Sine sweeps are used as reference tones to measure frequency responses. This one scans all audible frequencies from 20 Hz to 20 kHz in a 20 second time span. It differs from the classic engineering sweep, as it takes into account two fundamental non-linear properties of the human ear so that it can be used as a subjective test. Subjective testing means executing a test in which one uses his own hearing to judge the results, without the help of any audio equipment.
This test signal is truly unique, it is the only perceptually-compensated sweep test signal available on the Internet.
Our perceptual sweep compensates for two non-linear phenomena affecting human hearing.
First, human hearing has a lower frequency resolution at higher frequencies. So, it makes no sense to scan the high frequencies with the same level of detail as the lower ones. Our sine sweep uses the logarithmic time scale, which means that the sweeping speed exponentially increases as the frequency increases. This ensures that octaves are equally spaced in time, i.e. the time taken to travel from 20 Hz and 40 Hz (a factor of two, or one octave) is the same as from 10 kHz to 20 kHz (one octave). Because our hearing works logarithmically, you won't notice any increase in speed, but you will experience a constant-speed sweep. That's the first reason this sweep is called perceptual.
The second reason relates to the sensitivity of our hearing, which depends on the frequency. With a prominent sensitivity bump around the upper medium range, frequencies around 1-3 kHz will be perceived as much louder than any other frequency played at the same level. Our perceptual sine sweep compensates for this phenomenon, by decreasing the sound level in the areas where the ear is the most sensitive. Our goal is to keep the subjective level constant across all frequencies.
Unfortunately, such sensitivity not only depends on the frequency but on the sound level too, and we have no idea how loud you are playing the file at your end! So, unless we can find a way to measure the loudness at your end, we cannot adjust our sweep tone to sound perceptually flat on your system.
Those familiar with Audiocheck, know we have a few tricks in our bag. Here is another one: we will ask you to play the sweep file at your hearing thresholds, which means as quietly as possible, but not quieter ;-) The tone should remain audible, just faintly. Now, we have an exact idea of the sound level achieved at your end, and we will use the human sensitivities around hearing thresholds - which are well documented - to turn our sweep perceptually flat!
Keep in mind though that these sensitivities as found in the literature, are averages measured across a group of (healthy) people. By definition, an average may not exactly match your own hearing sensitivities, but will often provide a good approximation of what your hearing is supposed to be, unless you suffer from a hearing loss.
Sine sweeps play one frequency at a time over a wide range of frequencies. They are used as reference tones to check frequency responses, such as the frequency response of you system, speakers, or ears, or determine resonant frequencies in your room.
This sweep is not meant to be used along with test devices, but only with your own ears. When played at hearing thresholds, its perceived loudness is supposedly flat. Any deviation in loudness comes either from your audio system or your ears.
(20Hz-20kHz, Hearing Thresholds, 20s)
When checking by ear, the sound should evolve smoothly from the lowest frequency to the highest, without any change in perceived loudness. Loudness changes result either from your ears having a different sensitivity as the average (a hearing loss maybe? Check here), or your equipment frequency response being biased.
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