88 Lecture 5 
This choice has several important consequences. To begin with, a pulse-modu- 
lated carrier has now no advantage over the much simpler video pulse even though 
the distortion of the former would be much less* Further, it becomes essential 
that the pulse rise rapidly. This is no problem so far as the input pulse is 
concerned, but the output rises, relatively, very slowly. The fast rise is restored 
by amplification; nevertheless there is introduced an unknown delay equal to the 
time which the output pulse spends below the noise. This delay depends on the 
attenuation characteristics of the liquid. It is primarily for this reason that the 
velocimeter must be calibrated and used ona class of liquids within which the 
attenuation characteristics are not too variable. 
The timing is automatic. The received pulse after suitable amplification and 
reshaping is again applied to the sender; thus the device regenerates and the 
pulse repetition frequency (prf) depends upon the speedof sound in the liquid and 
to some extent upon electrical and other delays. The principle is not new. The 
earliest description we have found occurs ina patent [3] filed in 1937 by Shepard. 
Similar systems are described in later patents by Kock [4] and by Larsen [5]. 
The designation "sing-around" appears to have been coined by Hanson [6]. Hanson, 
Barrett and Suomi [7], and several others (for references see a recent paper by 
Ficken and Hiedemann [8]) have constructed apparatus similar to that described 
here. These instruments were not of high precision. 
A major source of difficulty is the existence of multiple echoes between the 
transducers. The various sets of echoes, each set arising from a different pri- 
mary pulse, are not synchronous because of the electrical time delay. Various 
means of eliminating the reflections have been used. In the case of a straight path, 
the transducers are tilted slightly out of parallel so that all received pulses but 
the first are lost in the noise. In the case of a bent path, where the sound is 
reflected back nearly on itself, the transducers and reflector occupy their ge- 
ometrically correct positions, but the reflection coefficient is rather small. The 
first received pulse is attenuated by reflection once and the second three times; 
the result is that all received pulses after the first are negligible. Hard rubber, 
Teflon, and perforated metal are suitable materials for the reflector. 
A bent path minimizes errors which arise from mass motion of the liquid 
and is preferred for field models, and in the laboratory in cases where a large 
volume of sample, which necessitates vigorous stirring for maintenance of ther- 
mal equilibrium, is used. In cases where the liquid is contained in a small tank 
immersed in a temperature-controlled bath the straight path is satisfactory. 
The advantages of a bent path are even greater for a doubly bent path with 
two reflectors. This arrangement, which is used on all recent models, will be 
described later in more detail. 
We recapitulate briefly the principle of operation. A block diagram of the 
sing-around principle is shown in Fig. 5.1. The input transducer is energized 
by a trigger-type, pulse-forming circuit which produces short fast pulses. This 
circuit is adjusted to run free ataprf somewhat less than the expected minimum 
operating prf. The pulses of pressure produced by the input transducer travel 
*However, the choice of a video pulse restricts operation to nondispersive liquids. An instrument based on 
a pulse-modulated carrier could, in principle, be usedon dispersive liquids, but at a single frequency only. 
