A different way to build a hybrid ESL speaker

[Janek36]

Many decades ago I had the opportunity to live with Beveridge electrostatic speakers. Even after trying other ESL's, I always thought that the Bev's presented sound better than any other type of speaker. They had their faults particularly in bass integration but the overall presentation of music was unique. As a DIY type I always thought I could build my own equivalent and maybe simplify the design enough for DIY construction. So, I constructed ESL panels similar to Jazzman53's original design (1'X4') and designed the cabinet to include a 6.5" mid woofer (Scanspeak 18W8531G). After several months of experimentation with different configurations of the Beveridge lens, I found that a modified lens projecting between 120-140 degrees worked well (The original Bev's were 180 degrees) and when integrated with the mid woofer projected sound very similar to the original Beveridge speakers. If you have the correct room configuration you get a wall of sound with no beaming typical of classic ESL panels and yet I can still localize where instruments are playing. The mid woofers do a decent job of blending with the ESL's and are usable to 60-70Hz. I use a subwoofer below that. All drivers are crossed over electronically (a passive PLLXO ). The 2d photo is a rear view of the lens. The 1st photo shows the finished speakers on their optimum side wall placement in my living room. If anyone is interested in more detailed information and pursuing their own construction of this state of the art design I will be happy to provide that.

 

[Duke]

After several months of experimentation with different configurations of the Beveridge lens, I found that a modified lens projecting between 120-140 degrees worked well (The original Bev's were 180 degrees)... If anyone is interested in more detailed information and pursuing their own construction of this state of the art design I will be happy to provide that.

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Ime there is enormous merit in the unique radiation pattern pioneered by Harold Beveridge, not to mention that he accomplished it with an electrostatic. Kudos to you for your DIY implementation!

I'm not an ESL DIYer (though among other thing I'm SoundLab dealer), but I would be very interested in hearing more about your experiments and your findings and how you turned your findings into a pair of speakers.

 

[Janek36]

It was basically an evolution of trial and error. I did download the original Beveridge patent data so I could better understand the thinking behind his design. From that starting point I also knew from personal experience what characteristics I favored in ESL speakers. Coincidently, I also lived with Soundlab speakers for several years. While they were genuinely excellent full range speakers, deep bass from electrostatic speakers in general does not have the same impact as in live concerts or as it can be on selected dynamic woofers. So my first design criterion was to create a hybrid ESL. Then I made the assumption that to integrate well with an ESL which has a diaphragm at or below the mass of the air around it and very low harmonic distortion, the best dynamic driver would have to have the lowest harmonic distortion possible in the frequency range overlapping the ESL. In this regard, most if not all commercial hybrid ESL's use 10'-12' woofers which makes it harder to blend as their heavier cone will not respond as well in the critical crossover range as a small light cone which is specifically designed for low harmonic distortion. Thus I knew that if I wanted to achieve a reasonable facsimile of a concert hall experience, the overall system would include a separate subwoofer for deep bass. The next issue was what changes in the lens design I thought would work. The original Lens provided 180 degree projection. While useful in some settings, for a home environment with one or occasionally 2 or 3 listeners, something less than 180 degrees would suffice. This had the advantage of making the lens easier to construct. Building the cabinet was mostly sizing it to fit the 1X4 ESL panels and have enough room under the electrostatic drivers for a separate mid woofer cabinet. Once the cabinet was built, I left the lens area open so I could insert different lens designs for testing. Having built at least a dozen different lens configurations, I discovered that there is a range of both overall size and configuration that will work satisfactorily. There might be an optimum configuration but I am not a computer nerd and cannot code, so I do things the old fashioned way. Finally, I spent 3 months with a Behringer dsp xover (I know, not exactly a state of the art audio device) but it allowed me to determine what was the best crossover frequency and whether it should be 12, 24, or 48
db/octave.

 

[Jazzman53]

Good to see you here my friend. And still at it, I see!

 

[Janek36]

sure am. As I say to my local friends, it keeps me out of bars at night. Good to hear from you. If in your wanderings you come west, let me know and we can meet for dinner and some audio.

 

[carbidetooth]

I'd certainly be interested in following your journey. JaneK36. I'm a far better woodworker than I am a speaker designer so naturally curious in stuff like this. Share as much as you're willing to.

