Out of spec T/s Parameters

[oublie]

Hi Folks,

I've been measuring my 4" wharfedale drivers i've got for my line array project and the specs seem really strange.

The t/s params given with the drivers are as follows:

QTS: 0.52
QMs: 2.84
QES: 0.637
VAS: 1.62
FS: 89.98
RE: 4.8
MMS: 5.3g
SD: 43.9
PE: 75W

xmax is not given but im not too worried.

Now these drivers have a removable rear magnet so i measured them with and without the magnet attached to see what difference the extra magnet would make to the T/S params.

So now the weird bit,

I used a 13.3 Ohm resistor, Tried 2 different amplifiers, used a base frequency of 500hz in order to determine the testing voltage of approximately 500mv but all the Q parameters and FS are a bit strange!

here are the results i got

Driver name Wharfedale 4inch with magnet
Calculate Qms, Qes and Qts

Source voltage at amplifier terminals Vs 492.00 mV
Source DC resistance Rs 13.30 Ohms
Speaker voice coil DC resistance Re 4.80 Ohms

Adjust F to at least 2 octaves above resonance. Check that varying frequency causes
little or no change in voltage. Compare voltage reading with Vref - they should be within 10%

Voltage across Rs Vref 361.52 mV
Source current Is 27.18 Amps

Voltage across Rs at resonance Vm 234.00 mV
Resonance frequency Fs 64.40 Hertz

Speaker impedance at resonance (Rm) Rm 14.66 Ohms
Current flow at resonance Im 17.59 mA
-6dB current Ir 21.87 mA
r0 r0 1.54
-6dB voltage Vr 290.86 mV

Carefully adjust frequency until Vr is observed across Rs, above and below resonance. Enter the frequencies.
Frequency above Fs for stated voltage Fh 74.10 Hertz
Frequency below Fs for stated voltage Fl 56.50 Hertz
Fs sanity check (should equal Fs) 64.70 Hertz

Mechanical Q Qms 4.55
Electrical Q Qes 2.21
Total Q of driver Qts 1.49


An without the magnet

Driver name Wharfedale 4inch NOmagnet

Enter the values in the shaded boxes. Values in bold are calculated

Calculate Qms, Qes and Qts

Source voltage at amplifier terminals Vs 512.00 mV
Source DC resistance Rs 13.30 Ohms
Speaker voice coil DC resistance Re 4.80 Ohms

Adjust F to at least 2 octaves above resonance. Check that varying frequency causes
little or no change in voltage. Compare voltage reading with Vref - they should be within 10%

Voltage across Rs Vref 376.22 mV
Source current Is 28.29 Amps

Voltage across Rs at resonance Vm 247.00 mV
Resonance frequency Fs 64.50 Hertz

Speaker impedance at resonance (Rm) Rm 14.27 Ohms
Current flow at resonance Im 18.57 mA
-6dB current Ir 22.92 mA
r0 r0 1.52
-6dB voltage Vr 304.84 mV

Carefully adjust frequency until Vr is observed across Rs, above and below resonance. Enter the frequencies.
Frequency above Fs for stated voltage Fh 73.00 Hertz
Frequency below Fs for stated voltage Fl 56.90 Hertz
Fs sanity check (should equal Fs) 64.45 Hertz

Mechanical Q Qms 4.94
Electrical Q Qes 2.51
Total Q of driver Qts 1.66

As you can see these couldn't be further from the specs if i tried.

So have i measured a duff speaker (it sounds fine in an open baffle) or what am i doing wrong when i'm measuring?

Please bear in mind i haven't measured the vas yet but i would assume it is pretty close to that specced for a 4 inch driver.

Any help would be greatly appreciated.

Please note that the specs are so far out of whack that winisd cant even suggest a box size! But the original specs point to a ported box but the measured indicate closed.

I measured the speaker in the centre of my garage suspending from a wire with nothing nearby for approx 5 feet.

 

[AndrewT]

the lowest Qts of 0.52 (specified) would suit a closed box.
Your measured Qts is highly suspect, but if right, then your only sensible options are infinite baffle or open baffle

 

[richie00boy]

I see you are using Rod Elliots method. Try using a smaller resistor, less than 1 ohm. This may bring Qts down.

 

[oublie]

I see you are using Rod Elliots method. Try using a smaller resistor, less than 1 ohm. This may bring Qts down.

Thanks buddy I'll give that a go I went for the 13 ohm resistor as it was the closest to the 10 ohm suggested by rod. I'll try later with a lower resistance

 

[oublie]

Well I measured a different speaker tonight (same type) and my results are still out of whack! I won't post them here now as it's late and I'm typing this on my iPod.

