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]

Originally Posted by richie00boy

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]

Originally Posted by 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?

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