I've had a number of chats with ELAC's Andrew Jones about speaker design over the years. He started out as a research engineer for KEF in 1983, then moved to Infinity and Pioneer, the latter where he was chiefly responsible for their ultra high-end TAD Reference line of speakers that sell for upwards of $80,000. Jones not only designs speakers that sell for as much as a luxury Mercedes Benz, he's also made outstanding affordable speakers for Pioneer.
Jones moved to ELAC in 2015 where he again designed some of our. With Jones' help I've come to see speaker design as a balancing act, where every decision changes the sound and that leads to another decision.
You can watch the full twelve minute interview here on YouTube and I've consolidated the main points of our talk here, with some paraphrasing for brevity's sake.
Jones started the conversation discussing three design parameters: cabinet size, the desired low frequency (bass) extension, and sensitivity. The choices – big or small cabinet, how much bass, and how much power the speaker needs to play loud, which techies call sensitivity – those are the three parameters he juggles with every new project. As Jones put it, "Absolutely you're juggling, within the laws of physics, and you can't actually overcome that!"
Jones explained "You're free to choose any two of those parameters however you want, the third will be given to you." For example Jones could design a small speaker that's capable of producing fairly deep bass -- those are the two desirable pluses -- but that speaker would be highly insensitive so it would need a very powerful amplifier to sound its best.
Or he could stay with the small cabinet and design for high sensitivity, but then lose some deep bass. Or design for high sensitivity and deep bass, but then he would need to increase the size of the cabinet. That last option might please die-hard audiophiles, but the mainstream market wants smaller, not larger speakers. Jones again, "We constantly juggle, and different designers make different compromises."
So far so good, but as we continued the discussion Jones brought up another factor: speaker impedance (a.k.a. resistance). If the designer chooses to lower the speaker's impedance from 8 to 4 ohms the speaker's sensitivity would increase. That's great, but lower impedance can make for a more challenging "load" for the receiver or amplifier. So all other things being equal a 4 ohm speaker will have higher sensitivity than an 8 ohm speaker, but most affordable receivers and amps can't easily drive 4 ohm speakers, they prefer 6 or 8 ohm speakers. Ah, but higher-end speakers will more likely be mated with better amplifiers that can easily drive 4 ohm speakers. So again, the designer's decisions affect outcomes.
Jones concedes "There's theory and there's practice, so in theory I can make a one liter box that makes [extremely deep] 20 Hertz bass, but I can't fit a big driver in a small box so it won't produce any real sound pressure level at 20 Hz, so it will be unusable...How close to theory can you get with the choices you make, compared with what you wished you could make."
So when a designer is tasked with making a small speaker that produces very deep bass, they can do that, but that small speaker won't play as loud as a similar design that forfeits some deep bass power. Again, no designer can make a small speaker that produces room shaking bass, plays really loud, and with high sensitivity so it's easy to drive.
Jones is also very aware of how people in the real world use their speakers, and that leads to where the compromises are made, "In practice, how often do you play to the limit of the loudness capability of the speaker, compared with how much would you miss bass?" Jones feels most buyers would trade some ultimate loudness capability to get more bass.
Then I asked, since powerful receivers and amps are pretty affordable nowadays aren't you more likely to design for lower sensitivity to get deeper bass from a given cabinet size? Jones explained as sensitivity decreases more of the amplifier's power is converted to heat in the drivers voice coils.
For example, when a speaker is played really, really loud for more than a minute or so the woofer's voice coil heats up to as high as 400-500 degrees Fahrenheit, and the heat more than doubles the woofer's voice coil's resistance (impedance), which in turn lowers the speaker's sensitivity. Jones says when that happens, "The frequency response goes to pot, and you lose dynamics and sound quality suffers."
No worries, when the driver cools down the speaker's sound returns to normal. Jones mentioned that when they test drivers in the lab with lots of power he can't immediately touch the driver's frame, it's too darn hot!
While the midrange drivers and tweeters temperature rises may be smaller the loss of dynamics might be evident when music is played very loud for more than a few minutes, and that would temporarily reduce or compress the music's dynamics. The listener's subjective reaction to all of that is a loss of dynamic life to the sound of music or movies. If that happens with your speakers you need to turn down the volume or buy bigger speakers!
So it seems every aspect of speaker design affects the sound quality of the whole speaker. Of course the design of a speaker's woofer, midrange, and tweeter play huge roles in the overall sound. No wonder speaker design is so complex, every choice affects other choices.
Where many speaker designers don't also design the woofers, midranges, and tweeters used in their speakers, Jones is one of the few who designs entire speakers: the cabinet, crossover networks, and all of the drivers in his ELAC speakers! We ran out of time to discuss that in detail, maybe Andrew and I will chew on that next time.