Product Review - InnerSound Isis Hybrid Electrostatic Speakers - April, 2000

John E. Johnson, Jr.


InnerSound Isis Hybrid Electrostatic Speakers (Isis on the left in the photo, and their larger model the Eros, is on the right)

Electrostatic panel with cone driver for low frequencies

ESL Size: 39" H x 10" W

Cone Size:  8"

Crossover:  900 Hz

Nominal Impedance:  4 Ohms

Sensitivity:  90 dB/w/m

MFR:  34 Hz - 27 kHz ± 2 dB

Size:  62" H x 11 1/2" W x 14" D

Weight:  54 Pounds Each

MSRP:  $2,750/Pair USA

InnerSound, Inc., 1700 Hwy. 16, Whitesburg, Georgia 30185; Phone 770-838-1400; Fax 770-838-0111; E-Mail [email protected]; Web 


As I sat there, hands folded and eyes closed, wallowing in the salubrious effects of Giuseppe Verdi, I knew that I was listening to some great speakers. I am sort of putting the conclusion at the beginning, but I wanted to make the point right away about the importance of "enjoying the music." We hi-fi buffs tend to be compulsive and obsessed with the technology so much that we sometimes forget to enjoy the fruits of our search for audio nirvana. The InnerSound Isis Hybrid Electrostatic Speakers (hybrid means there is a cone speaker along with the electrostatic panel, to cover the low frequencies) represents one of those stopping points in the search, at least for me. It is not a small speaker and not a huge speaker, but it is pretty big. The photo above, from the InnerSound website, reminds me of a 1952 film, "The Quiet Man", starring John Wayne and Maureen O'Hara. In one scene, O'Hara's wedding veil flies above her head in the wind as she comes down the steps. Some people said director John Ford was lucky in the filming of that shot. To the contrary, Ford said, he had planned it that way, using fans. The point is that luck had nothing to do with the appearance of the Isis. They are transparently beautiful, both in looks and sound, as you will see below.

I have always been a fan of electrostatic speakers (ESLs). The first time I heard some was about 35 years ago. A stereo shop in Seattle, Washington had some KLH ESLs set up as full range speakers. This required them to actually have two sets, one pair on each side, sitting much like you might have room dividers. They were huge speakers, but gosh, what a sound. Back then, 70 watts per channel represented a big amplifier, and the store was driving the ESLs with a Marantz power amplifier of just that rating. I thought, "Wow, these things take a lot of power to drive them."

The Threshold ESLs I have in our laboratory are full range, and I listen to them with a 250 w/ch power amplifier, and the preamplifier turned up about half way, or more. However, full range ESLs always require a lot of power.

How they are made

ESLs are very simple in design, but difficult to build. A thin plastic membrane (diaphragm), often made of Mylar®, is stretched into a rectangular frame. The membrane in its frame is sandwiched between two metal perforated "stators" along with spacers (see photo below, left). At the base of the ESL is a transformer and power supply. The incoming music signal passes through the primary transformer winding, having been connected to the speaker binding posts on the power amplifier (photo below, right). The secondary winding is connected to the two stators, one in front and one in the rear. A high voltage DC power supply is connected to a center tap on the secondary winding (3,000 Volts in the Isis), with the + end going to the transformer center tap and the - end going to the diaphragm. The diaphragm has an electrically conductive coating, and when the power supply is turned on, the diaphragm gets highly charged with electrons. The electrons don't flow anywhere, but just sit there (electro static). The secondary winding increases the amplifier voltage from, say, 50 Volts to about 5,000 Volts, and when the music plays, the alternating + and - high voltage changes on the stators cause the negatively charged diaphragm to move back and forth, pushing air through the perforations in the stators, giving the sound we hear.

There are several things that make the InnerSound Isis unique among ESLs. One is that the crossover frequency between the ESL panel and the cone driver is 900 Hz. Usually, the crossover frequency is lower, say around 250 Hz. The reason for the higher crossover frequency is that, with a lower one, the crossover would be right in the middle of the human voice. Secondly, a higher overall sensitivity (90 dB/w/m) is achieved by using 900 Hz. As a result, I found that my 80 watt White Audio monoblocks would drive the Isis just fine, with some room to spare in fact. Third, the ESL panel is flat. Usually, the panels are curved (bowed outward towards the listener), the reason being that ESLs have very narrow dispersion. As a result, the flat Isis have a small sweet spot. So, why did they do this? Most speaker systems use cone drivers, and they have a wide dispersion. This works out nicely for home theater and other situations where a number of people are listening all at the same time.

