Nearfield Bass Shootout

[Carlspeak]

Ken, the 3a woofer curve that I posted starts rolling off in the general range of 600Hz. Would this driver do that without a crossover in play?

Am I missing something here or, has the discussion veered away from the most important aspect of Z's test data (i.e. the left side of the woofer curve) and has become more focused on the right side (and downward slope beyond Fc) which, I agree, could indeed by influenced by the presence of a LP component?

Really, isn't it the left side's Fc 'hump' what's influenced by the box's Q?

[speaker dave]

"One comment on damping factor, reverse EMF, etc. I think it is always a little confusing to think in those terms..."

That depends on how you look at it. If you think strictly in terms of Qe, Qm, Qt, Vas, you're probably right. If you think in terms of Newton's second law of motion which the T-S parameters are based on and the second law of thermodynamics, you realize that all of the energy pumped into a speaker will ultimately be turned into heat. How that happens depends on several factors. A small percentage will be first turned into sound, the useful work output that will eventually dissipate as heat in the surfaces the sound waves encounter. The rest will either be dissipated directly through frictional mechanical losses in the driver itself and in the enclosure, mostly through aerodynamic drag in the case of an AS speaker system or through electrical resistance heating. The kinetic energy is greatest when the cone is passing throught its zero crossing point and the moving mass has its highest velocity. At that point potential energy is zero. At its extreme excursion on each cycle, it's potential energy is greatest because the restoring force is at its maximum and its velocity and therefore its kinetic energy is zero because it is stopped to reverse direction. If the system Q is high as in the case of a ported speaker which effectively has a resonant air column, a frequency at which the resistance to air flow is very low, the mass of the cone is high, and the frictional losses in the speaker suspension itself is low, electrical damping will be the overriding factor. The driver wants to continue vibrating at the resonant frequency no matter what the driving frequency. In this case, small changes to the series (DC) resistance of the electrical circuit whether it's due to the internal amplifier output impedence or the DC resistance of the speaker wires can have a substantial effect at the rate of dissipation. In the case of a highly mechanically damped speaker, it is the frictional losses that will be most responsible for energy dissipation.

The principle is directly analagous to dynamic electrical braking of rotary motors for say cranes. When the motor is stopped, the voltage source is removed and the armature windings are connected to a shunt, a low resistance bar of copper that quickly converts the rotary momentum into heat from the reverse emf. This saves the life of any mechanical brakes that may also be used to slow the motor. Reverse EMF is a critical factor in motors including linear motors because it limits the current that flows through them. Without reverse EMF, the motor current is only limited by the DC resistance and for an AC motor the reactive inductance of the windings. This is the locked rotor current and would be analogous to the current draw in a speaker if you physically restrained the voice coil from moving. It's often about ten times the full speed running amperage. The critical role the stuffing plays in an AS speaker cannot be overemphasized. The size, shape, density, and amount of fibers is critical. The frictional loss is velocity related, exactly what you want. The finer and more fibers you have, the greater the surface area and the more effective the loss. The tradeoff is that as the volume increases, the amount of air you displace for a given surface area increases too. This increases the K in the equation which is the displacement related force or springiness of the air trapped inside. You can see one major advantage of the AS design over the ported design by realizing that for an AS design, K is independent of frequency because it depends only on the gas laws, P1*V1=P2*V2. In a ported design K is highly dependent on frequency, being very low at the resonant frequency and its multiples and very high at the midpoints between them. This is why it is very much harder to design a ported speaker with a flat bass output that does not have a high Q. About the best you can do is try to match the Q of the air column with the resonance frequency and Q of the driver so that they compliment or compensate for each other.

As I said, always a little confusing.

David

[genek]

Am I missing something here or, has the discussion veered away from the most important aspect of Z's test data (i.e. the left side of the woofer curve) and has become more focused on the right side (and downward slope beyond Fc) which, I agree, could indeed by influenced by the presence of a LF component?

Really, isn't it the left side's Fc 'hump' what's influenced by the box's Q?

Carl, the only question addressed by the right side of the curve is whether the AR woofer measurements were done with or without the crossover in place, because Ken expressed the view that the crossover would affect Fs.

Also, I asked if this nearfield method's results could somehow be correlated to the woofer curves obtained using Villchur's old bury-the-box method for pseudo-anechoic woofer measurement, because Vilchur's article states that his results were subsequently verified in an actual anechoic chamber.

