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Characteristics of Sound


When we hear two or more sounds of different loudness, it is hard to listen or we cannot listen to lower level sound. This phenomenon is called masking. We can hear conversations at a normal sound level in the office, but we cannot understand these voices at an intersection unless the loudness is increased. In contrast, when we set loudness of radio or TV in the home to a pleasing level during the day, we may decrease the sound level at night. These are caused by masking effect. To delivery articulate speech to listening public through the speaker, the loudness level of the speech must be higher than that of surrounding
environmental noise.
To deliver information through a public-address system, the sound pressure level of it at the received point should be higher than that of ambient noise, as shown in Table 1.

Voice(Paging, guide, etc.)5-10 dB
BGM3-5 dB
Music(concert, etc)15-20 dB




The sound level progressively decreases as it travels from its source. To extend the propagation distance sound output should be increased. When performing a public-address system through a sound system, the distance between the speaker and listening public is a major design factor.
As the sound travels from the source (point sound source) spherically, its sound pressure level decreases inversely with the square of distance, i.e. by 6 dB per square of the distance.



Sound in a room reflects off ceiling and floor but some frequencies penetrate through or are absorbed by these materials. Glass, concrete, painted plate, etc., have high reflectance. In a gymnasium we sometimes hear howling that is caused by reverberation of echo sounds.
In contrast, curtain, carpet, etc., easily absorb sound and decrease sound reflection. Apparel and clothes also have the same effects. Therefore, in a hall, the same music is heard differently when it is played without audience and with full of audience. Some music rooms, sound rooms and studios are provided with boards with holes on the walls. These are called perforated boards and function as acoustic absorbents and do not reflect sound. Acoustic absorption materials of various shapes and configurations have been developed.



Possible causes
Complicated combinations of direct sound from the sound source and sounds delayed due to reflections from walls, ceiling, floor, etc.
When the sound source radiates a sound, the direct sound from it first reaches the sound receiving point followed by sounds slightly delayed and tapered due to the repeated reflections from walls, ceiling, floor, furniture, etc., in the room.
When measuring a reverberating time, reference time is used. This is the time required for a sound continuously emitted until it reaches a fixed level and then turned off, decays by 60 dB.
Voice from a person making a lecture, talk, play or speech should be clearly heard to recognize the context, so that a longer reverberation time has ill effect. In contrast, longer reverberation time may have favorable effects while music is played because the reverberation may enrich the sound, depending on the type or nature of the music sound. Various reverberation times are proposed for creating proper effects depending on the capacity and usage of the room. Commonly used reverberation time settings are shown below.



The human ears judged the easiness of speech listening in the past. Such judgment is ambiguous and prone to be influenced by individual variations among listeners and the state of voice. The Speech Transmission Index (STI) provides the sound transmission with an objective value, focusing the physical phenomenon of sound mixing. In short, the easiness or difficulty of speech listening can be assessed by the resemblance of the original sound wave and transmitted sound wave to ears.
The STI values range from 0 to 1. If the transmitted sound to ears includes much more reflected sound than the original, in other words, the transmitted sound badly differs the original, the STI value decreases toward 0. (poor intelligibility)
Contrarily the transmitted sound resembles the original sound, containing less reflected sound, the STI value increases up to 1. (good intelligibility)
The STI value is generally required more than 0.6 for human speech to be intelligible.
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