Important Notice:
Error Correction on Decibels
Important TAKS Information about Harmonic Motion
Harmonic Motion Topics to Cover for TAKS
In a recent Physics meeting we noticed that the Holt Physics book contradicts the CPO Physics book (on which I based my worksheets) regarding the human perception of decibels. CPO incorrectly states that a 20 dB change of volume sounds twice as loud, when the actual number should be 10 dB. We checked with additional sources, including (embarassingly enough) my own experience with sound recording, from which I know a 6 dB change is very drastic.
For verification I then took a very loud sound file and electronically altered it to 10 dB softer and 20 dB softer. I must admit that much of this argument I find subjective. The 10 dB change was, again, very drastic. To my ears it was more than half as loud. The 20 dB change was MUCH, MUCH less than half as loud; maybe more than four times softer. (It changed a very loud sound to a very soft sound.)
I conclude that a 10 dB change is much closer to half as loud than a 20 dB sound. Exactly half as loud I consider subjective.
Therefore: please teach that a 10 dB change in sound either doubles (if increased) or halfs (if reduced) the sound intensity. Subjectivity set aside, it is most important that students understand that decibels and sound loudness is NOT a linear scale.
Thus 30 dB sounds twice as loud as 20 dB and 50 dB sounds half as loud as a 60 dB sound.
I have made the appropriate changes to two worksheets [13:1 (Sound) and Harmonic Motion Equations] and to a study help (decibels).
Important TAKS Information about Harmonic Motion:
Upon perusal of all released TAKS tests to date (early 2006), there have been NO questions on harmonics: none! Therefore, teach harmonics if you have time. Certainly demonstrate it to your students if you have the appropriate equipment (it's fun to see!), but don't squeeze it in believing it to be necessary.
Harmonic Motion Topics to Cover for TAKS
Wave Interactions: Absorption; Reflection; Refraction; Diffraction.
The Wave Equation (Speed of a Wave) - very common on the TAKS test.
Recognizing wavelength from a graphic. (Used once in relation to the wave equation.)