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Another Time and Phase thread

Hello guys I found this post on another site.Please explain as this stuff seems to get more and more confusing to me.Which is it time and phase or not.This was posted by an AA member.
Thanks in advance!

I see much stuff about phase and time with confusion. If there are two drivers mounted on a baffle, say a midrange and a tweeter, then it would be nice if the acoustic radiation from the two drivers were in phase. Linkwitz Reily 24dB/octave accomplish this when the two drivers are in the same acoustic plane (voice coil alignment is very close to this with an offset baffle) In this case, for one octave above and below the crossover, the electrical signal applied to the drivers are 360 degrees out of phase. For continuous signals applied to the two drivers through the crossover and at the crossover frequency the motion of the midrange is one cycle behind the motion of the tweeter. This allows the main acoustic radiation axis to stay on the same axis as the individual drivers. The problem with the LR crossover is that half the energy applied for those two octaves around the crossover is thrown away by the form of the crossover. The transfer function has 4 terms plus a constant. Only the first term and constant appear in the acoustic output.
As far as that minimum phase stuff. Everyone seems to forget that the drivers must acoustically sum (low and high add together) somewhere in front of the speakers in the acoustic environment. With out of phase drivers that summing point starts down (midrange below the tweeter with applied signal of midrange lagging tweeter signal) and then moves up relative to the axis of the speaker depending on frequency. If you do this in a circuit, the summing is literally a point and so no such physical axis even exist. Speakers are not points and are not circuits though. A 6dB/octave crossover has a phase of plus 45 degrees for the tweeter and minus 45 degrees for the midrange at the crossover point. This is why the crossover is -3dB. With the two drivers 90 degrees out of phase, cancellation must occur. In this case. Half the energy is canceled out by the destructive interference from the two drivers at the crossover frequency. Also, if the voice coils are aligned as before, at the crossover frequency the acoustic center of radiation for the tweeter has moved forward in phase (effectively may be thought about as moving forward in space for analysis purposes) and the acoustic center of the midrange has moved back. The axis of radiation where the two drives sum and are in phase has been tilted down. The angle of tilt is directly related to the distance between the two drivers and the crossover frequency. If the drivers are more than one wavelength apart at the crossover frequency, then the tilt is so much that a second radiation axis occurs. This axis point way up with a acoustic radiation null between these two axis. Wave length in inches is equal to 13500/frequency of interest. This part about radiation is all basic physics) physics 102 from radiation from multiple sources.

As far as driver frequency response is concerned. Let us take that midrange and start at 200Hz and go to 4000Hz. Also, let us say the mid has a cone plus half the surround diameter of 5 inches. So this may be a nominal 6.5 inch driver. As we increase frequency we observe that the response if pretty flat and as frequency is increase the angle of radiation decreases such that the response may be fairly flat until around 650Hz. At this point the radiation angle of the driver starts to look fairly constant. How can this be? The outer edge of the cone starts to act more like a surround with the center of the cone moving in and out. As all this works out in a real driver, the radiation angle slowly decreases as the effective radiating area decreases toward the center of the cone. Many aspects: effective moving mass; radiating area; angle of radiation; and other factors serve together to make the ON AXIS response flat. It is very important to under stand that acoustic radiation resistance increases as frequency increases. This is why a woofer is big and a tweeter is small. Radiation resistance has increased at the higher frequency. Basic physics tells us that when the driver diameter is equal to 1/4 wavelength that the angle of radiation will be reduced to 45 degrees from the original angle at 200Hz of 180 degrees. For our mythical driver this occurs at 1350Hz. As we increase frequency more, the angle of radiation must continue to shrink if on axis frequency response is to remain flat. At 2700Hz the angle of radiation would be much smaller if a real driver ever made it to that frequency without acting like a drumhead where the center moves forward and the outer parts move back. This is not cone breakup as it is a normal motion. Cone breakup refers to irregular patterns of motion. This is drum head motion. Above the frequency drum head mode sets in, the on axis radiation is out of phase (-180 degrees) with the drive signal. This is known as incoherent. The energy response may be perfectly flat but the time response causes the energy to be useless for listening or summing to the tweeter on our two speakers with a baffle.

As far as tweeters go, if you have a 1 inch dome radiating at 17,000Hz, if is pretty clear the source is much larger that the wavelength. If the tweeter is flat on axis at 17kHz, then the angle of radiation is less than 22 degrees and it is in drum head mode of motion causing all the radiated energy on axis to be out of phase. Only tweeters with horns attached, tweeters with some physical means to control radiation pattern, very small tweeters, (<1cm) or tweeters that become extremely directional have a chance of going high in frequency without getting out of phase. For our 1 inch dome to stay in phase at 20kHz the angle of radiation would be on the order of 10 degrees. This means that a reading of 30 degrees off axis of the output would be some 20dB down. A good rule of thumb is if a tweeter is flat above 12kHz then it is out of phase. Having tested about 700 manufactured tweeters from Scanspeak, Morel, Audax, Seas, Phillips, Accuton, Focal, Dynaudio, Vifa, Becker, and Heil, and others none of the domes stay in phase (go into drum head mode) above 8.5kHz. It is really easy to tell by looking at a frequency response graph because the tweeter will be flat. If the tweeter rolls off at -6dB per octave starting around 12kHz then it may stay in phase. There are a very few (I know of three direct radiating tweeters, not horns) that do stay in phase up to 20kHz but will not tell you other than the Isophon. PLEASE NOTE- the electrical phase graph published with so many loudspeakers in no way reflects the acoustic phase of the driver. Except near resonance frequency, these two different aspects may be and almost always are totally unrelated for all drivers!!! The electrical characteristics are useful for crossover design. The unpublished acoustic characteristics (time and phase) are required for the acoustic design. Lots of luck on that one.

