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Author Topic: Mixer summed mono voltage?  (Read 15508 times)

Art Welter

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Re: Sub Measurements
« Reply #30 on: August 16, 2014, 06:36:49 PM »

1)I mean, I'm gonna get in thermal trouble where current is highest, right?
2)So shouldn't I set RMS limiting based on a 3 ohm load, and play with raising it till I measure compression or distortion  ....particularly in the low 40's hz?
3)Peak is about mechanical failure, right?
4)Is mechanical distortion more likely to be audible at a particular frequency based on impedance, (or other criteria)?
5) I'm trying to reconcile the number of people who have reported using bridged amps on single labs (when in a bank), with wattage typically in the 3000 @ 4 ohm category. 
Maybe they just never got hammered with low 40 hz music?
1)The worst point for thermal is at Fc, where excursion and impedance are lowest.
2) The impedance drops below 3 ohms at Fc, closer to 2 ohms.
3) Or to limit distortion, hard to tear up a Lab 12.
4) If the horn is driven below Fc, distortion goes through the roof, exceeding 100% without much power at all.
5) 3000 watt peaks for a pair of drivers, no problem. Sustained 38 volt sine wave, and you can smell the adhesives cooking  within a few seconds.

Although this is about a tapped horn, rather than a FLH, there is some useful info about the Lab 12s in it:

http://www.diyaudio.com/forums/subwoofers/185588-keystone-sub-using-18-15-12-inch-speakers.html

As far as the flanking cabinets, they could be pieces of plywood and have the same effect, the SPL  increase due to the boundary is what I was suggesting you measure.
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Ivan Beaver

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Re: Sub Measurements
« Reply #31 on: August 17, 2014, 10:02:07 AM »


As far as the flanking cabinets, they could be pieces of plywood and have the same effect, the SPL  increase due to the boundary is what I was suggesting you measure.
Yet another "'ol timers thing" that works quite well.

We used to call them "barn doors".

Forward directivity is a GOOD thing.  It increases the level out front-reduces the level in the rear and is cheap. 

Acoustic issues are better addressed physically-rather than electrically. Yes today we can do all sorts of "steering" and such-but still the better SONIC result is done physically.  Every thing from less drivers with less interaction-to physical pattern control-to sub cabinets with directivity etc.

But I admit that is not always possible due to some restraints.
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Mark Wilkinson

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Re: Sub Measurements
« Reply #32 on: August 19, 2014, 11:14:51 AM »

1)The worst point for thermal is at Fc, where excursion and impedance are lowest.
2) The impedance drops below 3 ohms at Fc, closer to 2 ohms.
3) Or to limit distortion, hard to tear up a Lab 12.
4) If the horn is driven below Fc, distortion goes through the roof, exceeding 100% without much power at all.
5) 3000 watt peaks for a pair of drivers, no problem. Sustained 38 volt sine wave, and you can smell the adhesives cooking  within a few seconds.

Although this is about a tapped horn, rather than a FLH, there is some useful info about the Lab 12s in it:

http://www.diyaudio.com/forums/subwoofers/185588-keystone-sub-using-18-15-12-inch-speakers.html

As far as the flanking cabinets, they could be pieces of plywood and have the same effect, the SPL  increase due to the boundary is what I was suggesting you measure.

Art, thanks for your answers .... and the keystone threads are very interesting.

My deck is only 10 ft wide where it is most reflection free. .....
So I've focused on understanding impedance and power handling for now, until I figure out an easy, relatively reflection-free way to try measure different arrangements and baffles.

I've checked impedance using a rackmount Audio Tool Box to generate constant level sine waves, and have the amps deliver 1.0v  to the speakers. And then measure actual current being drawn at each freq.
The lowest impedance i can measure is 2.71 ohms at 39 hz, with readings below 3 ohms from 37 -41 hz. This is measuring 1 box in a bank of 3, all three driven the same.
Is the 2.2 ohm minimum you use, from the keystone or labhorns?
Just trying to make sure my impedance measurement technique is valid...

And I plain don't get how impedance can fall below the voice coils resistance.... pls help !

But I get what happens below Fc....

It seems from digging in old posts, that you guys have found that heat dissipation, particularly when trying to bump up the labhorns response at 40hz, is the labhorn's achilles heel.
I've put some temperature probes on a driver and hope to play around a little with em today.

Based on 38 volts frying,  I think I'll try 30 volts at  39hz and watch the temp....I get that should put about 5.5 amps on the coil...
Another I don't get........ how coils take all the current with their thin wiring.....  I know it comes down to how long do they have to take it, but still.......

