Subwoofer Phasing

[Jeff R.]

All I have always been confused about the phasing from 0-180 on my sub. What is the reason for it being on the sub and why and when should I be changing it.

 

[ParadigmDawg]

Depending on the absolute phase of your main speakers and amplifier and the distances of the subwoofer and the main speakers from the listening position, the bass in the crossover region may be smoother if you reverse the subwoofer’s phase. Typically, though, phase is left at 0° for most applications.

While seated in your listening sweet spot, play music with bass content that is familiar to you and then have someone switch the 0/180 phase switch on the sub to 180°. This will let you determine if the bass sounds louder in your seating position. The more bass-heavy setting is where the output of the subwoofer and the main speakers are most in phase. Use whatever position (0/180) is louder at your seating location. (Note: some subwoofers may contain a “variable” phase control, which provides variable control between 0 and 180 for even more precise phase control of your subwoofer.) However, if you do not notice any difference when changing the 0-180 setting, it only means there are no issues in your room and all is fine.

 

[j_garcia]

If you placed your sub at the back of the room, you might end up with the sound beng 180 degrees out of phase with the mains. The adjustmet allows you to compensate for that. If your sub is basically on the same end of the room as the mains, it won't likely be an issue.

 

[Midcow2]

phase 0 or 180

All I have always been confused about the phasing from 0-180 on my sub. What is the reason for it being on the sub and why and when should I be changing it.

Think of a sine wave. It has a period of 360 degrees.
If you have two sound waves and you shift them 180 degrees you get an exact mirror image and the sounds cancel to nothing.

If you have two subs you don't want the waves cancelling out each other, you would get no sound or very little. You want them to add or build on each other; sometimes a 180 degreee shift is required. With one sub( since nothing else has matching frequencies, well except maybe towers with a very dynamic range (or a powered sub) and you have LFE or Bass set to both you might also need to change the phase. Otherwise it has no effect.

Good Luck,

MidCow2

 

[Jeff R.]

Thanks guys appreciate it. Mine never really seemed to make a difference but as mentioned my sub always sits right next to my fronts. So that would make sense.

 

[Midcow2]

it is the same freuencies colliding when they are out of phase.

Thanks guys appreciate it. Mine never really seemed to make a difference but as mentioned my sub always sits right next to my fronts. So that would make sense.

Jeff,

It is not where it sits ,but if you have two sources that both produce the same frequency. Normally, there is very little overlap. The sub plays low frequencies and the fronts play mid and high frequencies. If you have a dynamic range front ( low, mid and high frequencies) then it is possible for the low frequencies from the front to be out of phase with the low frequencies of the sub. If they are out of phase by 180 degrees they can cancel each other out; this can also occur if you mis-wire a speaker polarity. Some of the automatic speaker setups will detect speakers out-of-phase or reverse polarity. This can also cause sound cancellation.

The newer corvettes use 180 degree phase shift for thier mufflers. A microphone detects the exhaust noise and through electronics it is shifted and played back resulting in noise cancellation and a very quiet exhaust.

Good luck!

MidCow2

 

[cerwinmad]

i...

I have to have my sub 180 out as the fronts are 3 way with twelve inch drivers and even with a crossover setting of 60hz i was getting some cancellation. i have the sub and fronts on the same wall.

 

[TLS Guy]

Thanks guys appreciate it. Mine never really seemed to make a difference but as mentioned my sub always sits right next to my fronts. So that would make sense.

It has to make a difference in that position and a big one. You can't have incorrect phase without wrecking the system performance.

Play a tone in the region of the crossover frequency to the sub, and set the phase control to the position for maximum spl. Then you will be correctly phased.

 

[MidnightSensi]

Phase changes with frequency because all frequencies have different wavelengths. So the phase of a subwoofer can only be correct at one particular frequency compared to the main speakers phase. Also phase changes with position.

The phase knob is basically useless, in other words, and that's why some manufacturers don't use them.

 

[TLS Guy]

Phase changes with frequency because all frequencies have different wavelengths. So the phase of a subwoofer can only be correct at one particular frequency compared to the main speakers phase. Also phase changes with position.

The phase knob is basically useless, in other words, and that's why some manufacturers don't use them.

Nonsense! A sub interface is a crossover, anyway you slice it.

Filters are not brick wall, so there is overlap. The point of the phasing knob is to phase the sub to the system at crossover frequency. If the sub is out of phase at crossover, there in cancellation, and a deep dip in the frequency response at crossover. It is no different to having any other speakers out of phase.

Correct phasing of ALL speakers in a set up, is a basic fundamental step to good results. In any event, phase only changes with frequency in a region in which a filter is active. That is the point, of the phasing knob, to correct the phase shift at crossover, so the drivers are reinforcing each other at crossover and not canceling each other.

 

[Midcow2]

Paul Harvey's "The rest of the Story"

You need to understand the relationship between frequency and wavelength.

The formula is : wavelength (meters) = 300 / frequency in MHz

---or --

To convert any frequency to a wavelength, divide the speed of light by it.

So, it's: wavelength = 299792458 divided by frequency.

