[Interview] An Interview with Dr. Earl Geddes

An Interview with Dr. Earl Geddes.

A Conversation on Psychoacoustics and High Fidelity. Part II

[Earl Geddes intervistato su TNT-Audio da Matteo Bruni]

MB You actively participate in DIY forums such as diyaudio.com where you generously share your expertise with others. How do you see the evolution of DIY today? What advice would you give to someone who wants to build a truly high-quality loudspeaker from scratch?

EG Unfortunately, I don't see DIY as a road to achieving a world-class audio system. That's because it takes so much work, knowledge and equipment that the DIYer is simply outclassed by the major manufacturers. One can make a decent system by following what others have proven out, but starting from scratch to do a design is almost impossible – it's simply too complicated. Modern software makes this more feasible, but it's still a challenge. And proving out a design through testing is a real problem. The best that a DIY can do is copy an existing proven design, but that isn't always effective. It's hard to know what details to pay attention to and without a lab and measurement equipment it's hard to evaluate the deigns effectiveness. In a sense, I was a DIYer but I also had a substantial professional background and enough equipment capability (finances) and software knowledge to make it work. But that's the exception, not the norm. Mostly I go to DIYAudio.com not as a supporter of DIY as a great way to build a worldclass system, but as way to help alleviate the community of the serious misconceptions and falsehoods that are present in audio. To wit the idea that compression drivers cannot be audiophile level components and are, in fact, ideal.

MB How do you envision the future of high-fidelity audio? Do you think audiophilia is heading in the right direction, or is there something we are losing along the way? How do you see the future evolution of audio technology, particularly regarding speakers and amplification?

EG I think that audiophilia is moving in the right direction, although I am way out of the mainstream. I know from my own kids that audio nirvana is not something that they crave like I/we did years ago, so in that sense I think that what I/we were looking for has changed with later generation. They use earphones and convenience is paramount. It's a different world for them. But what has remained is very impressive – for example the waveguide software development at DIYAudio is truly state-of-the-art. It's technology like this that will drive the evolution of speakers. Amplification? To me that's done. DSP has made electronics virtually perfect and vastly more capable that in the past.

MB You have worked, and continue to work, in the world of audio, and you are an active member of communities like diyAudio, I imagine that such enduring dedication is driven by a deep passion. What continues to motivate you today? And what does high fidelity mean to you on a personal level?

EG These things have changed dramatically over time. I grew up passionate about music and even performed in a rock-n-roll band. My brother went on to be a professional bass player. But I found myself attracted to the equipment – I had already made my first speakers at about 14. I have loved music ever since and regularly seek it out. But at home, these days, my interest has shifted to home theater. My setup – sound wise – is better than any commercial theater that I have been in, and in fact I haven't been to a commercial theater for many years somewhat because my last experience was so bad. Just atrocious sound quality. Inexcusable! Hence most of my leisure time these days - every day - I am in my HT. Since I have retired, however, my interest and passion are working with glass. I've always been interested in glass but didn't have the time until I retired. I'm not one of those people who tweaks their system ad-infinitum, I have it just the way I want it and haven't changed it for more than a decade, maybe two!

MB Is there a listening experience you remember with particular emotion? A moment when you thought: “Yes, this is the right sound” with the systems you have created?

EG The first set of Summas that I made some 30 years ago. The sound quality blew me away, I still have the incremental improvement version of that speaker now, but it's far more similar to the original than it is different. A few tweaks. I tried to make this same speaker ever smaller and lower cost, the smaller systems are good, but the larger NS-15 is still my preference. Smaller is always a compromise. And these speakers were always expensive to make owing to the waveguide casting and large radius edges. The cabinet material was sheets of polyurethane boards which cost several times as much as even 11 ply plywood.

MB Last question Dott. Geddes: our readers - myself included, of course - are curious to know what kind of system you personally use in your listening room. Could you share a few details with us?

EG As I said, I basically have three Summas across the front of the room behind the screen – not a commercial screen, one that I made myself. Commercial perforated screens are a disaster. Mine is a woven fabric – somewhat more absorbent but far better transmission frequency response. Sound output on my system isn't an issue so the higher absorption is not an issue, while the frequency response problem would be. The model that I am using now is the NS-15 (“New Summa”.) which is bi-amped in this iteration. The surrounds are four Harpers – a 10 inch unit with good response and directivity but no real low end. The system is augmented with three subs spaced around the room which are blended into the mains with DSP. The rest of the electronics is simply high-end Pioneer. I'm not a believer in electronics being a factor in sound quality.

MB In conclusion, please feel free to add any personal reflections or considerations.

EG The thing is that I could ramble on for pages and pages on the way we hear in small listening rooms based on looking at models of hearing. These models use Gammatone filter [A Gammatone filter is a band-pass filter whose frequency and time response is modeled to resemble that of a single auditory nerve fiber] sets to represent the critical bands in our hearing. If one looks at the impulse responses of these filters it becomes very evident how we process reflections – HFs very quickly and LFs very slowly. This is a key factor in any discussion of listening in small rooms. LFs are continuous, basically steady-state signals to our brain, while the HFs are very sensitive to the timing. Looking at the Gammatone impulse responses tells us everything that we need to know about what reflection timings effect which frequencies. It becomes clear that there is no distinct cutoff as is often referred to, but a continuum of influence depending on the reflection delay. It is also clear that minimizing the very earliest reflections in level, i.e. the use of high DI, will make the initial impulse response less muddied. Nearby diffraction effects from the speakers act as very early reflections as well. I could go on a great deal on the psychoacoustic specifics, but that would be far too far afield from what is necessary to understand why a high and smooth DI is a fundamental requirement for good sound quality.

MB Many thanks to Dr. Geddes for the time and energy he devoted to this valuable interview!

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