Phase offset audibility, particularly with mixed cabs

[agedhorse]

Simply put, the damping should be within 10-15% for predictable results, but even this is not hard and fast depending on personal tastes.

 

[fast_frank_d]

So just to be excrutiatingly clear for me, you are saying to select speakers with Qts within 10-15% of each other, correct?

 

[agedhorse]

No, not at all.

The SYSTEM response is comprised of several important parameters, including the filter response Q (totally different from Qts).

This is why I recommend listening to the combination to see if it works for you or not. All this technical stuff means nothing if you don't understand the big picture impacts of each factor. That's the kind of stuff we engineers go to school for, it's not easy to explain because it's complicated, heavily math oriented and frankly not terribly fun.

 

[ThisBass]

There is wiki for the curious.

The wiki stuff of course is totally right but does focus too much on steady sinusoidal signals which show infinite time characteristic.

In practice we have to deal with more "realistic" signals which begin at a distinct time stamp and do stop sometime.
As the signal is not "infinite" in time (neither for past nor any future consideration) we have to consider additional artifacts which come along with the start and stop of any signal.
I think the most common term just to call those artifcts by name is transient noise. Actually its an additional wide broad frequency "content" that adds to the start of a signal (and also this happens again when the signal does stop, or does fade out).

In System theory science we have got the step response that provides us with good information about the way how a signal turns in and also turns out whenever a signal passes a transfer channel.
At least in Germany these things are called "transient response".



Fortunatelly there is LTspice which helps to "demonstrate" what is going on under the hood
in the time domain.

-6dB/octave, phase shift equals -45 degree
Frequency 100Hz, time cycle equals 10msec

The first cycle looks noticeable distorted.
At 10msec of the time base (remeber 10msec fits to 1 cycle at 100Hz) the phase shift equals already -45 degree.
A trained eye may notce some (additional) lowish frequency content for the first couples of msec that overlays the (original) sine shape

Sample for 1 time cycle, just to demonstrate what happens with transient response if the signal does "hard stop"

btw the time base 0..1msec might look like some "delay" but its nothing but distortion to the (original" signal shape caused by the transient response.
In "rough" easy understanding words: actually a negative magnitude of phase, for example -90 degree of phase "tries" immediatelly to shift a sinoidal signal to become a (-1)cosine signal


Same -6dB/octave filter alignment but for "reverse" frequency response
Phase shift to the signal equals +45 degree

same situation as above the first cycle looks distorted but, in a different way than above.
At 10msec (after 1 cycle period is gone) it can be easily seen that the signal is phase shifted by +45 degree on the time base line.
The signal shape at 0..1msec looks like as if the signal "intends" to go back in time.
That's not surprising cause positive magnitude of phase shift "tends" to shift a sinoidal waveform to become cosine "shaped"

Transient response for the time domain

Thank's LTspice.
Hope its at least NOW understandable that phase on its own does never cause delay.

Of course phase deviation due to dfferent travel path in air is totally different animal.

 

[ThisBass]

@fast_frank_d
Science is a fascinating way to learn every day for a livetime cause there is no way to know everything.
I think you woun't stop to learn more of it.
But, as Andy already told you then it must be a very hard way to learn if you have to go on with basic skills to understand complex things sometime in the future.

About the phase thing, let me tell you a couple of examples which actually cause lots of phase but have never been noticed cause there is no audible artifact.

The alternating magnetic field of the pickup is 90 degree phase shifted versus the vibration of the string.
The voltage output of the pickup is -90 degree phase shifted versus the current running through the voice coil.
The SPL that a cabinet does "move" into the air is 90 degree phase shifted versus the cone movement.
Even the oscillation of the string on the instrument can be already shifted by 90 degree depending on the players fahsion how to plug a string.
Last but not least the polarity switch on a desk console does cause nothing but 180 degree phase shift
Actually it was possible to build a schematic that shifts 101 times the polarity respectively shifts 101 times the signal by 180 degree. The result was just the same as having "one" polarity switch

 

[xk49w]

The wiki stuff of course is totally right but does focus too much on steady sinusoidal signals which show infinite time characteristic.

In practice we have to deal with more "realistic" signals which begin at a distinct time stamp and do stop sometime.
As the signal is not "infinite" in time (neither for past nor any future consideration) we have to consider additional artifacts which come along with the start and stop of any signal.
I think the most common term just to call those artifcts by name is transient noise. Actually its an additional wide broad frequency "content" that adds to the start of a signal (and also this happens again when the signal does stop, or does fade out). ...

Acknowledged, but it has nothing to do with either group delay or phase delay, which are continuous signal measurements.

 

[ThisBass]

Acknowledged, but it has nothing to do with either group delay or phase delay, which are continuous signal measurements.

Sorry but, I think we don't discuss RF modulation respectively any other modulation principles !
For modulated (RF) signals its absolutely normal that the phase of the carrier travels with a higher speed than the signal itself.
And if the carriers phase WAS linear versus frequency THAN the group delay WAS of a steady number versus frequency AND in this case the modulation signal was NOT distorted due to group delay issues

On the (demodulated) LF side its no more valid to discuss a (carrier) phase delay versus group delay cause the carrier phase is already gone.


With amplitide modulation the phase delay versus group delay issue is quite easy to understand. This topic becomes way more complex with frequency modulation, phase modulation, pulse width modulation...

 

[xk49w]

Sorry but, I think we don't discuss RF modulation respectively any other modulation principles !
For modulated (RF) signals its absolutely normal that the phase of the carrier travels with a higher speed than the signal itself.
And if the carriers phase WAS linear versus frequency THAN the group delay WAS of a steady number versus frequency AND in this case the modulation signal was NOT distorted due to group delay issues

On the (demodulated) LF side its no more valid to discuss a (carrier) phase delay versus group delay cause the carrier phase is already gone.


