Amplitude testing question

[Simon Coffin]

Hi Folks,

After a very long hiatus I'm starting to take another look at some powered wedges I made a few years ago. Shortly after I made them my business changed and I shifted to multi use boxes. But now, with the move last year to Pennsylvania and less shows as well as different shows on the calander, I have both the time to test them and the need for a good powered wedge.

The wedges use a Radian Audio 5312 coax, more or less the same driver used in the Microwedge, and a SpeakerPower dual channel amp with dsp. They sound reasonable, and can certainly get very loud, but I've always had the sense they could do with some optimization, especially in the crossover region which is a weak spot for this driver.

Two weeks ago I got FuzzMeasure, an acoustic meaurement app for the Mac and started to learn how to use it and interpret the data it was showing. So far I've adjusted the delays between the drivers to match and done some near field testing of the ports, which interestingly enough have a distinct peak at 50hz, right where I planned it. Right now I'm looking at the low freq driver to see how it is reacting with the ports and the cabinet.

Can someone explain to me how to best measure a driver's impedance and how to interpret the results? I remember seeing a schematic for a circuit that you put in-line between the amp and driver which allowed you to measure impedance as a function of voltage, but haven't been able to find it again. Are the standalone Impedance testers the best route, or are there other methods?

Thanks,
Simon Coffin

 

[Jason Lavoie]

Re: Impedance testing question

this Dayton Audio Dayton Audio Test System | DATS has been very handy. and seems to be the cheapest way to get an accurate impedance sweep.
plugs into USB and is small enough that I keep it in my laptop bag at all times
you can find them through resellers often for less than list price also.

Jason

 

[John Roberts]

Re: Impedance testing question

You can directly measure impedance vs frequency by driving the speaker with a known resistor in series with a voltage source, and then measuring the amplitude drop formed by the resistance divider between the known fixed R and unknown shunt leg to ground (like a pad).

I actually built this into my old TS-1 audio test set. As i recall I used a 51 ohm resistor in series with the sine wave generator output, that was normalized to the dB meter input so the drop would show up automatically. Just set it to 0VU with no load, then look for how much it drops with the speaker connected. In my owners manual I had a look up table for dB drop to ohms. But if you work with volts to volts, the linear voltage divider math should be easier.

JR

 

[Art Welter]

Re: Impedance testing question

Can someone explain to me how to best measure a driver's impedance and how to interpret the results? I remember seeing a schematic for a circuit that you put in-line between the amp and driver which allowed you to measure impedance as a function of voltage, but haven't been able to find it again. Are the standalone Impedance testers the best route, or are there other methods?

Simon,

One can measure impedance with a resistor, a VOM, a sine wave generator with frequency readout, amplifier and a calculator, but because most VOM measure differently at different frequencies that method requires re-calibration for accuracy over a wide range , quite a lot of work.
Rob explains how to do it here:
http://sound.westhost.com/tsp.htm

I'd go with Jason's suggestion of DATS.
In either case make sure the room is quiet when you test.

Phase, amplitude and impedance are important for getting passive crossovers correct, but since you are using DSP, why is measuring impedance important to you?

Art

 

[Simon Coffin]

Re: Impedance testing question

Simon,

Phase, amplitude and impedance are important for getting passive crossovers correct, but since you are using DSP, why is measuring impedance important to you?

Art

Hey Art,
Thanks for the link. Lots of good background information. Yes, you're right, I am using DSP. I'm interested in the impedance to try and see how the driver is relating to the ports and any other resonances in the cabinet. Am I correct in saying that there should be an impedance peak at a resonant point? It seems that most ported cabinets will exhibit a double impedance peak, although I'm not certain why.

Is there anything to learn by comparing the free air impedance graph of the driver and its in-cabinet graph?

Phase is the next thing I want to look at as it seems important in the crossover region to ensure the drivers sum constructively. This driver already has a weakness at x-over, the specs show a ~10db drop at 900hz. I've tried eq boosts, but the results are ambigious, (an 8db boost gives me a 2db result) which makes me think I have some destructive summing going on. Given this, I want to get the drivers summing as well as possible acoustically, before I start boosting anything.

The consensus seems to be that DATS is the way to go. Does seem like a very useful tool.

Thanks,
Simon

 

[Robert Lofgren]

Re: Impedance testing question

Simon,

One can measure impedance with a resistor, a VOM, a sine wave generator with frequency readout, amplifier and a calculator, but because most VOM measure differently at different frequencies that method requires re-calibration for accuracy over a wide range , quite a lot of work.

While you can do it that way it is much easier to add an extra full-bridge rectifier and a capacitor to smooth the signal. Then you can use any dc-capable volt meter and measure it accuratly.

 

[John Roberts]

Re: Impedance testing question

While you can do it that way it is much easier to add an extra full-bridge rectifier and a capacitor to smooth the signal. Then you can use any dc-capable volt meter and measure it accuratly.

Except for the diode drops....

The diode forward voltage will be almost constant vs the voltage divider, making small outputs look even smaller.

perhaps if you use large enough voltages the diode drop will be relatively small relative to the rest but then the series resistor may need to handle some serious power.