If Zilch repeats his 3a woofer measurements without crossover and the results are not significantly different from what he got with, I think the question of whether there was one connected in Villchur's buried box will be rendered pretty much irrelevant.

[soundminded]

As I said, always a little confusing.

David

If you're not accostomed to looking at it that way it can be. A motor and a generator are the same machine. Minor differences optomize them for one purpose or the other but they really are the same thing. A loudspeaker is merely a linear motor and is also a generator. It's the same as a dynamic microphone and in many old intercoms built in the 1940s or 1950s, they used the same device as both the microphone and the speaker.

There is a trend now for exploiting this idea with flywheels for energy storage units as environmentally friendly substitutes for batteries. When there is commercial power available, the device operates as a motor storing energy as angular momentum in the flywheel. When power fails, the same device becomes a generator supplying electrical power from the stored energy to a load. It is very reliable but the cost is high and the available energy for the space it takes up is much lower than batteries. The problem of undamped bass resonance in speakers is often one of getting rid of stored energy.

[speaker dave]

If you're not accostomed to looking at it that way it can be. A motor and a generator are the same machine. Minor differences optomize them for one purpose or the other but they really are the same thing. A loudspeaker is merely a linear motor and is also a generator. It's the same as a dynamic microphone and in many old intercoms built in the 1940s or 1950s, they used the same device as both the microphone and the speaker.

There is a trend now for exploiting this idea with flywheels for energy storage units as environmentally friendly substitutes for batteries. When there is commercial power available, the device operates as a motor storing energy as angular momentum in the flywheel. When power fails, the same device becomes a generator supplying electrical power from the stored energy to a load. It is very reliable but the cost is high and the available energy for the space it takes up is much lower than batteries. The problem of undamped bass resonance in speakers is often one of getting rid of stored energy.

Yes, I agree that the more comfortable point of view comes from your particular background and knowledge.

Regarding motors and damping, I recall as a kid watching my brother and a grand experiment with our Sears Craftsmen radial arm saw. He thought that electrical braking would be more sensible than the twist knob friction brake. He rigged up a multiway switch that, in the off position, shorted out the motor for electrical braking. Being a bright spark he even put 100 ohms or so in as the shunt rather than have a dead short across the motor.

Well he ran it up to speed and flipped the switch....KACHUNG!! Instant braking. I'm amazed the blade didn't spin off and chase us around the room.

We decided the mechanical brake wasn't so bad after all.

David

[Zilch]

Carl, the only question addressed by the right side of the curve is whether the AR woofer measurements were done with or without the crossover in place, because Ken expressed the view that the crossover would affect Fs.

Also, I asked if this nearfield method's results could somehow be correlated to the woofer curves obtained using Villchur's old bury-the-box method for pseudo-anechoic woofer measurement, because Vilchur's article states that his results were subsequently verified in an actual anechoic chamber.

If Zilch repeats his 3a woofer measurements without crossover and the results are not significantly different from what he got with, I think the question of whether there was one connected in Villchur's buried box will be rendered pretty much irrelevant.

So might we erroneously assume.

A teaser:

[soundminded]

So might we erroneously assume.

A teaser:

OK Zilch, I'll play.

All seem to me to be AS designs due to their 12 db per octave rolloff. Orange with its peculiar dip and then rise again around 700 hz looks like the original Advent. The others all look like 3 way systems because their high ends roll of too low for 2 way systems. Red has the same shape as Orange in its bass region but its highs roll off more quickly and is not as sensitive. Looks like AR3a. The others with higher Fs could be from the AR2 family of drivers and one may be KLH model 5. One might even be from a 3 way Advent like Heritage. Did I get any right?

[Zilch]

OK Zilch, I'll play.

Did I get any right?

All are AR3a drivers.

Orange is the red one without a lowpass filter.

It's in a different cabinet for that measurement, which variable I am proceeding to eliminate presently, rewiring input terminal 2.

The notch at ~650 Hz is a nearfield measurement artifact, presumably.

Yes, I'm gathering Fc and Qtc for each, as well....

[kkantor]

Ken, the 3a woofer curve that I posted starts rolling off in the general range of 600Hz. Would this driver do that without a crossover in play?

