ultrasonic ambient noise in tropical jungles

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Page 1: Ultrasonic Ambient Noise in Tropical Jungles

THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA VOLUME 18, NUMBER 2 OCTOBER, 1946

Ultrasonic Ambient Noise in Tropical Jungles •

JOHN S. SAB¾ AND HOWARD A. THORPE Department of Physics, The Pennsylvania State College, State College, Pennsylvania

(Received July 20, 1946)

INTRODUCTION

HE intensity level of ultrasonic ambient noise, and its spectral distribution up to

30 kc, have been measured in the jungles of Panama, during the rainy season, for all times of the day, at many representative locations. It was found that contrary to popular supposition, the jungle ordinarily is not extremely noisy either in the audible or ultrasonic range. The calls of awakening mammals and birds at dawn and of nesting birds at sunset often seem much louder than they really are because they are readily identified by the ear. In reality, most of the jungle background noise is caused by the nearly continuous and often almost unnoticed insect

chorus. The ultrasonic intensity seems to be caused largely by the high frequency com- ponents of the more strident insect noises which usually are still more intense in the audible region. Such insects as the cicada, for example, have a broad noise peak in the audible, but also have considerable output up through 30 kc.

Since jungles were found to be less noisy than anticipated, the question arose as to whether the data obtained at the particular locations we had chosen truly represented "typical" virgin jungle, or whether perhaps the animal and insect life was frightened into partial silence by the presence of the observers, or by natives living nearby, or by other human disturbances such as the distant sounds of motor vehicles and aircraft. Observa-

tions were therefore made for several days at the Biological Preserve, on Barro Colorado Island in Gatun Lake, Canal Zone, where the wild life has been relatively unmolested for many years and where at the time disturbances were at a

minimum.

There, most of the measurements were made from a shelter so constructed that the observer

was almost completely hidden from view, and located sufficiently deep in the jungle to guaran- tee freedom from any disturbances caused by the

x The work reported in this paper was done under con- tract with OSRD, Contract No. OEMsr-1210, SC-105.

small permanent laboratory located at the edge of the jungle.

It was found that even when the observers

came into full view, only the nearest insects stopped their "singing," and that the visible presence of observers never measurably affected the noise level. Furthermore, this undisturbed jungle location yielded data indistinguishable from typical data from the jungles elsewhere on the Isthmus, except that the high frequency noise level was sometimes higher in low, dense, second-growth jungles because of the greater abundance of harsh-voiced insects.

EXPERIMENTAL METHODS

Data were obtained using a Western Electric 640AA microphone calibrated up to 30 kc in con- nection with a portable tuned amplifier and output meter. Numerous observations were made in rapid succession with the amplifier tuned successively to different frequencies. Typical levels corresponding to each frequency setting were thus obtained at various times of the day, sometimes over a continuous period of several days.

The raw instrument data were analyzed into spectral intensity distributions by dividing the acoustic spectrum into arbitrary frequency bands, and considering each instrument reading to be composed of the intensity contribution from the "principal" band to which the instru- ment had been tuned, plus contributions from all the other bands. A weighting factor for each term was determined by the receiver sensitivity, and its discrimination against each of the other bands when tuned to the frequency-of the principal band.

Each output meter reading was thus set equal to a sum of terms involving the set of unknown intensities, and all readings together thus com- prised a set of linear simultaneous equations, solvable for the intensities. The intensities were

converted to acoustic intensity (db above 10 -16 watt/cm •) per c.p.s. (as plotted in Fig. 2).

271

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Page 2: Ultrasonic Ambient Noise in Tropical Jungles

272 J. S. SABY AND H. A. THORPE

The values for "integrated intensity" above 15 kc, as plotted in Fig. 1, were obtained from the spectral distribution data by expressing the average intensity over each band in watts/cm 2, adding contributions from each band from 15 to 25 kc, and finally re-expressing this'•integrated intensity in db above 10 -1ø watt/cm 2.

Integrated intensities over the audible range, up to nearly 10 kc, were measured by a General Radio type 759-B Sound Level Meter.