JR

 

[Mark DeArman]

Re: Impedance testing question

My preferred method for measuring impedance, is still the high-CMR Instrumentation amp, dual channel method. Calibration of it is easy. Provides coherent results. Find an non-inductive current sense resistor of value 0.01 - 1 Ohm in your required power. Find an instrumentation amplifier which will reject the differential voltage up to your desired max power. Measure the voltage drop across the current sense resistor. Measure the voltage across the loudspeaker. Cost to build is very low. I think LinearX sells a box also.

 

[Ivan Beaver]

Re: Impedance testing question

I prefer to use TEF-as it will also show the phase of the impedance.

Not such a big deal for port tuning-but is good for making sure a crossover is not reactive one way or the other.

But it is not a cheap "rig".

 

[David Sturzenbecher]

Amplitude testing question

I think LinearX sells a box also.

Yes, this is the LinearX V-I Box. I have used this with very good results.

However the OP is in a unique situation with the speaker power amp as the output is essentially bridged and what I will call biased (constant DC output on both pos and neg). So if you plan to use the amp as a source for your test, the gear will need to account for that. The VIBox will not.


Sent from my iPhone using Tapatalk

 

[Art Welter]

Re: Impedance testing question

I'm interested in the impedance to try and see how the driver is relating to the ports and any other resonances in the cabinet. Am I correct in saying that there should be an impedance peak at a resonant point? It seems that most ported cabinets will exhibit a double impedance peak, although I'm not certain why.

Is there anything to learn by comparing the free air impedance graph of the driver and its in-cabinet graph?

Phase is the next thing I want to look at as it seems important in the crossover region to ensure the drivers sum constructively. This driver already has a weakness at x-over, the specs show a ~10db drop at 900hz. I've tried eq boosts, but the results are ambigious, (an 8db boost gives me a 2db result) which makes me think I have some destructive summing going on. Given this, I want to get the drivers summing as well as possible acoustically, before I start boosting anything.

Simon,

A ported cabinet basically splits the single less extreme impedance peak of a sealed cabinet in to two larger peaks, the impedance minima between the two also corresponds to the excursion minima which occurs at Fb. You can see them in the attached simulation.

I use a sine wave tone sweep to determine by excursion (a white dot on the cone makes excursion easy to see) that Fb is as designed, the impedance is really not much of a concern to me, as I know it will be at minima at the excursion minima, and can assume it will be near the voice coil DCR.

As far as your statement "This driver already has a weakness at x-over, the specs show a ~10db drop at 900hz" , the "driver" is a coax, two drivers and a horn.
If both drivers individually (unprocessed) show a 10 dB drop in the crossover region, the "driver" is broken ;^).

More likely you mean that the combined response results in a dip at crossover, which could indicate that there is a polarity reversal or time alignment problem.
First try reversing the HF polarity, and see if the 900 Hz hole improves.
Generally the HF driver needs to be delayed, if you incrementally delay the Hf driver and the dip becomes less, you are going in the right direction. Be careful not to mistake a polarity reversal for a time alignment problem, or the crossover could be "in phase", but the HF could lag by one or more cycles in the crossover region. Remember that the speed of sound is around 1130 feet per second, so 900 Hz is 1.25 feet, and a foot takes sound a little less than one millisecond to travel.

Since the natural rolloff of each driver is usually quite different, to make the acoustic crossover balanced may require different slopes and out of band EQ on each, and may also require underlap or overlap, the LF for instance at 900 Hz and the Hf 925 Hz.

The process requires juggling a lot of different parameters, different slopes may affect the polarity and delay time differences.

Ultimately, when everything is aligned properly the phase will be flat or a smooth transition throughout the crossover region.
After alignment, reversing polarity of one driver will result in the maximum reduction at crossover, which in theory would be a complete cancellation, but in the real world will usually be somewhere between 10-20 dB.

Art

 

[Mark DeArman]

Re: Impedance testing question

I prefer to use TEF-as it will also show the phase of the impedance.

Not such a big deal for port tuning-but is good for making sure a crossover is not reactive one way or the other.

But it is not a cheap "rig".

I'm pretty sure the TEF uses the method I spoke of above. CLIO QC and SoundCheck also have software that works this way.

 

[Mark DeArman]

Re: Amplitude testing question

So if you plan to use the amp as a source for your test, the gear will need to account for that. The VIBox will not.

Yes. The VIBox is missing the instrumentation amplifier part and can be very dangerous to your gear if you just plug it in willy-nilly. But it has XLRs on it

 

[Mark DeArman]

Re: Amplitude testing question

However the OP is in a unique situation with the speaker power amp as the output is essentially bridged and what I will call biased (constant DC output on both pos and neg).
Sent from my iPhone using Tapatalk

Why does it have a constant DC output on both positive and negative and why is it bridged?

Isn't the speakerpower module just an BO IcePower module and DSP? It has differential amp outputs I believe.

Edit: I guess not.

