Originally Posted by TimVG
Why not simply look at the directivity indices?
Below is an excerpt from an interview with David Smith. He's an engineer who has worked with several speaker companies, including JBL, Snell, KEF, McIntosh, and PSB.
RA: There has been a lot of research with regard to psycho-acoustics and Toole's book is a great summary of this research. You have not only worked with some of the leading researchers but also contributed to the research itself. What is your take on the findings and how they apply to the home environment?
DS: Toole’s book is a groundbreaking work, worth reading over and over. It summarizes his decades of research on the subject of loudspeakers, their measurements and the applicable psychoacoustics. I think it is the new bible on sound system priorities, covering the audibility of the many measurable aberrations found in loudspeaker, including the effects of the room. If we place a speaker in the typical lively room and do a high resolution measurement, the result is such messy resonant picture that we can’t possibly discern whether it should sound good or not. Over the years we’ve developed a lot of approaches to smoothing out the view to simplify the picture, but without having any real justification for these approaches.
Toole’s classic paper, published in 2 parts in 1982 (part 1 and part 2), had a carefully run listening panel rank order 20 loudspeakers for preference and quality and at the same time made a large variety of measurements on them. It showed which measurement correlated with listener preference and which didn’t. This is great stuff for a speaker designer because you can ask: “what should I concentrate on when designing speakers”. Everything costs money and it is a competitive market. Should I spend money to get the phase response flat? Should I add extra components to flatten the system impedance curve? How low does distortion need to be? In other words, what measurable parameters correlate with our subjective impression?
This is a fundamental difference between commercially built speakers and DIY efforts. The enthusiast constructer can pick any design aspect and beat it to death. But if you are trying to survive in the market place you need to think about “bang for the buck”. Do your cost choices give the end user an audible benefit? Or are you creating straw men to knock down, say, reducing distortion to levels way lower than audible, over a misguided belief that that aspects overrides others.
Well, what I think Toole’s early research shows is that axial frequency response is the number one criterion. Power response, or overall directivity, seems to be a poor correlation with subjective impression. Why this is important is because it shows how room curves (strongly influenced by later arriving off axis response) can be misleading. The larger the room the more power response determines the room curve and the more misleading they will be. Still, I think he has drifted slightly from some of his earlier findings. In spite of what Toole’s early works clearly showed he is now placing importance on room curves and extending that from small rooms to large rooms as well.
I’m currently on one of the SMPTE committees that is looking to replace the X Curve approach to Cinema equalization. For years we’ve known that going into a large space, plunking down a microphone, feeding pink noise into the speakers and adjusting to flat response gave bad results, it is always too bright.
Now what does that mean? Something that measures flat sounds too bright? This isn’t an issue with amplifiers or record/playback systems, where flat sounds flat. I think there are a lot of clues to the reason for this, it is tied into human hearing and our ability to focus on earlier arriving sounds while ignoring later arriving sounds, but this is still a new concept to many in the industry.
With domestic listening rooms, the difference between the anechoic performance and the steady state, in-room performance is fairly minor for upper frequencies. But as the room gets to auditorium or cinema size, the differences enlarge and the steady state curve becomes very misleading.
There are a number of studies, by Kates, Salmi, Lip****z and Vanderkooy, Bech and others, that suggest that we judge frequency balance with largely a time windowed approach. Late arriving sound is ignored. Also, this time window is long for low frequencies and short for high frequencies. In effect it is the steady state or room response for low frequencies and typically just the direct (anechoic) response for high frequencies. At mid frequencies it might contain the first floor or back wall bounce, but later reflections are generally under the level required for audibility.
"Wouldn’t it be nice to have a measuring system that perfectly mimicked human hearing and the way we perceive frequency response? It could take all the subjectivity out of it."
Viewing perception this way answers a lot of questions, including why we need to roll off the response of a system in a large room: the early response is inherently brighter than the later response, due to rising speaker directivity, rising room absorption, even the absorption of the air. Flat steady state response would give very bright early sound, and so we reject it.
Why this has always fascinated me as a speaker designer is that it holds out the promise of our being able to design speakers that are perfectly balanced, if only we can figure out how our hearing works. Remember, if we accept as our goal that the speaker shouldn’t add anything, it should be a neutral lens for the recorded sound to come through, we still have to figure out what neutral or flat means? Is it flat anechoic response, flat in-room response, flat power response, flat time windowed response? In smaller domestic listening rooms the direct response from the speaker and the room response (including all reflections and reverberation) aren’t that far apart, perhaps 2-3dB of shelving in the room response when the direct component is flat. As rooms get bigger it becomes a major issue and steady state curves need to roll about ten dB (the Cinema X Curve).
My current approach is to design for flat anechoic response for midrange and up and then see how the low end interacts with the room, do a lot of listening and fine tuning until it seems right across a broad spectrum of software. Still, there is always a feeling that a little more tweaking could give a better result, and a suspicion that I am tuning to give a pleasant result with music I like, rather than achieving verifiable accuracy. Wouldn’t it be nice to have a measuring system that perfectly mimicked human hearing and the way we perceive frequency response? It could take all the subjectivity out of it.