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 Now online (2)dariusgriffin ijonspeches ...and 380 guests Last 5 registered Oplandisks nothingstar N_loop yipe foxtrotromeo Browse members... |
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The Audio Spectrum. "A major criterion of a good sound system is its frequency response. The usual frequency range considered "hi-fi" is 20-20,000 Hz. Hear sample tones at 100 Hz (12 kb wav file) and 10,000 Hz (44 kb wav file). Yesterday I did a test using the very accurate signal generator built into my CLIO system. I can clearly hear, and certainly can feel, a 10 Hz tone. My sound system totally poops out below 10 Hz, so I can't test any lower than that. The lowest notes on organs and pianos are 16.4 and 24.5 Hz respectively. Testing at the other extreme, as a 61 year-old male I can hear a 13,500 Hz tone, but no higher. (It is generally agreed that women are more sensitive to high frequencies). However, good high frequency response is required to produce sharp transients, such as a snap of the fingers. I performed a test using a Ry Cooder CD, "Talking Timbuktu." Track 10 on this disk has some very sharp transients that just leap out at you from a good sound system. My pre-amp has a filter that cuts off frequencies above 12,000 Hz. With this filter in, the transients limp out rather than leap out. This shows that even though I cannot hear a pure tone in most of the range of frequencies cut out by the filter, I can clearly hear the difference in the sound quality of the transients. I recently recorded a segment of this cut as a .wav file, and digitally processed it with a 12kHz filter. You can compare the filtered and un-filtered sound yourself in the section on sound demos. James Boyk at Caltech has posted an interesting paper on the frequencies generated by musical instruments between 20kHz and 102 kHz! He also cites a paper that states that people react to sounds above 26 kHz even when they cannot consciously hear the sound. Jourdain (page 42) states that sound can be heard up to 40 kHz if sufficiently loud (Although a knowledgeable reviewer is skeptical). The ear tends to combine the sound within critical bandwidths, which are about 1/6 octave wide (historically thought to be 1/3 octave). This |
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This has led to the practice of averaging frequency response over 1/3 octave bands to produce beautiful-looking frequency response curves. In my opinion this is misleading. Suppose a loudspeaker has a bad dropout (very weak response) over a narrow frequency range; the dropout will be totally obscured by averaging. But when a musical instrument plays a note that just happens to fall in the dropout notch, you will not be able to hear the note. See the example of a warts-and-all response (28.2 kb) vs. a 1/3 octave smoothed response (24.5 kb) from my final system measurements section. Since we can barely hear a 2-dB difference in sound level, it is reasonable to accept ±2 dB as an excellent level of performance for frequency response. In fact this is impossible to achieve in the real world, due to room acoustics. (see the section on room acoustics). Personally I would say a more-or-less practical goal for a sound system installed in a room is a frequency response ±5 dB from 200-20,000 Hz, and maybe ±10 dB from 10-200 Hz. It is also worth noting that the ear itself has a quite variable frequency response, as shown by measured data on head-related transfer functions, and as discussed in the next section. What is the minimum audible change in frequency? I created two .wav files: case #1 was a series of 1/2 second tone bursts, all at a frequency of 800 Hz; for case #2 the bursts alternated between 800 and 805 Hz. I can reliably distinguish between these two cases in a double-blind test. This difference in frequency is less than 1/100 of an octave. I could also distinguish between 400 and 402 Hz. According to Jourdain (page 18) this is about normal for a young person; at age 61 I'm not supposed to be able to detect a difference of less than about 8 Hz at 400 Hz. But I can." |
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interessting indeed |
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Intresting reading: http://www.silcom.com/~aludwig/EARS.htm |
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interesting... thanks archrival, i didn't know you were 63! |
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audio theory is always a fascinating subject. |
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