Just to make sure I redid all my measurements with a 1.2 ohm resistor and a 3.3 ohm resistor but I still got the same results more or less. I did however find that when using a low ohm resistor it was quote difficult to get accurate measurements as my multimeter only do a minimum scale of 2V and 2 decimal places.

Just to confirm I'm not doing something silly I would like to list my procedure as I have done it for confirmation please.
1. Measure RE across speaker terminals
2. Measure resistance of resistor to be used e.g 13.3 ohm
3. Connect positive of the amp directly to speaker positive
4. Connect negative of amp to resistor and then to negative of speaker
5. Connect + and - of multimeter either side of resistor
6. Set frequency to 500 hertz and amp to a small signal e.g. 500 mv
7. Set frequency to that where the voltage
Is lowest I.e. 61 hertz and record the voltage e.g. 121mv
Confirm results by setting frequency above and below fs in order to check result accuracy.

Sorry if this is hard to read typing in bed on an iPod is a pita

Let me know if this sounds right please

Thanks

 

[wintermute]

It looks like you are using the constant voltage method for measuring.. I have not done this ( I use speaker workshop) but I have Joe D'Appolito's testing loudspeakers in front of me 🙂

on the AC Voltmeter he says

"Since we are interested only in the shape of the impedance curve, neither meter need be absolutely accurate. However, the voltmeter must have flat freq response to assure that the applied voltage is constant with freq and High Imput impedance so as not to corrupt the current measurement."

He also says

If an AC voltmeter is used to measure the voice coil current, select the smallest value sampling resistor that will produce reliable readings with your meter in order to maximise the electrical damping. A value of 1 ohm or less is preferable.

Also note that as you change the freq you should ensure that the voltage across the speaker terminals remains constant!

Note that the procedure you have posted seems to be a bit simplified compared to what I am reading, but hopefully this might give you something to go on 🙂

Tony.

 

[oublie]

Thanks Wintermute,

I have tested with a resistor around one ohm, i got the contant voltage level by setting the frequency to 500hz and then to 600hz and down to 400hz to ensure there was no major voltage change which there didn't seem to be.

The strange thing is everything points to my measurements being fairly accurate considering the basic equipment im using. The Impendence curve does seem to reflect what im measuring as the voltage level rises above and below the FS i found.

I think i'll redo a full set of tests tonight if i get the chance with a very low ohm resistor and a higher voltage rating e.g. 750mv to see what i can find.

The speaker were originally used in the Wharfedale Modus Cube Satellite speakers and a few of the other Modus range where the specs of the Modus cube indicate a range of 100hz to 15khz with around 30w RMS power handling. The Modus box itself is very small (not much bigger than the speaker) so it would indicate that the stated T/S values are more realistic and that my measurements are up the left.

 

[richie00boy]

Higher voltage won't help, the voltage needs to be small. Does your meter have a 200mV setting? As that used with a low ohm resistor should give good results.

Also doing your setup at 500Hz does not feel right to me, that's a frequency way out of band. If I remember correctly Rod advises around 200Hz to try to be in the linear region. 500Hz would be getting near the inductive region if not already in it.

Personally I don't like the method and would prefer to substitute the speaker for a known resistor e.g. 8.2 ohm during setup. Then at least you know your setup is accurate and not relying on the assumption that your voicecoil DCR reading and calibration voltage match.

Where are you in UK?

 

[wintermute]

Hi Richieboy, yes that is what Joe D'appolitos book says too... set the initial voltage using the known high precision resistor (edit: close to the value of Re) and then substitue in the driver, being careful to make sure the voltage accross the terminals stays constant as you change the freq.

I hadn't read Rod's guide so didn't comment, on that aspect.

Oublie just check the voltage across the speaker terminals when you are at the lower frequencies to be on the safe side 🙂 It may not show a difference at 400Hz or 600Hz but could well do at 60 Hz 🙂

Certainly your sanity checks seem to support that your measurements are good, are you sure that the freq generator is accurate?

regards,

Tony.

 

[oublie]

Higher voltage won't help, the voltage needs to be small. Does your meter have a 200mV setting? As that used with a low ohm resistor should give good results.

Also doing your setup at 500Hz does not feel right to me, that's a frequency way out of band. If I remember correctly Rod advises around 200Hz to try to be in the linear region. 500Hz would be getting near the inductive region if not already in it.

Personally I don't like the method and would prefer to substitute the speaker for a known resistor e.g. 8.2 ohm during setup. Then at least you know your setup is accurate and not relying on the assumption that your voicecoil DCR reading and calibration voltage match.

Where are you in UK?

Richie,

I'm in Northern Ireland so a long way from you,

I should have enough resistors about to cobble together a resistance similar to the re of the driver and then take some measurements.

