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Audio Physics 101 - Transients Part 2


Promoted to article from the 'SQ and Technical' board of the forum, as it needed keeping for posterier-ty. Crisly written by Black Cat, who works in our world.


In the second part of our look at transients we will look at the system requirements needed to get good transient reproduction. Many people ask about how to get fast accurate bass from their sub. It's a bit more complex than that and actually it needs more than just a sub.

Many thanks to Puggie for supplying a nice uncompressed and ungated kick and snare drum track. These are about as close to a transient as you will find in music and make for nice clear diagrams.

The setup for this test is pretty simple. Using my PC to play out to a Behringer ultradrive crossover and recording the results.

I have made the following settings:
2 way active + sub
Sub crossover at 80Hz using Linkwitz-Riley 24dB slope
Midbass 80Hz to 3kHz again using Linkwitz-Riley 24dB slopes
Tweeter 3kHz using Linkwitz-Riley 24dB slope.

This equally applies to using a passive midbass/tweeter setup.

Lets start with the kickdrum:
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Kick Drum

As you can see thats a nice thump with a little bit of tail-off.

So lets look at the output from the crossover.

The sub output:
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Kick Drum Sub

As you can see it's been rounded off and the amplitude has been reduced quite significantly. If you listen to the filtered output it is now quite wooly and has lost all of the 'snap' you expect from a good kick drum.

The midbass output:
Posted Image
Kick Drum Midbass

Here you can see a much more jagged waveform as the midbass is playing a very wide range of frequencies. You will also note the amplitude is higher than the sub output. This shows just how important the midbass is to reproducing good music. The other key is the transition from sub to midbass.

The tweeter output:
Posted Image
Kick Drum Tweeter

Just to round out the system here is the tweeter output. You might think it a bit odd, needing a tweeter to reproduce a kickdrum but it's true. It may be a small waveform but it puts the final edge to the sound we're trying to reproduce.

So what does this all mean in reality? If your bass sounds slow there are more factors than just your sub that may be at fault. You can hear quite clearly that the sub output sounds wooly by nature, so what is the fault? The most common cause is simply down to the waves from the sub and midbass reaching your ears at the wrong time.

The most common causes are:
* Incorrect phasing of speakers, either by wiring or crossover selection (more on this later)
* relative distance between speakers is large (sub in the boot, midbass in the door)
* high group delay from your sub enclosure

Group delay affects all speakers but it affects large displacement subs the most. In a nutshell it's a phenomenon where different frqeuencies produced by the speaker are delayed by different amounts. All enclosure simulation software will show group delay. If you're using a sealed enclosure, the group delay is generally very low (only a few ms) so can pretty much be ignored. Ported and bandpass enclosures can suffer from very high group delay depending on how you tune them. Delays in excess of 50ms are not uncommon. Box design is a whole topic on its own and many others have covered it before. EF Max for one.

The distance beween speakers can be tuned out to an extent when using modern digital processors using time alignment. Basically delaying signals to different speakers so that all the waves reach your ear at the same time.

There are more basic methods such as the old trick of swapping the phase of all the speakers on the other side of the car to get better midbass. i.e. on the driver's side wire + on the amp to + on the speaker, on the passenger side wire + on the amp to - on the speaker.

It's worth noting that when setting up an active system, wiring + to + everywhere is quite often not the right way. However there is no set rule as it's all setup-dependant.

And while we are looking at crossovers, here is a visual demonstration of the differences between crossover slopes.
Posted Image

This shows a key difference between using 24dB and 12dB slopes. The last waveform is half a cycle delayed. 12dB crossovers are quite common as they are simpler and cheaper to produce. But when setting up an active system using such crossovers it's worth noting that you may need to switch the phases of some speakers.

Most sound check CDs will have suitable test tracks. Such as the 'my voice is in phase, my voice is out of phase' or tracks using drums. The best way to set up midbass is to just have the midbass speakers playing, in one phase they will sound wooly and the other phase they will sound sharp and give a kick. There is no need for an RTA, your ear will tell you when it's right. If in doubt, listen to the sounds on headphones and compare.

In conclusion, a sub with low group delay will help but if you want fast, accurate bass you need more than just a sub and to spend some time on set up.

Happy listening!


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