Acoustical Society of America
ASA/EAA/DAGA '99 Meeting
Lay Language Papers

Acoustic Weapons?
Sources, Propagation, and Effects of Strong Sound

Jürgen Altmann

Experimentelle Physik III, Universität Dortmund/Institut für Experimentalphysik III, Ruhr-Universität Bochum
D-44780 Bochum, Germany
(The first part of this work was done at the Peace Studies Program, Cornell University, Ithaca NY, USA.)

Popular version of Posters 2aPPb10 and 3aPPb12
Presented Tuesday morning, March 16, and Wednesday morning, March 17, 1999
ASA/EAA/DAGA '99 Meeting, Berlin, Germany

Military Press: Strong Allegations, Few Facts

How can one turn a threatening gunman into a retching bundle of nerves, suffering simultaneously from bowel spasms and loss of courage before surrendering to police? Simply use infrasound on him, i.e., sound loo low in frequency to be heard. This is at least what some military journals promise as the virtue of so-called "acoustic weapons." What is more, these reports always tend to claim something like, "The effect ceases as soon as the generator is turned off, with no lingering physical or environmental damage." Potentially more harm is ascribed to "acoustic bullets," (either low or high frequency), baseball-sized objects which propogate to hundreds of meters, causing incremental effects from discomfort to death.

Reliable information on the science and technology of such weapons and their effects is hard to come by, however. One cannot avoid the impression that much of what is written on acoustic weapons is based on hearsay and misunderstandings. Nonetheless, the U.S. Army has a few acoustic weapon research and development programs underway, some mounted on a helicopter or other vehicle, others fixed to protect certain sites. Both low and high frequencies are being studied, with powers of tens of kilowatts and ranges up to kilometers. Use against both crowds and soldiers is foreseen. Scientific publications are not available, however.

In order to provide reliable information for future assessments of how acoustic-based weapons should be addressed by international laws of warfare and of human rights, I have done an exhaustive literature survey and conducted theoretical analyses. My questions included: What kind of sound sources could be used? How far does strong sound propagate? What are the effects on humans? Is there a danger of permanent damage?

Table 1 Relevant sound frequency ranges.
Frequency is measured in cycles per second = Hertz (Hz). Depending on age, people cannot hear sound above 14-18 kilohertz. Infrasound between 1 and 20 Hertz can be heard, however, if the level is high enough.
20 Hz 500 Hz 20,000 Hz = 20 kHz
Infrasound Low Audio High Audio Ultrasound

Sirens, Whistles, Explosions: Large Sources

Banks of loudspeakers, such as those used in open-air rock concerts, are not very efficient. Higher power levels can be achieved with a siren - already in 1941 37 kilowatts were produced using two combustion engines of 71 and 15 kW, respectively. The device had a mouth of 0.7 m diameter and was mounted on a small truck. With its low audio frequency of 460 Hertz (close to the concert pitch of 440 Hz of the well-known tuning fork), the sound level was 137 decibels, about the ear-pain threshold, at 30 m distance. At 100 m, the level had decreased to 127 dB - comparable to an aircraft jet engine at about the same distance.

If one enlarges a whistle to one meter or more and blows it with a strong air pump, acoustic powers of tens of kilowatts can be generated in the low-frequency range as well.

For higher frequencies (e.g, 1 kilohertz to ultrasound, above 20 kHz), the devices become smaller and deliver less power, but several can be used in parallel.

For explosion-produced shock waves, there is no limit on sound power or level - if a person is close and the source strong enough, not only can the ear be injured, but the lung can rupture and death can ensue. Repetitive explosions, as in a combustion engine, could be used to produce extremely strong sound, e.g., at megawatt powers, at low frequencies.

Except for small acoustic munitions which would work over a range of one meter or so and could be shot or thrown to the vicinity of a victim - like a whistling or an exploding firecracker - acoustic weapons would be quite large and heavy. Because they are likely to be rather bulky, there is a good chance that such weapons would need to be mounted on a pickup truck or helicopter.

Beam Widening and Strong Absorption: Obstacles to Long-range Propagation

Producing sound is not enough to make an acoustic weapon- the waves have to propagate to the intended target. But there are two effects hampering high sound levels at some distance. At low frequencies, the waves expand from a source essentially to all directions, reducing the power per area with the inverse squared distance from the beginning. (For each doubling of the distance, the level decreases by 6 dB.)

