Gunshots and audio equipment.

In a typical television scenario, a "wired" undercover officer accompanies a suspected criminal to a secluded location, where the officer talks the suspect into revealing details of illegal activity. All along, the operative is confident that backup officers positioned just out of sight can monitor every sound made during the encounter.

Unfortunately, in the real world, this may not be the case. A growing number of studies reveal that the sound waves produced by gunshots may not always be detected by body-worn transmitters or audio recorders. This problem--commonly referred to as audio shutdown--raises significant safety and legal concerns for the law enforcement community.

Gunshot Noise

In an auditory sense, gunshot noise is a rather unique phenomenon. A gunshot produces a very short, but very loud, sound impulse. The blast of gunfire strains the capacity of even the human ear, which detects noise more efficiently and distinguishes sounds from a much wider spectrum than most audio equipment. This is why shooters at firearms ranges wear ear protection. The same sharp sound fluctuation that causes auditory discomfort at a firing range (and that can lead to permanent hearing loss) can cause audio equipment to momentarily "shutdown" or fail to register the sounds of a gunshot.

Measuring Sound Levels

In three separate tests, using a wide variety of weapon/ammunition combinations at both indoor and outdoor firing ranges, researchers measured the peak pressure levels--the highest sound pressure level achieved--of different firearms.(1) In all three tests, the peak levels registered between 130 and 153 decibels (dB).(2)

As part of the final study, researchers also exposed transmitters, receivers, and recorders from various manufacturers to sound pressure levels exceeding 140 dB.(3) Although researchers found it difficult to re-create the shutdown phenomena in the controlled tests, they did identify some shortcomings in the audio equipment.

During the tests, rapid, multiple gunshots, as from a semiautomatic weapon, were masked to sound like a single shot on some recorders. In addition, gunshots did not always sound like gunshots when played back on the recorders. To further complicate matters, other loud noises, such as handclaps or doors slamming in close proximity to the microphone, sounded more like gunfire than did the actual gunshots.

The results of these preliminary studies led researchers to conduct further tests under more realistic conditions in the practical exercise area at the Federal Law Enforcement Training Center, in Glynco, Georgia. During these tests, one of the four body-worn transmitters used clearly exhibited a tendency to shut down when shots were fired.

Graphic evidence of audio shutdown was also tragically chronicled in the January 1991, shooting death of a Nacogdoches, Texas, law enforcement officer. The shooting was visually recorded by his patrol car video camera. However, the wireless microphone system worn by the officer did not record a gunshot. As the tests illustrate, this is not an isolated problem.

Tracing the Cause

What causes some audio systems to shut down when shots are fired? To find the answer, researchers interviewed technical representatives from various manufacturers, as well as technical experts in the law enforcement community. These authorities generally agreed that no single cause is responsible. Rather, microphone design, the automatic gain control circuitry (AGC) in individual transmitters, receivers, and recorders, the bandwidth of the equipment, and other factors may all contribute to the phenomenon.

The Microphone--The electret condenser microphone is the most commonly used microphone in surveillance equipment. At sound pressure levels in the range of 120 to 150 dB, the microphone's element may freeze in place temporarily and generate no signal.

The AGC Circuitry--The automatic gain control circuit boosts low-level signals and attenuates (slightly diminishes) high-level signals. This allows a fairly constant signal level with a minimum of distortion. When the circuit is exposed to the high-level signals produced by a gunshot, it attempts to attenuate the signals. This greatly reduces the signal level and causes the circuit to become "deaf" to lower-level signals during this time.

Bandwidth--Most technical surveillance equipment has narrow signal bandwidths. Gunshots generate sound frequencies that produce signal bandwidths much wider than operational equipment accepts. Therefore, gunshots that are recorded by electronic equipment still may not be accurately reproduced.

Considerations

Variations in the individual components used in the manufacture of these devices and the quality control of each particular product must also be considered. Slight variations in these highly sensitive devices explains why identical models of a transmitter may behave differently--one may shut down while the other does not.

These factors make it difficult to predict whether specific surveillance equipment will shut down. A number of conditions--including component quality, microphone design, the proximity of a blast to the microphone, and the setting in which the equipment is used--contribute to the quality of audio transmission. However, because of the problems detected in these studies, as well as a growing number of personal accounts, police departments may consider conducting their own tests. These tests need not be complicated. Equipment managers may conduct simple analysis during routine firearms qualifications.

Department Responsibility

Considering the above research and findings, supervisors should ensure that all officers understand that they may not hear gunfire when listening to surveillance equipment. Therefore, when monitoring undercover officers or informants, backup personnel should also rely on visual information and conversation clues to ensure safety.

The potential for audio shutdown makes preplanning in this area essential. Operatives wearing transmitters should clearly state (perhaps with prearranged code words) what is occurring, particularly if suspects produce weapons.

In addition to the safety issues involved, audio shutdown may also have legal implications. For this reason, technicians and legal advisors should be prepared to explain in legal proceedings the nature and limitations of the audio equipment used by their agencies.

Conclusion

Modern audio equipment provides law enforcement with a valuable asset in the battle against crime. However, like any tool, it has inherent limitations. Only by knowing and understanding these limitations can law enforcement personnel use these devices to their best advantage.

Endnotes

1 In December 1980, industrial hygienists with the Occupational Environmental Health Service, Naval Regional Medical Center, Jacksonville, Florida, conducted impact noise measurements during weapons firing at the Federal Law Enforcement Training Center (FLETC), Glynco, Georgia (outdoor range); in April 1984, a private firm measured peak sound pressure levels in the FLETC indoor firearms range; in July 1991, the authors conducted similar tests at the U.S. Customs firearms facilities, Fort Benning, Georgia.

2 A decibel is a measure of sound intensity equal to one-tenth of a bel, which is a unit of sound measurement expressing the logarithmic ratio of the values of two amounts of power. For purposes of these tests, the two amounts of power being compared were silence (ambience) and the peak pressure level of a gunshot blast.

3 Researchers tested the standard types of audio devices commonly used by law enforcement undercover officers. Transmitters convert sound waves into electronic waves, then modulate and transmit them to an antenna. Receivers convert transmitted electronic waves back into audio waves. Recorders preserve audio impulses on electromagnetic tape.

Special Agent Glenn E. Brazil, U.S. Customs Service Academy, Glynco, Georgia, and Special Agent David R. Montalbano, U.S. Forest Service, Atlanta, Georgia, provided information for this Research Forum.