The other day I kept hearing this noise from my neighbor. I couldn’t quite figure
it out, and naturally it was annoying. I didn’t do anything about it, but it
got me thinking about some random facts about sound and noise.
Medium
|
Velocity
|
(m/s)
|
(ft/s)
|
Aluminum
|
4877
|
16000
|
Brass
|
3475
|
11400
|
Brick
|
4176
|
13700
|
Concrete
|
3200 - 3600
|
10500 - 11800
|
Copper
|
3901
|
12800
|
Cork
|
366 - 518
|
1200 - 1700
|
Diamond
|
12000
|
39400
|
Glass
|
3962
|
13000
|
Glass, Pyrex
|
5640
|
18500
|
Gold
|
3240
|
10630
|
Hardwood
|
3962
|
13000
|
Iron
|
5130
|
16830
|
Lead
|
1158
|
3800
|
Lucite
|
2680
|
8790
|
Rubber
|
40 - 150
|
130 - 492
|
Steel
|
6100
|
20000
|
Water
|
1433
|
4700
|
Wood (hard)
|
3960
|
13000
|
Wood
|
3300 - 3600
|
10820 - 11810
|
-
The range of human hearing is 20 Hz – 20,000 Hz, however most people can only hear
between 40 Hz – 16,000 Hz
-
All frequencies are not equal. Our ears perceive certain frequencies to be louder
than others (found at Wikipedia http://en.wikipedia.org/wiki/Equal-loudness_contour):
-
Sound travels a smidge less than 1 foot per second at standard temperature and pressure.
Therefore if you need to place speakers in front of other speakers, you need to delay
them based on distance… 40 feet = ~40ms of delay.
-
If a speaker is placed in front of another without a delay, the sound from the speaker
farthest from you will sound similar to an echo. This is called the Haas effect.
However, most people don’t notice this until there is a 40ms gap between sounds, or
roughly 40 feet. After about 40ms of delay, the intelligibility of the sound
also decreases. I.e. it starts to degrade the quality, and you start having
trouble understanding what you hear.
Mostly useless facts, but they are fun to know.
Interesting error found in explorer.exe. I tried hitting [Windows] + [E] and
got this message:
Kinda bizarre. I blame solar flares.
Holy crap this is cool:
Someone once told me that a balanced audio connection works because of polarity. I
wish I had a rolled up newspaper so I could swat him with it on the nose. Balanced
systems are used to keep noise and interference out of systems. It is a common myth
that balancing a system involves polarity. It does not. Polarity plays a part in keeping
interference out, but the real reason balanced systems work has to do with impedance.
This type of connection is known as an unbalanced system. There is only one connection
leaving the Op-Amp in device A. The second connection is ground. In device B the signal
is brought in on one leg of the Op-Amp and the the second leg is a replica signal
sourced to ground (or reference). In other words the signal is the same except opposite
(polarity). There is absolutely nothing preventing noise and interference from entering
this system.
This is a balanced input. Notice how the input connector has 3 connections instead
of 2.
This a balanced output. Notice the 2 Op-Amps and 3 connections. The balanced system
has both connections equally referenced to ground. How this prevents interference
is an idea called Common-Mode Rejection (CMR). Because interference hits all three
wires in a cable at once, they will all have an equal level of extra noise. It is
voltage essentially. When the signal enters the Op-Amp at the input it looks at the
ground line and sees what’s on it. It then compares what it sees on the two signal
lines. It kicks out what all three have in common. Hence Common-Mode Rejection.
This is theory though. Not all inputs are perfect, and because all cables have something
called cable capacitance, voltages differ minutely on each wire within the
cable and the rejection doesn’t work as well as the theory states it should. But it
still works pretty darn well. There is a whole science devoted to developing a standard
for getting better CMR. One of my favorite resources is Jensen Transformers’ Bill
Whitlock. He is a freakin genius. Here is his seminar
handbook on balanced and unbalanced connections. This is where it all started
making sense to me.
Enjoy!