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-   -   Ported mains / Sealed sub (http://www.diyaudio.com/forums/multi-way/1604-ported-mains-sealed-sub.html)

Javaman 27th December 2001 02:48 AM

Ported mains / Sealed sub
 
Noticed that Elliot Sound Products has a (sealed) ELF EQed sub project where he makes a big point of warning not to use it with ported main speakers due to 'unpredictable phase response'.

Wouldn't this be the case with any sealed sub and ported mains? My main speakers are ported - should I stick with a ported sub design?

If a ported and a sealed enclosure receiving the same signal are out of phase then clearly one of them has an accuracy problem, no?

Super 27th December 2001 03:25 AM

I would tend to agree that there would be an accuracy problem. However, I tend to disagree with Mr Elliot, in that the phase response of a ported or sealed system can really be categorized in this manner. I've heard sealed subs that dont match up with sealed mains, but do with ported mains, and vice versa. Come to think of it, I've yet to hear <i>any</i> sub integrate seamlessly with a set of mains without having phase correction...

newmz 27th December 2001 04:55 PM

It seems to me (without knowing enough to be sure) that any ported enclosure can be potentially a big phase problem, since you are getting some sound pressure waves from the port that are opposite in phase to the radiation from the fronts of the cones. Is that what the porrt does though? - Invert the phase of the tuned frequencies? Iisn't it possible to use the Linkwitz Riley 4-th order type crossover so you're phase problems are minimised? I don't understand the concepts very well but I know I keep reading about how they are "phase coherant". I also read that the "jury is out" as far as just how much phase is percieved by us when we listen to complicated audio like music. It's obvious when I run a sinewave test tone through my speakers but I don't think it bothers me so much in music, except when the phase problems are in the lower bass ranges.

GRollins 27th December 2001 09:56 PM

You get into phase problems in at least three unrelated ways:
1) Absolute phase. When the drum head moves towards the microphone, does the speaker move towards you or away from you? After years of 'golden ears' insistence that it was audible (and scorn from those who thought it irrelevant), they finally proved that it is, indeed, audible. The effect is subtle and not of importance to most people.
2) Relative phase. Are the speakers both moving towards (& away from) you at the same time? This one is so obvious that you'd have to be seriously hearing-impared (i.e. deaf, for those of us not inclined towards being politically correct) not to be able to tell the difference between the two options.
3) Phase shift. As an example, an instrument with a very broad frequency range, say, a pipe organ, plays a note. Do all frequencies arrive at your ear at the same time or does one frequency extreme arrive in advance of another? This can be readily demonstrated on an oscilloscope using something like a square wave. Hearing it is a more suble process.
A number of people confuse these three. I'm just making sure that we're all reading from the same page.
Bryan is speaking of phase shift when he's speaking of electronic phase correction for subs. His point is a valid one.
The phase shift through the port in a speaker is a tricky item, as it varies with frequency. Another ugly variable is the positioning of the port relative to the driver. At some frequencies (hence wavelengths) you'll get cancellations, yet at others you'll get reinforcement. Use the same cabinet, the same driver, in fact the same everything, but change the physical location of the port relative to the driver, and you change the speaker.
It's enough to drive a man to drink (I've got my keys, you ready to go?).
A point that tends to get lost in the shuffle is the Q of the (sub)woofer. Critically damped (i.e. .707) is a maximally flat Butterworth alignment. Bessel will give you .5, etc. The thing is that some people go for a .5 alignment because it 'gives you more bass.' Hmmm...well, yes...and no. It rolls off more slowly, but begins rolling off sooner (i.e. higher up). A properly designed .707 cabinet will roll off with a sharper slope (which some chose to interpret as 'less bass') but stays within a tighter range in the pass band and the rolloff begins at a lower frequency. Your choice. But there's a catch. Going for a Q of .5 to get 'more bass' means a compromise--the bass you get is less tight. You want <i>really</i> tight bass? Try a Q of 1 or more. Of course, now you've got a pronounced hump just before rolloff, but hey, you wanted tight bass.
The stereo gods giveth and the stereo gods taketh away.
Balancing frequency response against tightness is part of the 'art' of building speakers. Just be aware of the tradeoffs.
There's no law that says that you should use ported with ported, sealed with sealed, or ported with sealed.

Grey


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