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[Len Zenith Jr]

Hi Tom, trying to understand what you are saying...

So as a start, I thought to maybe just test the effect on impedance in going from full space to quarter space..

While DATS is a program I use often and useful for finding the frequency of impedance peaks and impedance minima I don't have any faith in its ability to determine absolute impedance. I have ran 3 or 4 consecutive tests in a row and impedance maxima varies conciderably between concecutive tests in a row. Results are only as good as your tools. Give a man a watch and he will be absolutely cetain of the time. Give a man 2 watches and he will never know what time it is.

[Mark Wilkinson]

While DATS is a program I use often and useful for finding the frequency of impedance peaks and impedance minima I don't have any faith in its ability to determine absolute impedance. I have ran 3 or 4 consecutive tests in a row and impedance maxima varies conciderably between concecutive tests in a row. Results are only as good as your tools. Give a man a watch and he will be absolutely cetain of the time. Give a man 2 watches and he will never know what time it is.

Sure do agree about 2 watches (meters) !

I've had 2 DATS units fail outright, but other than that I've found repeated consecutive tests to have little if any variance.
And when I've checked DATS against an audiotoolbox's impedance sweeps, measurements looked to be the same.....I have to say "looked to be" because the ATB gives data in 1/12 oct vs the DATS continuous curve...

Just now I checked the speaker with the ATB, same conditions as yesterdays posting.  Measurements look near identical.

But that's not saying either one is accurate 

[Ivan Beaver]

Results are only as good as your tools. Give a man a watch and he will be absolutely cetain of the time. Give a man 2 watches and he will never know what time it is.

I use the term "meters" instead of watches.

Same result

[Art Welter]

Results are only as good as your tools. Give a man a watch and he will be absolutely cetain of the time. Give a man 2 watches and he will never know what time it is.

I use the term "meters" instead of watches.

Same result

And fill the same box with two different brand 18" drivers and you know it's time to change the spec sheet  .

Looks like 4-5 dB difference at 31.5 Hz to me, what's it look like to you?

Art

[Jay Barracato]

Give an engineer two meters and they will trot out an old comparison with watches and prove they really don't understand the concept of calibration...

Sent from my XT1254 using Tapatalk

[Scott Carneval]

Results are only as good as your tools. Give a man a watch and he will be absolutely cetain of the time. Give a man 2 watches and he will never know what time it is.And fill the same box with two different brand 18" drivers and you know it's time to change the spec sheet  .

Looks like 4-5 dB difference at 31.5 Hz to me, what's it look like to you?

Art

Given that the sensitivity is rated at 108db, I would say the -3 point is 40hz and it's more like -7 or -8 at 30/31.5 (depending on which graph you're looking at) 

But the box is rated at 40hz f3 on the website and 35hz f4 on the downloadable spec sheet. Not sure why these two differ, probably just an oversight. They're not necessarily conflicting specs, but different ways of rating it.

[David Sturzenbecher]

Not to spam Ivans thread, but here is an interesting white paper from David Gunness
http://fulcrum-acoustic.com/assets/pdf/whitepapers/comments-on-half-space.pdf  exploring constructive and destructive interference from the ground bounce.

We often discuss these different spaces as if the sum from a bounce is flat, but it isn't always and can have audible frequency response effects.

I now return this discussion to the speaker geniuses (not me).

JR

Very interesting. Thank you for posting.

[John Rutirasiri]

John Rutirasiri,

From your response I am assuming that you do not understand the principle of reciprocity. Basically it states that if you swap the positions of the source and receiver (microphone/ear) the result is the same, it doesn’t matter which way round they are.

Good to learn something new everyday.  However, wouldn't duality only occur under ideal conditions, such as anechoic chamber and 100% omni responses (so omnidirectional radiation, omni directional polar pattern on the mic, etc.)?

In this case of Ivan's experiment and using your logic, the measurement mic would be put on the ground to simulate half space, and hanging in the air to simulate whole space.  Say you put the foam under the mic to avoid movement, and use guy wires when hanging the mic to prevent it from swinging.  In the case of the mic on the ground, I'm not sure if the omnidirectional pattern of the measurement mic is maintained anymore.  In the case of the mic being in the air, the guy wires to prevent it from swinging would be a PITA to set up. 

IMO it is easier to just lift the sub up with a forklift, which simulates what you'd find in the real world when subs are flown.

Best,
John R.

[Ivan Beaver]

Results are only as good as your tools. Give a man a watch and he will be absolutely cetain of the time. Give a man 2 watches and he will never know what time it is.And fill the same box with two different brand 18" drivers and you know it's time to change the spec sheet  .



Looks like 4-5 dB difference at 31.5 Hz to me, what's it look like to you?

Art

When we changed drivers in the TH118 (very early on in its history), the measured freq response change was so minimal (around 0.5dB), it was not considered to be worth changing the spec sheet.  Especially when you consider that the driver to driver variance is often more than that.

But I guess if you do like a lot of manufacturers and don't publish ANY response graphs, you don't have to worry about such things. There is nothing to be held accountable to.

Seems to be REAL popular these days

A simple number that is not referenced to anything seems to be the "norm".

Yes, you would "expect" the performance to change with different drivers, but this is not always the case.  In the case of some horns, you can end up with very similar results with different drivers.  The horn itself is often the biggest governing factor.  In a ported cabinet, the driver is often the biggest governing factor.

And sometimes a slight change in driver parameters will throw the whole design off a lot.

It depends

The tapped horn I used in this measurement/experiment was part of a "experimental" project for a new product idea. Not a TH118.  I was playing around with it in the "project" and just decided to do the quick measurement outside-the bay door was open, it was a nice day, I got hit with the inspiration/idea  yada yada----------

What we have found during that project is that (in some cases) drivers with very different parameters can "even out" and the response be almost identical.

On a side note-I think some people think I used the eq between the measurements.  NO.  I used it (just a quick throw something in to even out the response a little) on the ground, and then (without changing anything electrically) we lifted the cabinet in the air.  So the SAME eq was used in both the air and ground measurements.

HOWEVER, it was not the same eq point between the different cabinets.

I just mentioned it during the post so as to be "full disclosure" of what I was doing.

[Art Welter]

When we changed drivers in the TH118 (very early on in its history), the measured freq response change was so minimal (around 0.5dB), it was not considered to be worth changing the spec sheet. 

The tapped horn I used in this measurement/experiment was part of a "experimental" project for a new product idea. Not a TH118. 

Since the chart was labeled TH118, I thought it was a TH118.

[ProSoundWeb Community]