info on phase switch please

[HTADDICT]

The phase switch seems like a bonehead thing but....

I know it is primarilly used to tailor your sub to your room, but what does it actually do. 3 questions

1.What does it do when you switch it from 0 to 180, I know it's to keep it in fase with the rest of your speakers so you don't cancel out but how does it do this?

2. If 180 fits the room best would it always be the best or would 0 suit different materials better (say 90 persent of cd's sound best at 0 10% sound better at 180 same for movies. also would movies sound better at 180 and music at 0 ? Stupid questions huh

3.If your running multiple subs in different locations could one sound best at 0 and the other at 180 just because of room placement or are you better off at setting them the same?

I just got my ksw15 to go with my 12 and started to wonder about this, I just set mine to what sounded best and left it but was thinking that maybe I should put some more thought into it.

I'm sorry if i'm insulting some of your intelligence but I learned at a along time ago to ask questions no matter how stupid if you don't know the CORRECT answers. Read alot about other stuff but the phase switch doesn't usually come up (maybe cause it's a no brainer?)and would just like some more knowledge on it. So if you guys know more than I do would like some info on it.

This message has been edited by HTADDICT on 07-19-2001 at 11:14 PM

[boa12]

ht, keep on askin til u start askin ?s nobody can answer

1) switching the phase to 180 is like switching the leads on the neg/pos terminals on a speaker. iow based on the same signal the sub cone then moves out whereas at 0 it would be moving in. the idea is to invert the bass wave from your sub in case u are getting cancelation w/ your mains &/or other speakers.

2) should not matter what material you're playing. it's a matter of the speakers' properties, positioning,

& their physical environment.

3) bass waves are much affected by speaker locations & room characteristics, so the overall bass may sound better w/ one sub at 0 and the other at 180. that's a good ?. 1st time i've heard it asked. you'll need to experiment & may need two helpers to switch while u listen.

------------------

Klipsch KLF 30 (front), KLF C-7, Cornwall I (rear)

Velodyne HGS-18 sub woofer

Monsterbass 400 sub interconnect & Monster CX-2 biwire & Z-12 cable

Marantz SR-8000 receiver

Sony DVP-C650D 5-disk cd/dvd player

Sony Trinitron 27" stereo tv

Toshiba hi-fi stereo vcr

Technics dual cassette deck

Scientific Atlanta Explorer 2000 digital cable box

Boa's Listenin Lounge:

Klipsch RF-3 (front), RC-3, cheap little Technics (rear)

Monster MCX Biwires

Sony STR-DE935 a/v receiver

Kenwood KR-9600 AM/FM stereo receiver (vintage 1975)

Russound AB-2 receiver switch to RF-3

Teac PD-D1200 5-disk cd changer

Technics SL-1950 turntable/AT LS500 cartridge

Sega Genesis game player

Sub: None yet

rock on!

[WMcD]

This is a bit of a simplification.

But consider what happens when the sub and the rest of the bass system cross over at 100 Hz. The wavelength there is about 10 feet. Half a wavelength is 5 feet.

Therefore, if the sub is 5 feet more distant than the woofers from the listening position, it is half a wavelength different in phase, at that frequency. This means the 100 Hz acoustic outputs are cancelling. If you reverse polarity with the switch, they are back "in phase".

This is simplistic because it only addresses that exact frequency and that exact difference in distance.

Room acoustics, the roll off rate, crossover point, actual spacing all conspire to make it a more complex set of interactions. Therefore, it is important to experiment with "phase" (actually polarity) and placement of the sub.

If you've got exactly the right problem, the switch of polarity might solve the problem. Otherwise, you might have close to the right problem, and it is an improvement, or maybe makes things worse overall.

In short, it gives you an extra tool to work with, without being a universal cure.

This shouldn't be specific to any type of music. However it is possible that a given combination of factors (polarity, cross over point, speaker locations) produce a good result at one point (say 100 Hz plus) in the bass response, and a bad one in another (say below 100 Hz). Therefore, music heavy in bass in one point might sound better or worse.

Klipsch FAQs point out that one way of reseaching the placement of the sub is to put the sub at your listening position, and then walk around the room to find a place where the bass sounds optimum (whatever that is for you). Then put the sub at that location.

In theory, this can't take into account the effect of bass from the mains, and the interaction which the polarity switch is supposed to address.