RESULTS

Figures l(a) and l(b) depict the integrated intensity for two frequency bands, 0-10 kc, and 15-25 kc, as a function of time for two locations, namely in the jungle at Barro Colorado, and in a typical grass-and-bush second growth jungle near Gamboa. For the former location the inten-

sities plotted for any given time are average values for two days. A diurnal cycle of ambient noise is clearly evident. The ultrasonic level at both sites is seen to follow essentially the same pattern as the audible noise level, i.e., it is noisiest during the early evening, and quietest during the heat of the day. However, at Gamboa, in the evening, the intensity in the higher fre- quency band slightly exceeded that of the lower frequency band. While this was not an un-

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6AM 12N • PM TIME OF DAY

Fid. 1. Diurnal variation 'of integrated intensity in frequency bands 0 to 10 kc and 15 to 25 kc; (a) on Barro Colorado Island, (b) near Gamboa.

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I0 15 20 25

FREOUENCY (Kc/sec.)

Fid. 2. Spectral distribution of ambient noise, 8 to 25 kc, for three different times of the day; (a) on Barro Colorado Island, (b) near Gamboa.

common occurrence, it seems not to have been altogether typical.

In these figures the crosshatched areas in- dicate that with the available apparatus in- tensity levels could not be measured below about 32 db for the range above 15 kc. It should be noted that in both locations the ultrasonic

intensity dropped below the instrumental thresh- old at noon. At Gamboa it was below that

threshold most of the day and above it during the night. The Barro Colorado data are typical of wooded jungle locations on the Isthmus. The Gamboa data are typical of newer, second growth jungles whose bushy nature attracts certain of the more noisy species of insects, accounting for the high frequency noise peak of Fig. 1 (b).

Figure 2(a) shows typical spectral distribu- tions, from 8 to 25 kc, of noise in the jungle at Barro Colorado for evening, early morning, and midday. Corresponding data for the jungle near Gamboa appear in Fig. 2(b). Again it is apparent that the ultrasonic intensity is greatest in the early evening. Furthermore, this is true through- out the measured frequency band. It is least near noon, when for all frequencies it is either below or very near the instrumental threshold. The general conclusion is that the ambient noise level in Panamanian jungles during the rainy

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Page 3: Ultrasonic Ambient Noise in Tropical Jungles

ULTRASONIC NOISE IN TROPICAL JUNGLES 273

season is of the order of 0 to 10 db above 10 -•ø

watt/cm 2 per c.p.s. for frequencies between 15 and 25 kc.

Attempts were made to discover whether any appreciable part of the ultrasonic noise was caused by insects whose output was wholly above the audible range. •' The results were negative in that no such insects were observed.

In the first place, it was always possible to correlate significant fluctuations in the instan- taneous ultrasonic level, as indicated by a meter, with corresponding activity of individual insect voices heard by ear. Then the ultrasonic noise was made "audible" and explored by a hetero- dyne arrangement whereby beat notes between ultrasonic noise up to 28 kc and a local oscillator were amplified and made audible in a headset. The ultrasonic output of particular insects, previously "observed" by correlating ultrasonic meter readings with audible noises, were thus detected by ear. Their tone quality was rasping and irregular, resembling random noise. At no

• Such insects have been observed in the United States. See W. H. Pielemeier, J. Acous. Soc. Am. 17, 337 (1946).

time did we observe any ultrasonic noise of natural origin which had a definite pitch or easily identified timbre such as is associated with bird

calls, or the chirps of the more musical insects in the audible range. We should have been able to detect such sounds, had they been present, for by that method the tones of ultrasonic whistles could be readily distinguished by definite pitch and characteristic quality.

A brief search was made also in the jungle for possible ultrasonic sources other than insects. However, none was discovered. In particular, wind noises or the rustling of foliage do not appear to contribute measurably to the ultra- sonic background. Nor does rain seem to con- tribute much except when it falls upon the surface of water, say a lake, or when it strikes the metal roof of a shelter--as it did on a few

occasions on Barro Colorado Island.

We are indebted to Professor Merit Scott for

direction and aid in the construction of the

apparatus, and to Professor Harold K. Schilling for guidance and supervision of the experimental work.

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