So with these things in mind it is pretty clear that our two drivers need to be at most one wave length of physical separation at the crossover frequency with less separation being desirable, say at most one wave length at double the crossover frequency. It also appears that some method to keep the midrange and tweeter in phase through the crossover region is desired to allow proper acoustic summing and keep the main axis of radiation on the same axis as the tweeter and midrange are away from the crossover. This can possibly be accomplished by an all pass filter aligned to cause lag in the tweeter signal around the crossover to match the midrange. If acoustic summing is to be deemed "minimum phase" then the criteria of no axis of radiation tilting (and therefore no cancellation) must be followed. Any speaker with more than one wavelength of driver separation at crossover frequency or with drivers canceling by being out of phase cannot be a minimum phase system. Several of us take in phase to mean less than 22 degrees of error between drivers. This concept is thrown around a lot but never appears to apply. High order crossovers often fail also. One approach was built by Bang and Olsen (sp?) covered by the paper in AES about 1975 using a "filler" driver between the midrange and the tweeter which corrects the phase error and provides the necessary acoustic output to achieve minimum phase using three drivers in what was essentially a two way speaker. This is a novel a valid approach.

This is not to say some speaker may or may not sound pretty good but do not pretend that some absurd claim about minimum phase or flat response means very much. Absurd in this case can easily be identified by crossover frequency compared to driver separation. In general, all the small two ways I have tested from 100-10,000Hz (almost 7 octaves) have at least half the energy radiating on the axis as incoherent. At least the good ones did, the rest were far worse. One recently tested, popular, and very widely used loudspeaker was incoherent from 430-4,000 and 5,600-10,000 Hertz. This same speaker appears in many recording studios!!! Remember, this is the age of marketingism, advertise what you don't have as your prime feature. Find, create, or academically publish misinformation which supports your claims. And most of all, smile when you deceive.
gmood1
sean
6,231 posts
 
Bigtee: You bring up some valid points for typical "full range single driver" systems. What you may have forgotten to take into account is how a Walsh driver works and is shaped.

Due to the shape and size of the cone, the section of cone that is closest to the motor is furthest away from the ear in terms of actual physical distance. Yet at the same time, it is closest to the ear in terms of a direct path when one is seated in a normal listening position. The lower section of the cone is larger, making it physically closer to the ear, yet it is further away due to the higher position of ones' ears at a seated listening position.

The end result is a relatively consistent wavefront in terms of arrival time vs frequency. That's why this is called a "bending wave" transducer. It takes into account the length, path and arrival times of the various frequencies being reproduced and where they are emanated from on the driver itself. Obviously, some attention needs to be paid in terms of speaker height vs seated listening height, but this is true of any speaker system.

As a side note, Ohm had to play with the shape of the driver quite a bit when redesigning the original Walsh driven Ohm A to come up with the Ohm F. If you look at pictures of them side by side, the shapes of the cones on the A and the F are quite different. Whereas the A looks like an upside-down funnel with a much broader flare near the base, the F uses the same basic "upside-down funnel" design but maintains a consistent rate of flare from top to bottom. If these were potentiometers, you could say that the Ohm A had an "audio taper" whereas the Ohm F had a "linear taper". Sean
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PS... The Walsh's are FAR, FAR, FAR from perfect drivers, but they do some things VERY well.

PPS... Mburns: E-stat's and any other "panel" type speaker have their own set of problems. Each speaker design has its' own benefits and baggage. Plasma drivers are the "best" in terms of linearity, but they are just about the worst in terms of spl capacity when taking safety into account : )
royj's avatar
royj
151 posts
 
At the urging of several parties, I decided to post a response on this over at Audio Asylum.