A wavelength calculator : http://www.mcsquared.com/wavelength.htm

50 Hz = 22.6 feet

100 Hz = 11.3 feet

200 Hz = 5.65 feet

500 Hz = 2.26 feet

1000 Hz = 1.13 feet

2000 Hz = 6.78 inches

5000 Hz = 2.72 inches

20K Hz = .678 inches


As you can see the lower frequencies have a wavelength in the range of typical listening distances and therefore you can get out of phase. At the higher frequencies this is not an issue becuase the wavelength is very short compared to the listening distance. This is why you only see phase adjustment on lower frequencies, especially on subwoofers. ... And now you know the rest of the story!

Good Luck ( said in a deep baritone voice )

MidCow2

 

[MidnightSensi]

I didn't say you can't get out of phase.

Nonsense! A sub interface is a crossover, anyway you slice it.

Filters are not brick wall, so there is overlap. The point of the phasing knob is to phase the sub to the system at crossover frequency. If the sub is out of phase at crossover, there in cancellation, and a deep dip in the frequency response at crossover. It is no different to having any other speakers out of phase.

Correct phasing of ALL speakers in a set up, is a basic fundamental step to good results. In any event, phase only changes with frequency in a region in which a filter is active. That is the point, of the phasing knob, to correct the phase shift at crossover, so the drivers are reinforcing each other at crossover and not canceling each other.

Standing near the sub and turning the phase knob will not have the same effect as doing it from the listening position with a friend moving the knob. Since the relative distance between the sub and the speakers back to the listening position affects the phase. The correction will only work in a very narrow band of frequencies.

 

[TLS Guy]

I didn't say you can't get out of phase.



Standing near the sub and turning the phase knob will not have the same effect as doing it from the listening position with a friend moving the knob. Since the relative distance between the sub and the speakers back to the listening position affects the phase. The correction will only work in a very narrow band of frequencies.

I was at my friend's house today, integrating his new sub. Out of interest I measures the difference between correct phase and 180 degrees out of phase. The difference was 10 to 12db over a whole octave. So yes correct phasing is essential and job 1 installing any sub.

 

[MidnightSensi]

I was at my friend's house today, integrating his new sub. Out of interest I measures the difference between correct phase and 180 degrees out of phase. The difference was 10 to 12db over a whole octave. So yes correct phasing is essential and job 1 installing any sub.

10 to 12dB over an entire octave?

Now I'm positive your seeing a lurking variable. The phase knob can't fix that, it could fix a very very tight frequency range at a very specific location. You can't possibly get the bottom of an octive in phase with the top, the wavelengths are simply different lengths at that frequency.

 

[TLS Guy]

10 to 12dB over an entire octave?

Now I'm positive your seeing a lurking variable. The phase knob can't fix that, it could fix a very very tight frequency range at a very specific location. You can't possibly get the bottom of an octive in phase with the top, the wavelengths are simply different lengths at that frequency.

Oh yes it will! with mains at an F3 of 32, and the crossover properly set to roll off the sub at 60 Hz, with the mains full range it will. Even if the mains had been had been shelved at 60 Hz, with second order crossovers, I don't think the result would be different.

I was paying steady tones, and every time phase was reversed the dial went down 10 to 12 db. That is exactly what you would expect.

Anyhow, phase is crucial, and if you think it isn't you are dead wrong, and science is against you.

 

[TLS Guy]

10 to 12dB over an entire octave?

Now I'm positive your seeing a lurking variable. The phase knob can't fix that, it could fix a very very tight frequency range at a very specific location. You can't possibly get the bottom of an octive in phase with the top, the wavelengths are simply different lengths at that frequency.

Oh yes it will! with mains at an F3 of 32, and the crossover properly set to roll off the sub at 60 Hz, with the mains full range it will. Even if the mains had been had been shelved at 60 Hz, with second order crossovers, I don't think the result would be different.

I was paying steady tones, and every time phase was reversed the dial went down 10 to 12 db. That is exactly what you would expect.

Anyhow, phase is crucial, and if you think it isn't you are dead wrong, and science is against you.

I have designed enough speakers and crossovers to know that phasing errors at crossover are what ruins an otherwise good design.

 

[highfigh]

You need to understand the relationship between frequency and wavelength.

The formula is : wavelength (meters) = 300 / frequency in MHz

---or --

To convert any frequency to a wavelength, divide the speed of light by it.

Speed of light has nothing to do with this and the speed of sound in dry air at 68° is 1125ft/sec, or 343 m/sec. c is used to designate the speed of sound, too.

So, wavelength(λ) = c/f, so for 5Hz, λ=1125/55, or 22.5', or 22.6 is 1130'/sec is used for c.

 

[highfigh]

The thing about a crossover "point" is that it's not at the same amplitude as the rest of the curve outside of the crossover's effective range. The F3 for a HP filter is not that close to where the filter starts working and if a LP and HP coincide exactly, cancellations can easily make a huge valley in the response curve and how broad the effect is, depends completely on how much the filters overlap and what the frequency range is. The slope of the filters matters a lot, too. Even if it's 12dB/octave, the energy from one will affect the output from the other. Also, phase controls only affect the LP.