August 18th, 2002
I wrote this essay / monologue for a classroom session with some teenaged would-be audio engineers. Mike Eschman.
Physics - The Reproduction of Sound.
The Social Impact of Reproduced Sound (including Video).
Reproduced music is commonplace.
It's live music that is unusual today.
Even in a club, you hear the band through microphones, amplifers
and speakers. With Dolby 5.1, the acoustics of a room can be
overcome, so that the soundstage has no apparent relationship
to the room.
A peculiar situation, because our notion of a soundstage is based,
on first person accounts of live music. But our experience, to some
is fundamentally third person. That is why reviews in a publication,
say, Consumer Reports, is more likely to influence the purchase of
audio and video
equipment for the home than listening test. Most word-of-mouth
comes from reviews.
That is a social endorsement of a third person "preference in selection",
to a first person bias. Somehow, to some unknown degree, we have
lost absolute faith
in first hand experience. Disturbingly, that is occurring in
tandem with growing
isolation, as discussed in the essay on "Human Diversity".
Compounding all other problems, the notion that costs and prices should
have put dead musicians in direct competition with the living.
That could drive live
musicians into a system of patronage, where the few decide what the
many will hear.
In the past, listening to music played by someone other than you and
played a relatively minor role in daily life for most Americans.
Now, it occupies a
greater portion of our day, and we are more isolated from each other.
Automobiles make suburbs possible by encouraging mobility, which weakens
I can't put numbers to that, but, I would be embarrassed for anyone
who argued that automobiles
discouraged mobility and were good for inner city neighborhoods.
Orson Welle's film adaption
of Booth Tarkington's "The Magnificent Ambersons" is a fine exposition
on this view of recent
What are the changes to the listening experience with reproduced sound?
(1) Room reverberation detracts from a recorded performance, but enhances
a live performance.
We want to hear a recording that sounds like the Hall the band is playing
in, we do not
wish to hear the room we are sitting in.
(2) Each listener added to the room changes the performance in some
fundamental way. In a live
performance, if the audience and the band share some bond, it can add
a great deal of excitement,
possibly even insight, into the performance. For most people,
most of the time, at least in my
experience, that is not typically the result of a home listening experience
with more than
ten participants. For working adults, most listening to recorded
music is done one-on-one.
(3) Each listener must maintain a geometric relationship to the speakers
in the room, even in multi-
channel systems, in order to hear EXACTLY what was recorded.
This is about the same risk as buying
a seat in a new concert hall, but the frequency and duration of events
where you hear DISTORTION of
one type or another is probably most of your adult musical experience,
because opportunities to listen
to recorded music permeate our daily lives.
Within these limitations, a stunning range of human accomplishment is
available to a huge worldwide
audience with a "reasonable" fidelity to the original. Most of
that audience exists because reproduced
What is "reasonable" fidelity?
(1) "Good" distortion is slow and sneaks up on you, and bad distortion
leaps out at you.
(2) Any distortion at a frequency that was silent in the original sounds
Example: A 60 cycle hum, or any other constant buzzing in the
Example : When a radio station starts to "break up".
(3) Reproducing the sound quality of voices and instruments is very
Example : The Trumpet and the Clarinet must sound different from each
If I can't tell one from the other, the reproduced sound is BAD.
That quality is known as "Timbre". It is related to the spectrum
of sound produced by
an instrument or voice. It can always be heard, even in a recording
that is BAD.
For any instrument or voice, the timbre is different when a player
plays LOUD than when
singing SOFTLY. Furthermore, if a recording of a player playing
SOFTLY is played LOUDLY,
the listener can still tell that the player is playing SOFTLY.
If you listen to a
LOUD passage at a low level, you still know they are playing really
LOUD - you can hear it -
so it works in both directions.
You should make a grid with all the combinations and label each possibility
true or false.
Using instruments which display a video, that shows clearly WHEN frequencies
change in volume,
it is possible to see how an instrument or voice sends sound into a
room. These SHAPES demonstrate
that a single musical note is composed of MANY frequencies, and that
each instrument, including voice,
has a distinctive shape on the video screen, because the note must
start at 0 and change until it
reaches the desired note.
