Eleven Audiophiles from the Bay Area Audio Video Society participated in a double blind low frequency hearing test conducted using two TRW -17's in a home theater setting. Here is the low down on the test.
The test used four wave files. The wave files are downloadable here:
This is the first file BAAVS hearing experiment 1a.wav: file one
This is the second file BAAVS hearing experiment 2a.wav: file two
This is file three BAAVS hearing experiment 3a.wav: file three
This is the fourth file BAAVS hearing experiment 4a.wav: file Four
Each file consisted of 7 four second tone series. There was .2 second silent pause between each tone series. All tones were sine waves.
The first tone was always a 256Hz sine wave which had a duration of four seconds. To create the descending tone series the second four second tone was a 256Hz sine wave added to a 128hz sine wave. The third four second tone was 256Hz, 128Hz, & 64Hz. The fourth four second tone group was 256Hz,128Hz,64Hz, &32Hz. The fifth tone group was the same as the fourth adding 16Hz to the mix. The sixth added 8Hz and the seventh group added 4hz. When played the total length of the wave file was about 29 seconds.
The second wave file was identical to the first except the last tone group was omitted and the sixth tone group which included 8Hz was repeated twice. The duration of this wave file was also about 29 seconds
The third wave file was identical to the first except the last three files were the same and only included 256Hz,128Hz,64Hz, 32Hz and 16Hz which was repeated three times.
The fourth wave file was identical to the first except the last four files were the same and included 256Hz,128hz,64Hz, and 32Hz repeated four times.
To account for hearing sensitivity changes the playback sound levels for the subjects (encoded in the sound track for each series) were: 256Hz @ 68dB, 128Hz @ 73dB, 64Hz @ 78dB, 32Hz @ 83dB, 16Hz @ 88dB, 8Hz at 96dB and 4Hz @ 104dB.
When you listen to the series you first hear a 256Hz tone at low levels, four second duration, then you hear 256Hz plus 128Hz four second duration and the tones keep going down in frequency until you no longer hear any change.
This test is not a sine wave audibility test which would require higher sound levels. This is a test of our perception of the FM effects of very low frequency sounds on higher ones and if present, do they change the tonal balance or character of the sound.
The four wave files were played to ten participants in two different sessions. We created a score sheet with a series of vertical check boxes to check off the frequencies which were thought to be percieved.
Here is an example of a scoresheet:
As the frequencies stepped down, if you thought you heard an additional tone you checked the box, if you did not hear anything you stopped checking boxes. The participants were not asked to identify a specific frequency, only check a box if a lower frequency was percieved.
We called on a random letter generator to name the files so no one knew which file was playing. We used I-tunes in its "shuffle" mode to randomize the playlist. We did not know which file contained which tone series or the order that they would be presented to the subjects. If fact the same file was accidently renamed twice.
Participants were given a practice run by playing two wave files with known content and explaning what they should hear.
There were problems. We could not remove room cues, noises, sqeeks, and wall motion. We ended up playing the test at near threshold levels and there were still room cues possibly at 8Hz and definitely at 4Hz. To a degree the results depended on where you sat in the room, one participant said they could not hear 64Hz and another 32Hz.
The files were named: LWC included 4Hz,CMM included 8Hz,ACN included 16hz and BZL which included 32Hz. The results are excellent considering most people believe we cannot perceive anything below 20Hz and further, if we do, we are really hearing harmonics.
LWC - 10 of 11 recorded hearing of 4Hz, one participant only identified frequencies down to 8hz.
ACN - 10 of 11 recorded hearing of 8Hz, none recorded anything lower. One did not hear or percieve 8Hz but did hear 16Hz in this file. Another did not hear 16Hz, but did hear 8Hz.
CMM - 11 of 11 recorded hearing 16Hz.
BZL - 11 of 11 recorded hearing 32Hz, however, 9 of 11 also recorded hearing 16Hz in this file.
The Bottom Line:
There was no difficulty in distinguishing between the tracks that included 4, 8 or 16Hz. The order of file playback did not appear to have any affect on the outcome.
Because of room wall/door vibration this test might have validity to 8Hz, certainly to the frequencies above that. The room could not take a clean 4Hz sine wave at 104dB so the perception at this frequency was likely the result of room motion.
We cannot claim that we can hear 4Hz but we can conclude that there is a perceivable difference for whatever reason with mixed sine waves between 4Hz, 8Hz and 16Hz in this home theater.