Linear Phase

[Betomon]

Hello all,

I'm curious as to the theory behind all these claims of "time alignment" and "linear phasing"

I was looking at some old, highly regarded technics speakers where the drivers were in a sort of cascade, with the woofer being physically closest to the listener. What's the theory behind this?

I realize there are lots of marketing-hype / dubious claims made by the manufacturers, but, can we go a little more in depth with the physics or theories being put forward?

Is it correct to say that the "best case scenario" would be for all waves, regardless of Hz, to reach our ears at the same time?

If so...

Does this not go against the way sound is created naturally in the real world without recording / reproducing anything? (i.e, a live acoustic band playing in front of you).

Thanks!

 

[62vauxhall]

Low frequencies move slower than high frequencies. The idea behind time aligned speaker enclosures is that output of the drivers are coherent on arrival at the listener’s ear.

 

[Coolhandjjl]

Low frequencies move slower than high frequencies. The idea behind time aligned speaker enclosures is that output of the drivers are coherent on arrival at the listener’s ear.

Low frequencies are not slower. Myth.

 

[62vauxhall]

Better you should tell that to those who wrote the promo copy for time aligned speakers back in the old days.

 

[jazzmans]

Low frequencies are not slower. Myth.

unless you are on mars....

...."On Earth, sounds typically travel at 767 mph (343 meters per second). But on Mars, low-pitched sounds travel at about 537 mph (240 meters per second), while higher-pitched sounds move at 559 mph (250 meters per second)", concluded NASA.....

 

[Lavane]

unless you are on mars....

...."On Earth, sounds typically travel at 767 mph (343 meters per second). But on Mars, low-pitched sounds travel at about 537 mph (240 meters per second), while higher-pitched sounds move at 559 mph (250 meters per second)", concluded NASA.....

And a nano-second is 11" long. That's how far an electronics travels down a wire in a non-second. I love fun facts.

 

[Akustic]

Beware Ye who enter the realm of psycho-acoustics ...
You just asked the most complicated question in audio ...

And I'm going to answer with a question ..
Why don't you like the sound of your system when you point the speakers at the wall instead of at your listening position ??
Phase and timing ..

I think that it begins with the fact that ears have about 50,000 years worth of evolutionary improvement at positioning a sound in space ..
We use small differences in the timing of the sounds to establish the sound's position
Ears are incredibly sensitive to timing ..
@CGchief can probably throw a little light in here ..

 

[SleeperSupra]

You want the center of each voice coil lined up with each other. The theory is that it helps with phase alignment. I makes them sound more coherent, almost as if they are coming from a single point.

 

[highend64]

Hello all,

I'm curious as to the theory behind all these claims of "time alignment" and "linear phasing"

I was looking at some old, highly regarded technics speakers where the drivers were in a sort of cascade, with the woofer being physically closest to the listener. What's the theory behind this?

I realize there are lots of marketing-hype / dubious claims made by the manufacturers, but, can we go a little more in depth with the physics or theories being put forward?

Is it correct to say that the "best case scenario" would be for all waves, regardless of Hz, to reach our ears at the same time?

If so...

Does this not go against the way sound is created naturally in the real world without recording / reproducing anything? (i.e, a live acoustic band playing in front of you).

Thanks!

Time alignment and phase alignment are related but tend to have different purpose. Phase alignment is used when two speakers with different frequency response are corrected, usually at the crossover point(sub and main or in a 3 way crossover low, mid, high). Time alignment in the other hand is used when you use all frequencies from the same source are matched.

 

[Betomon]

You want the center of each voice coil lined up with each other. The theory is that it helps with phase alignment. I makes them sound more coherent, almost as if they are coming from a single point.

Is this why my buddy's Urei co-axial speakers sounded so... "accurate / clean" (the only way I can describe it simply)?

Time alignment and phase alignment are related but tend to have different purpose. Phase alignment is used when two speakers with different frequency response are corrected, usually at the crossover point(sub and main or in a 3 way crossover low, mid, high). Time alignment in the other hand is used when you use all frequencies from the same source are matched.

