Page 240 - The Master Handbook Of Acoustics
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                                                                                ABSORPTION OF SOUND


                      expanded metal, metal lath, hardware cloth, or even perforated vinyl
                      wall covering can be used as a cover. Do not be surprised by absorption
                      coefficients greater than 1.0. This manufacturer elected to publish the
                      coefficients actually obtained from laboratory measurements rather
                      than arbitrarily reducing those over 1.0 to 0.99 or 1.0. The greater
                      absorption of the standard 8 	 9 ft sample results from edge diffraction
                      and other effects that make the sample appear larger acoustically than
                      it really is.


                      Helmholtz Resonators

                      The Helmholtz type of resonator is widely used to achieve adequate
                      absorption at lower audio frequencies. There is nothing particularly
                      mysterious about such resonators; in fact they pop up in various forms
                      in everyday life. Blowing across the mouth of any bottle or jug pro-
                      duces a tone at its natural frequency of resonance. The air in the cav-
                      ity is springy, and the mass of the air in the neck of the jug reacts with
                      this springiness to form a resonating system, much as a weight on a
                      spring vibrating at its natural period. Change the volume of the air cav-
                      ity, or the length or diameter of the neck, and you change the fre-
                      quency of resonance. Such a Helmholtz resonator has some very
                      interesting characteristics. For instance, sound is absorbed at the fre-
                      quency of resonance and at nearby frequencies. The width of this
                      absorption band depends on the friction of the system. A glass jug
                      offers little friction to the vibrating air and would have a very narrow
                      absorption band. Adding a bit of gauze across the mouth of the jug or
                      stuffing a wisp of cotton into the neck, the amplitude of vibration is
                      reduced and the width of the absorption band is increased.
                         The sound impinging on a Helmholtz resonator that is not absorbed
                      is reradiated. As the sound is reradiated from the resonator opening, it
                      tends to be radiated in a hemisphere. This means that unabsorbed
                      energy is diffused, and diffusion of sound is a very desirable thing in a
                      studio or listening room.
                         Bottles and jugs are not appropriate forms of a Helmholtz resonator
                      with which to apply the resonance principle in studios. An interesting
                      experiment conducted many years ago at Riverbank Acoustical Labo-
                                           10
                      ratories bears this out. To demonstrate the effectiveness of a continu-
                      ously swept narrow-band technique of measuring sound absorption
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