That's a good question. I've been thinking about it since before my last comment. But, I am having a difficult time settling it, either theoretically or in the paperwork I have been searching.

On one hand, I have seen some old, felted paper, heavy cone, high Le 12" woofers that roll off fairly cleanly below 1K at 1m. (Especially true if the mic is a bit off axis, or there is spatial averaging going on.) On the other hand, I agree with you that the curve under discussion seems a bit too clean to be raw.

-k

[kkantor]

It is crossover controlled. I have a graph of the earlier AR-3 woofer curve somewhere (run by the company on a brochure that also has the 3a woofer curve) and the woofer extends beyond 600 Hz to 1 kHz. The AR-1 woofer did the same thing. To get the 3a woofer to do the lower-frequency rolloff in its design, low-pass filtering had to be applied.

I believe that all of the AR driver response-curve graphs show the results with the system networks in operation. Allison also did that with his Allison Acoustics curves.

Howard Ferstler

I sit corrected!

If you can ever dig out that raw 12" data, it would be great. I can't find mine.

-k

[genek]

All are AR3a drivers.

Are there any dates stamped on the frames?

[Zilch]

Are there any dates stamped on the frames?

No markings. #1 is shortwire, #2, longwire, and #3 & 4, yellow chromate, have integrally stamped lands for the surrounds.

Here's the data:

With lowpass:

Re = 3.37, 3.15, 3.36, 3.34, M = 3.31 Ohms

Fc = 36.92, 34.46, 35.25, M = 35.67 Hz

Qtc = 0.83, 0.57, 0.59, 0.59, M = 0.65

No filter:

Re = 2.60, 2.40, 2.63, 2.62, M = 2.56, Delta = -0.74 Ohms

Fc = 43.89, 41.89, 43.56, 44.94, M = 43.57, Delta = +7.90 hz

Qtc = 0.63, 0.42, 0.46, 0.46, M = 0.49, Delta = -0.15

Measured using three methods:

[Zilch]

AR3a lowpass filter SPICEd.

Maybe someone can do a more sophisticated job of it than me; Le of #4 is 0.76 mH. It's clear how AR3a gets from point A to B:

[Rlowe]

I sit corrected!

If you can ever dig out that raw 12" data, it would be great. I can't find mine.

-k

Does this help?

[speaker dave]

No markings. #1 is shortwire, #2, longwire, and #3 & 4, yellow chromate, have integrally stamped lands for the surrounds.

Here's the data:

With lowpass:

Re = 3.37, 3.15, 3.36, 3.34, M = 3.31 Ohms

Fc = 36.92, 34.46, 35.25, M = 35.67 Hz

Qtc = 0.83, 0.57, 0.59, 0.59, M = 0.65

No filter:

Re = 2.60, 2.40, 2.63, 2.62, M = 2.56, Delta = -0.74 Ohms

Fc = 43.89, 41.89, 43.56, 44.94, M = 43.57, Delta = +7.90 hz

Qtc = 0.63, 0.42, 0.46, 0.46, M = 0.49, Delta = -0.15

Measured using three methods:

Interesting curves.

You can see that the considerable crossover inductance changes the corner shape significantly by pushing the corner down, and sharpening it up.

David

[speaker dave]

Does this help?

Not the raw curve (without crossover) but still interesting.

Guys, 89 dB for 2.83 volts? Almost qualifies as high efficiency! (Okay, 86 for 1 watt into 4 ohms.)

David

[Carlspeak]

So now we've learned from Z's fine work and Lowe's post that the inclusion of a LP filter truncates the right side of the curve and lowers the Fc.

What does that mean for Z's original post comparing the various speakers which had the LP filters included? I guess perhaps if the filters were different on each speaker, the filter could have influenced the differences seen? If so, so what? Their inherent performances are still different regardless which, I suspect, Z was trying to point out.

[genek]

So now we've learned from Z's fine work and Lowe's post that the inclusion of a LP filter truncates the right side of the curve and lowers the Fc.

What does that mean for Z's original post comparing the various speakers which had the LP filters included? I guess perhaps if the filters were different on each speaker, the filter could have influenced the differences seen? If so, so what? Their inherent performances are still different regardless which, I suspect, Z was trying to point out.

The discussion of crossovers was largely the result of trying to correlate Zilch's 3a curve to similar ones in old AR literature, and if we now know that both were done with filter in place, that would eliminate with/without crossover as a possible explanation for the differences...?

