In previous portions of this five-part series, we discussed the acoustics of musical instruments and in most cases we saw these same general acoustic principles applied to the realm of hearing aids and speech acoustics. This week, I would like to turn to the performance venue and how the lowly pinna effect can be of assistance.
Close your eyes and imagine that you are sitting on a beach with your back to a large rock with the water to your rear… actually don’t close your eyes because you won’t be able to read this. But imagine that you are sitting on this beach leaning up against a large boulder, with the water to the rear. It’s getting a bit windy and the water is getting choppy. Some waves crash up against the shore and hit the water-side of the large boulder. Not to worry, the waves hit the boulder and bounce off back toward the ocean. You are still dry and still enjoying an exciting book about vestibular rehabilitation,while sipping a rather large Margarita.
Suddenly, a long rolling wave is blown up on shore, and, unlike the previous choppy waves, this one soaks you completely. The boulder was no help at all!
After chugging back the last Margarita in the pitcher, it all becomes clear. Choppy, short wavelengths are obstructed more easily than long wavelengths and with this revelation comes another pitcher of Margaritas! Once you sober up, you realize that this is just another manifestation of the pinna effect. Short wavelength, higher frequency (sound) waves see obstructions; longer wavelength, lower frequency (sound) waves do not see the obstruction.
When you cup your hand behind your ear, or just happen to have a pinna on the side of your head, the lower frequency, long wavelength sounds go right through the pinna and hand as if they were not even there – for lower frequency sounds, the pinna effect is zero. The pinna and hand are acoustically transparent.
However, for higher frequency sounds that have shorter wavelengths, not only is there an incident sound coming to your ear, but the reflection of this sound off the pinna or hand constructively combines with the incident sound to provide a net high-frequency boost. We know this from the study of hearing aids; placing the hearing aid microphone in a location that will benefit from the pinna effect means that the hearing aid does not have to generate as much high-frequency sound since the sound has been pre-emphasized by the pinna (and hand).
As a general acoustical rule of thumb, the way to best remember this is if an obstruction is on the order of half the wavelength or greater, then the sound will be obstructed (and in the case of the pinna effect, reflected). Low-frequency sounds have wavelengths that are just too long to have any significant attenuation or reflection due to an obstruction. The diameter of such an obstruction would have to be gigantic.
This is why sound-treated enclosures attenuate the higher frequency sounds more than the lower frequency sounds. An excellent sound booth may have 40 dB of attenuation at 125 Hz, but maybe 70 dB at 4000 Hz. Hearing protection is another example. Even the best hearing protection will provide attenuations of 25 dB in the lower frequency region, but can provide up to 40 dB in the mid- and higher frequency region. It is almost impossible to isolate oneself from long wavelength, low-frequency sounds.
Acoustical engineers understand this as well. A frequent complaint from clients may be that they are bothered by their next-door neighbors talking at night. It is an easy task to get rid of the higher frequency vocal energy, but quite a different and much more difficult task to rid their client of the lower frequency sounds from their neighbors.
A strategy that is frequently used in musical venues is to create a pinna effect on stage. This is something that can easily be implemented even in a high school music class. If the orchestra or band is moved back 2 meters (about 6-7 feet) from the lip of the stage, not only is much of its energy directed to the audience, but the sound is also reflected off the unoccupied section of the stage. This adds up constructively to yield a greater sound level (but still lets the band get away with playing at a softer, arguably safer level). That is, the stage functions as an acoustic mirror (or pinna reflector). The sound at the back of the audience is sufficiently loud despite the musicians playing more softly.