I've ended up running each driver off parallel amp channels, to keep loads above 4 ohms.   Seems to sound the same as running drivers in parallel and amps in bridge mode, at low volume levels. But when pushed, the channel per driver seems to stay cleaner longer.  Maybe I'm giving up a little power, but clean trumps IMO. (Using a plx 3402 or equiv per box.)

I like the idea of Wayne Parham's cooling plug http://www.audioroundtable.com/PiSpeakers/messages/17999.html 
But I don't see how much heat it would remove since labhorn cover plates always end up abutting each other unless you separate them...   
Hey can we put baffles between labhorns to any good acoustic effect, and give the plates space to dissipate heat?
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Ivan Beaver

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Re: Sub Measurements
« Reply #33 on: August 19, 2014, 12:52:55 PM »

Art, thanks for your answers .... and the keystone threads are very interesting.

My deck is only 10 ft wide where it is most reflection free. .....
So I've focused on understanding impedance and power handling for now, until I figure out an easy, relatively reflection-free way to try measure different arrangements and baffles.

I've checked impedance using a rackmount Audio Tool Box to generate constant level sine waves, and have the amps deliver 1.0v  to the speakers. And then measure actual current being drawn at each freq.
The lowest impedance i can measure is 2.71 ohms at 39 hz, with readings below 3 ohms from 37 -41 hz. This is measuring 1 box in a bank of 3, all three driven the same.
Is the 2.2 ohm minimum you use, from the keystone or labhorns?
Just trying to make sure my impedance measurement technique is valid...

And I plain don't get how impedance can fall below the voice coils resistance.... pls help !

But I get what happens below Fc....

It seems from digging in old posts, that you guys have found that heat dissipation, particularly when trying to bump up the labhorns response at 40hz, is the labhorn's achilles heel.
I've put some temperature probes on a driver and hope to play around a little with em today.

Based on 38 volts frying,  I think I'll try 30 volts at  39hz and watch the temp....I get that should put about 5.5 amps on the coil...
Another I don't get........ how coils take all the current with their thin wiring.....  I know it comes down to how long do they have to take it, but still.......

I've ended up running each driver off parallel amp channels, to keep loads above 4 ohms.   Seems to sound the same as running drivers in parallel and amps in bridge mode, at low volume levels. But when pushed, the channel per driver seems to stay cleaner longer.  Maybe I'm giving up a little power, but clean trumps IMO. (Using a plx 3402 or equiv per box.)

I like the idea of Wayne Parham's cooling plug http://www.audioroundtable.com/PiSpeakers/messages/17999.html 
But I don't see how much heat it would remove since labhorn cover plates always end up abutting each other unless you separate them...   
Hey can we put baffles between labhorns to any good acoustic effect, and give the plates space to dissipate heat?

Since you have an audio toolbox-why not just use the impedance sweep to get the impedance.  That way it is faster-you get more steps etc

I have found it to be pretty accurate-at least accurate enough for most uses.

How are you putting the temp probes on?  Remember that the rear chamber has to remain SEALED or else the impedance/tuning etc will change.

Also remember that it takes time to heat up the magnet assembly and the voice coil could die long before the magnet gets hot.

Yes any boundary will help give directivity to low freq.
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Ivan Beaver
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Mark Wilkinson

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Re: Sub Measurements
« Reply #34 on: August 19, 2014, 03:33:39 PM »

Since you have an audio toolbox-why not just use the impedance sweep to get the impedance.  That way it is faster-you get more steps etc

I have found it to be pretty accurate-at least accurate enough for most uses.

How are you putting the temp probes on?  Remember that the rear chamber has to remain SEALED or else the impedance/tuning etc will change.

Also remember that it takes time to heat up the magnet assembly and the voice coil could die long before the magnet gets hot.

Yes any boundary will help give directivity to low freq.

I ran individual sine waves because I wanted to get as accurate as possible, and compare manual measurements to the sweep function's 1/3 or 1/12 octave averaging.
Plus, making manual measurements has helped restore a bit of needed familiarity with ohms law and other basic audio math....which my rusty mellon sorely needs :)

I ran the temp probe leads through one of the cover plate bolt holes and sealed it up with silicone.

Yep, I've already seen that the magnets heat too slowly and that the coil will die long before I read much of a temp change. Gonna relocate a probe as deep in the vent hole as i can, but still think it will lag way to much for determining imminent danger.

This has me rethinking that what I really want to find is the onset of thermal compression. I mean, my really interest is how loud can i get and maintain linearity, not where they smoke...
I'm thinking to try incrementally higher voltages and watch for when amperage from the amp starts to drop from coil heating..... looking for a drop within 1 minute as a first stab.. ......??

