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A Periodic PhD Course in Loudspeaker Design from PSB’s Paul Barton

I've been an audiophile since 1981 and a reviewer since 1995, yet I'd be hard pressed to tell you that I've ever had a conversation with PSB's founder and chief engineer, Paul Barton, without learning something new or coming away with something to think about. In fact, Paul's talks almost always remind me of how little I really know compared to someone like him, who has been designing speakers for more than 40 years. But I also feel grateful, since these talks are like periodically receiving personalized PhD-level courses in loudspeaker design from one of the greatest speaker-engineering minds in the world.

Usually our discussions occur at Canada's National Research Council (NRC), because we both use these facilities to measure speakers. It's sort of fitting, because we both consider the place more or less a learning institution. In fact, that's one of the reasons Paul still likes to go there -- knowledge abounds there, which is mainly due to the number of PhDs working in the various departments. But other times we will talk at audio shows or on the phone.

I could bring up numerous examples of things I've learned from these discussions, including the very first one, which I vividly recall because I learned exactly what diffraction means, and how you can easily identify it in a set of frequency-response curves. But this article will focus on our latest two talks, which happened very recently and involved topics I think will interest some readers.

SoundStage! InSight filmingPaul Barton being filmed for SoundStage! InSight

One was last Friday, when I met Paul at NRC with our video team to shoot the interview portion for a future segment of SoundStage! InSight, which will focus on PSB's history and its newest loudspeaker, the Imagine T3 (look for it to be released in May). It was after the interview when, for reasons I can't remember (topics can lead from one thing to another and another within seconds, as well as just spring out of the air), Paul began talking about speaker spikes and exactly why they change the nature of a speaker's sound. What he said surprised me.

I'd always thought that spikes simply give a speaker a firmer footing so the drivers have a sturdier foundation to push off against. That's some of it, but not all. Paul explained how, if the spikes are coupled very securely to the floor, a significant amount of energy gets transferred through to the floor, which causes the floor to vibrate -- much like your cell phone vibrating on a table -- and those vibrations reach the listener as sound. In other words, you're hearing the sound radiating not only from the speakers but also from the floor (and I'm not talking about reflections; I'm talking about the floor acting as another transducer). Depending on what the floor is made of -- hardwood, for example, particularly with no rug or padding on top -- the sound produced from it can be quite profound. With that explanation in mind, Paul posed this question to me: Which sound is right? (Meaning with or without spikes.) Paul didn't have an answer at that moment -- he just wanted me to think about it. And I have since . . .

Then there was what we talked about yesterday, when I phoned Paul to ask him something about the T3. Our discussion led to a general talk about measurements, since he knows I lead our speaker-measurement program and like to discuss the topic not only with him but with other designers who are really into it as well. For reasons I can't remember again, the conversation turned into him talking about typical 1" dome tweeters and the reasons some sound bright. You'd think the only reason would be excess tweeter energy towards 20kHz, which is the top of the audioband. According to Paul, maybe . . . or maybe not.

Paul reflected back to listening sessions done in the 1980s by the Canadian Broadcasting Corporation (CBC), where a particular speaker was scoring well overall but most thought it a little bright. Paul looked at the measurements for that speaker, and although the on-axis response looked neutral and didn't indicate that the speaker would sound bright, when he looked off-axis from the tweeter (meaning out to the sides), a "knee" in the responses was apparent at about 5kHz. In other words, there was more energy around that region than in the frequencies above or below. In turn, that meant the overall radiated response (often called "sound power") wasn't as neutral as the on-axis curve -- it showed a bump at about 5kHz, and that bump was what was causing the listeners to perceive the speaker as bright. Paul explained that the reason for the bump is that a typical 1" dome tweeter, which is what most designers use, radiates tremendous energy in all directions at around 5kHz, which simply has to do with the size of the diaphragm. If you look through our measurements, you can often see the "knee" that I mentioned with speakers that use a 1" dome.

From that experience at the CBC, Paul learned to design speakers so that the on- and off-axis responses combine, resulting in the overall radiated response being flat, even if that means compromising on-axis neutrality somewhat, which he'll do in some of his designs. This is also one of the reasons that you can't judge the sound of a speaker solely by its on-axis response (although many try), which is a topic I'm going to discuss in a future article, either here or on SoundStage! Hi-Fi.

Paul Barton and Doug SchneiderPaul Barton and Doug Schneider

These might seem like small details to some, but I think they're important because the small details are often what audiophiles sweat over. I also believe that the words of someone who’s been designing speakers for 40 years -- and who cares deeply not only about how something sounds, but why it sounds that way -- are worth listening to, and are important enough to share with our readers.

Doug Schneider
Founder, SoundStage!