However, there is a price to pay for this wide dispersion. The sound is directed not only at your head, but the side walls too. The high frequencies have a short wavelength, so the combination of the direct sound and the first reflection results in smeared sound. Since the details of the music are embedded in higher frequencies, we lose a big part of that detail. With the Isis, the dispersion is very narrow, so the high frequencies are directed towards us with very little in the way of first reflections smearing them. Of course, you must set up the Isis very carefully, as it is extremely important that they both are facing right at you in the sitting position. InnerSound has a detailed procedure for doing this. In practice, I found it very easy to set them up, just toeing them in (turning them so they faced me on the couch) until the sound was the clearest. InnerSound suggests that the distance from both speakers to you must be exactly the same in order to prevent phase problems. This might be true for mono music, but in practice, I found that unequal distances did not cause any problems with stereo music. InnerSound seems obsessed like the rest of us, and I guess it is time for them to enjoy the music too.

The 8" mid/bass driver in the Isis is configured as a transmission line (TL), with about 7 feet of passageway through which the sound must pass to reach the port (just below the woofer on the front of the enclosure). TLs have several advantages, including low cabinet resonance, powerful bass below 50 Hz, and not very much mid/bass sound coloration (peaks and valleys). As a result, the sound of the Isis is very natural (see below under "The sound"). A generic diagram of a transmission line design is shown on the right, above. It is not necessarily the layout of the Isis, and there are countless variations on this theme.

The sound

Once I had the Isis set up to my liking, I have to say that I cannot remember ever hearing such detail in recorded music. As I said, the sweet spot is very small, but in that spot, the imaging is pinpoint sharp. In fact, it was almost like wearing headphones, except that the sound came from in front instead of in my head. You see, headphones don't have the first reflection problem either. But, it is that "in your head" direction of sound that puts most people off from using headphones all the time.

There are four frequency areas that I always listen for when testing speakers. One is at 80 Hz (boominess), one at 120 Hz (chestiness), one at 800 Hz (nasality), and one at 6 kHz (sibilance). A peak in any of these regions can really destroy the sound. In the Isis, they were all very natural. I was particularly concerned that there might be a nasality problem since that is near the Isis crossover frequency. However, there was no nasality at all, and the transition to the panel was seamless. On the rear of the Isis there is a small rotary control (photo at right) for customizing the amount of sound coming from the 8" cone. Turning it up gives more "body" to the music, and InnerSound put it there due to the thin nature of ESL panel sound that is inherent to all ESL designs. I liked it all the way up, and I added the lowest octave (20 Hz - 40 Hz) with a Sunfire Subwoofer connected to the output jacks on my class A triode preamplifier with Y adapters.

There are two sets of metal binding posts on the Isis for bi-wiring or bi-amping. You do need to be aware that the speakers have to be near an AC receptacle for connection to the Isis power supply in each speaker.

My wife, whose ears are extremely critical, thought the Isis to be fabulous, with a very natural sound to them. Like most ESLs, they are not very high on the spouse approval for looks, because they are pretty big. ESLs have to be big in order to move enough air. That is just something we have to deal with. However, the Isis are not as big as some ESLs. Full range panels, especially, can be 7 feet tall. Perforations in the stators help to make ESLs not totally take over a room though.

Because the Isis excel in detail, it is very important to use good cables (the characteristic of poor cables seems to be a loss of high frequencies). In this case, I used Nordost Red Dawn interconnects and speaker cables. However, a mass market CD player seemed to give all the detail I needed to fully appreciate these speakers. I listened to them evening after evening, with the CD carousel full of my favorite music. My wife usually sat in the sweet spot, but I still just enjoyed the heck out of these speakers. Sometimes, when listening to many speakers, I find that I think I am enjoying myself, but after a half hour or so, I feel the need to turn it off and do something else. I suspect that some odd ordered harmonics are in there somewhere, not plainly obvious, but enough to cause listening fatigue. With the Isis, I listened far beyond my bedtime, never getting tired of them.

In conclusion, I previously thought I had heard all that ESLs have to offer (they all are transparent in sound, and none of the ESL panels have enclosures to resonate or color the sound), but the Isis are different. Very natural sound and superb detail are their hallmarks, more so than ESLs with curved panels. The price for this detail is less dispersion, and ESLs are not for everyone, but the Isis might very well be for a larger group of us than most designs.

- John E. Johnson, Jr. -


© Copyright 2000 Secrets of Home Theater & High Fidelity
Return to Table of Contents for this Issue.

Our Vault pages may have some display quirks. Let us know if we need to take a look at this page or fix a bug.
Connect with us
  • Instagram
  • Google+
  • YouTube
  • LinkedIn
  • Pinterest
Secrets "Cave"