Example : When a trumpet plays the musical note A = 440 hertz, a note
that changes in intensity as a
pressure wave in a room, exactly 440 times a minute, the trumpet must
reproduce all the frequencies
from 0 up to 440. As it moves towards it's stable frequency,
some intermediate frequencies have no
enharmonic relationship to that frequency, but others do.
What that means : The molecules in the materials from which an instrument
is made, including voice,
have geometric properties, one of these being length. When the
length of the space between the
pressure waves in a sound field, and the length of the molecules in
the material have some specific
numerical relationships to each other (don't worry about what those
are, it's well documented) then
the instrument adds a coloration to the sound, because while most of
the frequencies being run die
immediately, these resonate with the instrument and the musician's
body cavities, if you're a wind player.
So these make up part of the sound for an extended time period and
cause multiple frequencies to shape
the pressure waves making up the sound field.
Musical timbre depends on the relative loudness of these "partials",
when the target frequency is reached.
Phase relationships have an important role in determining the sound
of instruments, and the human voice.
What is a Phase relationship?
In this case, it is the time each partial took to reach it's stable
volume level, on the way to the target
If all the partials rise to the same volume at the same rate very quickly,
our instruments will display
pressure waves that have straight sides and a flat top. This
is a harsh, raspy sound.
It is most closely approached by plucked strings (guitar) and struck
But pianos and guitars can sound very smooth and expressive, not harsh
Let's take a look at bowed strings - violins.
The motion of a bowed string follows a sawtooth pattern. The bow
drags the string along for a
short distance, then the string slips back, only to be picked up again
and carried along with the
motion of the bow. This is abrupt, and should sound BAD.
But it can sound quite beautiful. How
can this be? Bowed instruments incorporate a type of amplifer
that smears the time domain in some
beneficial way. This amplifier is called a SOUNDBOARD.
Partials that are close to the resonances in the soundboard are made
louder than other frequencies,
even frequencies right next to a partial frequency. A soundboard can
also make a partial SOFTER, rather
Think about making an energy conservation argument that might explain
For bonus points, specify an experiment to test your hypothesis.
Partials affect timbre in unexpected ways. Closed organ pipes
have a "hollow" sound because
only odd harmonic partials are present.
Vowel sounds can be distinguished and perceived regardless of pitch
because they get their sound
from the partials in our personal soundboards, the human vocal cords.
Even in whispered speech,
where ALL FREQUENCIES are present, vowels are clearly distinguishable.
If you sing the "ah" phrase - a vowel phrase - for as long as you can,
two minutes? more?
you will hear the sound begin to lose its "ah-ness".
So a vowel sound looses its vowelness over time, if sustained.
If that's so, then whispering "ah", it should "disappear" into
the "hash" of ALL FREQUENCIES?
If so, if not ...
You can play SOFT clarinet sound LOUD on a radio, and STILL TELL, the
player played softly?
This is due to loudness being a function of the distribution of volume
among the partials.
Example : Loud trumpet has proportionally more energy in upper partials
than soft trumpet.
This type of distribution, volume across the partials in a sound, is
a type of "spectral energy
In all brass instruments, partials tend to rise late and fall early.
More so for a trumpet than a tuba -
i.e. more pronounced in higher register brasses.
The upper partials oscillate rapidly - this is called "brilliance".
Draw a "brilliant" spectral energy distribution that accounts for partial
volume as a function of time.
Hint : color coded partials vary volume over time.
In woodwinds partials rise and fall at the same time.
Flute and string players produce a small burst of noise just before
a musical note sounds. This noise
is important for the quality of sound the player rises too.
Bells and gongs produce partials which share no harmonic (i.e. the numerical
relationships we talked
about earlier) relationship. The harmonic series defines the
relationships between fundamental
frequencies and their related partials. This behavior can be
experimentally deduced using vibrating
Here's a real mystery : Bells and gongs don't exhibit consonance
and dissonance when you play a chord
sequence on them. But they are able to reproduce melody, as they
Write me with an experiment we can try in "the lab".
I'll videotape it and send it back to you.
mike eschman, etc ...
"Not just an afterthought ...