 demonstrated by stopping up one ear. difference effect is the major factor in sound location up to several It is then found that one becomes aware of extraneous noises, room hundred hertz, above which the head is big enough in relation to reverberation, etc. that one was largely unconscious of before. It is wavelength to begin to have an obstacle effect. Consequently there a well known fact that sound recordings made in a non-acoustically start to be amplitude differences at the ears and these can play a part treated room often contain disturbing amounts of unwanted noises in location. that were not apparent to the ears at the time of making the recor- ding. It is a common observation that steady tones are difficult or even impossible to locate and there needs to be a relatively low frequency envelope for the location process to occur. 8.6.3 The Haas Effect Sounds above or below the person, or in the arc behind the head, seem to be located by one or more of the following: The ears tend to judge the location of a sound according to the direction of the first arrival, even though the later arrival is higher 1. Successive head movements, often ending with the person in level. The Haas Effect, as it is known, is significant in high quality looking at the apparent source. sound reinforcement systems, where loudspeakers close to parts of 2. Visual indications. an audience may be fed with a delayed signal SO that the direct sound Common sense (bird song heard from inside a sitting room is from the stage, platforms etc. arrives first at the listeners. The unlikely to come from anywhere except a window!). illusion is then that the sound originates only from the stage, 4. The effect of the folds and ridges in the outer ear, as mentioned whereas the received level from the loudspeakers may be as much previously. as 10dB higher than the direct sound. Table 8.11 gives the approxi- Psychoacoustics 8/15 Threshold Shift, dB Figure 8.17 Noise masking by 400Hz tone mate relationship between level differences in dBs and the time a