$sound diffraction around an outside barrier$

However, the human ear is not equally sensitive to all frequencies. Most humans can hear in a range from 20 Hz to 20,000 Hz. Frequency is defined as the number of cycles of repetition per second, or the number of wavelengths that have passed by a stationary point in one second.įigure 6. The amplitude determines the strength, or loudness, of the wave.įinally, another characteristic of sound is its frequency, or tonality, measured in Hertz (Hz), or cycles per second. The length between wave repetitions is referred to as the wavelength (λ). The 'height' of the sine wave from peak to peak is referred to as its amplitude. For simplicity, the outward propagating waves can be approximated by the trigonometric sine function (see Figure 6). As the sources move and vibrate, surrounding atoms, or molecules, are temporarily displaced from their normal configurations thus forming a disturbance that moves away from the sound source in waves that pulsate out at equal intervals. As stated earlier, sound sources produce sound energy that, in turn, translates into tiny fluctuations in atmospheric pressure as the sources move and vibrate. The next characteristic of sound is its amplitude, or loudness. In the above example, the exact value would be computed as follows:Ħ0 dB + 60 dB + 65 dB + 75 dB = 10*log 10 First, rank the values from low to high, then add the values two at a time. The above table can also be used to approximate the sum of more than two decibel values. The following table may be used as an approximation to adding decibel levels (Note: Table approximations are within ☑ dB of the exact value). To combine decibels, they must first be converted to energy, then added or subtracted as appropriate, and reconverted back to decibels. They would, in fact, produce an SPL of 63 dB. For example, if a single vehicle pass-by produces an SPL of 60 dB at a distance of 15 m (50 ft) from a roadway, two identical vehicle pass-bys would not produce an SPL of 120 dB. That is, because decibels are expressed on a logarithmic scale, they cannot be combined by simple addition. The above relationships are important in understanding the way decibel levels are combined, i.e., added or subtracted. P ref is the reference sound pressure of 20 µPaĬonversely, sound energy is related to SPL as follows: To express a sound's energy, or sound pressure in terms of SPL, or dB, the following equation is used: The following figure shows a scale relating various sounds encountered in daily life and their approximate decibel values: A value of 140 dB is equal to an SPL of 200 million µPa, which is the threshold of pain for most humans.

$sound diffraction around an outside barrier$

On this scale, a value of 0 dB is equal to a sound pressure level (SPL) of 20 µPa and corresponds to the threshold of hearing for most humans. Because of this wide range, sound pressure is measured on a logarithmic scale known as the decibel (dB) scale. Typical sound pressure amplitudes can range from 20 to 200 million µPa. These sound pressure fluctuations are most commonly expressed as sound pressure and measured in units of micro Newtons per square meter (µN/m 2), or micro Pascals (µPa). These sources each produce sound energy that, in turn, translates into tiny fluctuations in atmospheric pressure as the sources move and vibrate. Highway traffic noise originates primarily from three discrete sources: truck exhaust stacks, vehicle engines, and tires interacting with the pavement. This section describes the acoustical considerations associated with highway noise barrier design, beginning with a brief technical discussion on the fundamentals of highway traffic noise.

$sound diffraction around an outside barrier$

Sound diffraction around an outside barrier