The first group of eight listeners (average 92 dB SPL playback level) ranged in age from 20 to 60 (+/-3 dB) and included a mix of male and female auditioners chosen from Axiom staff as well as a few guests who happened to be visiting the Axiom plant in Northern Ontario. All reported normal hearing acuity. Each listener participated in individual listening sessions that typically lasted about 20 to 25 minutes. The tests were then repeated with an additional eight listeners at an average level of 89 dB SPL, and a further group of eight was tested at 86 dB SPL. Interestingly, the results demonstrated that the average amplitude level of the music playback did not affect the noise detection results. At each test level, the ability of listeners to detect the “noise” distortion depended on the relative loudness of the “noise” signal to the music, not on the overall average loudness of the music.
The “noise” or distortion test signal consisted of pure tones at fixed frequencies of 20 Hz, 40 Hz, 80, 120, 160, 200, 240 Hz and so on up to a high-frequency limit of 10 kHz. The test signals were chosen to simulate what loudspeakers would do under normal operating conditions. The pure tones were played over the third loudspeaker and subwoofer at gradually increasing levels of loudness along with the music from the stereo speakers and subwoofer. The test tone would be left on for two or three seconds, then off for a similar period of time, the test tone gradually increasing in loudness until the listener detected the “noise,” signaled by raising his or her hand. The tone would then be reduced in level until the listener lowered his or her hand indicating that the tone couldn't be heard anymore. This would be repeated to verify the noise level at which the listener detected the “distortion.” Data were recorded for each listener and each test frequency for both of the musical selections and plotted on a graph. Figure 1 shows the final average result for all 24 listeners.
Initially, individual listeners were told to raise their hand only if they “heard something in the music that doesn't seem right.” As the tests proceeded, alternate listeners were told the exact nature of the tests—that we were going to introduce a series of pure tones along with the music at increasingly louder levels. We also specified the exact frequencies and the order of the noise signals, i.e., 20 Hz, 40 Hz, and so on. Amazingly, the results were the same whether the listeners were told the test procedure in advance or not. Tests were then repeated beginning at 10 kHz, moving down in frequency to a lower limit of 20 Hz. Again, the results were the same whether we started at the top and moved down, or began at 20 Hz and moved up to 10 kHz.
The tests were repeated with music at average listening levels of 86 dB SPL, 89 and 92 dB, measured at the listening seat with a professionally calibrated Sound Pressure Meter. Subjectively, these levels ranged from normal to quite loud. All the listeners were comfortable with these playback levels. In the graph in Figure 1, the horizontal line at 0 dB represents the average level of the music. Typical dynamic range between +5 and -5 dB could be measured during playback of the musical selections. The small squares on each curve represent the specific frequency points of the sine-wave “distortion” fed into the listening room along with the music. As can be seen in Figure 2, the results with the different musical selections track each other very closely, indicating that the nature of the particular music did not significantly change the results.
Figure 2, Results with Different Source Material (click image to enlarge)
Figure 1 shows the combined average of all results together with a sloping trend line representing the test subjects' average ability to detect the distortion at lower levels as the frequency increases. The graph in Figure 3 documents the individual detection curves for each of the eight listeners at the 92-dB average listening level. The congruity is remarkable. Only one obvious deviation at 10 kHz, for the oldest listener in the tests, shows any significant departure away from the other listeners' curves. Even at much higher frequencies, 5 kHz for example, the distortion tone had to be raised to an average of just 30 dB below the music level (about 3% distortion) before listeners could hear it along with the music.
The Results
While it is has been recognized for years that human hearing is not very sensitive to low bass frequencies, which must be reproduced with much more power and intensity in order to be heard, what these results show is that our detection threshold for “noise” (made up of harmonically related and non-harmonically related test tones) is practically non-existent at low frequencies. (The “noise” test tones are noise in the sense that they are not musically related to tones commonly found in musical instruments.) In fact, the “noise” tones at 20 Hz and 40 Hz had to be increased to levels louder than the music itself before we even noticed them. Put another way, our ability to hear the test frequency “noise” tones at frequencies of 40 Hz and below is extremely crude. Indeed, the results show we are virtually deaf to these distortions at those frequencies. Even in the mid-bass at 280 Hz and lower, the “noise” can be around -14 dB (20% distortion), about half as loud as the music itself, before we hear it.
Conclusion
Axiom's tests of a wide range of male and female listeners of various ages with normal hearing showed that low-frequency distortion from a subwoofer or wide-range speaker with music signals is undetectable until it reaches gross levels approaching or exceeding the music playback levels. Only in the midrange does our hearing threshold for distortion detection become more acute. For detecting distortion at levels of less than 10%, the test frequencies had to be greater than 500 Hz. At 40 Hz, listeners accepted 100% distortion before they complained. The noise test tones had to reach 8,000 Hz and above before 1% distortion became audible, such is the masking effect of music. Anecdotal reports of listeners' ability to hear low frequency distortion with music programming are unsupported by the Axiom tests, at least until the distortion meets or exceeds the actual music playback level. These results indicate that the “where” of distortion—at what frequency it occurs—is at least as important as the “how much” or overall level of distortion. For the designer, this presents an interesting paradox to beware of: Audible distortion may increase if distortion is lowered at the price of raising its occurrence frequency.
Next episode: The effects of harmonic distortion
The tests done in this experiment are essentially noise tests; things such as mechanical resonances and port noises that are not harmonically related to a specific fundamental contained in the music would be examples of noise distortion. Other types of distortion such as Harmonic Distortion and Intermodulation Distortion have a direct relationship to a frequency being reproduced as part of the music. These types of distortion may be harder to detect than straight noise distortion; a subject for a future round of experiments.
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