Two Chapter 9

· The intensity of sound is difficult to measure.

· Decibel (dB) – The unit used to measure intensity of sound.

· Intensity of Sound – The average rate of energy (power) flow per unit area across a surface perpendicular to the direction of propagation.

· The ear can detect sounds with intensity greater than 1 X 10^-12 Watts/m².

· To convert intensity measured in Watts/m² to decibels, use the following equation.

· With the above equation it should be noted that dB is the decibel difference between the intensity I₁ and the intensity I₂. The equation basically compares the intensity of two sounds.

· Therefore, if we want to determine the decibel reading of a sound we can hear, then I₁ is the minimum intensity we can hear. The formula then becomes:

· 130 dB is the level of sound that is painful to the ear.

· Sounds over 90 dB can cause ear damage if exposure is too long.

· For humans 0 dB = 1 X 10^-12 W/m².

o Therefore,

§ 130 dB is 1 X 10¹³ times more intense than 0dB.

§ 90 dB is 1 X 10⁹ times more intense than 0dB.

§ 50 dB is 1 X 10⁵ times more intense than 0dB.

§ 40 dB is 1 X 10⁴ times more intense than 0dB.

§ etc.

· The intensity of sound picked up by the ear depends upon

o the power of the source.

o the distance between the source and the person.

· Wave energy is spread over increasing area as distance from the source increases. Therefore, the decibel reading decreases as distance increases.

· Sound waves reflect as any other waves do. They obey the two laws of reflection.

· Echoes – Sounds that reflect off of a barrier back to the source.

o Can only be heard if the time delay is more than 0.1 s (i.e.: the barrier must be more than 17 m away)

· Reverberation – The apparent prolongation of a sound because the echo occurs less than 0.1 s later. (i.e. the barrier is less than 17 m away).

o some reverberation can enhance sound

o too much reverberation makes music and voice indistinct.

· Reverberation Time – The time required for a sound of standard intensity to die away and become inaudible.

o used to design concert halls.

o Depends upon

§ materials used

§ height of ceiling

§ length of hall

§ type of music

§ audience present

· Well-designed halls have reverberation time of 1 to 2 s for orchestral concerts.

o 2s to 5 s is best for choral

· Anechoic room – rooms that have reverberation times near zero

o used for studying the performance of sound devices such as telephones, microphones and loudspeakers.

· Sound reflects off of barriers, as do other waves.

o Parabolic reflectors reflect sound just as they do light and water waves (Turns wave fronts from spherical to planar and vice versa)

§ Used in band shells – The band is located at the focal point

§ used with microphones – Microphone is placed at the focal point of a parabolic reflector. This enhances sound from wild life and sporting events.

· Echo sounder – uses sound reflection to measure the depth of the sea.

o A transducer converts electrical energy into sound with a frequency of about 30 kHz.

o the signal bounces off the ocean floor and is picked up by a hydrophone (underwater microphone)

o A computer measures the time between the sent signal and the return signal, and calculates the depth.

o The result is a plotted graph.

· Similar equipment is used in the fishing industry to locate schools of fish.

· Similar methods are used by the armed forces to locate submarines.

· Sonar – Sound Navigation and Ranging

o Any of the above and other similar devices.

· Ultrasonic Sonar – A similar device to above used to

o detect flaws in railway tracks and pipelines.

o determine fat/lean ratio in cattle and pigs.

o diagnose brain damage.

o detect breast cancer.

o monitor growth of the unborn child.

· Radar – Radio Detection and Ranging

o Similar to sonar only it uses radio waves (a type of light).

o The time between sent and reflected signal gives the distance to the objects.

o Results are displayed on a cathode ray tube or monitor.

· Sound diffracts – bends around corners just like any wave does.

o E.G. You can hear noise in a class down the hall from the room you are in.

· Long wavelengths diffract more than short wavelengths.

· Low frequency (pitch) has a larger wavelength, therefore, these sounds diffract more.

· Sound refracts just as any other wave does. It obeys the law of refraction.

· If sound travels between air masses, of different temperature, then refraction occurs because speed changes.

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November 13, 2013