Resistance = Voltage divided by Amperage). If you want to plot a power or impedance curve, repeat the previous test for each frequency, i.e. 70, 65, 60, 55, 50Hz etc. Most standard or manufacturer recommended enclosures will lend a pretty substantial rise to the average impedance of your subwoofers, which means that the amplifier will push significantly less power than what you might expect. This is what is known as "box rise".
The commonly used term 'Box Rise' is the sum of your subwoofer(s) impedance and the additional acoustic impedance presented by the enclosure and the vehicle. You may wire 4 subs to a 1.0 ohm load on your amplifier, play the system and find out that your “Box Rise” is actually 4.5 ohms. That 1000w amplifier is now pushing between 200w and 350w! That does not mean that your car is not loud-as-hell; you could be breaking crap off your neighbor’s wall.
An informative experiment would be to test your box as it sits and then push it against the back of the trunk or cargo area…maybe 1” from the port opening and test it again. Your numbers will come out way different due to changing acoustic load presented by the new location. Moving the enclosure around the interior can greatly affect the SQ and SPL of your system.
--My truck: 2007 GMC 1500 crew cab, Kenwood DNX570HD, DLS front stage, SQ3500.1, 3x RD Alpha Max 15"s in 6th order blow through, Second Skin deadening.
--Wife's car: 2013 Chevrolet Impala LTZ, Kenwood DNX470, GermanMAESTRO components on CT125.2, 2x AT10"s on an AT800.1, RF Deadskin deadening.
--Work truck: 2014 GMC 2500 crew cab, Kenwood DNX470, Kenwood front stage, DD M1A on a pair of American Bass XD10s.