Page 20 - Winter 2011

DOWNLOAD THE AUDIO CLIP EXAMPLES
Shortly after the print copy of this issue is mailed, it will also be published in the Acoustical Society of America's Digital Library. The Table of Contents may be reached directly by going to your internet browser and typing the following Uniform Resource Locator (URL) in the address block: http://scitation.aip.org/dbt/dbt.jsp? KEY=ATCODK&Volume=7&Issue=1. At some point in the download process you will probably be asked for your ID and Password if you are using a computer that is not networked to an institution with a subscription to the ASA Digital Library. These are the same username and password that you use as an ASA member to access the Journal of the Acoustical Society of America. Open the abstract for this article by clicking on Abstract. At the bottom of the abstract will be a link. Click on the link and a zipped folder (DavidGriesinger.zip) will be downloaded to your computer (wherever you usually receive down- links). Unzip the entire folder (as a whole), save the unzipped folder to your desktop. There are 2 examples. Do not remove any file—simply open the file called 00_OpenDavidGriesinger. It is an interactive file. By clicking on any of the hyperlink clips in this file, you should hear the audio clip associated with that link. The best way to enjoy article is to open the magazine. Open the file on your computer. With mouse in hand, read the article from the magazine and when a example is men- tioned, click on the link associated with the clip. Good PC speakers or earphones will enhance your perception of the subtleties present in the examples. The audio clips were recorded in .mp3 format. If you have difficulty in playing them, you might download the PC or MAC ver- sion of VLC Media Player from www.videolan.org. This is a non-profit organization that has created a very power- ful, cross-platform, free and open source player that works with almost all video and audio formats. Questions? Email the Scitation Help Desk at help@scitation.org or call 1-800-874-6383.
loudness we seek, along with the envelopment we find so desirable.
Reverberation can be beautiful, and at least in a large hall. “louder is better.” But reverberation is by nature chaot- ic and blending. Information about which instrument played what, and from where, is lost. For some people this is OK. They have no desire to hear every instrument with the clari- ty of the score, and a rich blend of all the instruments is just fine. Other people want the kind of clarity that enables you to distinctly hear the way the composer has written each line, how it is played, and from where. Even without consciously listening for these details the brain reacts more strongly to the music when this clarity is present. Clear sound, sound perceived as close to the listener, demands attention. Attention creates drama—and drama in music is addictive. Surprisingly, this kind of clarity can co-exist with reverbera-
tion and envelopment, even when the reverberation is much stronger than the direct sound.
We need to appreciate that the direct sound—the brief segment of sound that arrives at a listener before being aug- mented by reflections—conveys most of the information about localization and timbre. We also need to understand that in most seats in most halls the direct sound is weaker than the sum of the early reflections and reverberation that quickly overtake it. But we propose that if the auditory nerve firings from the direct sound are more numerous than the nerve firings from the reflections in the first hundred mil- liseconds after the beginning of a note the brain stem can cre- ate separate neural streams for each musical line, and identi- fy which instrument played them. How can we test this pro- posal? How can we find the distance in a hall where the abil- ity to separate individual voices vanishes?
Binaural recordings of the eardrum pressure
Studying sound perception in halls is difficult because the brain suppresses the conscious perception of noise, reflections, and reverberation. Thus the sound quality in a hall is difficult to judge, and almost impossible to remember. The dominance of our vision further complicates the situation. If we see musi- cians playing we will perceive their sounds coming from the direction we see them—regardless of whether sonic localiza- tion is possible or not. For many people it takes practice to per- ceive a scene from sonic information alone. But differences can be startling when the sonic images from two different halls or two different seats in the same hall are rapidly compared in the absence of a visual image.
It is possible to make binaural recordings of the sound at a listener’s eardrums, and to reproduce it through head- phones also equalized at the eardrums. The result is nearly perfect reproduction of an auditory scene. Surprisingly recordings made at my own eardrums are convincingly real- istic for at least 50% of listeners, even without individual headphone equalization. They are particularly successful for people such as recording engineers who are accustomed to work without a visual image.
With the help of these recordings, we find that in all halls the location and timbre of individual instruments can be clearly identified when the listener (or the binaural recording position) is close to the musicians. As the listener moves back into the hall the location and timbre of individual instru- ments, and the ability to clearly hear their musical lines in the presence of all the other instruments, continues to be good up to a certain point. At this critical point the sound changes. Instead of perceiving a coherent image—where each instru- ment can be localized and identified—all the instruments blend together into a fuzzy ball of sound. Occasionally a solo instrument will be localizable, but when instruments play together they all fall into the same sonic blob. Timbre also changes dramatically. When instruments are localizable each timbre is distinctly perceived. When they are not localizable the whole ensemble takes on a darker color—one that sound engineers call “muddy”. This change in timbre is distinctly
perceived even when these binaural recordings are played
through loudspeakers.
16 Acoustics Today, January 2011
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