This is different at higher frequencies, where a beam can form, focusing the power into a cylinder and then a small cone. However, here the non-linearity of sound propagation at high power creates problems: From a certain distance on, the waves deform to the so-called "shocked state" in which they lose much more energy to the air, with a faster decrease of the level with range.

Calculating both effects is complicated; the level at a given distance depends on many details, such as source power and size, frequency, wavefront form, and humidity. Estimates show that, using sources of meter size and tens of kilowatts power, it may be possible to achieve levels of 150 decibels at 10 meters, but that at, say, 50 m, the level would decrease to 100-130 dB.

In my investigation of the literature I have not found a convincing mechanism for the so-called "acoustic bullets" - perhaps the originators of these reports got mixed up with invisible laser beams.

Thus, the range of an acoustic weapon is limited to few tens of meters. On the other hand, at shorter distances, extremely high levels are possible with the potential for permanent hearing damage. The same situation holds true within closed rooms, if the sound energy from the weapons can be efficiently transmitted into them.

Table 2 Sound levels and effects at audio frequencies.
Level is a logarithmic quantity, derived from the air pressure variation constituting the sound; it is measured in decibels (dB). 0 dB is at the threshold of hearing, 0.000,02 Pascals sound pressure at 1 kilohertz frequency. For every factor 10 in sound pressure, 20 dB are added. At 194 dB, the overpressure part of the wave is equal to the normal air pressure, 101,000 Pascals. In the infra- and ultrasound regions, effects are much weaker or do not exist at all.
0 dB 20 dB 40 dB 60 dB 80 dB 100 dB 120 dB 140 dB 160 dB 180 dB
Talking Car close Truck close Jet aircraft at tens of metres
Hearing threshold Discomfort Ear pain Eardrum rupture
Conditional hearing risk (long-term exposure) High hearing risk (short exposure)

Hearing and Non-auditory Effects on Humans

In the discussion of acoustic-weapons effects one deals with sound levels which are far above those occurring in daily life. Because of the danger to hearing, a level of 140 dB should never be exceeded; for long-term exposure at the workplace, much lower limits exist. Thus, only very few human experiments with sound levels above 120 dB were done. With animals, on the other hand, all aspects of temporary and permanent hearing loss have been studied. This is what can be extracted from the scientific literature:

In spite of the fuss made about infrasound in the military press, at 1-20 Hertz there is really no profound effect on humans. I have found no hard evidence for vomiting or uncontrolled defecation, even at levels of 170 dB or more.

In the audio region (20 Hz to 20 kHz), discomfort starts at 120 dB and pain in the ears at about 140 dB. Sounds at such levels affects hearing - at first in a reversible way, but after some time permanent damage will develop. At 140 dB, this can take only few tenths of a second.

Eardrum rupture occurs at about 160 dB.

(With a non-periodic explosive blast wave, eardrum rupture begins at 185 dB, and lung rupture at about 200 dB.)

Non-auditory effects are strongest at low audio frequencies of 50-100 Hertz; intolerable sensations mainly in the chest were observed, but only at levels above 150 dB.

Therapy can help with ruptured eardrums, but much cannot be done about permanent hearing loss.

Ear protection is possible, however - earplugs and earmuffs can reduce the level impinging on the eardrum by 15-45 dB.

What Follows from All This?

Acoustic weapons are not likely to fulfill the promises made in military journals. In particular, they will not render opponents incapable of action.

It is possible to produce pain-provoking sound levels for unprotected ears at few tens of meters range. Such levels involve an immediate risk of permanent damage to hearing. Thus, acoustic weapons are prone to producing lingering damage - contrary to the quote in the first paragraph. As a consequence, they need to be analyzed under aspects of the laws of warfare - keywords are "indiscriminate effects" and "unnecessary suffering." For law-enforcement use, e.g., by the police, general human rights and proportionality have to be taken into account.

There may be specific situations where acoustic weapons could add options for the application of legitimate force, e.g., when hostages were taken. On the other hand, the opponents could protect their ears and could afflict damage to the hostages nevertheless.

Because many types of acoustic weapons would be large and cumbersome, and because protection is easy, the military and police interest in them may turn out lower than expected. Agreement on preventive limits may be possible; one could, e.g., simply limit all systems to 120 dB at anybody's ears. Because of various application scenarios and many different conditions, however, more complex rules will probably be needed requiring detailed deliberations.

(A detailed article will appear in the journal Science and Global Security.)

The author can be reached on Monday via the Conference Secretariat, on Tuesday and Wednesday mornings at his posters.