How to fight Ohm Rise? (And testing for SPL in general)

[Redline Audio Performance]

So one of the guys has a 1508 in a 1CF with spec Port, We Clamped it today and were seeing 8 ohm's in what way should we change the box to get it lower?

Thanks

[JaniU]

He is competing in SPL with one 1508?
If not, then why do you care about impedance rise...
You might want to tell how it's tuned/where it peaks. You of course found the peak freq. and tested with that... (and better tell the whole testing process)
Someone might even like to know in which kind of car it is etc...

[Team BK]

A little Krazy 8's coming back on the scene??

[Redline Audio Performance]

Box is tuned to 40, Car Peaks at 54 with our best score of 140.9

He will be in MECA AS1 Car is a firebird.

[DD staff]

If you want to lower the impedance at a specific point; use a smaller enclosure. If you check around the world, the SPL competitors in the street-style classes use small enclosures. They are normally way smaller than the standard enclosures that DD recommends. You are not after a bandwidth, but rather a spike. For the impedance to stay close to DCR, you have to work with a smaller enclosure and start playing with what size and length of ports is best for that particular vehicle and the frequency it works best with. If you'd like a little insight on that, you can contact one of many folks out there. One person we know that tests and understands what is happening, rather than just guesses is Brad Firgard of dB Drag's Street B and Street C world championship notoriety. He no longer competes with his own vehicle, but stays up on some of the new techniques and knows how to test them methodically. He can be contacted by calling Digital Designs on Tuesdays or Thursdays. 405-239-2800. There is a limited amount of time he can spend helping an individual, as his duties there span more than SPL competitor tech questions all the way into R&D and production efficiency process evaluations.

Make sure that you atleast know what you are doing and exactly what question you need answered. Get ready to receive AN answer with directions to do A test. He can't give you 50 minutes worth of information and suggestions and expect you to remember 10% of it. One question, one answer with one set direction to help you move forward. Expect to do things in steps. As soon as you guys get the USB device connected to your hard drive driven brain, we'll just upload it all to you.....until then, you've got to do it in steps. He's good enough at going through things with folks so that even the most novice guy can learn fast and correctly how to test and understand a system.

We get calls from folks that insist they need to start off with a pair of 3kw amplifiers that really don't need to be allowed within a mile of a 1kw system.

If you want to have anything to do with SPL....here is a quick checklist of MANDATORY equipment.

SPL mic, SPL testing program, Laptop or PC that runs the program.

Paper and pencil with eraser. The bigger the paper, the better...there are many test columns to fill in!

3 digital Volt meters with appropriate ends that will secure a good connection before, during and after a test burp or run. One is for Battery Voltage readings@ the battery both before and during the run to log the battery voltage drop from maximum float voltage to the minimum voltage during the entire test process. The second meter is to measure the same thing AT the input terminals of the amplifier. This will help you to understand the relationship your amplifier(s) have with the transmission system you have to deliver the magic from your battery(s) to your amplifiers. Anything and everything between a battery and its load has resistance. That resistance lowers the voltage seen at the battery terminals. These two meters will help you to set up a more efficient delivery system. The third voltage meter is to be set on AC voltage and secured to the outputs of your amplifier(s) to determine the output voltage of your amplifier. You CANNOT determine how much power (wattage) your speakers are receiving without knowing the actual impedance of your subwoofer system in that specific set of variables without the AC voltage reading.

1 Clamp Meter or ammeter with clamping device. This is clamped over the speaker wire(s) coming out of ONE of the amplifier outputs; either + or -. This meter is to be set on AC amperage so you can later do some simple math to determine system impedance under those very specific set of variables.

Calculator.....for obvious reasons.

Friends. You want to have one per meter if at all possible. Don't pick friends with ADD....you want these folks to actually help you, not distract you or distract themselves and render an entire test useless and invalid. This means that I cannot help you, because I have more ADD than the entire ADD population combined (subtracting myself from that population of course). Why does the Abbreviation for Attention Deficit Disorder come out to ADD as in "in addition to"......ADD damn sure doesn't add anything positive!

Now, armed with your test equipment, pencil, paper and friends, you can jot down any measured or calculated information, such as:

Battery voltage before burp/test.
Battery voltage minimum during burp/test.
Voltage drop between the battery and the amplifier inputs.
Output AC voltage during burp/test.
Output AC amperage during burp/test.
Calculate system impedance during burp/test.
Calculate wattage.
Plot impedance graph which will help you find the efficiency curve of the system (with more math).
Seat positions (if applicable).
Head unit volume setting.
Amplifier gain setting.
Xover settings.
Changes in box location/position.
Any other interior changes.
Any exterior changes such as pushing/supporting.
Temperature changes (if you have the equipment).
Barometric pressure changes (if you have the equipment).
Altitude changes (if you have the equipment).
Name changes (if recently wed).
Neighbor comments (if your vehicle is actually loud enough!)
City citations (which help you determine the projected SPL or seismic projection of your vehicle).
Lost hours of sleep, sex or paycheck drops (due to spending x amount of hours on your vehicle).
Nerdiness level increase (if you have the equipment).

Feel free to add to the list! LOL.

/supersteve

[ddshredder]

now thats a mouthfull...lol.... and right from the horses mouth..... i will say i like the way your on the forums answering questions alot more..... nice improvments guys.... every bit helps.....

[Redline Audio Performance]

Nice wright up. I can handle all of it.

I have all of the equipment, including the Termlab.

So To start we should fully document the current setup and then drop the box size a bit and run it again.

