INTRODUCTION TO ACOUSTICS WAVE

Time

Any period motion will be have a frequency, we can say 60 cycles per second. This means if a one second time period was observed, 60 cycles would be present. However, it is not always practical to observe one second of time and count the numbers of cycles.

We can measure the time period for one cycle and calculate the frequency. We can also calculate the time period for one cycle if the frequency is known. Time and frequency are the reciprocal of each other. For example, if 60 cycles occur in one second, divide one by 60 to get the time period for one cycle. When determining the frequency from the time period for one cycle, divide the time period for one cycle into one.

If 60 cycles occur in one second and the time period for one cycle is 0,0167 seconds, the calculation can be verified by

f x τ = 1 or 60 x 0,0167 = 1. For note, that the time period for one cycle of all frequencies above 1 Hz will be less than one second. Also note that if frequency is in cycles per second, time must be measured in seconds.

Frequency

The number of cycles that occur in one time period, usually one second we call frequency. Until a few years ago, frequency was identified as cycles per second (cps). CPS was changed to hertz honoring the man who developed the frequency theory. Machine speed is measured in revolutions per minute (RPM), but the frequencies generated by those machines are measured in hertz.

From the above discussion, the formula for frequency and time can be derived.

For simple, these formulas can be concluded with a triangle like picture.

where f equals frequency or number of cycles that can occur in one second, τ equals the time period for one cycle and 1 equals 1 second in this case.

Acoustic Waves

Acoustic waves are pressure disturbances that propagate through a compressible fluid, such as air, and are interpreted by the human ear as sound. Normally, these pressure disturbances are very small compared with ambient pressures, but they can be measured using sophisticated microphones. These waves usually propagate uniformly in all directions, unless the wave encounters a difference in impedance. Acoustic waves, just like other mechanical waves, experience reflection, scattering, diffraction, refraction, and interference.

Sound Intensity

When analyzing enclosures, it will be useful to define the sound intensity and sound power of a source. The sound intensity, I, at a point is the time average of the instantaneous rate at which work is done by a sound wave as it travels. It is defined as

Where P is the sound power, A(r) is the area of a sphere of radius r. For spherical sound power sources the sound intensity are

Acoustic Impedance

The acoustic impedance plays a large role in the propagation of sound waves. By choosing materials of appropriate impedance, engineers can manipulate the sound transmission through a particular path. A wave will tend to continue uninterrupted in its path so long as the acoustical impedance in unchanged. By choosing materials with similar impedance characteristics, engineers can ensure the promotion of wave transmission, such as with ultrasonic testing. On the other hand, engineers can also suppress sound by choosing materials with much different impedances. For example, when a wave traveling through air (which has relatively low impedance) encounters a wall (which has very high impedance), the wave will reflect back on itself and much less of the wave will continue on in its previous path. Acoustical impedance is not merely a property of materials. Impedance also changes with changes in cross sectional area, bends, junctions, and openings. The acoustic impedance is defined as the ratio of the complex acoustic pressure to the complex particle velocity.

Where p is the sound wave pressure and u velocity of sound wave.

The noise level of household water pump we can measure easily. But for the more easily we define what that noise. The noise of a sound that is psychologically disturbing. Noise causes the listener to feel disturbed in their daily activities either at work, study or sleep. And even more fatal consequences will cause deafness in the listener's ears.The water pump is generally still widely used by households, especially in regions that are still sack groundwater needs. The use of water pumps generally produce noise. This will cause sleep disturbance when the water pump is operating at night.To reduce noise / loud this can be done by using active or passive methods. Active method is a method that uses sound waves of different phases with a wave of noise there. While the passive method is a method that uses acoustic material.The first step that must be done to control the noise level is certainly familiar sound sources produce noise. The next step measuring the sound pressure level (Sound Pressure Level, SPL) on the source of the sound. Finally new noise control methods can be applied accordingly.Measurement of the noise level of this water pump can be done easily. The tools used are sound level meter, the market price is still relatively cheap. But if there can be used an android Smartphone application that has installed a Sound Level Meter. These applications are generally still free.Noise measurement in done by first dividing the dimensions of the pump into several parts, namely, top, right side, left side and so on. Each section is divided into a number of measurement points. Each point can be measured SPL her big 3 times or more as shown in the picture

 

After the data obtained further spool water pump is made in the form of noise contour. Noise contours that have been completed can be seen in the following figure

 

From this noise contours then we can start designing a method for efficient and effective reduction shall we do to the water pump.

Post will later be followed by a simple experiment everyday materials that can be used to reduce the noise of the water pump.

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