Neutral frequency response & controlled directivity

Charles Sprinkle — Director of Acoustics for Kali Audio — on creating ideal loudspeakers through value engineering.

Written BY

Mike Dias

I founded Domo Audio because I love music. I love audio. And I believe in building relationships, not transactions.


November 21, 2019

Hi Charles — thank you for taking the time to talk with us today. Before we dive into what you’ve been working on lately at Kali Audio, let’s go back in time and talk about how you first started working with audio. Is it true that you got an internship at JBL through Dr. Sean Olive? 

Yes.  I was attending university at CSUN and met Sean at the local AES chapter.  Sean hired me to assist in the Multichannel Listening Laboratory.

What’s the most profound lesson that you learned while working with him?

I think the biggest eye-opener was how many nuisance variables skew subjective evaluation of loudspeakers.  When evaluations are double blind, it was apparent that frequency response and directivity were the biggest opportunity for improvement.

All the work I did with waveguides was in support of this.  The image control waveguide was created to resolve a directivity anomaly of the bi-radial designs.  The constant-directivity elliptical waveguides had the best directivity characteristics we could make.

I don't want to minimize the importance of low distortion and low noise.  It's just that frequency response and directivity were so bad in commonly available loudspeakers, that resolving these issues was a priority, especially given the fact they were two of the least expensive issues to improve.

Your waveguide work eventually led you to JBL Pro where you began working on the M2. How was the change from the consumer division to pro?

The thing that I liked most about the professional division —and especially in recording and broadcast — was that the end-user is extremely concerned about the sound quality.  The loudspeaker is a tool that they use to do their work and critical for them to make good decisions. 

I actually started working on the Waveguide for the M2 while in the consumer division.  There were not many projects going through the Northridge engineering group in consumer division so I had time for research.  The team working on the M2 was having issues with the bi-radial horn they were using at the time so I showed them my work. The geometry was unique in that it allowed the use of a compression transducer in a relatively shallow waveguide and also used a blended diffraction feature to ameliorate the high frequency beaming of a 38mm throat diameter.  It was able to acheive upper frequency directivity comparable with a 25mm dome tweeter. The waveguide worked very well and I assisted the system engineer in its implementation.

And from there, you moved onto the LSR 3 Series, no? What was the key development on this one that allowed it to become so massively popular?

The 3 Series was my first full project in Pro.  The key thing was everything!   What I mean by that is value engineering is done on the entire system in order to maximize the value to the customer.  In that system, an amplifier was selected that provided the maximum value with both DSP and amplification.  The power supply was designed to provide sufficient power to just run it to its full output, and transducers were custom designed to take exactly that much power before reaching their limit.  So amplifier, power supply, and transducers got to the limit simultaneously.  (Along with the port tube, by the way.)  This means that we did not spend too much (or too little) on any component.  The waveguide costs nothing but design time and since the amplifier had DSP, we had everything we needed to provide really good frequency response and directivity. 

So the short answer is I believe that the customers recognized the value we designed into the product.

And now — you’ve been able to build upon this huge legacy and knowledge-base. That clearly shows in your work on your LP-6’s. The reviews have been fantastic. How do you get that sound at that price point?

Once again, the answer is value engineering.  Some of the things didn't add too much cost, like the boundary tuning switches.  For other things, we had to creatively move costs around — an example is the woofer.  We wanted the lower distortion that comes from a large diameter two layer voice coil but that requires a significantly bigger magnet and that costs money. That’s a large addition to the BOM. So we opted for a plain white box — rather than a $10 five-color lithograph beauty-box that just gets thrown away.  And we used the savings to pay for the woofer motor improvement.

Tell us about your design choices on your IN-8's and how you ended up with the co-axial mid-range and tweeter. 

The LP-6 and LP-8 are very good loudspeakers and are reviewed as such, but they are still two way-speakers with non coincident transducers.  At the crossover, the sound from both transducers forms an interference pattern with an associated directivity anomaly in the vertical plane.  It's relatively small in frequency bandwidth but it is still there.  This by the way is the reason why we have stayed with two-way monitors in order to reduce directivity anomalies from additional crossovers.  With a coaxial midrange and tweeter, however, we were able to execute a three way design without the associated directivity issues.  The midrange and tweeter do not have an interference pattern.  Also, since the woofer and midrange are well within 1/4 wavelength at their crossover point, there is no significant directivity issue there either.  The three-way system also helps to lower distortion, since the tweeter is crossed over at 3kHz and is not working so hard.  Additionally, the midrange has flux stabilization rings that help to reduce flux modulation and linearize inductance.  This also reduces distortion.

Given everything that you have worked on over the years and all the colleagues that you’ve been able to learn from, how would you summarize your acoustical philosophy.

The guiding acoustic philosophy at Kali is to provide the most value for our customers.

In my mind, the ideal loudspeaker has neutral frequency response, controlled directivity, high output, wide bandwidth, low distortion and low noise.  The things that don't cost anything but design time should never be missed.  Improvements with an associated cost should be prioritized based on how important they are for the intended use.  This isn't focusing on any one detail, but all the details in balance.  

Charles Sprinkle is the Director of Acoustics for Kali Audio. He is an acoustical systems engineer with 16 years experience in audio product development. Prior to co-founding Kali Audio, Charles was a Principal Engineer for Loudspeaker Systems at JBL Pro.


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