I'll try using a lower frequency say around 200hz for calibration and bump it up and down 50hz just to check im in a linear region.


Wintermute,

The frequency generator is actually Sinegen the hardware chain is pc based - optical out - dac - amplifier

i've tried 2 different multimeters, a cheap and nasty from argos and a skytronic cheap but not so nasty.

Amplifiers are my old Nad 3020 set to lab mode and a 2020 t amp. Both are giving similar results.

i've tried 3 different drivers all the same wharfedale 4inch with the same model number and getting similar results for all.

Unfortunately i dont have a 200mv setting on my multimeters minimum setting is 2v with 3 decimal places which should be good enough.

I thought this was going to be simple 😱

 

[wintermute]

I'm wondering what you would get if you shifted all your measured frequencies by 25Hz (to match the stated fs of 89Hz... plug those in and see if you get something like the published specs... I suspect you won't

Here is another snippet from Joe D'appolito's book.

If an AC voltmeter is used for current measurement, keep the sampling resistor as small as possible while still getting reliable data from your meter. Since the voltage across the voice coil terminals must be kept constant for this test larger values of Rsamp will cause greater variation of Vtest with changing frequency, necessitating frequent adjustment of the signal-generator output level.

I really think your problem is going to be related to not keeping a constant voltage at the loudspeaker terminals 😉 Since you have two multimeters, try putting one across the terminals, and the other across your sampling resistor and see if it really is staying constant or not.

It should do with the substitute resistor (in place of the driver) but when the driver is substituted it will almost certainly change.

The other thing that he says is to do the initial reading of the current value (with the substitute resistor) at around the freq that you expect resonance to be (so in your case about 90Hz).

The procedure in Joe Appolito's book unfortunately only says how to do it using an ammeter to measure the current in the drivers voice coil, and doesn't explain how to calculate using the sampling resistor... I'm struggling with my good old V=IR at the moment (its late) and I can't work out how the sampling resistor will work... but I'm wondering... are you doing it the wrong way around? should you be looking at the max voltage across your sampling resistor not the minimum... reason I say this is that the Fs is at the lowest current through the voice coil Will the lowest current flowing through the voice coil also cause the lowest voltage drop on the sampling resistor? I guess it will as the voltage drop accross the larger resistance of the voice coil will be bigger than that over the smaller resistance of the sampling resistor... It's basic electronics but it is doing my head in tonight 😉 I think I'll sleep on it 🙂

Tony.

 

[richie00boy]

Fs will be the point of lowest current, which will also be lowest voltage across the resistor.

 

[wintermute]

yes I thought about it after I went off line.. used a simple example that I could work out in my head.. 1V across the speaker terminals and 1 ohm sampling resistor... The mv reading on the sampling resistor will = the ma flowing through the voicecoil. which can then be used to work out the resistance of the voice coil. Was just having a moment 😉

Anyway I still think if there is an issue it will be with not keeping the voltage across the voice coil constant 🙂

Tony.

 

[rcw]

The varyation in T-S parameters might be due to variations in compliance.
Even in the best drivers this is subject to variations of twenty percent.
rcw.

 

[pheonix358]

I have to ask, have you ran the drivers for a day or so before you start measuring. I am assuming they are new. They need to be run in so to speak.

Terry

 

[AndrewT]

the T/S parameters we work out are the small signal parameters.
I see mention of 500mV input (10Vac to 20Vac at the output) to the power amp and 1Vac across the speaker terminals.
These voltages are far too high.

 

[oublie]

I have to ask, have you ran the drivers for a day or so before you start measuring. I am assuming they are new. They need to be run in so to speak.

Terry

Ive had the drivers a few weeks the ones i've been testing have been well run in in an open baffle.

 

[Ron E]

Try the constant current method - use a 1kHz series resistor
search for epanorama impedometer for the method.
epanorama impedometer - Google Search

 

[rcw]

If you measure Vas, cms is the only physical driver parameter appart from diaphram area, and this is easy to measure and specify, and is not subject to large manufacturing tolerances, unlike cms.
If you measure fs, then appart from cms the only other physical driver parameter is mms, again this does not vary by much from sample to sample.
If you calculate a cms value from these two measurements these should be in close agreement, if not then the error is in the measurement tecnique.
rcw.

 

[richie00boy]

I should have enough resistors about to cobble together a resistance similar to the re of the driver and then take some measurements.

I'll try using a lower frequency say around 200hz for calibration and bump it up and down 50hz just to check im in a linear region.

Don't even bother with 200Hz, just do your calibration at 50Hz. After all this is around the kind of frequencies you are trying to measure. Also, your meter may not cope with frequencies higher than 120Hz very well.

Looking forward to your results.