From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Noise Meter (type TF500), its design, features, and various accessories for measuring vibration and analyzing waves.
| Identifier | ExFiles\Box 138\2\ scan0190 | |
| Date | 1st December 1938 guessed | |
| Noise Meter, type TF{T. Fisher}500.—Description and Design. The instrument measures noise levels in phons over practically the whole range encountered in industry and technology. The measurement is objective, so that there is no possibility of error due to the personal idiosyncrasies of the operator, but phones may be plugged in to listen to the noise being measured. The Meter covers a very wide range of applications, being suitable for the measurement both of steady and impulsive noises. Particular care has been taken to ensure an accurate evaluation of intermittent sounds. The Noise Meter, Type TF{T. Fisher}500, comprises a microphone, amplifier and calibrated indicator meter. The microphone is of the robust piezo-electric type, having a high sensitivity and flat frequency characteristic. It is very nearly non-directional. Following the microphone is a valve amplifier, the coupling circuits of which include weighting networks giving the instrument three alternative frequency characteristics. An external weighting network can also be used. A crest valve voltmeter is used to operate the indicator meter, the circuit arrangement being chosen to ensure an accurate assessment of intermittent noises. Two attenuators are incorporated in the amplifier circuit ; one has six 10 dB{Donald Bastow - Suspensions} steps and the other ten 1 dB{Donald Bastow - Suspensions} steps. These are calibrated to read directly in phons when the meter reading is set to a standard deflection. The power supply is from batteries which are housed inside the instrument in a separate compartment. FEATURES. The mechanical arrangement of the instrument is designed for maximum convenience in operation. The microphone is mounted in the detachable lid{A. J. Lidsey} for transport and is provided with 10 feet of flexible screened cable for connection to the unit. For measurements the microphone may be mounted either on the unit itself or on a telescopic tripod which when not in use is clipped to the panel of the unit. Weighting networks incorporated in the amplifier provide frequency characteristics which correspond with the 40 and 70 phon contours after Fletcher and Munson. A flat characteristic can also be selected. The Meter gives a reading which is very close to the level of an individual impulse, and which is not seriously affected by the frequency at which the impulses occur; it is, however, equally suitable for measuring steady sounds. Spurious mechanical and electrical pick-up is negligible in the working range. The instrument is housed in a light but rigid metal case and provided with a detachable cover. MEASURING VIBRATION. ACCESSORIES. Vibration Pick-Up Unit, TF{T. Fisher}481. This unit replaces the microphone of the Noise Meter for the investigation of mechanical vibration in solid materials. Apart from the simple measurement of vibration, the use of the Vibration Pick-Up makes it possible to locate the source of noises transmitted through the framework of buildings, or to separate a particular mechanically-produced noise from the general background. The output of the Vibration Pick-Up is large ; a small attenuator unit is available, reducing the output to a level suitable for application to the Noise Meter. Adjustable Tuning Unit, TF{T. Fisher}456. This unit can be connected by a plug link to the Noise Meter and makes the indication of the Meter selective, so that a noise of a particular frequency can be suppressed or emphasised. An example of the application of this unit is the investigation of gearbox noise in motor vehicles, this noise being emphasised by tuning the Unit to its frequency ; the exhaust and other noises are largely suppressed. This Unit is completely described in a separate leaflet, Com. D-7. Wave Analyser, TF{T. Fisher}455. This instrument is for the direct measurement of the components of complex voltage waves having an amplitude of 0.2 mV. to 200 volts in the frequency range 20 to 15,000 c.p.s. The Wave Analyser is a superheterodyne valve voltmeter with a very sharply tuned intermediate frequency amplifier. The test signal is mixed in a balanced rectifier bridge with a carrier signal adjustable in frequency from 35 kc. to 50 kc. This frequency is adjusted so that the sum of its frequency and that of one of the components of the test signal equals 50 kc/s. This signal is passed through a highly selective two section quartz crystal filter and calibrated attenuator to a direct reading valve voltmeter. This unit is completely described in a separate leaflet, Com. D-6. NOISE METER, TF{T. Fisher}397. A special high sensitivity Noise Meter, Type TF.{T. Fisher} 397, is available without the impulse feature. The range is 24 to 130 phons and an input signal of 3 µV applied to the microphone socket produces half scale deflection on the output meter at the 30 phon setting. This sensitivity can be increased still further by means of a directional microphone—the range then being 18-124 phons. A mains operated Noise Meter is available, also Tuning Units with special frequency ranges and detachable cover. | ||