 

[Simon Coffin]

Re: Impedance testing question

Hey Art,

If you look at the specs from Radian Audio, the response chart for the 5312 coax shows the ~10db dip at just under 1Khz that I talked about. This is caused by the natural rolloff of each driver. I tested them both full range and they are quite a few db down at 1khz. I find it a bit surprising that Radian's passive crossover can't bring up the response in the x-over region some more, but maybe it's really a driver limitation as opposed to a summing error. Interestingly enough their new Apex 1200HD wedge doesn't show the 1khz dip, so maybe it uses a different driver or crossover. The Biamp settings have the crossover point at 1.5khz and no significant boosts so the driver is probably different as my low driver is at least 15db down at that point.

In order to make the testing easier, I think I'm going to use my Omnidrive and a seperate amp, then translate the dsp settings to the Speakerpower amp. It takes way to long to make changes in the Speakerpower dsp.

I suspect getting the phase through the crossover region to behave well is going to be the real challange here.

Again, thanks for the info.

Simon

 

[Ivan Beaver]

Re: Impedance testing question

Hey Art,




I suspect getting the phase through the crossover region to behave well is going to be the real challange here.


Simon

Getting the phase right is one of the harder things about crossover design. Unless you don't mind a dip in amplitude at crossover.

In many cases if the amplitude is correct-then there will be a sharp phase shift at crossover. Or if the phase is correct there will be a dip in amplitude at crossover.

Yes there are ways around that-to get both correct-but it requires some "unconventional" approaches.

 

[Robert Lofgren]

Re: Impedance testing question

Except for the diode drops....

The diode forward voltage will be almost constant vs the voltage divider, making small outputs look even smaller.

perhaps if you use large enough voltages the diode drop will be relatively small relative to the rest but then the series resistor may need to handle some serious power.



JR

If we output 20v from the signal generator (should be enough for impedance measurements) the load is roughly 60ohm using your 50ohm resistor. Worst case scenario is therefor a load of 50ohm.

This give us Pmax=20*20/50=8W for the resistor and the speaker gets an average of 1.5W@8ohm to play with (pun intended ) This is not a problem for some 10W resistor or two 5W.

The crossover-range voltage being zero due to the diode forward voltage drop acting as a hysteris should be neglible for output voltages that are more than 5v since we're only interested in the peak voltage anyway.

The forward voltage drop can be further recuced by using germanium diodes or transistors in a diode bridge configuration.

 

[Simon Coffin]

Re: Impedance testing question

If we output 20v from the signal generator (should be enough for impedance measurements) the load is roughly 60ohm using your 50ohm resistor. Worst case scenario is therefor a load of 50ohm.

This give us Pmax=20*20/50=8W for the resistor and the speaker gets an average of 1.5W@8ohm to play with (pun intended ) This is not a problem for some 10W resistor or two 5W.

The crossover-range voltage being zero due to the diode forward voltage drop acting as a hysteris should be neglible for output voltages that are more than 5v since we're only interested in the peak voltage anyway.

The forward voltage drop can be further recuced by using germanium diodes or transistors in a diode bridge configuration.

Hi Robert and other Gents,

To be honest, much of the discussion here has gone over my head. I'm tracking with the basics, but the details of the various test methods and electronics needed are a bit beyond me. But, since I'm treating this tuning effort as a learning experience, that's ok.

Next week I'll have time to do some more testing and will post my results along with a herd more questions about how to interpret the results.

Ciao
Simon

 

[Simon Coffin]

Re: Impedance testing question

Getting the phase right is one of the harder things about crossover design. Unless you don't mind a dip in amplitude at crossover.

In many cases if the amplitude is correct-then there will be a sharp phase shift at crossover. Or if the phase is correct there will be a dip in amplitude at crossover.

Yes there are ways around that-to get both correct-but it requires some "unconventional" approaches.

Hey Ivan,

You posted a while ago about setting up the Speakerpower dsp and using a Behringer CDX2496 for the initial testing. Have you had any issues translating the Behringer dsp settings to the Speakerpower? Do they correlate closely enough, or do you have to adjust?

Ciao
Simon

 

[John Roberts]

Re: Impedance testing question

If we output 20v from the signal generator (should be enough for impedance measurements) the load is roughly 60ohm using your 50ohm resistor. Worst case scenario is therefor a load of 50ohm.

This give us Pmax=20*20/50=8W for the resistor and the speaker gets an average of 1.5W@8ohm to play with (pun intended ) This is not a problem for some 10W resistor or two 5W.

The crossover-range voltage being zero due to the diode forward voltage drop acting as a hysteris should be neglible for output voltages that are more than 5v since we're only interested in the peak voltage anyway.

The forward voltage drop can be further recuced by using germanium diodes or transistors in a diode bridge configuration.

Have you actually done that?

Driving the speaker with 20V may get louder than is comfortable, and require a power amp, not just a signal generator.

I actually built this impedance feature into a piece of value test equipment I designed and sold back in the '80s.. I used a modest stimulus voltage (0dBu) and only a 1/4W sense resistor. I did have a precision voltmeter (actually a dB meter). It was test equipment after all...

JR