Table 1Īpproximate Speed of Sound in Common Materials In gases, the higher the velocity of sound, the higher the pitch will be (Remember the "Mickey Mouse" sound when people talk after inhaling helium gas?). Table 1 lists the approximate velocity of sound in air and other media. These pressure variations travel through the air as sound waves ( Figure 1). Thus, as the drum surface vibrates, it creates alternating regions of higher and lower air pressure. When the surface moves in the opposite direction, it creates a negative (lower) pressure by decompressing the air. This creates a positive (higher) pressure by compressing the air. As it moves forward, it pushes the air in contact with the surface. The drum surface vibrates back and forth. To illustrate, imagine striking a drum surface with a stick. These air pressure changes travel as waves through the air and produce sound. When an object vibrates, it causes slight changes in air pressure. Sound is produced by vibrating objects and reaches the listener's ears as waves in the air or other media. In either case, it can be hazardous to a person's hearing if the sound is loud and if they are exposed long and often enough. Rock music can be pleasurable sound to one person and an annoying noise to another. The difference between sound and noise depends upon the listener and the circumstances. Logarithms (or logs for short) are simply a way of describing numbers which vary by very large amounts in a much smaller range.Sound is what we hear. Just about every piece of audio equipment (microphones, loudspeakers, sound cards, amplifiers, mixers, etc) will have specifications expressed as logarithms (i.e. We need to get familiar with the idea of a logarithm. The decibel is a better fit to how our brains perceive sound. The relationship between perceived loudness and the decibel is a straight line. On the next graph below, a logarithmic decibel scale is used and now the curve becomes a straight line. Our ’10x’ rule means that as the overall level increases, we need increasingly large changes in intensity to get a similar change in loudness. An intensity of 1,000 is also half as loud as an intensity of 10,000. So an intensity of 1,000 is twice as loud as an intensity of 100. To make a sound twice as loud, you need to multiply its intensity by about 10. In the graphs below, the x-axis represents the perceived loudness of a sound, and the y-axis represents the acoustic intensity needed to create that loudness. After analysis, I could offer various recommendations to rectify the issue − all distinctly individual from each other, as acoustics can be affected by nearly everything!” Rob Bungay, Acoustic Consultant, WSP What are decibels? To understand and try to reduce these causes of complaints, among other things I installed sound level meters throughout the room to record noise levels during the day and night, and used acoustic models of the space to assess changes within the room. “I recently worked on a multi-bed intensive care ward where patients and visitors were complaining about the intrusive noise made by the alarms from bedside monitors. In acoustical and audio engineering, decibels are used everywhere because knowing how loud a sound is important whether you’re assessing whether the noise from a new road is going to be a problem or setting the loudness of music in an arena. Graphic: Nick Maroulis Where are decibels used? When a sound is perceived to double in loudness, this corresponds to roughly an increase in 10 dB. It makes things easier if a logarithmic scale is used this is what the decibel scale is. The ratio of intensities between silence and ‘ow that hurts my ears’ is about 1:100 million million. The human ear is capable of hearing very quiet (low intensity) sounds and extremely loud (high intensity) sounds.
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