It seems like you're using two subs. That injects another level of complexity for which there are single guidelines. I'd definitely say to keep them both set to the same polarity. As an experiment you might try putting them next to each other and switch one to the inverted mode. You should recognize a marked reduction in bass.

You might find that putting both subs in the same corner gives lots of bass, maybe too much. Putting them in different corners might be too much too, but worth trying. Moving them to a central position on adjacent walls (e.g. left and back) might be a good choice too. No one can tell without experimenting.

The final issue with two subs is that common wisdom is that people can not locate bass below 120 or 80 Hz, meaning there is no "stereo" effect for bass. To get stereo bass it would have to be in the mix to begin with and you have to have the subs feeding off left and right outputs of the amp. Wendy Carlos claims to be able to detect it. But that assumes it's in the mix to begin with. Given Carlos's credentials and experience, who am I to argue.

Gil

This message has been edited by William F. Gil McDermott on 07-20-2001 at 07:18 PM

[HTADDICT]

Good point on the distance. SO assuming that you put the sub in the listening position checked the room for optimum sound and placed your sub accordingly. by the time you get it ther it could be out of phase? Would there be a different sweet spot if you switched from 0 to 180?

Right now I have my 15 up front and 12 in the rear, nothing dialed in just placed where they look good.

What I plan to do is this, place sub in listening position find the best spot in front of my room with the 15 and best spot in the rear of my room for my 12. But the dillema would be that by the time you would get them there they would possably be out of phase with each other or the fronts right, therfore you would get a degree of cancelation so next step would be to start flipping phase switches again. But if my question about there being a different sweet spot from 0 to 180 holds true then flipping the switches would eliminate that sweet spot alltogether.

So would a better way be to try each setting and find each sweet spot and mark them so that if you did have to flip a phase switch to correct it you could move it to the according optimum spot. I know this is starting to sound like a riddle but hey riddles are fun I'm just trying to think it out what would sound best on paper, a guidline for placement so to speak. There is more to this I know but someone is going to read this and say hey I think it should be this way and we'll have an answer (maybe)It'll be a lot easier I know if my answer as to wether there is a different sweet spot between 0 and 180 is no but I think there would be as the sound waves would be altered.

HMMMM riddle me this riddle me that. Anyone?

[boa12]

ht, i'd just leave the phase on 0 for the sub positioning tests, then when u have them placed best, get a helper to switch the phases on each while you're listening in the center listening point. there're only

4 phase settings for 2 subs. phase is just fine tuning; totally secondary to good sub placement.

if i had 2 subs like that, i'd probably take the smaller one & wire it into the rear surround loop. or maybe both stacked or next to each other in the same spot would sound better. only way to find is to experiment w/ them there w/ your stuff. trial & error.

or trial & mucho bass

------------------

Klipsch KLF 30 (front), KLF C-7, Cornwall I (rear)

Velodyne HGS-18 sub woofer

Monsterbass 400 sub interconnect & Monster CX-2 biwire & Z1 cable

Marantz SR-8000 receiver

Sony DVP-C650D 5-disk cd/dvd player

Sony Trinitron 27" stereo tv

Toshiba hi-fi stereo vcr

Technics dual cassette deck

Scientific Atlanta Explorer 2000 digital cable box

Boa's Listenin Lounge:

Klipsch RF-3 (front), RC-3, cheap little Technics (rear)

Monster MCX Biwires

Sony STR-DE935 a/v receiver

Kenwood KR-9600 AM/FM stereo receiver (vintage 1975)

Russound AB-2 receiver switch to RF-3

Teac PD-D1200 5-disk cd changer

Technics SL-1950 turntable/AT LS500 cartridge

Sega Genesis game player

Sub: None yet

rock on!

[WMcD]

This is a can of many worms; more like bucket of snakes. If you don't mind a lecture.... At least it is free of charge.

= = = = =

The switching of transmission and reception points relies on reciprocity. I.e. switching the locations of sub speaker and ears yields the same result regarding room reflections. The polarity switch should not alter results.

In case this hasn't been stated, the polarity switch just means the diaphragm is moving out and in, rather than in and out. Just like hooking up the wires wrong, black to red and vice versa. With a mono source of sound, i.e. one sub, you can't hear it. At least in the bass. (Perhaps we can in the treble, when things are right. A subject of debate.)

= = = = =

The polarity does come in to play when there are two or more sources. Which is what we were talking about.

