On 12/22/2015 11:53 AM, Tim R wrote:
On Tuesday, December 22, 2015 at 11:39:49 AM UTC-5, Phil Hobbs
wrote:
On 12/22/2015 11:02 AM, c4urs11 wrote:
On Tue, 22 Dec 2015 05:59:52 -0800, Tim R wrote:
However, my question is about how you would use a 1950s era
scope to determine a sine wave or the degree of harmonics
present.
Scopes from that era easily reached several MHz of bandwidth.
That should be considered adequate to inspect audio signals.
Cheers!
The eyeball is a really lousy detector of harmonics, though,
especially odd harmonics.
Plus he had to use a 1950s-era microphone, so the scope bandwidth
is irrelevant.
I've used an RTA, but those hadn't been invented yet. Before my
time, but wasn't there something called octave filters?
The experimenter wasn't real detailed but supposedly he could tell
from looking at the scope that it was a pure sine without harmonics.
Well, he was wrong about that. Even 10% third harmonic isn't easy to
spot unless you have a comparison sine wave on the screen at the same
time. (I'm thinking about zero degrees relative phase, so the peaks are
symmetrical. It's a bit easier to see at other phases.)
I was very skeptical that 1950s technology allowed that.
Unless he had a really expensive ribbon mic, his 1950s microphone had a
heavy diaphragm and rolled off really badly above about 5 kHz. (One of
the audio guys will correct this, but it's roughly right.) None of the
nice 40-kHz piezo film mics you can get nowadays. (I have a matched set
of Earthworks omni mics from about 15 years ago--their impulse response
is about 15 microseconds wide.)
He is a believer that the material a trumpet is made from determines
the sound, whereas many of us believe it is the shape of the air
column.
It's both.
I will quote the article: ****** At one time we ran an experiment in
which we used steel, aluminum, various plastics, glass, silver,
various combinations of brass and the last one we used was lead. To
demonstrate results as quickly as possible, I will choose the two
extremes. The steel bell, which we tempered so it was extremely
hard, gave possibly one of the most interesting results. Many people
test a bell by tapping it with their finger or knuckle and in tapping
the steel bell, it would emit a very ringing sound, truly like a
bell. However, when we played this instrument, the quality of sound
was extremely dead. On searching for the reason for this, we looked
at the oscilloscope when the performer played on the instrument and
found the sine pattern very faint but the distortion pattern, coming
from the vibration of the bell itself, going through at a very
jagged and rapid rate, killing the brilliance of sound of the true
tone.
Changing the material also moves all the mechanical resonances, which
will have a huge effect.
At the other extreme was the lead bell. This bell, if rapped with
your knuckle, emitted an extremely dead sound like rapping on a piece
of wood. However the sound that emanated when it was blown was
extremely brilliant, brilliant to the point of being mechanical. This
showed up on the oscilloscope as a perfectly true sine pattern, there
being no distortions in the harmonics either above or below, and, as
a result, the sound was absolutely pure but not usable musically,
except for a general effect such as a percussion instrument would
give.
The voice, you know, registering on an oscilloscope, gives harmonics
both above and below the note. These distortions, if we may call them
such, give warmth to the tone. We have to have that "distortion" in
order to have the sound acceptable to our ears as a musical sound.
Cheers
Phil "Not an audio guy" Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net