Home.
The
Acoustics Lab at Kettering
University is the home of Dan Russell 's work
with
guitars , pianos, and, more recently, baseball and
softball bats ! It's also an active
teaching
laboratory supporting our coursework (leading to a minor or
concentration)
in
Acoustics .
Recent renovation
of the lab space has opened up great opportunities. The
latest, and perhaps the largest improvement in the lab facilities is
the Anechoic Chamber, assembled in the Fall of 2006 with the assistance
of Herman Orgeron (AP/ME dual major).
My background is in wind instruments, so I am working to augment the
current capabilities and equipment (especially with our larger,
renovated
space)
with facilities for winds and air columns. While I enjoy work
with
trombones
and other brass instruments, more general capabilities will help us
teach
more and explore further. I also have an interest in extending
the
computer-based resources in the lab, both in data acquisition and
experimental
control, and in computational modeling of acoustic and dynamic systems.
The input impedance is defined as the ratio of the pressure to the
volume velocity at the input to the mouthpiece. In everyday
terms, this is the relationship between the "push" or acoustic effort
(pressure) and the result of that effort, the amplitude of the flow
(volume velocity quantifies the oscillating volume of air). For a
brass instrument, this ratio is quite useful for understanding the
behavior of the instrument - playability, intonation, and so on.
My
work in understanding the oscillations of the player's lips relied
on input impedance to characterize the instrument's effect on the
lip. The aerodynamic forces that regulate lip motion and cause a
periodic, musical tone are dependent on a well-designed
instrument. Most simply, the reflections of sound energy that
return to the mouthpiece to exert pressure on the lip need to be
well-timed to coincide with the period of the lip oscillation.
Put together, a well-regulated lip oscillation and well-timed pressure
waves to perform that regulation create a musical tone at the intended
frequency. This occurs at the resonances of the instrument, as
determined by the length and cross-section design of the pipe.
(There is much more about this in an excellent Introduction to the Acoustics of Brass Instruments,
written by Joe Wolfe of the University of New South Wales.) This
also happens to occur at the peaks of the input impedance
function. At those peaks, the instrument can exert quite a bit of
pressure on the lips for a given supply of volume velocity. This
nice, healthy pressure ensures that the oscillation of the lips is
timed to coincide with the standing wave of the instrument, and a solid
note can be sounded. Obviously, understanding the input
impedance for a particular instrument can be very important in creating
a good design. Two fine articles explain more about the input
impedance of brass instruments: one for a wide audience by Arthur
Benade [Benade, "The Physics of Brasses." Scientific American, July
24-35 (1973)] , and the other, more technical study Journal of the
Acoustical Society of America [Causse, et al., "Input impedance of
brass musical instruments - Comparison between experiment and numerical
models." J. Acoust. Soc. Am., 75, 241-254 (1984)].
Piezoelectric driver: One of the first projects of a general nature is the assembly of an impedance head. The original idea sprang from a description by Arthur Benade [Benade and Ibisi, "Survey of impedance methods and a new piezo-disk-driven impedance head for air column." J. Acoust. Soc. Am., 81, 1152-1167 (1987).] Begun in the Summer term of 2003 as an independent study with Tim Swieter and continued by Chidi Uhiara in Winter 2004, this project developed and built several inexpensive versions using a piezoelectric driver and electret microphone. After one term with Tim, we had our first impedance head, built for the dimensions of a trombone mouthpiece. Chidi refined the design somewhat, and began developing a calibration process and function to improve the precision of the device.
Microflown device: The Microflown is a
particle velocity sensor based on a MEMS device that measures
temperature differential between two tiny wires. Combined with a
very small precision microphone in a package the size of a match head,
we can measure the input impedance almost
directly (one only needs to convert particle velocity to volume
velocity with a known cross-sectional area. The device is part of
a system designed and built in the Acoustics Lab at Kettering
University to make input impedance measurements in a straightforward
and direct way. The project is made possible through a
partnership between Kettering and Microflown Technologies.
The Kalimba is a type of mbira made by Hugh Tracey in South
Africa. The mbira
(one of the group of instruments casually and generically called a
"thumb piano") is an ancient instrument of the Shona people of what is
now Zimbabwe. Its popularity reaches across Africa, and around
the world -- in traditional repertoire, amateur and recreational use,
and in contemporary, pop and Western-tuned compositions. The
acoustics of the kalimba come from two main structural features, the
metal keys and the wooden resonator box or plate. A thorough
investigation of the instrument, with the help of an independent study
student, William Rein, has yielded some understanding of these
structures. Experimental modal analysis of the resonator box,
laser vibration measurements of the keys, acoustic transfer function
measurements of the box cavity, and modeling of the keys all tell the
story written centuries ago by inventors in central Africa.
The Djembe (pronounced, and alternatively spelled Jembe or JEM-bay) is a drum that
originated in West Africa. Traditional forms of the instrument
have a goblet-shaped shell carved from a single piece of hardwood, over
which is stretched a specially-prepared goatskin. Tension in the
skin is maintained by a system of rope laced around iron loops which
sit at the top and middle of the shell. Web resources for more about the
djembe detail playing techniques, instrument care, and the social
aspects of the drum (dance and drum circles) abound.
In the Kettering Acoustics Lab, Dr. Russell and several students
have studied
the generation of the bass, tone, and slap sounds produced by the
djembe. My interest is to model the three acoustic parts of the
drum using multiphysics/finite element
software, and try to understand better how they work together: the
membrane, the air cavity, and the shell. Comparison of
experimental modal tests in the lab with results of eigenvalue analysis
in the model are looking promising, but much more work needs to be
done! Ultimately, we would like to inform the tuning/skin
tightening process and the playing techniques to achieve distinctive
bass, tone, and slap sounds by understanding the physics of the
instrument.
The middle of the 20th century saw a blossoming of new electrified
musical instruments. Many of these were modified versions of
traditional "acoustic" or non-electric instruments; the electric guitar
is a dominant example. Designs for portable electric pianos were
also popular. Among these the Rhodes keyboard and the Wurlitzer
were perhaps the most commercially successful. Both electric
pianos used tuned metal rods or bars instead of strings, eliminating
the need for a strong soundboard or frame to support string
tension. The metal rods were struck by hammers, much like piano
strings are struck, but the bars produce very little sound
output. A piano soundboard transmits string vibration into the
air through its large surface area. Electric pickups and
amplifiers did the job of the piano soundboard; the bars' vibration is
sensed, amplified, and coupled to an acoustic wave through a speaker.
The resonances of the bars are responsible, in part, for the tone of
the electric piano. A group of students in Acoustics
II: Sound and Vibration (Physics 482) investigated the
vibration characteristics of the bars to see how they compare to ideal
bars according to Bernoulli-Euler beam bending theory.
Copyright Daniel O. Ludwigsen, 2006