One instance where there ARE two or more sources of bass is when the mains (two sources) and the sub (the third source) are both generating bass. However, that should occur only in the cross over band of frequencies.

This is where, in a perfect world, the mains are not quite fully shut off, and the sub is not quite turned on. Roughly, each system is putting out half power at the cross over point. This is at about 80 to 120 Hz.

This is called "constructive interference".

However, if the distance difference is one half wavelength, you get destructive interference. And flipping polarity solves the problem. At least it causes constructive interference. (Perhaps an oximoron.)

Please note that once we move the subject musical note above the cross over frequency band, the sub is not working. When we move below the cross over frequency band, the mains are not working. Therefore, the polarity switch usually only addresses an issue which exists in frequencies near the cross over point. I.e. can we turn destructive interference into constructive interference, with a flip of the switch.

= = = = = = =

Of course, with two subs, we've got another problem. Now the two subs are working from the cross over frequency, and down to their lower limit; a wider band. I'd agree keeping them both the same, or normal polarity, is a good bet.

= = = = = = = =

I depart a bit from the original subject. You'll see how it all fits in.

"Phase" is a very tricky term. As the Tappet Brothers point out, explaining things is difficult. One must balance between being accurate and incomprehensible - - - or comprehensible and inaccurate. Maybe all four at once.

Phase is used to discuss phenomenona which can arise from several causes. Consider a sine wave. (We need diagrams. Imagination may suffice.) If you look at the second half of the cycle, it is the same as the first half, but inverted.

Therefore, if you can delay the arrival time of a sine wave by a time for half a cycle to occur, it is the same as inverting the sine wave.

Reversing polarity accomplishes the same thing, you invert it. If you've got the same delayed sine wave, inverting the polarity restores it to the original form.

Now, you say, this is simple, and Gil has been saying the same thing over and over. However, it is not quite so simple in real time at different frequencies.

A sine wave in air with a frequency of 100 Hz has a wavelength of 10 feet. It takes 10 milliseconds to go through a full cycle. 5 milliseconds delay is equavalent to 180 degree phase shift or reverse polarity overall.

But consider that a 50 Hz sine wave has a wavelength of 20 feet. A 5 millisecond delay is only 90 degrees of phase shift. Flipping the phase switch doesn't restore things (if we have a 5 mS offset). It becomes equivalent to just adding 180 degrees, and you wind up with a 270 degree phase problem instead of the 90 degree; and they're similar. You can't win.

I'm getting in deep water here, which is the goal. How do phase, time delay, and polarity all fit together? This is the deep water.

We see from the above that a uniform delay (5 milliseconds) causes different amounts of "phase shift" depending on the frequency of the sine wave. If the bass guitar player moves down the scale, the 5 mS delay now has caused a different amount of phase shift, actually reducing it!

Nothing has changed except the note. Specifically, at 100 Hz, 5 mS is worth 180 degrees (half a wavelength), at 50 Hz, it is worth 90 degrees (quarter of a wavelength). Therefore we see that if time delay is uniform, it causes a different phase shift depending on the frequency. A uniform delay seems to cause an increase in phase shift with frequency.

Is polarity the same as delay? In our primary example of 100 Hz, a 5 mS delay has caused a 180 degree phase shift, and we correct it with "polarity".

Big, big, peripetetic (Sp?) question. Does that mean that the polarity switch, which "inverts phase" causes a time delay? Think a while. No, it is just a simple switch which effectively wires the sub backwards.

In fact, since it alters phase by 180 degrees, it is always worth half a wavelenght. But at high frequencies, half a wavelength is equivalent to less absolute time delay. This is because the sine wave wavelength changes according to frequency.

= = =

Now you can see how the subject gets so confused. We know that the switch is not a delay at all. However, it acts in a way similar to a delay. Yet the delay it is eqivalent to is related to frequency / wavelength. Quite clever for a mechanical switch.

= = = = =

To close the loop, let's apply the concepts to two subs, near to each other. Flipping the polarity switch on one sub causes a 180 degree phase shift at ALL frequencies in that one relative to the other. (The same is true of one main speaker being wired "out of phase" with the other.) We see that we have destructive interference. Here, at all relevant frequencies there is no difference in arrival time (no delay).

So we have polarity equal to phase shift, but no time delay. In other cases, we had time delay equal to phase shift, depending on the frequency!

= = = =

Err. Does that help or just create confusion? Best I can do tonight.

Gil