Sound Transmission: How your windows keep your house quiet

Source Post: http://www.replacement-windows.com/windowbb/viewtopic.php?p=5720

Wow, three different posts about sound and noise and windows...

I am starting a fourth in the hope that I can answer some questions from all three previous posts in one place. I am thinking that this can get complicated very quickly, so I am going to throw in a quick course on basic sound and terms related to such so that if I use these terms later they may make some sense please bear with me here.

In order to have sound you need three things, a source, a medium, and a detector.

The source is, well, the source of the sound (kinda self-explanatory).

The medium is whatever is transmitting the sound from point A to point B.

The detector is you.

In order to eliminate the sound, you need to eliminate one of the three elements. Since removing the source is unlikely, and since eliminating the detector is somewhat self-defeating, that leaves the medium as the element that we have to deal with.

Sound frequencies are measured by the number of and are referred to as or Hz. One Hz = one cps (or cycle-per-second)...100 Hz = 100 cps 1000 Hz = 1000 cps and so on and so on.

As human beings, we are born with the ability to hear from 20 Hz up to about 20,000 Hz. By the time we are teenagers, we have lost the ability to hear above about 14,000 Hz or so (with exceptions, of course). We also don’t hear at the same level at different frequencies. For example, I mentioned that we hear down to about 20 Hz. Well, we don’t hear a 20 Hz frequency as well as we hear a 400 Hz frequency. We are tuned to hear specific frequency ranges better than we hear at other frequency ranges. Our best hearing range is from about 1000 Hz to 4000 Hz and, coincidently, the human voice tends to range from about 400 Hz to 4000 Hz. Go figure heck of a design feature. Our voice range can go higher and lower than the frequencies noted, but the 400 to 4000 Hz range is generally accepted as intelligible.

The lower the number the lower the frequency. Traffic noise is low frequency, whereas a child screaming in the aisle next to you at the grocery store tends to be a relatively high frequency often to the point of pain and distraction.

Sound amplitude, or pressure, or loudness, is measured using decibels or dB. A dB is simply a logarithmic unit used to describe a ratio or by the dictionary definition:

A unit used to express the intensity of a sound wave, equal to 20 times the common logarithm of the ratio of the pressure produced by the sound wave to a reference pressure (typically 1 micropascal at 1 meter).

Forgetting the mathematical definitions for a moment, for our purposes, the higher the dB the louder the sound is. I may reference back to this later because some questions are best answered mathematically (at least partially), but I will try to avoid going there!

A change of 3dB either higher or lower equals a doubling or halving of the sound level. And of course, being a logarithm, the increases and decreases are exponential meaning that a higher dB can be A LOT louder than a lower dB (and I won’t go into that unless someone has a specific question).

There is a little thing used in human acoustics called a . This is based on our ability to hear one frequency better than another even at different amplitude levels. Our hearing response may not be the same to an identical dB level at two different frequencies, and because of this, the folks that do such things have come up with a way to measure our response to various frequency stimuli at different dB levels. They call this weighting or dBA.

dBA is used when designing acoustical systems.

A few definitions:

Amplification increases the sound pressure level (SPL).

Amplitude is the loudness of the sound or power of the SPL.

Attenuation decreases the sound pressure level.

Sound Transmission Loss (TL) is a measurement of the amount of sound energy that is attenuated, or lost, when sound passes thru a specific medium actually, not quite that simple, but it works for what we are doing.

Now I have to leave again, but I do anticipate being home later this evening, and at that point I will continue this thing on another post - this one is already long enough!

Have a great day all! And PLEASE, if you have questions about this stuff - ASK! I know that this stuff becomes complex very quickly and I also know that I don't always explain things in the clearest way possible when writing these things... Confused

Source Post: Sound and Noise and Stuff
http://www.replacement-windows.com/windowbb/viewtopic.php?p=5720
Posted: Wed Feb 08, 2006 8:51 am