Who made the Decibel Scale4/10/2024 Likewise our hearing does not have the same sensitivity at all frequencies. No microphone has the same sensitivity to all frequencies and no speaker reproduces all frequencies equally well, as we will see in Chapter 18 on electronics. The above curves are very much like the frequency response curves of microphones and speakers. Medium loudness doesn't change the perceived pitch very much. Low frequencies are perceived to be slightly lower than expected if they are very loud. High frequencies are perceived to be a slightly higher pitch than normal if they are very loud. Decibel values are thus only linear at first glance 120 dB seem to be twice as loud as 60 dB. dB values are most often given in dB (A) the A standing for the use of the A filter, otherwise known as the sound level evaluation curve A. It is also the case that intensity has an effect on perceived frequency the same laboratory frequency will appear to be a slightly different frequency if the intensity is different. The value on the scale thus corresponds more with our perceptions and is made measurable. This scale is used particularly in applications where sound travels in water.\( \newcommand\) is due to the tube resonance of the auditory canal (see chapter 12 for tube resonance and chapter 10 for a picture of the auditory canal). It should be noted at this point that there is another decibel scale in use, called the sound pressure level, based on the ratio of the pressure amplitude to a reference pressure. For example, a 56.0 dB sound is twice as intense as a 53.0 dB sound, a 97.0 dB sound is half as intense as a 100 dB sound, and so on. It’s mind-boggling how quickly the logarithmic scale of decibels goes up from there: Basically, for every 10 dB increase, we’re adding a zero to the amount of intensity versus the threshold of hearing. In equation form, intensity I is I=\frac\\ is given (and not the actual intensities), this result is true for any intensities that differ by a factor of two. Zero on the decibel scale is the threshold of human hearing: it’s the least intense sound a human can detect. Power is the rate at which energy is transferred by the wave. There is also a (rarely used) B weighting scale, intermediate between A and C. Measurements made on this scale are expressed as dB(C). The C scale varies little over several octaves and is thus suitable for subjective measurements only for moderate to high sound levels. Intensity is defined to be the power per unit area carried by a wave. Measurements made on this scale are expressed as dBA. The relevant physical quantity is sound intensity, a concept that is valid for all sounds whether or not they are in the audible range. High noise exposure is hazardous to hearing, and it is common for musicians to have hearing losses that are sufficiently severe that they interfere with the musicians’ abilities to perform. In cartoons depicting a screaming person (or an animal making a loud noise), the cartoonist often shows an open mouth with a vibrating uvula, the hanging tissue at the back of the mouth, to suggest a loud sound coming from the throat Figure 2. We are all very familiar with the loudness of sounds and aware that they are related to how energetically the source is vibrating. But when a passing motorist has his stereo turned up, you cannot even hear what the person next to you in your car is saying. After settling into bed, you may hear your blood pulsing through your ears. In a quiet forest, you can sometimes hear a single leaf fall to the ground. The term became accepted in the 1920s and since then noise measurement has generally come to rely on the decibel scale and others derived from it. Noise on crowded roadways like this one in Delhi makes it hard to hear others unless they shout. Decibels may be qualified as dBA, dBB, dBC, indicating the weighting network of the SOUND LEVEL METER with which the measurement was made.
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