Four weighted filter functions, A, B, C, and D, are used to simplify and apply regions of the loudness contours that are most meaningful for describing the frequency response of the human ear toward real world applications.
Refer to Figure 3 for the following discussion. A-weighting defines the shape of the filter and the human ear response at low sound pressure levels, namely the 40 phon loudness contour curve. Sound level measurements in decibels relating to A-weighting are denoted with the units — dB A. Shaping for this curve means that low frequencies are attenuated and the speech frequencies are amplified within the measuring equipment. B-weighting describes an intermediate level approximating the 70 phon curve.
Notice how the ear's response begins to flatten. C-weighting utilizes the phon curve, which describes the nearly flat response of the ear for high levels. The C-weighting response is most useful for typical home theater listening levels and for evaluating system performance for flat response characteristics. The D-weighting Curve is a special case developed for aircraft fly-over noise testing, which penalizes high frequencies.
The A and C weightings are most often used since the former relates to normal everyday sound pressure levels and the latter relates to higher listening levels where the ear's response is nearly flat. We've covered some significant background, but how does all of that relate to the loudness control feature on an audio system?
Understanding how the ear perceives sound intensity versus frequency leads us directly to that loudness feature. The loudness control is simply intended to significantly boost low and high frequencies when listening at low levels so that the ear perceives an overall flatter sound pressure level. In other words, if the loudness contouring control is not enabled at low volume levels, bass and treble appear to be lacking.
This effect corresponds to the recently described A-weighted condition where low and high frequencies require additional amplification so the audio "sounds good. Since the ear's frequency response is relatively flat at high sound levels, the compensating effect of the loudness contouring control is not required.
The loudness feature is a kind of equalizing function that, ideally, should adjust itself to have greater compensation effect at low sound pressure levels and less effect as sound pressure increases. From Figure 4, you can see that the amount of power needed green shaded area bounded by LA curve to compensate for low frequencies is significant. For this reason, in home theater audio system design, it is not uncommon to use fairly large, separate amplification just for the low frequency channel.
The shaded area within the high frequency range indicates relative compensation required for this portion of the spectrum when at a lower volume level. At high loudness levels, where the ear's response is nearly flat, compensation requirements decrease to nearly zero as shown by the LC curve.
The issue is whether the implementation of the loudness control feature merely boosts lows and highs using one fixed setting as some simplistic designs might do; or is it dynamic and capable of modifying the amount of equalization depending on the setting of the volume control? Historically, most loudness controls were analog implementations using discrete resistors, capacitors, and even inductors intended to approximate the compensation curve curve LA in Figure 4 for the A-weighting function.
Most were designed around the volume control. Figure 5 illustrates one simple approach using a volume control incorporating a fourth tap located about halfway through rotation. This value is designated as dB A. The dB A is often used as it reflects more accurately the frequency response of the human ear. Weighting networks are often incorporated in measuring equipments to give readings in dB A. Copyright : Environmental Protection Department. All rights reserved.
Sound Pressure expressed in. Click on the demo button, you can learn how to add the sound levels using the chart. A normal human ear is able to hear sounds with frequencies from 20 Hz to 20, Hz. The range of 20 Hz to 20, Hz is called the audible frequency range. The sounds we hear comprise of various frequencies. A particular sound or noise can be seen to be having different strengths or sound pressure levels in the frequency bands, as illustrated by the following diagram. Characteristics of Sound and the Decibel Scale.
There are two important characteristics of sound or noise - frequency and loudness. Frequency of Sound. Sound is the quickly varying pressure wave travelling through a medium. When sound travels through air, the atmospheric pressure varies periodically. The number of pressure variations per second is called the frequency of sound, and is measured in Hertz Hz which is defined as cycles per second.
Admittedly, this is a rather simple explanation of a complex process. Sound frequency is an important aspect of how we interpret sounds, but it is not the only one. A sound wave has five characteristics: Wavelength, time-period, amplitude, frequency and speed. While amplitude is perceived as loudness, the frequency of a sound wave is perceived as its pitch. As you see, sound frequency is determined by the way in which sound waves oscillate whilst travelling to our ears, meaning that they alternate between compressing and stretching the medium, which in most cases is air.
In the same medium, all sound waves travel at the same speed. Squeaky sounds, like the blow of a whistle or a screaming child, oscillate at a high frequency, resulting in oftentimes deafening high-pitched sounds. The low rumbling of a nearing storm or a bass drum, on the other hand, is produced by low-frequency oscillation, so we hear it as a very low-pitched noise.
How is sound frequency measured? The total number of waves produced in one second is called the frequency of the wave. The number of vibrations counted per second is called frequency. Here is a simple example: If five complete waves are produced in one second then the frequency of the waves will be 5 hertz Hz or 5 cycles per second.
Also called infrasound, low-frequency sounds stand for sound waves with a frequency below the lower limit of audibility which is generally at about 20 Hz. Low-frequency sounds are all sounds measured at about Hz and under. A high-frequency sound is measured at about Hz and higher. You'll have come across the terms frequency and decibels by now.
While both are obviously associated with sounds, do you know what each one really means? Decibels, commonly shortened to dB, are a unit to measure the volume of sound. The decibel scale starts from 0 dB, which is the threshold of sound for the human ear. When looking at the measurement of dB, it depends on context. Sound at 0 dB doesn't mean that no noise is present, rather, it's the quietest sound that the human ear can pick up.
In order to accurately calculate the volume of sounds we hear into digits, a logarithmic scale is used rather than a linear one. That means if a noise reads at 3 decibels higher, it's double the sound intensity. At 0 db, it's almost complete silence. A whisper will read around 15 dB, whilst a normal conversation is approximately 60 dB. To scale, a rock band's volume can hit dB, and though it reads double on the decibel scale, a rock concert is not twice as loud as a mere normal conversation!
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