[2003F150]

HAHAHA Steve funny stuff...well some of it ....some is very true. SPL is all about testing, testing, testing and then more testing. I have never actually took the time to take spl this seriously. WOW!!! Brad is a lucky man to be able to work with the guys/gals at DD.

[Charger]

Kickass post Steve got a few chuckles from me too :P

[Redline Audio Performance]

Don't know why you guys are Laffing every bit of it is true.


We know we have built a good system when the nebigor behind and two to the left calls and asks us to turn it down, Stuff was moving on her Kitchen Table.

[1508NC]

sorry matthew i will turn it down when i pull in from now on

[2003F150]

Some of the shit Steve says is funny as hell..I was laughing at the ADD part...

[Jeremiah25]

Steve......you left out the case of beer..............it always worked for us.

[SilverSPL]

Quote: Originally Posted by DD staff 1 Clamp Meter or ammeter with clamping device. This is clamped over the speaker wire(s) coming out of ONE of the amplifier outputs; either + or -. This meter is to be set on AC amperage so you can later do some simple math to determine system impedance under those very specific set of variables. /supersteve

Great writeup steve, just one note to anyone getting a clampOn amp meter. your gona want a true rms clamp meter if your measusing any other frequency besides 60htz because your reading will be off. true rms meter can detect what frequency is going through the wire and give an amp draw reading based on that frequency.
I have the fluke 337 and it does both ac and dc amp draw readings and is true rms

[SPL15]

When I made SPL boxes, I used to go the opposite, bigger box tuned higher (50-60Hz) for more efficiency with the medium power levels I was running. If your running stupid high power then smaller box is better to get the peak efficiency. the smaller box helps keep a load on the sub to handle the power. If your running stupid high power then your ports become less linear in how they operate due to velocity of the air and non linear compression of the air. Smaller boxes help to keep the pressure behind the sub to keep it from unloading as much when this happens (well that's the overly simplified reason of why smaller boxes work better)

My personal philosophy is bigger box tuned high. You get better efficiency with a larger box and the impedance curve is more predictable, but it also has LOTS more peaks.

Your tuning frequency of the box should be tuned to the peak of the vehicle. The way I find this is to run my computer subwoofer in the car which is a TC sounds sub in a small sealed 0.5 CU FT box. The box is small enough that the bass region is all below the resonant frequency of the box/sub. This gives a perfectly predicitable impedance and with active equalization it gives a completely flat response to 15 hz - 80 hz in open air measured in my back yard. It however is extremely inefficient so it mates well with the 1400 watt QSC pro audio amp it's powered with. The reason for all of this is to see what the car is adding to the sound. If you put a perfectly flat subwoofer with perfectly flat frequency response in the car, you can see what your car is adding to the bass. Also by looking at the impedance curves of the sub while in the car, you can judge what frequencies add or subtract to the loading of the sub in the car. Also since the sub is small, you can position is any which way to see what difference angle, position, etc have on the bass response. Usually, the peak frequency of the car coincides with an impedance oddity with the sub. This poorly describes how I do it, but It works well to find out exactly what your car does to the bass response. However, when you start getting to stupid pressure levels, things become MUCH more unpredictable due to cabin leaks, panel vibration leaks, and even atmospheric pressure differences in these cases, testing and measuring over and over again is the only way to get that perfect edge.

With ported boxes playing in their linear range, the impedance will dip to near DC resistance levels at the tuning frequency if the port is large enough to stay linear. On each side of the frequency band before and after the tuning frequency you'll have a massive peak in impedance around 10 to 40 to even 50 ohms depending on how your ported box is aligned. Avoid these areas because well, your amp wont put out as much power into 10 to 40 ohms... Also, after the peak above the tuning fequency, the impedance dips back down to more normal levels but not quite as low as the tuning frequency. This dip is much larger on larger boxes. After this dip the impedance slowly rises back up to 10 to whatever ohms do to regular coil inductance.

For maximum power delivery and efficiency of the box/speaker make your tuning frequency the frequency which you burp at. This does two thing, it reduces excursion of the woofer as the port loads the rear of the speaker 180 degrees out of phase and it also has the lowest useable impedance letting your amp put maximum power to the sub. This frequency is the most efficient frequency that your box/speaker will play at. It can get loud with other frequencies, but it is more a function of the cabin gain, not the box efficiency.

Also of minor importance, the port will reinforce frequencies that are an even harmonic of the tuning frequency more than the other frequencies. However, its not that much more reinforcement and can usually be ignored.

Best scenario would be to tune your box to the frequency your car excites at. However, all the calculations in the world wont get you peak efficiency because as soon as you load your box into your car and slam 3K to 4K or more watts to the sub, things change. The calculations will get you close, actual measurements and UNDERSTANDING of these measurements will get you there. I had one car where I would measure where I needed to be, built the box to do what I had previously measured, and then put it in the car and found out things shifted because of the displacement of the actual box and proximity of the ports to walls and the change in tuning frequency of the box and loading of the sub... I did this measure, build, then test sequence 3 times before I got it right, each change of the box changed parameters in the car which shifted the peak efficiency frequency each time.

With this system, I hit 153.3 to 153.4 at 44hz every time in a station wagon with one 9515D on one MMATS D200HC in an 8 cu ft box tuned to 44 hz for competition. That's pretty damn good for 1200 watts and one 9515 with big 3 done on stock 90 amp alternator electrical system. You can either slam several kilowatts of power into your subs on a marginally efficient system, or you can tune for absolute peak efficiency and get good numbers with less than 2000 watts.