Okay, shot my mouth off, remarked off the cuff.... Some people wrote and
asked, "okay, who is Helmholtz and what has he got to do with mufflers?"
The original question and list comment, with regard to Flowmaster,
concerned a "Helmholtz resonator," and its purported effectiveness as a
high-throughput muffler design.
In actuality, the Helmholtz resonator has nothing to do with car
mufflers, as I suggested. Its origination stems from Helmholtz's
interest in developing a sonic-frequency _detector_:
"HELMHOLTZ RESONATOR"
"A cavity-type RESONATOR so constructed that it will vibrate only at a
particular FREQUENCY, giving off little energy to the outer medium, and
therefore resonating for a considerable length of time.
"It was developed by the German physicist Hermann von Helmholtz in the
19th century to analyze the HARMONIC components of a COMPLEX TONE, but
has since been superseded by
more sensitive electronic devices.
"However, large-scale Helmholtz resonators have been used recently by
acousticians in
perfecting the design of the celebrated Sydney Opera House. The basic
design of the
resonator as an enclosed air space with a single aperture is similar to
the body of the violin or guitar." [quote from the audio lab of San
Francisco Univ.]
In short, the Helmholtz resonator is a free-standing closed box with a
single opening, and the size and shape of the box and its single
aperture must be tuned to resonate at a particular frequency--if the
design is correct, the box will resonate at a specific sound frequency.
How that principle applies to a modern car exhaust system is beyond me.
The closest automotive application to a true Helmholtz resonator, in my
memory, is the blanked branch trap suggested by John Morrison, in his
book on intake and exhaust systems, for engines with siamesed exhaust
ports, as was used on many MGs.
Otherwise, I believe the Helmholtz resonator, as described above, has no
real application for automotive use, and Flowmaster has simply adopted
the term as a fancy, and misleading, means of describing a blanked
expansion chamber in an otherwise standard muffler. Since the size of
the chamber and the opening to the chamber are fixed, it will resonate
only at a specific frequency, and as everyone knows, modern engines must
operate at widely variable rpm, and the exhaust system of such engines
is a compromise to accept highly variable gas speeds and exhaust gas
sonic properties.
Further, the chamber must be designed to extract gases more efficiently
to be marketed as a power-producing device, based on sonic frequencies,
rather than to simply resonate at specific frequencies, as the reference
to a Helmholtz resonator suggests. My general guess about such a chamber
is that, at a specific rpm and a specific frequency, it could be
designed as a noise-cancelling device by creating negative sinewaves
which, when added to the input frequency, create beat frequencies with a
resultant of near-zero amplitude, but would have no significant
improvement on overall engine power, or if it did, would only produce
such power increase at a specific and limited rpm range and gas speed.
If one still doubts, simply call Flowmaster and ask for the formulae by
which the muffler was designed. With those, one can make sound
determinations of the true efficacy of their muffler design. But, you
won't get them directly from Flowmaster. If you encounter opposition,
simply ask for the patent numbers of the device and go to the US gov.
patent office site and search by patent number. The reasoning and
formulae have to be there, by law, for the patents to be issued. One
will find from such a search that the application of Helmholtz's
resonance principles is likely general and that the device works as
described over a very narrow rpm and gas speed range. That's the nature
of exhaust gas sonics.
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto: mporter@zianet.com]
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
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