Have you used the ATB's distortion meter?  Accurate enough for general purposes, like with the impedance sweep?

Thx,   Mark
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Art Welter

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Re: Sub Measurements
« Reply #35 on: August 19, 2014, 06:28:11 PM »

Art, thanks for your answers .... and the keystone threads are very interesting.

1)Is the 2.2 ohm minimum you use, from the keystone or labhorns?
2)And I plain don't get how impedance can fall below the voice coils resistance.... pls help !
3)It seems from digging in old posts, that you guys have found that heat dissipation, particularly when trying to bump up the labhorns response at 40hz, is the labhorn's achilles heel.
4)Another I don't get........ how coils take all the current with their thin wiring.....  I know it comes down to how long do they have to take it, but still.......
5)But when pushed, the channel per driver seems to stay cleaner longer.  Maybe I'm giving up a little power, but clean trumps IMO. (Using a plx 3402 or equiv per box.)
6)Hey can we put baffles between labhorns to any good acoustic effect, and give the plates space to dissipate heat?
1)From all the sims and measurements I've seen, whether TH, FLH, or BR, the impedance minima is usually near DCR.
http://forums.prosoundweb.com/index.php/topic,134223.0.html
Silas Pradetto said he measured LabHorn dips at certain frequencies that are at about 2 ohms, actually they probably about half the Lab 12 DCR of 4.29 since two drivers are in parallel. 2.2 ohms is "close enough for rock and roll", depending on the current limiting scheme in the amp you use.
2) It does not.
3) The Lab 12 is definitely not good at getting rid of heat compared to many modern drivers, and when closed up in a tiny compression chamber, that heat is contained.
4) But still, with most music (that I like...) the average heat is 1/10th of the peak power or less, the thin wire can take it when it is moving and can wick to the magnet structure. Once the magnet structure and the box air heats up, the voice coil temp rises faster.
Even with the magnets exposed, with the Speakerpower SP4000 running pink noise (12 dB crest factor) the Lab 12s lost headroom in minutes.
5) Start using an amp that does not current limit like the PLX and you will hear the thermal compression faster :^).
6) You could, but the thermal dissipation capability still won't be what it would be with larger voice coils designed with better thermal dissipation capability. Wayne's thermal device is kind of like hot rodding a Model T engine, when there are 426 Hemis available.

As far as the Audio Toolbox distortion readings, I found them quite limited compared to the DIY approach outlined in the Keystone (and other) threads. That said, there are free programs now that do a good job of THD measurement without the bother, but they would require using a PC rather than Mac, which leaves me out.

Art
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Ivan Beaver

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Re: Sub Measurements
« Reply #36 on: August 19, 2014, 08:10:01 PM »


This has me rethinking that what I really want to find is the onset of thermal compression. I mean, my really interest is how loud can i get and maintain linearity, not where they smoke...

Thermal compression and how loud can often be very different things.  It really depends on the dynamic range of the material-the duration etc.

I started doing a dual FFT measurement for power compression long before it got popular-back around 2000  (and is now part of an AES standard-but I didn't get an credit for it :(  ).

This allows you to find exactly the point of power compression at any  dB reduction you want.  3dB is "kinda standard".

Think of  heating of the voice coil like this-it is  like a candle flame.  You can hold your hand over it for quite awhile-if it is a long distance away and the closer you get it-the shorter that time becomes.

It is heat over time that kills most speakers.  So a lot for a little while or a little for a long while and everything inbetween.

There is NO easy or correct answer.

If you set your limits for the point of power compression- you are going to need a lot more rig to do the gig-because you will be missing the peaks.

As a "general rule" if you are doing bass intensive work (EDM-Dance-rap etc) then setting a slow attach (2 or 3 seconds) limiter at about 1/2 of the continuous rating of the speaker is a good starting point.  Sine waves can quickly kill a loudspeaker.

It is not a bad idea to have another limiter around the voltage of the continuous rating (with an attack time of maybe 20-30ms) and then a fast attack limiter at the peak rating (or 4 times the power or twice the voltage) of the continuous rating.

But you still have low freq excursion issues to deal with-so a freq selective limiter (as a side chain) is not a bad idea.

For example you can drive it really low if you don't drive it hard.  The harder you drive it, the less low freq you can get out of it.

