[Patrick Turner]

For those left wondering what the heck the facts are about sensitivity,
here they are.

But first I must describe the conditions of the test.

A reliable source of pink noise was used.

To ascertain what signal voltages are involved,
an osciliscope was set up so that 10Vrms of a 1kHz sine wave from 300
watt SS amp
registered 1/4 of the screen vertical room, ie, 40Vrms will just fill
the screen completely.
This is 56 peak ac volts.

Pink noise was fed to the amp and adjusted for level until
the peaks in the noise were equal to the height of the screen, ie, the
maximum
peak signals were 56Vpk.

No amplifier clipping was visible.

The maximum Vrms within the noise could be up to 40Vrms.

Using a variety of DMM meters in their Vac ranges all gave 12.1Vac when
in fact there
was a maximum of 40Vrms present.
So the ratio of maximum Vrms within the noise was 40 / 12.1 = 3.3:1

The pink noise was adjusted down to measure 0.85 Vac on my Fluke, ( 2.80
Vrms max )

This was fed to the ER Audio speaker which give a very quiet output and
the
SPL metering level was set at -12dB.

The speaker leads were then changed to a pair of average sensitivity
speakers i built,
the Sublimes, shown and described at
http://www.turneraudio.com.au/loudspeakers-new.html

Now the sensitivity stated at my site for these speakers is 90dB/W/M,
but
at present I have a series resistor in front of the midrange/treble unit
which drops the sensitivity to 88dB with the same applied voltage.
Impedance is 5.6 ohms average.

The level attained with my speakers was 0.0dB at 1kHz with the 0.85 vac
of pink noise.

The SPL meter registered +/- 2dB level changes along the band for both
ERA and my own,
so there was no need to do complete response tests to confirm what was
so
plain to my ears which is that the ERA ESLs produce an SPL of 12dB
lower than my own for the same input voltage.

So to power my speakers to healthy levels of listening I would use most
days an amp capable of putting 5.6Vrms into 5.6 ohms would work, which
is 5.6 watts maximum peak power.
In practice, a 5.6 watt amp will begin to sound weak so a 25 watter is
the lowest power I can get by well on.
50 watts is a luxury, 300 watts an excess.
When measuring busy music, the Vac in my meters is rarely more than
0.85Vac, and allowing for
Vac meter reading to maximum Vrms ratio of 1:4, I get maybe 3.4Vrms, so
allowing to go to 5.6Vrms is
being generous to the music.

With clasical orchestral music, a far higher ratio between the solo
passages and all musicians occurs,
and I need the 25 watts some days for peaks.

So with 0.85Vrms applied to my 5.6 ohms speakers I have 2.8Vrms max =
1.4 Watts max.

With the ERA ESLs, I need to apply 4 times the dynamic speaker signal
voltage
to get the same SPL level, ie, 2.8 x 4 = 11.2Vrms.

The average speaker Z = approx 14 ohms, so power max = 8.96 watts.

So ERA speakers need 8.96/1.4 = 6.4 times the power to produce the same
SPL meter reading.
The SPL meter merely measures the amplified microphone voltage.

So if my dynamic speakers produce 88dB for 1 watt @ 1M,
then the ERA need 6.4 watts for the same job, or if you like,

1 WATT FED INTO AN ER AUDIO ESL-IIIB makes an SPL = 80dB at 1M.

At 3 metres away, I measured a 6 dB drop in SPL levels for both speakers
in the room,
so 1 watt max doesn't produce a huge SPL volume in the ESLs.

This is what I consider to be the official maximum possible safe working
sensitivity
attainable when they are engineered for a flat response and to ensure
absence of arcing problems,
by setting the EHT at the maximum safe level of -2,700V.

The only way to improve the sensitivity is to use much less series
resistance
between the step up transformer secondaries and the bass panels, and
then the
response at 100Hz and 10kHz will barely change, but the middle of the
band will
be as arched as you want, and the sound will become just midrange with
nothing else,
ie, sound like crap.


Now most amplifiers are set up to run best with 8 ohms.

The Musical Fidelity A3 which I tried with the ERA ESLs is good for
120W into 8 ohms, and 240W into 4.
But this implies 31Vrms is available and will make only 68 watts into 14
ohms.


But the Z of ESL panels varies greatly, and in the case of ERA ESL
I have here with the necessary input filtering to prevent
the worst of membrane flapping and banging,
Z at 50Hz = 20ohms, falling to 10ohms at 100hz, then to a peak at 20ohms
at 300Hz and
then falling to 3 ohms at 20kHz.

So where most of the audio energy is located between 50Hz and 1 kHz,
the 120watt amp is barely enough.

Now much music doesn't have a convenient nice ratio between average
comfortable levels and peaks
and I have a good recording here where some laid back jazz
has a guy on a double bass going a bit beserko, in a nice exciting kind
of way.
As you all know modern bass levels in music are often very high,
and when i played this music to a friend last saturday using an amp
capable of 22Vrms into any load above 8 ohms,
the ERA speakers just made horrible noises when the bass player got
keen,
although the rest of the music seemed at a very relaxed level, so
the bass signals present went probably way over 22Vrms, max, and the amp
clipped.
Later in the day when my customer came for a listen I tried the MF A3,
and the same thing happened, but this time
it was a combination of speakers and amps clipping, and finally
the 300 watter gave the best performance but was occasionally triggering
its clipping indicators.

My customer wasn't impressed, because he knows his 25 watt SEUL will not
be able to adequately drive these ESL,
even if I altered the OPT windings to give a match to 13.4 ohms instead
of to
8 ohms, and thus lift the available voltage possible.

So although the sensitivity testing I have conducted tonight describes
what sensitivity is possible,
the real maximum signal capability at frequency has yet to be graphed.
It would appear the ESL just don't like a large bass signal voltage.

If I changed to a higher ratio for the step up transformer, say from
the existing 1:90 to 1:180, thus allowing a drive voltage = 1/2 what is
now needed,
the impedance of the speaker would be reduced from 14 ohms average to
about 2.8 ohms average.
At HF the speaker would indeed become horrible to drive!!!

So the maximum practical step up ratio = about 1:140.

Tonight I also phoned my colleague in Sydney who also does audio repairs
and
builds the occasional new speakers and amps and he wants to give me a
complete unassembled ERA kit
bought some years ago by a prominent audiophile in the
Audiophile Society of NSW. He was given it by a Mr L
because Mr L had bought it while a friend was constructing such a kit.
The friend's efforts were found to be very unsatisfactory, and Mr L
traded away
his unbuilt kit for chassis and other amplifier parts supplied by my
colleague, Mr S.

So despite what idiots like Basset are saying, I didn't have to ask
around
much amoung the few people I know before I found someone who wasted his
money with ERA.
I also have a second contact in Sydney who has been doing his own
experiments with ESL panels now
for 20 years, and he is the one man in the ASON club who knows a lot
about ESL,
and has acted as a partner to another Sydney man who have restored many
Quad ESL57.

So before writing off the ERA ESL, its fair that I research a little
more
and measure the levels of applied voltages which get the ESLs into
trouble,
which are at the bass frequencies.
As I said a month ago, it may be quite pointless to expect to get decent
bass
headroom and sound quality, and to build bass speakers in boxes under
the panels like Martin Logan.
Both guys who listened to the ESL and my dynamics last saturday
concluded the
bass from my speakers was far better sounding, and overall distortion
was lower.


Patrick Turner.