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Facts about Sound you Probably didn’t Know

by Steve Syfuhs / August 11, 2010 04:00 PM

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):

Lindos1.svg

  • 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.

Bizarre Error Message from Explorer

by Steve Syfuhs / August 02, 2010 04:00 PM

Interesting error found in explorer.exe.  I tried hitting [Windows] + [E] and got this message:

image

Kinda bizarre.  I blame solar flares.

The Known Universe

by Steve Syfuhs / February 09, 2010 04:00 PM

Holy crap this is cool:

> > >

Balanced Audio Connections

by Steve Syfuhs / January 31, 2009 04:00 PM

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.

image

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.

image

This is a balanced input. Notice how the input connector has 3 connections instead of 2.

image

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!

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Steve is a renaissance kid when it comes to technology. He spends his time in the security stack.