The Erhu


 

Intensity

When sound waves hit a person's eardrum, they cause it to vibrate at the same frequency as the waves. Likewise, the larger a sound wave's amplitude or compression is, the larger the ear drum's vibrations will be, and the more neural impulses will be sent to the brain, resulting in a perception of louder sound. With sound waves, we have to talk about the amount of "compression" when discussing amplitude because sound waves are longitudinal-- particles in the medium (in this case, air) move in the direction of the wave, so that air is continuously being compressed in front of the wave.

To create a sound wave of higher amplitude, more energy must be put into the strings, causing the sound waves to be more compressed at their condensations and less compressed at their rarefactions, as shown in the diagram. The condensations are analogous to peaks and rarefactions are analogous to troughs in a transverse wave.

Thus, the sound that reaches the ear will be louder. Similarly, a smaller amplitude of string vibration will compress the air less and the sound will be softer. The conversion from mechanical energy in the form of the strings' vibration to sound energy in air is discussed in the Amplification section.

Intensity is measured as the power (energy per second) per unit area or the energy flow past a certain area over a certain time. It is measured in Watts per meter squared. The sound intensity level is the magnitude of intensity, measured in decibels. The formula is where LI is the sound intensity level, I is the intensity, and Io is the reference intensity, 10-12 W/m^2, which is the Threshold of Hearing-- if you plug it into the equation as I, you get 0 dB. Most humans have a sound intensity level range of about 120 decibels, with 0 decibels being the softest audible sound. A normal conversation is about 60 dB. On the loud side, a jet plane heard up close has a sound intensity level of 110 dB. Our erhu has a similar sound intensity level to a violin, which is about 80 dB.

The decibel scale is used much more widely to measure intensity than the standard W/m2 unit of intensity because the human range of hearing is very large; to accommodate this, the decibel system is actually logarithmic. This means that 100 dB is not twice as intense as 50 dB; it is actually 10,000 times as loud. This results in numbers that are easier to handle, so that we don't always have intensities with big exponents everywhere; 0 dB is a lot easier to work with than 10-12 W/m^2.

The intensity of a sound decreases with distance because the energy of the sound spreads out in a larger area. Since the power, which depends on energy, is divided by area to calculate intensity, this will create a smaller intensity. Because distance squared is proportional to the area, if a person is twice as far from a sound source as another person, he will hear an intensity one fourth that of the intensity the other person hears. This can be easily scene in the schematic drawing below.

 

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Copyright 2008 Harvest Zhang & Karen Kaminsky. All Rights Reserved.