James Grebe R.P.T. from St. Louis pianoman@inlink.com "I am only as good as my last tuning" ---------- > From: ralph m martin <rmartin30@juno.com> > To: pianotech@ptg.org > Subject: Re: The Revenaugh Lower Lid in New York Times reply > Date: Friday, December 12, 1997 5:45 PM > > Hi James > The original name for the bass reflex enclosure was : a phase inverter so > you are absolutely correct. I've been designing loudspeaker enclosures > for years so this is a subject close to my old heart. > > About the phasing of sound leaving the grand, though, I think you may be > mistaken. It's safe to say that anything below 300hz is non-directional > and all frequencies above that frequency leave their source in a beam > effect. This is why apartment dwellers hear mostly bass sounds from a > piano played on the other side of the wall belonging to an adjacent > apartment. > > The lid on a grand, when it is open does not affect the phasing of the > sound, but actually redirects it or reflects it out toward the listener > or whatever else is out there. If the sound is being reflected off the back wall of the enclosure how is that different than inside a piano where the sound is reflected off the lid? If we are to assume that, like > loudspeakers, the SB is a transducer speaking from both sides of it's > surface, then a drop lid of sorts on the bottom of the piano would do > exactly the same thing as the lid (providing, of course, that it was > reflecting the sound in the same direction). Any bass frequencies below > 300 hz you will hear anyway, despite the lid direction. Would the distance the bottom lid from the soundboard being different from the upper lid and soundboard distance cause any difference in phasing or sound delay or would it actually be enhancement and a fattening of the sound? > > While reading all the posts on SB technology, I have tried to visualize > any connection with loudspeaker design. I know that the loudspeaker, (if > it is a single, extended range type) is conical in shape so that the > entire diaphram acts like a piston for low frequencies and vibrates in > partials for all the other frequencies.This would seem to be a good > argument in favor of the crowned SB and it's resultant performance. While > there have been some flat loudspeakers manufactured, they seem unable to > transmit any strong bass content no matter how large. > > In Loudspeakers, the basic problem, when speaking from no baffle, is wave > cancellation. The bass frequencies, being non-directional, tend to cancel > themselves out by restricting themselves in the rear of the speaker. > Just mounting a speaker on a flat board helps simply by delaying slightly > the front emanations from the rear. The next best approach is the phase > inverter you mentioned. Would the effect be different if the hole were mounted in the back or sides of the enclosure? It, through delay and tuning, places the signal > 180 degrees out of phase with the frontal and reinforces the frontal > signal through an opening in the box or an open tube. The tube allows for > smaller enclosures. The box then becomes a Hermoltz resonator tuned (in > it's simplest form) to the free air resonance of the speaker. Rarely, > however do we tune the enclosure to the exact frequency due to other > factors such as the Q total of the speaker and other > parameters as well. > > Yet another type of enclosure is the infinate baffle and the acoustic > suspension baffle, both of which are sealed, totally enclosed boxes which > operate in two different manners. > > While I have not really "scratched the surface" of the subject here, I > though it may be well to see if there is any correlation in these factors > that could be helpful when thinking about the common SB we are all so > concerned about > > This is obviously something Del could jump into. > > best regards > Ralph Martin > > On Fri, 12 Dec 1997 07:08:48 -0600 "pianoman" <pianoman@inlink.com> > writes: > >Hi Zen, > >It seems like I remember that once sound waves bounce off of something > >like > >a panel or inside a speaker box the phase is reversed and the waves > >will be > >in phase again. I think that this is how the bass reflex cabinet > >works. > >Actually sound coming from the top is a mixture of phasing. The sound > >straight from the soundboard going into the room and the sound bounced > >off > >the underside of the lid out of phase. There is also the time delay > >effect > >of this. It would seem like a grand would sound better with no lid > >at > >all, thence no phasing effects and no time delay.. > >Just my opinion. > >James Grebe > >R.P.T. from St. Louis > >pianoman@inlink.com > >"I am only as good as my last tuning" > > > >---------- > >> From: Zen Reinhardt <diskladame@provide.net> > >> To: pianotech@ptg.org > >> Subject: Re: The Revenaugh Lower Lid in New York Times > >> Date: Tuesday, December 09, 1997 7:25 PM > >> > >> Anybody out there familiar with the physics of acoustics? > >> > >> My dad (who has an undergraduate degree in physics) suspects that > >the > >lower > >> lid may give rise to some out-of-phase cancellation in certain > >directions. > >> I'll try to look further into this with some other physicists I know > >.... > >> > >> ZR! RPT > >> Ann Arbor MI > >> diskladame@provide.net > >> > >> ---------- > >> > From: Delwin D Fandrich <pianobuilders@olynet.com> > >> > To: pianotech@ptg.org > >> > Subject: Re: The Revenaugh Lower Lid in New York Times > >> > Date: Tuesday, December 09, 1997 2:02 AM > >> > > >> > > >> > > >> > Benjamin Treuhaft wrote: > >> > > >> > > Today's NYT has a front-page story on a new invention: a > >lower > >lid > >> > > which collects and reflects outward the tone from the bottom of > >the > >> > > soundboard - tone which normally just bounces around down there. > > The > >> > > only part of the idea I don't like is that with the lower lid in > >place > >> > > there is no room for me. I love to sit under pianos when > >they're > >being > >> > > played. You have to pretend you are checking the substructure > >or the > >> > > customer will think you're nuts. > >> > > Benjamin Treuhaft > >> > > Berkeley, Calif. > >> > > >> > ----------------------------- > >> > > >> > I looked up the story in the NYT and found it to be quite > >interesting. > >> Especially the part that said Mr Revenaugh held the > >> > patent on the device. > >> > > >> > Odd. I'm sure I've came across an old patent on this idea before. > >As > >in, > >> from several decades back. Anyone living near a > >> > library housing the proper reference works up to doing a quick > >patent > >> search? > >> > > >> > Personally, I'm not sure just how effective the device would be in > >a > >> concert hall. I should think that if you were standing > >> > close to the instrument on the treble side you might notice some > >> difference. Back ten rows, I kind of doubt you'd hear much, > >> > if any, change, let alone any improvement. > >> > > >> > I should think, however, that this might be somewhat like > >positioning > >the > >> instrument, or deciding on voicing: shouldn't it > >> > kind of be up to the musicians? The pianist and the conductor. > >Even, > >> perhaps, the piano manufacturer. Surely it's not a > >> > decision for the "artistic director" alone to make. > >> > > >> > Del. > >> > > >> > > >
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