Villchur claimed that his curve was consistent with repeat measurements taken in an actual anechoic chamber, so I guess the next step would be to repeat Zilch's in one and see what happens.

I hope the entire point of this thread hasn't been just to point out that the speakers tested have different performances, because I don't recall anyone being under the impression that they were all the same.

[Pete B]

No markings. #1 is shortwire, #2, longwire, and #3 & 4, yellow chromate, have integrally stamped lands for the surrounds.

Here's the data:

With lowpass:

Re = 3.37, 3.15, 3.36, 3.34, M = 3.31 Ohms

Fc = 36.92, 34.46, 35.25, M = 35.67 Hz

Qtc = 0.83, 0.57, 0.59, 0.59, M = 0.65

No filter:

Re = 2.60, 2.40, 2.63, 2.62, M = 2.56, Delta = -0.74 Ohms

Fc = 43.89, 41.89, 43.56, 44.94, M = 43.57, Delta = +7.90 hz

Qtc = 0.63, 0.42, 0.46, 0.46, M = 0.49, Delta = -0.15

Measured using three methods:

Nice work, did you use a good DVM to measure the DC resistance, or was

that reported by the WT?

My interpretation is that the no filter case provides the true Fc since the XO

inductance shifts the phase measurement lower. However, the with filter

measurement provides a better measure of Qtc, since the DC resistance of

the XO inductor alters Qts by way of Qe. You could meausure the DC resistance

of the XO inductor, then use that value in series to the WT to confirm this point.

I believe that AR claimed a Qtc of about .7 for the 12" woofer driven by a low

impedance source so your .65 value is close, and suggests that your amplitude

measurements should be closer to the curve provided by AR above.

Do your systems have the 2.85 mH/150uF values or the older ones?

Thanks for the data.

[Pete B]

Not the raw curve (without crossover) but still interesting.

Guys, 89 dB for 2.83 volts? Almost qualifies as high efficiency! (Okay, 86 for 1 watt into 4 ohms.)

David

I agree and noticed years ago that the AR system is not really low efficiency by todays standards.

It is 2pi which provides significant gain over 4pi but this is how most modern drivers are measured.

[Zilch]

Nice work, did you use a good DVM to measure the DC resistance, or was

that reported by the WT?

Re is as reported by WT2. I do have a couple of Flukes here, and CLIO has a high-res multimeter, as well, so I could confirm these values via other means if they are at issue.

What does that mean for Z's original post comparing the various speakers which had the LP filters included? I guess perhaps if the filters were different on each speaker, the filter could have influenced the differences seen? If so, so what? Their inherent performances are still different regardless which, I suspect, Z was trying to point out.

I wanted to illustrate the comparative bass performance of various acoustic suspension speakers I have on hand, using multiples of each, typically, and the spectrum is clear. Surprisingly, (or not, perhaps,) the Advents perform similarly to AR3a.

I don't believe anyone has ever done this, or similar, even. In any case, the data is now available as baseline for others to replicate ... or not.

Do your systems have the 2.85 mH/150uF values or the older ones?

Both have the larger inductor, #9 is it?

Now that I'm running direct to the woofer, I can put an external filter on to verify that the original is working correctly, but given how they measure with none, I don't see how they could ever match the performance shown in the T. Holl curve; they do not have the requisite rising response.

I'm thinking only Ken can resolve this discrepancy, in his lab and/or the AR chamber.

Are there any dates stamped on the frames?

There IS this on the short wire:

[genek]

Surprisingly, the Advents perform similarly to AR3a.

I never saw similar measurements compared, but back in the 70's, listening comparisons were frequently made between the 3a, Large Advent and KLH 5 and the differences cited were seldom related to their bass, so it isn't particularly surprising to me.

Are you thinking of moving on to the upper frequencies next? This is where the distinctions always were to my ears.

Now that I'm running direct to the woofer, I can put an external filter on to verify that the original is working correctly, but given how they measure with none, I don't see how they could ever match the performance shown in the T. Holl curve; they do not have the requisite rising response.

I'm thinking only Ken can resolve this discrepancy, in his lab and/or the AR chamber.

What are the odds that refoaming has altered performance? Unfortunately, there's probably zero chance of finding a pristine original 60's-70's woofer whose original surround is in new condition to compare to.

