Technical Reviews - 1954-1950

Tube Measurements and Sound Insulation

Mobile Laboratories

First, we discuss the kind of information which must be obtained so as to judge fully a sound reproducer's tecnical characteristics. Secondly, a review is made of the pros and cons of the two analyzer principles which can be applied to the measument of distortion in sound reproducing equipment where frequency modulation arises. Next, we describe a new semiautomatic measuring method where both the frequency characteristics and the different harmonics in relation to the fundamental can be recorded on one and the same piece of recording paper. Finally, we show how the same apparatus is used for estimating »wow« in sound reproducers.

In this issue we continue discussing the application possibilities of the Audio Frequency Spectrometer type 2109 and the Automatic Spectrum Recorder type 2311.

This number of our Technical Review is completely devoted to a new instrument, extending our series of apparatus for acoustic measurements, the Audio Frequency Spectrometer type 2109, This instrument, together with our High Speed Level Recorder type 2334, also results in a new type Recorder, the Automatic Spectrum Recorder type 2311, which makes possible the automatic recording of all kinds of noise spectrograms, and analyses of vibrations. A description of both instruments will be given, together with a number of typical applications.

To save the tedious job to calculate the F.L (expressed in db\c\s) from a room response curve, two different methods for obtaining direct recording of the F.L as a Junction of frequency are developed. The room response is recorded via a microphone, amplifier and level recorder. On the recorder is fixed a contact strip with 100 silver lamellae which via an electronic sealer registers the total "length" of the room response curve in db. . The other method uses the output voltage from the moving coil in the level recorder. This voltage is proportional to the F.L in dbjcjs and recorded on a second level recorder with extremely low recording speed. The two sets of apparatus have been used for measuring the diffusion effect of three different types of diffusers in a small model room.

Some Experiences with The Deviation Test Bridge Types 1502 and 1507.

Suitable measuring equipment for acoustic tests on auto-horns is briefly described. The conditions of quality for auto-horns are laid down, and in the case of three different types of cars measurements have been carried out on the car's sound insulation as a function of frequency, on the horn's sound intensity and frequency composition and the sound's distribution characteristic in both horizontal and vertical direction, as well as the difference in sound intensity between the outside and the inside of the car.

A New Megohmmeter For 100 Million MΩ.

B.B.C. Impulse Glide Method.

A special selector switch is described which in conjunction with the high speed level recorder makes it possible to record a complete sequence of reverberation curves on the one diagram. It is shown how these can be interpreted to judge the quality of at room's acoustics, the presence of flutter echo, and other phenomena. It is further shown how the universal selector can be used for investigation of room acoustics in accordance with the "Impulse Glide Method" developed by the British Broadcasting Corporation. Further, it is shown how the universal selector can be used for recording articulation diagrams in large lecture halls, for the demonstration of artificial reverberation produced by resonators, and for the automatic recording of the sound insulating properties of walls. Finally, a completely new method for the evaluation of a loudspeaker's quality is shown, the frequency characterictics and dynamic qualities being demonstrated on the same record with the help of the universal selector.

The Hetrodyne Voltmeter Type 2002.

Measurements on Fluorescent Tubes.