Page 453 echo sounding 5163 



Some echo receivers are constructed with frequency discrimination so they will 

 respond only to the frequencies of the echo and be insensitive to noises of other 

 frequencies. 



The transmitting and echo-receiving units, used in conjunction with the various 

 types of echo-sounding instruments of the Coast and Geodetic Survey, are listed in 

 table 12. 



5163. Echo Amplifiers 



When the echo receiver converts the acoustic energy to electric energy, the resulting 

 amount is so small it must be increased to a usable value. This is one of the principal 

 purposes of the echo amplifier. The desired amplification is obtained by the use of 

 vacuum tubes similar to those used in radio receivers, the number and kinds of tubes 

 used depending on the amplification required. In addition, this amplifier must possess 

 other important characteristics, which may be listed as follows: (1) frequency discrimi- 

 nation; (2) stability; and (3) a small and constant time lag, or the property of passing 

 the echo signal tlu"ough the amplifier in a short and regular time interval. 



Frequency discrimination is usually obtained by the use of highly resonant electric 

 circuits which, by their filtering action, pass frequencies corresponding to the echo 

 signal and attenuate spurious noises of other frequencies. 



Apparent time lag through the echo amplifier is of minor consequence so long as it 

 remains constant. Variations in the time lag may result in errors of serious magnitude, 

 especially in shoal-water soundings. Instability in the amplifier is usually responsible 

 for unpredictable changes in this lag. Gain-control variations also result in changes 

 in the time lag, so it is important always to use high values of gain, since the varia- 

 tion in time lag is smaller for higher values of amplifier gain. 



Most echo amplifiers are composed of a series of thermionic vacuum tubes with 

 filters, tuned to the echo frequency, between the tube stages. Where high gain is 

 needed a superheterodyne circuit is sometimes used. This type of circuit combines 

 high gain with stability of an order which is difficult to obtain with single-frequency 

 amplification. 



5164. Registering Devices 



The purpose of the depth-registering device is to indicate visually, or record graphi- 

 cally, the depth of the water. ' All echo-sounding instruments operate on the same 

 principle — by measuring the elapsed time between the emission of an acoustic signal and 

 its return as an echo. The primary purpose of the registering device is to measure this 

 time interval and convert it into some unit of linear distance (feet, fathoms, or meters). 



Certain other parts are essential to the operation of the registering part of the 

 equipment and will be considered as integral parts of the registering device. These 

 essential parts are; the speed control of the driving motor, indication of motor speed, 

 and the keying circuit that brings about the transmission of the acoustic signal. 



Depth-registering devices are of two general types, the visual indicator and the 

 graphic recorder. 



A. VISUAL INDICATOR 



A visual indicator usually consists of a scale graduated to read in units of depth and an index to 

 indicate the measured depth on the scale. The type of visual indicator now most frequently used 

 is a circular transparent depth scale, behind which the index revolves. The index is a slot cut in a 

 disk, or radial arm, behind which a neon tube is placed so that its flash is visible adjacent to the 

 depth scale. The neon tube is made to flash by the amplified energy resulting from the echo. Since 

 the index travels around the scale at a known uniform rate of speed, the depth of water corresponding 