 

[Janek36]

the simplest way to assist you is for you to ask specific questions and I will try to answer them. I have some photos but not of the whole process. But as an experienced woodworker, a detailed discussion may be enough for you to get started.

 

[jamescarter1982]

could we have some more information about the device that disperses the sound ? could this be used in front of normal loudspeaker drivers

 

[Janek36]

In theory you can but esthetics would be a problem. The original Beveridge towers do not sound that good when used face forward and were really designed for side wall mounting. However some rooms do not allow for side wall placement. I experimented with more standard speaker placements with a hybrid esl. What I found was the extreme beaming of a flat 1’x2’ esl mid tweeter mostly disappears when using a Beveridge lens on the back to widely disperse the rear output. The photos show how such a design works and the surprisingly satisfactory esthetics. The bass of that speaker is a 6.5” midwoofer mounted in a classic Bailey transmission line. The xover is 1000hz.

 

[jamescarter1982]

thankyou have you ant photos that explain how the sound from ghe front of the driver is effected if at all . from the original post I assumed it was some kind of dispersion improving device

 

[Janek36]

The front of the speakers in the photos is not modified and by itself is very beamy. The rear disperses the sound widely off the back wall and has enough effect to hide the beaming. A commercial speaker which has a similar effect is the Janszen which has a forward facing esl with a sealed back and uses dynamic mid-hi drivers in the rear and sides to reflect sound of the back wall. There is very little sense of beaming in that excellent design. The Beveridge lens lets one do the same thing without multiple speakers

 

[egellings]

They sure are beautifully made.

 

[jamescarter1982]

so it disperses the rear wave along the wall behind the speaker ?

 

[Janek36]

It was basically an evolution of trial and error. I did download the original Beveridge patent data so I could better understand the thinking behind his design. From that starting point I also knew from personal experience what characteristics I favored in ESL speakers. Coincidently, I also lived with Soundlab speakers for several years. While they were genuinely excellent full range speakers, deep bass from electrostatic speakers in general does not have the same impact as in live concerts or as it can be on selected dynamic woofers. So my first design criterion was to create a hybrid ESL. Then I made the assumption that to integrate well with an ESL which has a diaphragm at or below the mass of the air around it and very low harmonic distortion, the best dynamic driver would have to have the lowest harmonic distortion possible in the frequency range overlapping the ESL. In this regard, most if not all commercial hybrid ESL's use 10'-12' woofers which makes it harder to blend as their heavier cone will not respond as well in the critical crossover range as a small light cone which is specifically designed for low harmonic distortion. Thus I knew that if I wanted to achieve a reasonable facsimile of a concert hall experience, the overall system would include a separate subwoofer for deep bass. The next issue was what changes in the lens design I thought would work. The original Lens provided 180 degree projection. While useful in some settings, for a home environment with one or occasionally 2 or 3 listeners, something less than 180 degrees would suffice. This had the advantage of making the lens easier to construct. Building the cabinet was mostly sizing it to fit the 1X4 ESL panels and have enough room under the electrostatic drivers for a separate mid woofer cabinet. Once the cabinet was built, I left the lens area open so I could insert different lens designs for testing. Having built at least a dozen different lens configurations, I discovered that there is a range of both overall size and configuration that will work satisfactorily. There might be an optimum configuration but I am not a computer nerd and cannot code, so I do things the old fashioned way. Finally, I spent 3 months with a Behringer dsp xover (I know, not exactly a state of the art audio device) but it allowed me to determine what was the best crossover frequency and whether it should be 12, 24, or 48
db/octave.

so it disperses the rear wave along the wall behind the speaker ?

so it disperses the rear wave along the wall behind the speaker ?

yes and because of the lens widely and it has the effect of hiding the beaming of the front speaker output

 

[jamescarter1982]

yes and because of the lens widely and it has the effect of hiding the beaming of the front speaker output

interesting could I have an look closer up at the lens ?

 

[Janek36]

the picture on the left shows a typical layout of the 1/4" horizontal plywood spacers in which the vertical dividers slide into. the next 2 photos show a closer view of the back and front of the lens. the far right photo shows the back of the mid-high esl panel mounted on its frame. the lens was specifically built to fit into the back of the speaker up against the esl.

 

[jamescarter1982]

I wonder if one on the front would work as effectively? what are the vertical sliders made of bendy play ? hardboard?