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8/23/2005 P108 Lab 9a: Page 1 9 A : A DDING SOUND SIGNALS I NTRODUCTION Both loudness and pitch are complicated subjective phenomena. The purpose of this lab is to give you some direct personal experience of some of the hearing phenomena. This lab should also give you more practice using the oscilloscope as a measurement tool. N OTES ON THE E QUIPMENT At your station you will find some of the equipment which has already been hooked up for you: a dual-function generator, a headphone box, an oscilloscope, and a frequency counter. The function generators can be connected to a mixing amplifier, which combines (adds) two signals so you can hear them at the same time. The combined signals are displayed on channel 1 of the oscilloscope (and audible through the headphones). You can control the frequency and amplitude of each independently, or eliminate them from the combined signal by selecting "OUT." Another thing you need to know is that the mixing amplifier can only put out so much power. If you ask for more power, the amplifier will simply clip off the top and bottom of the output signal, which you will hear as a nasty buzzing sound. You can solve this problem by turning down the amplitudes of the generators. D ECIBELS As you should know from the previous lab, a decibel is a number (with no units) which compares two numbers. A decibel is always a comparison to some reference value, and is meaningless unless you know what that reference value is. A typical reference value for sound pressures is pressure of the softest sound the average person can hear. To get the decibel value, you divide your value by the reference value, take the logarithm, and multiply by 20. dB = 20 log (p/p 0 ) For example, if a sound pressure is 2000 times the reference pressure, it has a value of dB = 20 log (p/p 0 ) = 20 log (2000) = 20 x 3.3 = 66 dB, which is about the level of sound near a busy street when compared to the threshold of hearing.
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P108 Lab 9a: Page 2 With voltages, it is common to take the largest typical voltage as the reference, which means that the other voltages will be a negative number of decibels in comparison. For example, if the reference signal has an amplitude of 3 volts, then a signal with an amplitude of 1 V is dB = 20 log (V/V 0 ) = 20 log (1/3) = 20 x –0.477 = –9.5 dB compared to the fundamental. When you put a number less than 1 into your calculator and push “log,” it will automatically give you a negative number, but it is important that you know what positive and negative decibel mean. P
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This note was uploaded on 01/20/2012 for the course PHYSICS 108 taught by Professor Kesmodel during the Fall '08 term at Indiana.

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