General Car Audio Questions

[MunkeyQ]

Hi all. I'm new here...got a few questions about car audio. Not to sound big headed, but I've got a pretty damn good knowledge of speaker design when in comes to home/PA stuff but there's a few things I'm not sure about when it comes to applying it to car audio. It's something I'd like to learn more about.

1) Cabin gain - is it normal to calculate the transfer function of the car's interior and design the sub's box to compensate for the increased output below the function's fundamental? I'd imagine it would help...I just don't know whether it's that necessary as it doesn't seem to be commonly done.

2) Power ratings - what brands are most honest? I've benched a couple of supposedly decent Kicker amps into a dummy load with pink noise, and THD just got silly at about 2/3 rated power output. The dummy load was a creation I made to emulate a reasonably nice speaker to drive - not very inductive and mainly resistive. Hooking up the nasty dummy load resulted in the amp shutting down at half power, but there again it's a bit of an inductive bastard to drive with a very reactive impedance function. About as horrible to drive as Infinity's IRS series. Note that both loads are 8 ohm nominal, and the reactive load was corrected for phase by multiplying by the cosine of the phase difference between current and voltage.

Shoving 500w RMS filtered pink noise into a Kicker sub supposedly rated for 1000w resulted in lots of smoke - this was in a sealed box tuned to the driver so it didn't unload and over-Xmax. I'm guessing it's just like the home audio world...some brands are more honest than others. Speaker of Kickers, I do like their Solobaric concept though...pseudo-isobaric alignment in a single drive unit is very interesting. I need to get my greasy mitts on one to play with.

3) Amp classes - I've noticed that the Class AB designs have rather small heatsinks compared to what I'm used to. Any idea typically what point bias shifts into Class B operation for, say, a 200w amp?

4) Crossovers - why aren't active designs more popular? With the setups which have many, many drivers, maintaining absolute phase at crossover frequencies would be easier with an active...and you'd need less power too.

There's a few more questions I can't remember right now - I'll add them later.

Thanks for the help!

[Bunker]

1) I can only name a hand full of people who understand what the up front bass effect really does as well as corner loading many people don't look for it they just throw it in and say were done, if we need more we will just add more power. Oh and you have the horn effect in hatchbacks but most won't get the full output of their design because they don't understand the two effects. All effects of cabin gain.

2) You pay for what you get in any amp.

3) I have no clue.

4) Depends on what you are looking for.

[Boon]

1. I usually only get in-depth with cabin gain and transfer function when building an SPL car where every tenth of a dB is critical. Otherwise I tend to err on the side of caution when designing a box - any extended frequency response thanks to the environment is a bonus (and generally assumed )

Furthermore most people aren't looking for a flat response

There's also the fact that due to most car audio drivers rolling off at about 35hz any environmental gains below this point are very helpful if you're trying to play low with a relatively small enclosure.

2. Are you sure you weren't looking at max ratings with both the sub and amp? There's a lot of difference between max ratings and RMS ratings, often you have to climb into the fine-print to find the true ratings.

Also most of them are rated at what is optimal for the product - pink noise will always give shitty results, most full range amps are done at 1khz/1% (or 5% or even 10%!!!) THD on a pure sine wave.

A '1000w' sub will often have a real continuous power handling of about 300wrms.

There are a few brands where the opposite is true and they are very conservatively rated, eg my DD9515 is rated at 1500wrms but I have given it 4000 clipped watts before and it took it like a champ.

3. AFAIK Class AB car amps don't shift their bias - they run in push-pull operation at any level of output. Each channel has 1 or more complimentary pairs of NPN/PNP transistors, each handles half the wave regardless of output - there is only a very minimal amount of bias in a Class AB amp, I think it's basically just enough to avoid the transistors ever completely turning off (and the associated noise involved)

With regards to heatsink size, given that the entire chassis acts as a heatsink there is actually a pretty decent amount of metal considering the parts in the amp. A good 2x100wrms amp will probably only have 4 output transistors and maybe 8 or 12 power FETs yet will usually have a relatively large chassis.

4. Crossovers... I think active is less common because passive has such a huge degree of simplicity to it. Active is easy enough to do badly (especially in an environment as bad as a vehicle's cabin) so for simplicity of wiring and installation most speaker sets use passives. There's also considerations such as sufficient amplification - both in terms of power and available channels. A 4x50w amp will run actives nicely but if you bridge it into passives you'll get about 60% more power to each side which usually appeals more to people (despite the losses inherent in a passive crossover, it's numbers on paper that count to most people)

Last of all you have the difficulty vs benefit problem - 99% of people are running their setups with totally different path lengths, half the speakers are on axis, half are off axis and there's a whole lot of horrible reflective surfaces all over the place so even if you keep perfect phase all the way to the drivers your waves are going to get all mish-mashed regardless