Aim: The aim of the experiment was to discover the difference between loudness and sound pressure level, gain knowledge of typical sound level values in common environments and to be able to make rough estimations of what sound level measurements could be in different places.
Method: The CEL-440 sound level meter was used to measure sound pressure levels in dbz and loudness levels in dBA. It was decided to base the experiment on learning environments around the university, ranging from areas of study inside and outside. This was done because, as the areas that are considered in the experiement are ones that the group would use in everyday life so it would be interesting to see what loudness and sound pressure levels most people live and sit in during there every day lives.
Loudness is a psychological quality which relies on human interpretation. Loudness is hard to measure as it depends on how the human ear responds to it. Sound Pressure Level is linked to loudness as it is a physical quantity which can be measured. Sound Pressure Level can be measured, when loudness cannot.
Sound pressure level ranges from the smallest detectable sound which is measured in pa (pascals). Sound is heard, by air follicles vibrating on the ear. The highest sound is atmospheric pressure around 1,000,000pa that can not physically be heard. If it was any louder then there would not be any atmospheric pressure left so everything would be destroyed. Threshold of human aural perception is the pressure of the air molecules colliding with the ear drum which is close to the threshold of perception.
Young people with sensitive hearing are able to hear as little as 20pa, but most people will not be able to hear this measurement. The just noticeable difference depends on factors such as overall levels and frequencies. However, it is conveniently remembered as 1dB, one decibel. When something is measured in decibels it is refering to ratio (10log power difference, 20 log voltage difference) between the amount of two levels and the level being measured. How do people hear sound?
When a sound is made, it is dependant on the response of the human ear and every ear responds in a different manner. Everybody has a different individual shape to their ear. The ear canals are unique to every individual, and ear canals resonances occur at approximately 3.4khz and 13khz, but this depends on age and health. There is no accurate way of measuring sound as everybody interprets is differently. The diagram below shows the basic structure of the ear and where the ear canal is situated.
Fletcher and Munson Experience 1993 was the breakthrough of an investigation that explains a great deal. They investigated the perception of loudness in human subjects. The loudness level estimated by the subject for different stimulus levels and different frequencies was recorded. Humans have a limited range of hearing whereas dogs can hear things much louder. Fletcher and Munson discovered a graph of equal loudness contours. Shown below.
When the ear hears sound it is not equally sensitive to all frequencies, especially the low and high frequency ranges. Fletcher and Munson charted the response to frequencies over the entire audio range and this is shown in the graph above. The set of curves show the sound pressure level of pure tones that are percieved as being equally loud. The graph has been plotted for each 10 decibel rise in level when the reference tone is 1kHz. This is also referred to as loudness level contours. From 1 to 5kHz the curves are lowest in the range. There is a slight dip at 4kHz which probably indicates that the ear is most sensitive to frequencies in this particular range. The levels of intensity for higher and lower tones had to be raised in order to create the equal impression of loudness.
The results were accurate to the predictions made for the different locations. The sound pressure level in the places such as the computer labs and lecture theatres were thought to be quieter. However there were many computers and equipment which were left on standby, appearing off and very quiet. It was thought that the the library upstairs should have been quieter than downstairs as it is where books are kept and downstairs is where groups congrigate. Although it seems fairly quiet in the upstairs section, the air conditioning fitted can effect the sound pressure level recorded.
The recording booths and live room were exactly as predicted which is very good as it means they have been built correctly and are one hundred percent sound proof. The relationship between the dBa results that were collected and the dBz were more or less as expected. The measurements that stood out were the library upstairs as the sound pressure level was much higher than the loudness along with the Lecture room. The lecture room was empty until it was entered by the group and there were no lights on either. The lights turned on automatically when people entered.
It was thought that in both the library and the lecture theatre there are a lot of frequencies that exist but cannot physically be heard. This would include certain machinary such as air conditioning, automated light response system, fans from computers that seem very quiet, anything that is left on standby which appears off. Loudness is a subjective quality in which is depends a huge amount on sound pressure level as well as the frequency spectrum and amplitude envelope of sound. It also depends on the environmental conditions under which it is heard and the auditory of the listener.
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