There IS this on the short wire:

I'm mobile today, so I can't make out that last photo, not that I'd necessarily be able to tell anyhing from speaker codes.

[speaker dave]

No markings. #1 is shortwire, #2, longwire, and #3 & 4, yellow chromate, have integrally stamped lands for the surrounds.

Here's the data:

With lowpass:

Re = 3.37, 3.15, 3.36, 3.34, M = 3.31 Ohms

Fc = 36.92, 34.46, 35.25, M = 35.67 Hz

Qtc = 0.83, 0.57, 0.59, 0.59, M = 0.65

No filter:

Re = 2.60, 2.40, 2.63, 2.62, M = 2.56, Delta = -0.74 Ohms

Fc = 43.89, 41.89, 43.56, 44.94, M = 43.57, Delta = +7.90 hz

Qtc = 0.63, 0.42, 0.46, 0.46, M = 0.49, Delta = -0.15

Measured using three methods:

I've been staring at the curves a little more and something just doesn't add up. Most of your curves show a corner frequency around 60Hz but the calculated Fc's are in the mid 30s. The no-filter case shows a similar difference. There is also a 10dB or so droop from 100 to 500 that is at odds with any of the published AR 2pi curves. If you were to compensate for the 10dB down slope then the apparent corner frequencies would shift upwards, making the discrepency greater.

Any ideas about this? Do all of your near field curves show similar midrange slope? If you draw away in a few steps to a meter or so, does the curve flatten out (before the room effects take over)?

Regards,

David

[Pete B]

I've been staring at the curves a little more and something just doesn't add up. Most of your curves show a corner frequency around 60Hz but the calculated Fc's are in the mid 30s. The no-filter case shows a similar difference. There is also a 10dB or so droop from 100 to 500 that is at odds with any of the published AR 2pi curves. If you were to compensate for the 10dB down slope then the apparent corner frequencies would shift upwards, making the discrepency greater.

Any ideas about this? Do all of your near field curves show similar midrange slope? If you draw away in a few steps to a meter or so, does the curve flatten out (before the room effects take over)?

Regards,

David

I noted the issue and commented about the rise in this thread:

http://www.classicspeakerpages.net/IP.Boar...amp;#entry57392

It simply does not agree with the theory. Most of these measurement systems, even when using a sine test, are

windowed, because a tone burst of fairly short length is not the same a setting a signal generator to a fequency

letting the system settle and then taking a reading from a meter. A rectangular burst has a complex spectrum.

I believe that there is an issue with the gating of the signal and the window being used. Just an untested theory.

It is interesting that his cyan curve here looks to have a reasonably flat passband, why did this one end up passing

a sanity check, how was the measurement different? Is it repeatable?

http://www.classicspeakerpages.net/IP.Boar...ost&p=81313

[Zilch]

Are you thinking of moving on to the upper frequencies next? This is where the distinctions always were to my ears.

Nope. All done with that.

What are the odds that refoaming has altered performance? Unfortunately, there's probably zero chance of finding a pristine original 60's-70's woofer whose original surround is in new condition to compare to.

I doubt different surrounds would account for these discrepancies....

Any ideas about this? Do all of your near field curves show similar midrange slope? If you draw away in a few steps to a meter or so, does the curve flatten out (before the room effects take over)?

I can certainly do a quick draw-away study, and also measure with a Behringer RTA here, but there have been many instances such as the KLH-17 shown in Post #11 where the curves have departed significantly from this pattern.

Pete and I have had this debate previously with respect to Large Advents and whether they already have BSC built in or not, and then, as here, I cited the SPICE transfer function of the lowpass filter. Tell me how it is possible for a woofer that does not have significantly rising response to measure flat once the illustrated lowpass filter is applied.

Otherwise, what are we thinking, microphone proximity effect?

Does the response peak necessarily occur at Fc in closed-box alignments?

I note that Atkinson characterizes the Smaller Advent as "slightly underdamped." With a nearfield peak of 5 dB? Further, while Fc = 46 Hz, the response peak occurs at ~80 Hz. I'm not sure we're talking the same language here, and it's the reason I asked if you were correcting to anechoic response before applying the concept of "Q." It all makes sense if that is done. I'm going to have to go back and read Small again, apparently, as well....