Source Bibl. Bodlein MS Rawl. Letters 58
In the Chapter of Echoes etc. It would bee of noe small ornament and curiositie to contrive a whispering-place; for if the Arching bee ellepticall made by a line of a double center denoting the 2 focus of the ellepsis, these whispering places may bee made. For in the longest diameter of an elleipsis there are 2 poynts named the foci, always equidistant from the center, from one thereof if a line bee drawne unto the circumference soe reflecting that the angle of reflection bee equall unto that of incidence, they will reflect unto the other focus, and soe the sound bee conveyed unto him whose eare lyeth at it. And therefore if wee whisper at one focus, all the vocall rayes wch are caryed unto the circumference of the elleipsis are by reflexion all united in the other focus, and by the multitude and union of these reflexed rayes the voyce bee strogly heard at the other extreme or focus, not easily in the middle unto wch one direct and single ray only arriveth.
Nor to rest in the bare Architect or fabrick, butt upon the same to inscribe the mechanicall draught wherin lye the causes and reasons of this Admirable Effect, the figure being drawne in red or blewe extending the whole length of the arch, and each focus denoted by some mark or speciall colour wherat may stand 2 figures of Cupids, boyes or handsome draughts wth the mouth to the one focus the eare unto the other, according to this draught the rule wch containeth the mysterie of this effect.
Neere the same in other spaces may bee drawne the best acustick instruments contrived for advantage of hearing, such as may bee seen in Bellinis