So if my understanding is correct, we can never get the time aligned perfectly.. but then again, there is a threshold to how much we can perceive. So, do those technics and others' spacing of speakers on/in the cabinets actually make a difference?

Also, aren't all speakers with decent crossover designs supposed to be designed with the phase being correct? (i.e, not being out of phase or wired in reverse). I don't understand what "phase alignment" does if its not related to the timing, can you please elaborate for me?

I remember a brochure for my Avids talking about diffraction, which makes sense, I suppose our ears/brain may be able to perceive a box, so they tried to mitigate that with their box, the driver coupling devices, and the covers.. But then I saw a picture of their "listening room" and it was like a carpeted living room with the speakers all up against a wall (which would cause unwanted reflections etc according to current audiophile theory) - so.. ?? That was funny

Anyway - my question went unanswered though - aren't sounds in real life coming at us at different times and possibly in / out of phase etc, and the combination of everything is what we hear and love in a live performance.. So why are the speakers trying to adjust for all these variables? Wouldn't they be changing the sound?

Psychoacoustics.. so cool..

 

[Lavane]

Ya just have to ask yourself, "when is good good enough?"

 

[Dr. Ears]

I have two sets of linear phase speakers, Green Mountain Audio, used to advertise their speakers as time aligned and phase coherent and in the case of their "Imagos", they accomplish that using only first order cross-overs.
I bought a set of Technics Linear Phase SB-6060 as rear surrounds because I like how they sounded with the GMA's.
Do any of these terms mean anything, I am clueless, just like how these speakers sound compared to speakers with the drivers not aligned like these.
The late Roy Johnson, owner/founder of Green Mountain Audio started mounting the drivers in his later models in reformulated marble, making pound for pound some of the heaviest speakers out there.
Roy was a research nuclear physicist, who literally had his hands on components of our nuclear weapons, wearing asbestos lined ceramic gloves.
When Roy got too technical, which he frequently did, we reduce his explanations to "Roy Magic".
Roy is gone now, but for those of us lucky to own a pair of his speakers, his magic survives him.

 

[Powertech]

Time alignment by the use of stepped baffles on loudspeakers is employed to ensure that the focal point (energy source) of the drivers are vertically aligned and are the same distance from the listener. Where the tweeter (usually a dome) is placed forward of the centre of an associated cone driver the sound waves from the two sources will reach the listener at different times (microseconds apart). The theory goes that this leads to a smearing of the fine detail of the sound.. Human hearing is quite sensitive to sounds arriving at different times, especially at different frequencies. In severe cases it causes an echo type effect. This is not to be confused with room echo effects (where the same sound appears more than once) which the human brain is quite good at compensating for.
With large scale PA system for example, where very large deep bass bins are used, the energy centre of the bass driver is so far back behind the other speakers that time delays are used in the active crossovers. The effect is worse and of course the distance to the listener is much greater than at home, but the principle still applies.

Phase alignment is a different phenomenon. The most common form of phase correction in loudspeakers is reversing the polarity of the tweeter in a 2 way 12dB/octave crossover because the 2 drivers end up 180degrees out of phase at the crossover frequency. It also generates a large dip in the overall response at that frequency.
In general though, various research projects over the years have found that the human ear is not very sensitive to differences in absolute phase (i.e constant phase misalignment), only to changing phase (this is how a guitar waa-waa pedal works). If a waa-waa pedal is held at any constant position, no one notices the phase difference, but when the phase is changed quickly it is very noticeable. The doppler effect is another example of this. When you hear a police car siren approaching you quickly the tone of it's siren appears to rise, This effect is due to rapid phase change due to the distance between source and listener reducing quickly. The tone of the siren itself hasn't actually changed at all.

 

[Tom Brennan]

Time alignment started back in the 1930s at MGM pictures. John Hilliard, a sound engineer at MGM and subsequently the Zeus of the Great Southern California Horn Speaker Gods discovered that the 12' path difference to the listener between the bass and treble horns of a Western Electric monitoring system was causing a recording of Eleanore Powell tapdancing to have double taps where there should have been a single tap.