http://www.audioasylum.com/audio/speakers/messages/133397.html

Best,
Roy
Green Mountain Audio
bigtee
803 posts
 
Hey Sean, I wasn't refering back to you or the Walsh speakers but attempting to point out that typical single driver speakers are no where near perfect. I'll try to be more specific. I did enjoy reading your explanation, however. I agree that the Walsh was a good attempt but, as you said, not perfect by a long shot. That brings me to my next point.
Are we buying speakers that are designed as accurate transducers or are we buying speakers that we perceive as sounding good to use as an individual? I have noted in the past that most of the popular electrostatics test so poorly in frequency response test that if they were box speakers, they wouldn't even be considered. I believe if you don't start off with a flat frequency response, then everything else is a moot point(from a accuracy point) We barely scratch the surface on this sight as to speaker design with its complexities. ( I have enjoyed Roy's contributions)
One of the reasons I believe in phase coherent speakers is there is no way a speaker can be accurate without having the phase relationships match those as recorded. I also believe in time alignment for the same reason. B&W did a test once using a phase corrected CD to time align their speakers. The panel they used prefered the time aligned presentation and perceived it as more natural(and accurate) However, B&W continues to make non phase aligned speakers.
This is a business for designers now. They have to convince people their idea is best. Advertising rules! But, what is it doing to our hobby and our striving for something that is more accurate. If accuracy is not the goal, then I'm throwing everything out and buying some Bose!
Post removed 
sean
6,231 posts
 
Viridian: I agree with your observations about the majority of stock F's "ringing like a bell". There are ways to DRASTICALLY improve their performance in this area though. Since "ringing" has to do with transient response and internal damping, any improvements made in these areas improve the performance in all other areas.

1) F's have a LOT of driver mass. Unless the cabinet is properly damped and tuned for the individual driver being used ( most from the factory are not even remotely close ), the driver will tend to overshoot on large peaks and ring once it is "thoroughly excited". By fine tuning / altering the damping within the cabinet, you effectively:

A) lower the Q of the system
B) reduce the impedance peak at resonance
C) reduce the rate of roll-off below resonance
D) improve the low frequency characteristics
E) improve the transient response of the driver as a whole
F) increase power transfer
G) improve the amplifier / speaker interface

2) F's are a very tough load. Besides producing a very low impedance at very low frequencies ( appr 1 ohm at DC and close to 2 ohms at low and mid-frequencies ), they are also VERY in-efficient. From what i've been able to calculate, i have one set that runs about 82-83 dB's @ 1 watt and the other set is somewhere around 80-81 dB's or so. The early Ohm A's were supposedly somewhere around 78 dB's or so !!! As such, you need an amp that is TRULY capable of a GREAT amount of voltage and current. On top of this, it must be capable of delivering that power into very low impedances at very low frequencies i.e. where you already need gobs of power to begin with. Many "big" and "well respected" amps aren't "quite as big" or "quite as respectable" as one might think when trying to drive this type of load.

3) Besides the low efficiency and low impedance situation, these speakers produce a great amount of reflected EMF. This is due to their very large motor structure. This makes them hard to control and load into as the amp literally has to "muscle" past the "back-pressure" that the speaker itself is generating. As such, all but the sturdiest of amps will cough up a lung when trying to drive these speakers. I can't think of an SS amp made "way back when" that could really make the F's sing. By the time that amps of this calibre started to show up, the F's were either out of production or in the last stages of production. Due to the low impedance situation and lack of bass damping, tubes are pretty much out of the question with this speaker. Even "professional" type amps like the big Mesa Baron don't work too good for too long. High current draw due to their low impedances tends to pull quite hard on the tubes, shortening their life span quite noticeably.

4) The factory wiring is attrocious to say the least. One needs to disconnect the factory wiring at the Walsh driver and bypass it altogether. Feeding signal from the amp via high quality ( low impedance ) speaker cables directly into the Walsh makes a world of difference. Not only does this change the amount of control that the amp has over the driver, high frequency response is drastically improved in terms of both quality and extension. The use of a "normal" speaker cable with its' higher impedance ( most speaker cables are at least 40+ ohms ) will somewhat reduce these benefits, but it will be a big step forward as compared to using the internal OEM wiring that was supplied within the F's. For best results, speaker cabling with a nominal impedance below 20 ohms ( at most ) works best.

With all of that in mind, i sincerely doubt that more than a few handfuls of people have ever really heard what a well set up pair of F's are capable of. Quite honestly, i don't know if the guys that actually built and designed these things way back when had ever really heard what these things are capable of under "optimum" condititions. If i did not have the test equipment that i have, know how to interpret the data that it presented to me and impliment the necessary changes that i thought that they needed and have amps that were up to the task, i myself would not have the high opinion of these drivers that i do.

Having said that, the two sets of F's that i have vary quite markedly in measurements and sonics from one another. While Viridian is probably aware of this, most others probably don't know that all Walsh drivers were made piece by piece using hand labor. Each driver is comprised of three different types of materials ( titanium, aluminum and paper ) and are divided up into individual sections along the length of the driver. Once these different materials were hand glued together to form one large cone, varying levels of foam damping material were applied to the driver inside the cone. This was done by means of adhesive to the back of the metal sections, which helped to damp the ringing that all undamped metals tend to display once excited. As such, variances from unit to unit were not abnormal due to all of the hand labor. This is especially true when they first started production and were refining the design and near the end of production. Given just the variances in performance between the two sets that i have ( even their frames are quite different ), i do not doubt that one person could have a set that works reasonably well whereas the next person has a set that was a disaster out of the factory.

Sorry to side-track this thread, but i thought that others that own or are interested in what is an "out of the ordinary" speaker system might find this interesting. I'll shut up now : ) Sean
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