Letters
Voxativ Ampeggio Impedance Peaks #1
Editor: Horn loudspeakers was my main subject as B.Sc.E.E. in 1980. . . Ideally a sound should travel from the diaphragm through a horn and into the air without any sound reflected, and traveling back to the diaphragm. A sudden change in the horn shape will cause a sudden change in acoustic impedance, and sound will travel back to the diaphragm. The end of the horn wall, at the mouth, does make such a sudden change. But if the mouth is large compared to the wavelength, the acoustic impedance has almost changed to that of free air, and the reflected sound is small. This is similar to reflections in HF cables caused by impedance mismatch. . . The Voxativ Ampeggio mouth is small compared to a 70Hz wavelength, so there is a big change in acoustic impedance from the mouth into free air. Secondly the horn is made of conical sections and bendt, so the traveling sound meets many smaller sudden impedance changes. All this causes sound to travel back to the woofer cone and this is measured in the electrical impedance.—Ole Lund Christensen, olelundchristensen@yahoo.dk Voxativ Ampeggio Impedance Peaks #2
Editor: In the August 2011 issue of Stereophile, JA measured the impedance of the Voxativ Ampeggio loudspeaker and noted impedance peaks at about 80, 140 and 190Hz, which he stated are "unexplained." (The peak around 27Hz is presumably the driver resonance peak.) There are also measured peaks in the acoustic responses at either the woofer or horn mouth near those frequencies.
I found a schematic cross-section picture of the Ampeggio cabinet on the 6 Moons website, and measuring the length of the three straight sections of the folded horn gives approximate distances of 18", 24" and 40". If we assume these sections behave like quarter-wave pipes (, like organ pipes), this gives resonant frequencies of 190Hz, 140Hz, and 85Hz, respectively, which is close to the resonant peaks measured, so I strongly suspect that that is the mechanism involved.
As a completely unrelated comment, I have played a couple Schimmel pianos in
stores and they are very fine sounding and playing instruments.—James Lin, jlin712@earthlink.net
Editor: Horn loudspeakers was my main subject as B.Sc.E.E. in 1980. . . Ideally a sound should travel from the diaphragm through a horn and into the air without any sound reflected, and traveling back to the diaphragm. A sudden change in the horn shape will cause a sudden change in acoustic impedance, and sound will travel back to the diaphragm. The end of the horn wall, at the mouth, does make such a sudden change. But if the mouth is large compared to the wavelength, the acoustic impedance has almost changed to that of free air, and the reflected sound is small. This is similar to reflections in HF cables caused by impedance mismatch. . . The Voxativ Ampeggio mouth is small compared to a 70Hz wavelength, so there is a big change in acoustic impedance from the mouth into free air. Secondly the horn is made of conical sections and bendt, so the traveling sound meets many smaller sudden impedance changes. All this causes sound to travel back to the woofer cone and this is measured in the electrical impedance.—Ole Lund Christensen, olelundchristensen@yahoo.dk Voxativ Ampeggio Impedance Peaks #2
Editor: In the August 2011 issue of Stereophile, JA measured the impedance of the Voxativ Ampeggio loudspeaker and noted impedance peaks at about 80, 140 and 190Hz, which he stated are "unexplained." (The peak around 27Hz is presumably the driver resonance peak.) There are also measured peaks in the acoustic responses at either the woofer or horn mouth near those frequencies.
