Possible reasons for low sensitivity in a full range ESL
I'm having a sensitivity "problem" with a full ranger ESLs. Their specs are:
- The radiating area is approx 67cm x 200cm
- Panels' capacitance is around 2,7nF
- 45-49% open area carbon steel stators
- Step-up trafo is 1:75 from Plitron
- D/S spacing is 3-3.5mm
- bias supply is variable between 3.5 ... 7kV
- there is a 100meg HV resistor in series with the membrane
- membrane is aluminum coated mylar
- Polycarbonate/acryl spacers
- Nylon screws clamp the stators together
The measured sensitivity is around 75dB/2.83Vrms/1kHz/1m with ~5kV bias, which seems very low for this big stats. I measure the sensitivity in my room, so room modes affect the readings, I can get the readings peak at ~78dB if I keep changing the meter's place in front of the speaker. Membranes do not leak, the "Quad light bulb tweak" flashes in every ~3-4 seconds.
I would much appreciate speculation of possible reasons for low sensitivity. The DC bias on the membrane is somehow lower that it should be, some cancellation between the stators, the step-up trafo cannot drive capacitance this big or, or...?
Or are these readings norrmal and I just have to increase the step-up ratio in order to get more sensitivity?
The low sensitivity of your setup can easily be because of relatively large diaphragm - stator spacing and somewhat low step-up ratio.
1 : 75 is in my opinion more suited to a D/S spacing of around 1 mm.
5 kV bias is also not much for a 3 mm spacing.
Another possible source of problems is diaphragm not charged fully - problem with coating.
However in my opinion 75 db/1W is about to be expected in your case with step up ratio and voltages you mentioned.
In my experience, verifying that your ESLs are putting out the SPL they should be based on Vbias, Vsig, and diaphragm to stator spacing is best done with a NF(near field) measurement at the center of the panel. Note that panel size(height and width) does not figure into the calculation.
P = e0*Vbias*Vsig/d^2
SPL(NF) = 20*LOG(P/.00002) – 6
P = sound pressure (N/m^2)
e0 = vacuum permittivity = 8.8e-12 (F/m)
Vbias = bias voltage
Vsig = signal voltage applied between stators
d = diaphragm to stator spacing (m)
So, for your case:
P = (8.8e-12)*5000*(2.83*75)/(.003)^2 = 1.0377
SPL(NF) = 20*LOG(1.0377/.00002) – 6 = 88.3 dB
Alternatively, you can use arend-jan’s handy calculator for the generalized Walker equation:
Electrostatic Loudspeaker (ESL) Simulator
Attachment shows resulting summations match very well with theoretical calculation above.
The 75dB you mention measuring at 1m sounds a little low to me, but not that far off.
See what SPL you measure 1cm away from the middle of your panel.
If all is working well, you should expect to see output about -2dB to -3dB below theoretical output of 88.3dB.
BTW, what microphone measurement setup are you using? Have you checked its calibration with a known source?
More discussion on this topic here:
I think you are saying that you are still getting big changes in NF(near field) SPL readings with small changes in mic position which you are attributing to room modes. You could try repeating the measurement outside to test this theory, or try some other frequencies from 500Hz – 1Khz to see if this helps with possible room mode issue. SPL should be the same. If you do try measuring outdoors, you might consider bumping up the signal level by a factor of 2 or 4 to boost output by 6dB or 12dB to raise the output a bit more above ambient noise level.
Assuming your 81dB measurement is correct, this is a bit lower than I would have expected.
The discrepancy could be related to the panel, or the measurement technique.
Questions concerning your panel:
- I used D/S = 3mm when estimating the output level. I just noticed you specified a tolerance of 3mm – 3.5mm. A 3.5mm panel with 5kV bias would have a theoretical NF output level of 85.6dB. So, you can see that small changes in D/S will change the output level significantly. Can you estimate the D/S more accurately?
- What are your stators coated with, and how thick is the coating? Thicker coatings and ones with lower dielectric constant and higher bulk resistivity will lower the SPL more than the 2 – 3dB I estimated.
- a common reason for low SPL is poorly conducting diaphragms or leakage. But, you mention aluminum coated mylar and a slow blinking charge indicator. The only problem you could have here is if the diaphragm contact is poor.
- how are you measuring your bias voltage? are you confident the 5kV is accurate.
Questions concerning measurement technique:
- Yes, the digital RS meter is known to yield fairly accurate SPL readings below 1kHz. Were you using the “C” weighting setting? At 1khz it shouldn’t change much with the other settings, but you might check it if you were using the “A” weighting setting.
- How are you measuring/setting the 2.83Vrms level?
Oh, I forgot to ask, are your panels flat? or curved/faceted.
Sounds like curved ..... :confused:
The stators are double powder coated with epoxy based deep shiny black powder coat and there is sprayed 2.1kV/mil epoxy based conformal coating on the backside (the side facing the membrane) of the stator for extra insulation. Maybe couple of mils of the conformal coating, but I'm planning to add some more.
The contact to the membrane is done by soldering the lead to a copper tape, that has conductive glue, and it has been glued to the membrane.
I have not actually measured the bias voltage, in that I'm counting on the person who made the bias supplies.
I measured the SPL with C-weighting, "max" setting and fast reaction time (125ms average). The 2.83Vrms is measured with a multimeter, I have checked that it's reading stays constant to several kilohertz, but it does not measure 20kHz sine's amplitude correctly anymore (the reading is lower than it should). The meter is UNI-T UT33A. I also have a 500MHz digiscope but I have trusted the meter to be fairly accurate.
Sounds like you have all your measurement techniques and equipment in good shape, and diaphragm contact isn't an issue.
Two thinks that stick out to me from your description.
1) You don't know for sure what your bias voltage is: Even if you don't have a HV probe, you should be able to get a pretty good idea of what it is by measuring the DC voltage across the first capacitor in the multiplier chain with your DVM, and then multiplying by the number of capacitors in the multiplier. Do you have a picture or schematic of the HV supply?
2) Epoxy powder coating: I am not an expert on powder coating materials for ESL stators, but from reading on this forum I believe Nylon 66 is the powder coating material of choice. Epoxy coatings are not conductive enough and much of your bias supply voltage winds up across the coating rather than in the air gap where you want it. You want a coating that is an insulator, just not a really good one. This may account for the additional 4dB - 5dB discrepancy. You should be able to conteract this by cranking up the bias voltage to compensate.
Hopefully Calvin or one of the other guys with coating experience will chime in with their opinion on epoxy coatings .vs. alternatives.
Is the multiplier ladder fed with low frequency(50 Hz) current?
If so, the multiplier can be significantly loaded by the input impedance of your voltmeter - even when measured at first stage.
So you can see incorrect reading.
Simulation shows this should be a problem(with 0.01uf caps) if multimeter has an input impedance of 1 megaohm, but in case of 10 megaohms or more the effect is small.
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