196 Unit 3 Waves and Sound Copyright © 2002 Nelson Thomson Learning

8.7 ELECTRICAL INSTRUMENTS AND AUDIO REPRODUCTION

PRACTICE (Page 302-303)

Understanding Concepts 1. An electric guitar does not have a resonant box. The overtone structure and any resonant effects are produced

electronically. Thus, an acoustic guitar must be used without amplification. An electric guitar will sound very tinny if unplugged.

2. Woofers produce lower frequencies and longer wavelengths than tweeters. The cone in the speaker must be larger to produce a longer wavelength. Also more energy is required to produce lower frequencies since more air is required to be moved.

3. When old recordings are remastered for digital sound, the recording is first changed from analog into digital sound. Next the recording is ‘cleaned’. This process removes most of the clicks, hisses, and pops inherent in disc recordings. The next step is to use computer software to widen the frequency response; original recordings had a limited frequency range particularly in the high frequencies. The intensity range is electronically broadened since the intensities in the origin recording were limited for louder sounds by the movement of the record needle in the groove. Finally, the recording is ‘burned’ onto a CD.

Activity 8.7.1 Evaluating Headphones (Page 303)

Observations Experimental Set-up

Figure 1 Using the Pasco 500 Interface and Sound Sensor

Copyright © 2002 Nelson Thomson Learning Chapter 8 Music, Musical Instruments, and Acoustics 197

Figure 2 Using a dB meter

To eliminate resonance inside the Styrofoam head, one earphone was disconnected. The following observations were obtained with a dB meter, set at the dBb setting. The three readings were taken at the following settings, all peak-to-peak: 0.2 V, 0.4 V, 0.8 V. Background noise was approximately 55 dB.

Frequency (Hz) dB (0.2 V) dB (0.4 V) dB (0.8 V) 50 35 38 40 100 52 55 59 200 56 59 64 300 58 62 68 400 60 65 70 500 62 68 73 750 66 72 78 1000 70 76 83 1250 73 80 86 1500 76 83 89 1750 78 85 91 2000 77 85 92 2500 77 85 89 3000 76 84 90 4000 79 85 92 5000 79 85 92 6000 78 86 93 7000 75 83 90 8000 70 78 85 9000 69 76 83 10000 65 71 78 12000 55 58 63 15000 55 58 63 18000 48 49 50 20000 27 30 34

198 Unit 3 Waves and Sound Copyright © 2002 Nelson Thomson Learning

The data for 0.4 V sensitivity has been drawn (Figure 3). Since the dB meter was used, log graph paper is not necessary.

Figure 3

Analysis (a) The frequency response for the earphones is relatively flat from 500 to 8 000 Hz, only dropping off significantly below

100 Hz and above 10 000 Hz. This is to be expected for earphone, particularly at the low end, since the diagram of the speaker is small. Also, since the earphones were manufactured in 1998 the bass response is probably lower than newer units, where better methods are used to produce the low frequencies.

(b) For the same earphones, the results would be similar to those found in the chart. (c) When searching the Nelson Web site the site http://www.headwize.com will provide most of the information for this

section.

Section 8.7 Questions (Page 304)

Understanding Concepts 1. The sounds from a tweeter are more directional because the wavelengths are short and do not diffract much around large

objects. Therefore the listener must be in front of the tweeter. 2. The higher the sample rate, the greater the detail to reconstruct the waveform (see Figure 3, page 302). Sampling at

40 000 versus 20 000 times per second would produce twice as many “bars” providing a much better approximation of the original wave.