Western Electric was uninterested in supplying improved speakers and Hilliard got the OK from Doug Shearer, the head of the MGM sound department to design an improved speaker system for picture theaters. Hilliard went to work employing James Lansing and Robert Stephens among others and even Harry Olson from RCA got involved. The result was the vastly improved Shearer Horn, which was manufactured by Lansing Corporation, RCA and even Western Electric.

Hilliard was later the head engineering honcho at Altec where he developed the Voice of the Theatre speakers, which had better time alignment than the Shearers as well as improved midrange clarity.

Shearer Horn



An early Western Electric theater system. Note the long curved horn carrying most of the sound, the open baffle woofers and the Bostwick tweeters on the baffle above the woofers.

 

[highend64]

So if my understanding is correct, we can never get the time aligned perfectly

Pretty close but keep in mind that alot of factors involve in time alignment. Electrically, acoustically, and material has an in impact how you time align speakers. Say that you have two speakers(woofer and tweeter) in a vertical plane. If the voice coil are at the same distance on the woofer and tweeter, theoretically that should produce the same wavelength in time at the crossover frequency because acoustically the wavelength start at the same time for both drivers. In reality this is not true and I will tell you why. when you produce a test tone say 1k and say both woofer and tweeter are capable of doing this tone. If we measure both the woofer and the tweeter at the same distance at at different time by themself with the same test tone, I can assure you that the peaks of the waveform are not going to be the align in any given time. As I mentioned earlier, the factors that affect electrically(latency), acoustically(physical placement of the drivers), and material of the drivers(mass T/S parameters).

Also, aren't all speakers with decent crossover designs supposed to be designed with the phase being correct? (i.e, not being out of phase or wired in reverse). I don't understand what "phase alignment" does if its not related to the timing, can you please elaborate for me?

Phase alignment is very important in crossover networks. Phase shift occurs when one phase response is lagging and one leading at the crossover point are depending at what filter slope you are using. Phase alignment is very important and most obvious used when aligning sub/main because the wavelength are much longer in the bass region. Say you have the LPF at 80hz and the speakers HPF at 80hz and we use a LR 12db/oct for both, the signal going the sub is going to have a -3db rolloff at that frequency and same with with the speakers. For a 12db/oct network, you are going to have a 180 degree phase shift because the LPF is lagging 90 and the HPF is advanced making a 180. This method applys for the mid/tweeter also. That is why most of the time if the crossover network incorporates a LR 12db/oct filter, the tweeter is inverted.

 

[goodolpg]

I did some Quick Googling a couple days ago when I first saw this thread.
It seems some of the reason for alignment coherence is "lobing".

 

[highend64]

I did some Quick Googling a couple days ago when I first saw this thread.
It seems some of the reason for alignment coherence is "lobing".

Lobing occurs when the speakers in(usually in a TM configuration) are place offset. Projection will shift downward in a TM and opposite in a MT. Usually to correct this is physically align the drivers or electrically.

 

[goodolpg]

Most of it was over my head. As you mentioned there were examples as far placement of drivers (placement on the baffle, staggered offset, etc) and crossover type adjustments.
Often if my head starts spinning I just back away...and let those better informed carry the ball.

 

[Pio1980]

The ideal arrangement can produce a single impulse of the correct polarity simultaneously from all the drivers at the listening position when a polarized test impulse is provided from the power amp.
This means all drivers are working together at the same instant to produce sounds with maximum clarity and focus, instead of arriving at different times at differing polarities in a messy blurry fashion.

 

[Powertech]

The ideal arrangement can produce a single impulse of the correct polarity simultaneously from all the drivers at the listening position when a polarized test impulse is provided from the power amp.
This means all drivers are working together at the same instant to produce sounds with maximum clarity and focus, instead of arriving at different times at differing polarities in a messy blurry fashion.

Got it in 2 short sentences.