M. Greenspan and C. E. Tschiegg 99 
Piano Company, filed for a similar US patent; this was granted in 1946 [4]. 
Huntgren and Hallman [12] discussed possible applications to radar (they used 
the term "ring-around") in 1947; in the same year, M. J. Larsen [5] filed for a 
US patent, granted in 1949, covering a sing-around echo-location system for the 
blind. In 1948, R.D. Holbrook [13], working at Brown University, made what so 
far as we know was the first serious laboratory application of the principle; this 
was for measurement of small changes in the speed of sound in solids. It was in 
1948, also, that R. L. Hanson [6] readhis paper, based on Kock's patent, in which 
the term "sing-around" was coined; and at the same meeting, W.E. Kock and 
F.K. Harvey demonstrated a system using loud speakers in air. This demon- 
stration and Hanson's paper introduced the subject to us. Barrett and Suomi [7] 
in 1949 experimented with a balloon-borne sing-around device for the measure- 
ment of air temperature. They used a thyratron pulser and a 16-in. path. The 
electrical time delay was 34 y»sec. Holbrook's work at Brown was continued by 
Cedrone and Curran [14], who by 1954 had produced an instrument employing a 
pulse-modulated 10-Mc carrier with an accuracy in liquids of about 0.1%. A 
much simpler instrument, utilizing video pulses and good to about 1%, was de- 
scribed in 1956 by Ficken and Hiedemann [8]. 
Our own work began in 1952 and by the end of the year a prototype model of 
high stability, described in NBS Report 2702, Jan. 2, 1953, was in operation. The 
first operational model had a straight path and quartz transducers, and was 
field tested in June, 1953. This instrument was used by the Chesapeake Bay 
Institute for several years. All succeeding vacuum-tube velocimeters, of which 
more than a dozen were built, had ceramic transducers and a singly bent path 
with a reflector of hard rubber or perforated metal. These were described in 
1957 [9] but were first announced in 1955 [15]. The transistorized version was 
developed in 1957 to meet the need for a deep-sea instrument. At the present 
time (August, 1961), 65 of these instruments have been manufactured and we 
know of current invitations to bid on 61 more. Of the 65, three were made by 
NBS, two by US Coast and Geodetic Survey, two by the Woods Hole Oceanographic 
Institute, and the remainder by three different commercial manufacturers. Two 
instruments are in England, two in Norway, one is at the Saclant ASW Research 
Center in Italy; most of the rest are owned by various naval or oceanographic 
installations in the United States. 
In addition, about 25 instruments are being made with the same sound head 
but a different timing mechanism. There are also three instruments with a 10- 
cm path for 1'4-v operation. 
In 1958, A. Lutsch of the NPL ofthe Union of South Africa reported on an in- 
strument [16] similar to that of Cedrone and Curran [14], but of much higher 
accuracy. 
5.6. ACKNOWLEDGMENTS 
We are indebted to our section's shop support group headed by Henry A. 
Schmidt, Jr., and particularly to Marshall A. Pickett who performed many of the 
necessary pressure tests. 
The initial phases of the work were supported by the Office of Basic Instru- 
mentation of NBS. After 1952, most ofour support came from the ONR. Numerous 
