Sound Demo Files, HRTFsAll of these files were lost at one point, and I am finally (July 2006) in process of re-creating them. Each file is about 2Mb in size, and to avoid a surcharge by my ISP, I am not directly posting them on this site. Unfortunately COX, the web host for other site where the files were posted, has stopped hosting and erased all of the files. I don't know if or when I will try to re-creat them again.
I recommend good quality headphones for listening to these files (I use Sennheiser HD-545's). Otherwise effects will be washed out by the quality of computer speakers or the room you are sitting in. Standard computer sound cards are not the ultimate in fidelity, but they are adequate.
Arny Kruger has posted free downloadable software for performing ABX tests to compare two .wav files. His site also includes a selection of sample files to illustrate the effect of filtering out high frequencies, truncating bits, etc. One way to make A-B comparisons of multiple sound files is to open multiple Media Player windows, one for each file, and then play the files quickly in sequence. But Arny's ABX tester is much better.
All demos are based on a comparison of a reference source vs. the same music perturbed in some way. The two reference sources are: (1) a tidbit from Vivaldi's Four Seasons, [1.9Mb] Archiv Produktion CD 400 045-2; and (2) a jazz selection from Joe Sample, [1.7Mb] Warner Bros. CD 946572-2. Both files are 16-bit 44,100 sample rate .wav files, each about 10 seconds long.
Any decent tweeter will produce frequencies up to 20kHz. A lot of music doesn't have much spectral content above 10kHz, but the two reference files used here do have a lot of high frequency content, right up to the 22kHz limit. A good way to test the high-frequency response of your ears is to compare two files containing tones at different frequencies. Obviously if you can distinguish between the two, you can hear at least one of them. I recently did this (July 2006) with tones at 13kHz and 14kHz. I am happy to say that my 67-year old male ears still function up to 13kHz, but that's it.
I applied a brick-wall filter to the two reference files that totally eliminates frequencies above 6, 7, 8, or 10 kHz. I found the 6kHz cutoff very easy to detect, and I can reliably detect the 7kHz cutoff as well. For the 8kHz cutoff my success rate is not statistically significant.
This is a test of an ideal analog crossover, meaning that: (1) each filter sees a pure resistive load, and (2) the filter outputs sum perfectly and coherently. The first assumption is fairly realistic for an active crossover, but not for a passive crossover. The second assumption requires perfect time-alignment, and then is still only true for a small "sweet spot." Graphs of the crossover responses and more design details can be found here. An ideal Linkwitz-Riley crossover only effects the phase of the summed signals. When I apply the phase change to the reference files I can't hear any difference. Arny also has crossover test files on his site.
General Stuff Regarding Sound Processing
The computer age has provided us with incredible capabilities for manipulating sound files. This is my procedure: (1) sound from a music CD is recorded into my computer; (2) the sound .wav file is then read into Matlab, and manipulated to simulate one or more effects on the sound; and (3) a new .wav file is created so it can be listened to in comparison with the original file.
Once in the computer, the sound can also be analyzed up the kazoo (see, for example, peak vs. RMS power in the section on music and ears). All kinds of sound system designs can be mathematically simulated, and a new sound file written out which can be listened to, to actually hear the influence of the design on a particular piece of music. Crossover networks can be designed and heard without physically constructing them. The acoustic effects of various room shapes can be heard. Different levels of distortion produced by tube and transistor amps can be compared. Wow!! I feel like I'm in engineers heaven!
One of the most sophisticated and exciting aspects of current research involves modeling human hearing. The relationship between a sound wave traveling in open air and the sound at the eardrum is represented mathematically by a head-related transfer function (HRTF). There are research efforts dedicated to both numerically computing, and measuring, HRTFs. Bill Gardner and Keith Martin of the MIT Media Lab have generously made their measurements of head-related transfer functions available to the world by posting them on the web (link sometimes disappears). All of my results involving HRTFs are derived from their diffuse-field equalized data. The HTRF is a function of frequency and the angle at which the sound originates, with respect to the head. The angle dependence for the right-ear HRTF is shown here [35.7 kb]. Zero degrees elevation means the source is at ear level; 90 degrees means the source is directly overhead. Zero degrees azimuth means the source is in front of your nose; 180 degrees behind you. For most elevation angles there is nearly 20 dB isolation between the left ear at -90 degrees and the right ear at +90 degrees azimuth - which is a lot. In principle these data can be used to synthesize a virtual listening space. In other words, the sound at the eardrum can be made to mimic what you would hear if you were inside an Egyptian pyramid, or wherever. In fact there are several commercially available software suites for doing exactly this. Three that I am aware of are EASE/EARS, Ramsete/Aurora, and Catt. The Odeon site contains sound demos and other material. The University of Southampton has an interesting sound demo file which is designed for two closely spaced stereo speakers, and creates a sound stage much wider than the speaker spacing. Many other interesting links are also available regarding virtual 3D sound. This seems to be a very active research field, and some fascinating stuff is going on.
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