Erwin Meyer

Pulsation Oscillations of Cavities in Rubber

The oscillation behavior of a spherical cavity in an infinite elastic medium is calculated for the case of an incident spherical dilatational wave entirely reflected at the cavity walls. It is shown that there exists for every Poissons constant 0⩽σ⩽0.5 a frequency for which the amplitude of the oscillating cavity walls becomes a maximum. It is also shown that the amplitude resonance curves are symmetrical and that, assuming loss‐free material, they have a finite half‐width, which is caused by radiation losses and depends only on the ratio of shear wave and dilatational wave velocity.Pulsation oscillations of spherical, rotational‐elliptical, cylindrical, and cubic cavities in rubber blocks were examined experimentally. The pulsation oscillations were excited by means of a sound source coupled to the surface of the rubber block. The sound pressure inside the cavities was measured with a probe microphone. The experimental results for spherical cavities are compared with the theory. Good agreement is found ...

Experiments on the Influence of Flow on Sound Attenuation in Absorbing Ducts

The influence of a turbulent air stream on the sound attenuation in ducts, lined with sound absorbing materials or structures of different kinds, has been experimentally investigated. For linings consisting either of porous materials or sufficiently damped Helmholtz resonators, decrease of absorption with increasing flow velocity has been observed, together with an increase of the frequency of maximum absorption in the case of the resonators. Somewhat surprising results have been obtained for undamped or weakly damped Helmholtz resonators, where an attenuation minimum is observed above the resonant frequency, the frequency of which increases linearly with increasing flow velocity, and which—for undamped resonators—even reaches negative attenuation values. Aside from the negative attenuation, a self‐excitation of the duct is observed under certain conditions, the frequency of which is not identical with the attenuation minimum.

Some New Measurements on Sonically Induced Cavitation

This paper reports experimental investigations of the behavior of single cavitation bubbles produced in water. In the first set high intensity sound of frequency 14.6 kilocycles is focused on a hydrophone. Oscillograms were made of the natural oscillations of the cavitation bubbles. In the second series of observations motion pictures were made of cavitation bubbles at the rate of 105 000 frames a second. This enables the actual oscillations of the bubbles to be observed visually.

Experiments on cm Waves in Analogy with Acoustic Techniques Made in Göttingen

In the past four years a series of experiments has been carried out which serve to demonstrate the success of analogies between acoustical systems and electromagnetic wave systems in the microwave frequency region. Measuring techniques in the two disciplines are compared, with particular emphasis on the use of reverberation chambers. Analogous directive microphones and electromagnetic devices have been constructed. The design and performance of absorbing structures of both nonresonant and resonant types is discussed at length.

Some Measurements of Cavitation

In previous publications we have discussed a series of experiments on the critical pressure for the onset of cavitation and the spectrum of the cavitation noise. The behavior of an individual cavitation bubble is of even greater interest, and has been studied in two types of experiments. (1) In the study conducted by L. Bohn, the sound of a parabolic array of magnetostrictive transducers at 22 kc/sec is picked up at the focal point by a small probe microphone, a very thin nickel wire with all but the tip shielded against the incident sound by a plastic tube (frequency range up to 1 Mc/sec). The reaction of the oscillating bubble on the tip is recorded and its spectrum is analyzed. (2) In the study conducted by E. Mundry a vibrating nickel rod (2.5 kc/sec) is used, one end of which dips into water. The bubbles originating and oscillating at the lower surface are photographed by a high speed movie camera (100 000 frames per sec) and the sound waves of the collapsing bubbles are recorded by a Schlieren‐metho...

Some Experiments on Sound Propagation in Damped Air Ducts with Flow Velocities up to 80 m/sec

In a rectangular duct with the cross section 35×100 mm2 one larger side is lined with sound absorbing devices. The used absorbers are rock wool (layer‐thickness 6 cm) with and without perforated coverings, and undamped as well as damped resonators of the Helmholtz type. The flow velocities have been varied between 0 and 80 m/sec. The microphone used in the measurements has a probe tube with a front opening in the form of a slot around the tube in a distance of 5 tube diameters from the front end. The opening is covered by copper gauze. Measurements have been made with respect to sound attenuation and sound velocity as functions of frequency for different flow velocities, the signal to noise ratio being about 30 db. In general the sound attenuation in the duct decreases with increasing flow velocity. For absorbers without resonance character this is observed over a comparatively large frequency range. In the case of resonance absorbers the resonant frequency is shifted towards higher frequencies with incre...

Transmission of Sound and Vibrations in Buildings

Three problems are discussed: The insulation from air‐borne sound by multiple walls. Structure‐borne sound transmission. Measurements of vibrations by electrical apparatus.It is known that a single wall vibrates almost as a mass, provided its lowest natural period is much lower than the sound frequencies. Therefore a series of single walls with air‐spaces between them act as a mechanical low pass filter, with a cut‐off frequency and sound dispersion. But these properties are only present, if the component vibrations in the air‐space, parallel to the wall are damped. A multiple wall of this kind with a low cut‐off frequency has a very large transmission loss.Building materials propagate sound very well. Their small transmission losses (hysteresis) are investigated by a resonance method. To obtain a loss of 1 db material lengths from 10–100 m or more are required. Materials such as rubber or cork are used to avoid the sound propagation in buildings. Their dynamical properties (elasticity modulus and loss fa...

Reverberation and Absorption

We distinguish between geometrical and dynamical acoustics of rooms, which both have to be considered to obtain good quality of speech and music. Using a simple automatic method of recording the sound decay on an instrument with a logarithmic scale, concert and auditory rooms with and without audience are acoustically investigated.Generally the sound absorption materials are tested in reverberation chambers. The results of different laboratories are not in agreement, the reasons being probably, that the rooms are small and that geometrical instead of diffuse sound reflection occurs. Comparison tests therefore have been made in the reverberation chamber of the Institute and in the intake chamber (Wasserschloss) of a big hydroelectric power station: this chamber had a size of 13,000 m3 and an average reverberation time (60 db) of 40 sec. The results are discussed. It is interesting to make similar experiments in optics with “Ulbricht” balls. These experiments show that it is only possible to measure absorpt...