An RTA (at least 1/6th octave) and a sine wave generator is a good setup to look at the distortion of the lower freq.  Put in a sine wave  (let's say 30Hz) and look at it on the RTA.  Look for the levels of the distortion components (60-90-120Hz) and see how quickly they rise as you increase the level of the fundamental tone.
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Ivan Beaver
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Mark Wilkinson

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Re: Sub Measurements
« Reply #37 on: August 20, 2014, 12:48:40 PM »

1)From all the sims and measurements I've seen, whether TH, FLH, or BR, the impedance minima is usually near DCR.
http://forums.prosoundweb.com/index.php/topic,134223.0.html
Silas Pradetto said he measured LabHorn dips at certain frequencies that are at about 2 ohms, actually they probably about half the Lab 12 DCR of 4.29 since two drivers are in parallel. 2.2 ohms is "close enough for rock and roll", depending on the current limiting scheme in the amp you use.
2) It does not.
5) Start using an amp that does not current limit like the PLX and you will hear the thermal compression faster :^).
6) You could, but the thermal dissipation capability still won't be what it would be with larger voice coils designed with better thermal dissipation capability. Wayne's thermal device is kind of like hot rodding a Model T engine, when there are 426 Hemis available.

As far as the Audio Toolbox distortion readings, I found them quite limited compared to the DIY approach outlined in the Keystone (and other) threads. That said, there are free programs now that do a good job of THD measurement without the bother, but they would require using a PC rather than Mac, which leaves me out.

Art

1&2.  Sorry, I went brain dead in even wondering if it were possible for impedance to drop below dcr..

5. What are some amps that don't current limit so quick as plx's ?  I looked at the speakerpower stuff, but not available to non oems....
      (I have 2 plx 3402, a RMX4050HD, and a PL340...I've been thinking they are all about the same animal having very close sensitivity, and +32gain to +36.)

6 & toolbox comment.  Yea, the keystone thread is letting me see how much has happened since labs came out....
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Mark Wilkinson

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Re: Sub Measurements
« Reply #38 on: August 20, 2014, 12:56:58 PM »

Thermal compression and how loud can often be very different things.  It really depends on the dynamic range of the material-the duration etc.

I started doing a dual FFT measurement for power compression long before it got popular-back around 2000  (and is now part of an AES standard-but I didn't get an credit for it :(  ).

This allows you to find exactly the point of power compression at any  dB reduction you want.  3dB is "kinda standard".

Think of  heating of the voice coil like this-it is  like a candle flame.  You can hold your hand over it for quite awhile-if it is a long distance away and the closer you get it-the shorter that time becomes.

It is heat over time that kills most speakers.  So a lot for a little while or a little for a long while and everything inbetween.

There is NO easy or correct answer.

If you set your limits for the point of power compression- you are going to need a lot more rig to do the gig-because you will be missing the peaks.

As a "general rule" if you are doing bass intensive work (EDM-Dance-rap etc) then setting a slow attach (2 or 3 seconds) limiter at about 1/2 of the continuous rating of the speaker is a good starting point.  Sine waves can quickly kill a loudspeaker.

It is not a bad idea to have another limiter around the voltage of the continuous rating (with an attack time of maybe 20-30ms) and then a fast attack limiter at the peak rating (or 4 times the power or twice the voltage) of the continuous rating.

But you still have low freq excursion issues to deal with-so a freq selective limiter (as a side chain) is not a bad idea.

For example you can drive it really low if you don't drive it hard.  The harder you drive it, the less low freq you can get out of it.

An RTA (at least 1/6th octave) and a sine wave generator is a good setup to look at the distortion of the lower freq.  Put in a sine wave  (let's say 30Hz) and look at it on the RTA.  Look for the levels of the distortion components (60-90-120Hz) and see how quickly they rise as you increase the level of the fundamental tone.

Thx Ivan, good clear useful info.   I'm gonna turn attention to measuring compression and distortion now.....
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Art Welter

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Re: Sub Measurements
« Reply #39 on: August 20, 2014, 08:59:39 PM »

5. What are some amps that don't current limit so quick as plx's ?  I looked at the speakerpower stuff, but not available to non oems....
      (I have 2 plx 3402, a RMX4050HD, and a PL340...I've been thinking they are all about the same animal having very close sensitivity, and +32gain to +36.)
The Speakerpower SP4000 is great, and is available to individuals. It is only the Speakerpower DSP units that require a large purchase.

If you actually measure the musical SPL output of subs driven by the different amps you have, you will probably find they are different animals, even if their output ratings are the same.

http://www.diyaudio.com/forums/subwoofers/187472-speakerpower-torpedo-sp1-4000-plate-amp-review.html
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ProSoundWeb Community

Re: Sub Measurements
« Reply #39 on: August 20, 2014, 08:59:39 PM »


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