Date: Wed, 07 Nov 2001 16:33:46 -0600 From: Ron Nossaman <RNossaman@KSCABLE.com> Subject: Re: Tuned front duplexes >So, to me, at least, the question is unanswered. What constitutes >'projection' anyway? That would be an arbitrary perceived volume at X distance, X being projection, but not otherwise quantified by any specific proximity measurement. I hope that clears it up some. Now I'm not much on large hall acoustics, being more of a dimly lit small airless enclosure kind of guy, but doesn't low frequency sound carry farther than higher frequencies? Isn't a harmonic spectrum with lots of low frequency partials going to "project" farther back into the cheap seats than something with a lot of high frequency noise - like for instance clangy trebles or those front duplex noises that enhance projection? Can't I hear the bass line from Spike's stereo, through the house walls, while he's still three blocks away, but can't hear his high range stuff until he opens the car door in the driveway? No wonder the most common phrase I hear regarding piano tone is "But just listen to that bass"! Well yea, that's bone conductance. So I wonder why even harder hammers and more painful trebles still don't get to that back row. ----- Ron, A bit of a slow response on this. I thought I used to know some of this stuff but found I needed to consult a text. The first acoustics text I laid my hands on had so many double integrals and Bessel functions that my eyes started to glaze over so I had to go in search of a more rudimentary text (amazing the things that one could do in college that one can't do now). Let me preface all of this with the caveat that I am not an acoustician and I may be skewing or misinterpreting some of this. If there is an acoustician out there please correct me. As I read it acoustic power goes up with frequency at the same amplitude of vibration. I believe perceived loudness is closely related to power (I don't have a psychoacoustics text handy). I believe a high frequency tone with the same energy as a low frequency tone will sound louder. A tone with a preponderance of high frequency information with the same energy as a tone with a preponderance of low frequency information will sound louder. How the power is dissipated over distance relative to frequency I'm still not clear on. More study will be required. But, transmission through the air from a vibrating source is different than transmission through a barrier (such as the body of a car) or what the acousticians like to call a partition. I'm not sure that the reason you hear the bass tones from the rap music in the passing car is because lower tones 'carry further'. One thing is, I believe, clear from my reading. Higher frequency tones are much more directional than low frequency tones. Everyone in the hall can hear the low frequency information but you have to be in the right seat to hear the high frequency information. This could perhaps explain the results from Richard's not very scientific 'test' between the Yamaha and the Steinway. Perhaps the 'testers' were in the wrong seats. If my reading is correct the Yamaha, which I assume would have a preponderance of higher partials in its tone, would have more perceived loudness if you are in the right spot, than the Steinway, which I assume would have more energy concentrated in the lower partials. The corollary to this is that the variation in perceived loudness of the Yamaha is going to be greater than for the Steinway as you move around the hall. If you're in the right spot to get the high frequency information the Yamaha will sound louder ('project more') but if you're in the 'wrong' spot you'll only be getting the low frequency information, and since the Steinway presumably has more of that, the Steinway will sound louder. The percentage of the hall that is the 'right spot' for getting the high frequency information is I believe small. I believe this also means that the perceived tone quality of the Yamaha will have more variation as you move around the hall. Since the tone is made up of lower frequency information and higher frequency information, if you're in the spot to get the high frequency information the perceived tone quality will be different than if you are in a spot not to receive as much high frequency information. So this means to me: 1. The Yamaha is potentially louder ('projects better') than the Steinway but this potential is only realized in a few spots in the hall. 2. The Steinway is not as potentially loud as the Yamaha if you're in the right spot but in a majority of the hall it will be perceived as louder ('project better'). 3. The Steinway will have less variation in perceived loudness and tone quality than the Yamaha around the hall. If I were a designer I'd like the conclusions about the Steinway better than the conclusions about the Yamaha, but that's just me. What does this mean to tuned duplexes? For the front duplex, if the presence of the duplex adds high frequency information or causes the string to vibrate in such a way that it contains more high frequency information then it seems you would be increasing the potential projection. If we could just 'add on' the front duplex then we would, I think, be increasing projection potential. In some places in the hall we would realize this potential, but in most places in the hall we wouldn't. However, we can't just 'add on' the duplex. The duplex gets its energy from the string. So, once again, by concentrating more of the vibrational energy in the upper partials we have increased projection potential in a few places in the hall. But we have done this at the expense of lower frequency information in the string. So in most of the hall we have sacrificed projection. I think we have also contributed to more loudness and tonal variations around the hall as well which I don't think of as a good thing. The difference between a tuned and untuned duplex is unclear to me. Both would be putting string energy into higher partials. Perhaps the tuned duplex, by interacting with the speaking length causes the string to want to vibrate more strongly in the partial corresponding to the duplex and concentrate more of its energy in this partial. In that case the tuned duplex would make the phenomenon that we're discussing here more pronounced. The conclusion that I draw is that as a designer, if I want better projection in a majority of a hall, and want to minimize tonal and loudness variations around the hall, the front duplex should be eliminated altogether. I think the pianists would thank us as well since they wouldn't have to listen to annoying noises that go along with a front duplex. As to the rear duplex. I would draw the same conclusion. Eliminate it. Unfortunately we don't have that option. There has to be a back scale in order for the bridge to move. So we really have the option of a tuned or untuned system, although with either system the duplex could be felted out. Either system is putting high frequency information into the sound. So the same things apply here that were talked about above. It's hard for me to see that there would be any difference between a tuned and untuned system as far as projection is concerned. Sustain is another issue not talked about here, but without looking into it further I can't see why a tuned system would give better sustain. I believe there will be a difference in tone quality or color with the tuned and untuned systems. I'm not sure which I would choose if the two were side by side, but I'm not convinced it would be the tuned system. I think an argument could be made in favor of felting out rear duplex. Based on the things talked about above if you want less variation in perceived tone quality around a hall you might be better off not adding this additional higher frequency information. Also, if you believe a beautiful piano tone contains more fundamental relative to upper partials then you might want to felt out these additional 'partials'. I suspect all of this is a matter of taste. Some people will prefer one system and some another. Using different systems would allow us to have differences among our pianos which in my opinion would be a good thing. Phil --- Phillip Ford Piano Service & Restoration 1777 Yosemite Ave - 215 San Francisco, CA 94124
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