With amplitide modulation the phase delay versus group delay issue is quite easy to understand. This topic becomes way more complex with frequency modulation, phase modulation, pulse width modulation...

I don't understand your response in context and would like to take the discussion private, if you have no objection.

 

[ThisBass]

I don't understand your response in context and would like to take the discussion private, if you have no objection.

Of course I'm in.

 

[Downunderwonder]

Just a caution, sometimes models are not all that accurate (or even remotely accurate).

To get a phase difference of greater than 45 degrees it pretty tough UNTIL you start mixing different TYPES of cabinets (sealed with ported with folded horn with bandpass), and sometimes with different driver types (large diameter, heavy cones with high inductance motors can rapidly depart from phase coherency as frequency increases. The problem won't necessarily be in the low bass but more in the mid bass to midrange. I don't think these models address anthing except the lower frequencies. I typically will listen to cabinet pairings, and if something doesn't sound right, I will run phase coherency testing (it's harder and more time consuming than it sounds, because mic placement and any acoustic boundaries come into play)

There's more to it than purely phase response, but having coherent phase response between two cabinets generally improves the odds of the cabinets working well together over the same bandwidth.

Phase coherence isn't predicted by phase response?

 

[agedhorse]

Phase coherence isn't predicted by phase response?

Not completely, in practice. For coherency to be an issue, deviations have to occur at levels where each contribution has an effect on the total. For example a source that's 45 degrees different but also 12dB down at that frequency isn't going to matter much at all.

There is an analogy of sorts in control theory for stability criteria. In order for a (negative feedback) system to be stable the feedback signal times the gain must be less than 1 where the phase response becomes 0 degrees if the phase is 180 degrees there's no problem, and if the phase was 20 degrees there may be a problem as the feedback signal increases beyond the 0 degree stability point. It's a continuous function with boundaries.

(sorry for the mediocre explanation of stability, it's a difficult concept to simplify, and today's a quiet shop cleaning day so my brain is not wanting to remain fully engaged).

 

[Downunderwonder]

Woosh.


I think I am going to stick with mixed cabs sometimes sound mushy.

 

[agedhorse]

Woosh.

I think I am going to stick with mixed cabs sometimes sound mushy.

... and sometimes they sound better than what you can achieve doubling up one or the other. It depends.

 

[ThisBass]

If I was nitpicking enough then I would claim that even for four brand new drivers out of the box a matching phase coherence was hardly to be ensured.
Some folks try to claim phase issues as a reason for mud sound even if phase deviation was not bigger than kinda 5 degree. Sometimes I think those folks only feel happy so far (on paper) if phase deviation does equal 0 degree, right?

Even for "matching" drivers of a "matching" charge that also does "match" in month/day/year requirement, there is natural dispersion in parameter numbers on a current production line for the "next" driver tested as "good" and packed into the box.

Some cab designs show some sensitivity versus small variation of parameters while other designs are more "robust" in this regard. Manufacturers (sometimes) change driver parameters without notice. Sometimes the change of parameters is published later when a big charge of drivers is already shipped. Modelling software sometimes relates to a distinct production year (if alltogether) if there are different revisions available.


All that said, if little phase deviation respectively "coherence" was really a sensitive parameter then we would have absolutely NO MORE chance to design cabs anymore.

 

[ThisBass]

Let me do a supplement to the topic "phase offset audibility".

Ported cabs show a characteristic that equals a 4th order filter. 4th order filters show a strong dependency of phase versus frequency in the range of cut off frequency. Interestingly a highish dependency of phase deviation versus frequency relates into highish group delay and which takes place into large numbers at swing off (and transient response).

infinite signals like a steady (time infinite) sinoidal waveform are unsuitable to study artifacts which actually happen in practice with time limited signals.
So in science we have got signals like dirac pulse or step function or burst, that is because these signals transfer already information in the meaning of Shannon Theorem rather tnan meaningless time infinite conditions that tell nothing but nothing.

Here you are with a 4th order Butterworth HPF 100Hz cut off .
The chart shows the phase "delay" at 100Hz for an infinite time consideration.

Clearly the output phase "yellow" looks like as was 180 degree delayed.


Here you are with just the same 4th order Butterworth but now the output shows the transient response for a 100Hz burst of 5 cycle periods.
Again the blue graph shows the input, yellow shows the response at the output.
We can notice that the "information" at the output looks kinda delayed versus the input. And furthermore the output envelope looks clearly "distorted".
We can also notice that there is an oscillating tail outcoming added to the end at just the moment the input stops.

I think there are a couple of cabs out there which are designed for a given driver to gain extended low end response (large cab volume and lowish tuning) and thus may show little ripple within the pass band.
So here you are with the chart of a 4th order Tschebyscheff 0.5dB ripple.
Same cut off frequency 100Hz of course

Tschebyscheff filters show a higher steepness at cut off frequency but these filter type shows a higher group delay which relates into degraded distortion characteristics as it can seen with the graph.

As long as an amplifier powers only ONE distinct Type of Family the amplifier can (almost always) easily control the cabinets "smut" but, both cabinets in combination runs both, the amplifier and the cabinets" into issues.

The phase issue in this context means nothing cause the number was way too small to cause issues at 100Hz.

 

[morebass!]

"Type of Family" meaning sealed vs. ported???

 

[agedhorse]

"Type of Family" meaning sealed vs. ported???

Yes, but there are other type or family identifiers, such a s band pass boxes, very different ported box alignments, physical driver displacements from t=0, folded horns, etc.