ESL IV Vs Acorn MkII

We are often asked what the sonic differences are between the Acorn MkII and the ESL IV so I thought I’d detail it below.

The design philosophy for the speakers is quite different.

ESL IV

The ESL IV uses 2 panels each with a single diaphragm to reproduce the full range of audio. The two panels are wired together to act as a single transducer. This array is segmented into 35 electrically separate, vertical sections, the audio signal is fed to the central section first and then progresses through a resistive transmission line to the last section at each side. As the signal progresses across the panel, the reactance of the resistor and capacitance of the section creates a low pass filter which causes the high frequencies to be rolled off at each step. The result is the central segments reproduce full range signal including high frequencies and all of the panel reproduces low frequencies. This gives excellent bass response and because the high frequencies are being emitted by a very narrow segment, excellent horizontal dispersion at high frequencies.

Other than the transmission line resistors, there is no crossover to affect the signal.

The mid-range and treble response is excellent both from an objective and subjective perspective. Bass response is outstanding.

Now the compromises that have to be made to achieve this.

A single diaphragm that reproduces full range signal will suffer a degree of IMD (intermodulation distortion). This is common to all single full range driver systems, worse in some, better in others. Because the diaphragm is required to reproduce say 100 Hz and 4 kHz at the same time the only time the 4 kHz tone is truly accurate is as the diaphragm passes through the centre between the stators. This effect is minimal but it does exist.

To drive a transmission line that has significant insertion losses a very high step up ratio transformer is required. This becomes difficult to produce because, as the turns ratio increases the ability to deliver high frequencies decreases so the winding complexity increases, great care must be taken over the winding layout of the transformer. We use 2x such transformers in a parallel / series arrangement to give the desired step up ratio (turns ratio) while maintaining good high frequency response. However, this combination of transformers will only go out to around 22 kHz, high enough but not as high as we would like.

Diaphragm tension has to be low enough for good low frequency reproduction but high enough for stability. This is not necessarily optimum for either range of frequencies.

Acorn MkII

The Acorn MkII operates in a different manner to the ESL IV. This speaker has 3 mechanically separate sections with 4 electrically separate sections. There is a discrete bass section that reproduces signal from around 32 Hz to 260 Hz, the mid-range section reproduces signal from around 260 Hz to 6 kHz. The treble section has 3 electrically separate sections but uses a common diaphragm for all of them. The central section reproduces frequencies from around 6 kHz to 35 kHz. The sections each side of the central strip reproduce frequencies from 6 kHz to 18 kHz.

Each section has its own transformer to drive it, the transformers have been optimised for that specific application.

What are the benefits of doing it this way.

Because each section has a narrow band of frequencies to reproduce, IMD is reduced to very low levels.

The mid range and treble sections can have a smaller gap between the stator and the diaphragm because they are not required to create low frequencies which requires large diaphragm excursion. This gives the speaker the ability to reproduce high frequency dynamics and low level detail very accurately.

The diaphragm tension can be optimised for each section.

The audio transformers can be optimised for their individual application and high frequency response can be extended by a dedicated transformer that does not have to deliver low frequencies.

The narrow super tweeter strip can reproduce very high frequencies with good horizontal dispersion.

The response can be tailored by adjusting the drive to each section.

Compromises.

A crossover has to be used. Whether this is done actively or passively is up to the end user. We recommend the passive approach having found it to perform sonically better than active.

Although the Acorn is around 30% larger in area than the ESL IV, bass reproduction is limited to the area provided by the bass panels which is less than the 2x panel ESL IV. However, the transformer has a higher turns ration than the combined transformers of the ESL IV and the crossover has been designed to provide some equalisation at low frequencies to compensate for this.

Both speakers have the same tonal qualities with very fast transient response and excellent mid and high frequency reproduction, you will only notice a difference if you A -B compare them together.