Noise characteristics of some sound sources of technical origin used in forensic sound environment investigations.

Characteristics of noise of some sound sources of technical origin, used in forensic studies of the sound environment.

Kaganov A.Sh.
Russian Federal Center for Forensic Examinations

A criminal event should always be defined as an event existing in a specific environment, conditioned by the characteristics of space and time.

The tasks of forensic examination of the sound environment, as well as forensic knowledge of the crime event as a whole, are determined by the tasks of procedural proof, that is, in a criminal case, the crime event, time, place, method of committing the crime, the guilty person must be proven.

V.L. Sharshunsky [1] considered the sound environment as the texture of the surrounding environment, which consists of a complex of very different sounds.

In this case, the features of such a set of sounds will characterize the acoustic conditions in which the recording of the studied phonogram was carried out.

From a legal point of view, the peculiarity of the analysis of the sound environment of the incident under investigation is that such research allows us to prove many facts of criminal activity that occur face to face, without witnesses.

Thus, the sound environment of a crime event is a single system of sound traces of natural and artificial sources, the subject-communicative activity of the criminal, the tools, instruments and technical means used by him, united by a single goal.

The achievement of this goal is determined by the nature and time of the criminal attack on the object, as well as the unity of place (space) and the method of achieving it [2].

The fact that this research task is traditional for forensic science can be found in the work of Hans Gross, who back in 1905 pointed out the need to use audio information during investigative actions and wrote that there is no reason to consider «what is seen» more reliable and valuable than «what is heard» [3].

But if earlier the task of studying the sound environment was solved by means of tactics, now (thanks to the possibility of recording sound traces) it is solved in the process of producing forensic examination of sound recordings.

Identification of sound sources of technical origin is based on methods and techniques of spectral analysis and, in terms of the technology of conducting the research, overlaps with the tasks of forensic identification of sound recording devices and (partially) the speaker's identity.

At the same time, issues of forensic diagnostics of the sound environment require special consideration.

Let us recall that the most common questions that are put to experts by the investigation or the court may include the following questions:

–            does the sound environment recorded on the presented soundtrack correspond to the circumstances described in the protocol of investigative actions;

–            what are the main characteristics of the sound sources, the noise of which is recorded on the studied phonogram;

–            what is the distance between the sources of sound traces;

–            what is the direction of movement of the sound source;

–            determine the class of sources generating sound traces accompanying the subject of conversation, etc.

Answers to these and other questions of forensic diagnostics are based on a comprehensive analysis of the characteristics of those sound sources (mainly of technical origin) that form the sound environment of a crime event.

Let's take a closer look at some of the above characteristics.

 Transformer noise

The total noise level of transformers is determined, as a rule, by the magnetic component, and in any case it is significant in the frequency range up to approximately 800 Hz, that is, the noise levels in this range are significantly higher than at higher frequencies.

Transformer noise is predominantly composed of tonal components, the frequency of which corresponds to double the network frequency (at a network frequency of 50 Hz, the frequency of the tonal component is 100 Hz), and their multiple harmonics, which is perceived by ear as a low hum.

Internal combustion engines

Exhaust noise occurs during the pulsating flow of exhaust gases.

In the absence of a muffler, it is the most intense component of the total engine noise, the sound power of which is from 0.01 to 0.1% of the engine power.

At first glance, such power seems insignificant. However, it should be taken into account that 1 W (1/736 hp) of acoustic power creates a sound pressure level of 92 dB at a distance of 10 m.

The noise emitted directly by the engine (cylinder block, crankcase with sump, etc.) is two to three orders of magnitude lower in sound power, i.e. 20–30 dB, than the exhaust noise.

Nevertheless, in practice, the analysis of this noise should not be neglected, since it is interesting from the point of view of analyzing investigative situations.

 Aircraft noise

This type of noise is caused by the following: jet stream, turbine, compressor, blower.

The jet stream and turbine emit sound mainly behind themselves, and the compressor and blower — in front and behind.

The noise spectrum of compressors and blowers consists of broadband noise and tonal components.

The cause of broadband noise is the turbulent flow incident on the rotor and stator blades, as well as the uneven (including in the radial direction) shedding of vortices from the blades themselves.

The presence of tonal components in the spectrum is due to periodic oscillations of aerodynamic forces. These forces are mainly explained by the interaction of the blades with the flow that has passed through the guide vane.

The specified sound sources are of a dipole[1] nature. The fundamental tone has a frequency of B•N, where B is the number of rotor blades, and N is the rotor speed.

Helicopters. The main sources of helicopter noise in the far field are rotors and rotary engines.

The spectrum of noise emitted by the rotor is formed by tonal components (rotational noise) with a fundamental frequency of B•N, where B is the number of rotor blades, and N is the rotor speed, and broadband noise. Up to 50 harmonics of the fundamental tone can be found in the spectrum of helicopter rotor noise.

The reason for the appearance of tonal components are the aerodynamic forces acting on the blade. They can be decomposed into stationary (lift and drag) and variable components.

The former, due to the rotation of the blades, cause sound emission. For aircraft propellers, these forces are the main cause of the appearance of tonal components in noise. For a helicopter rotor, they determine only the fundamental tone and the lowest harmonics. Higher harmonics are caused mainly by variable forces.

The main cause of variable aerodynamic forces is the interaction of the rotor blade with the vortex wake of the previous blade.

Broadband noise is caused by non-uniformly changing aerodynamic forces acting on the blade, which are formed due to the turbulent wake caused by the previous blade.

Railway noise

Metro.

The average noise level in a subway train car (at a height of 1.2 m from the car floor) when moving in open space on tracks laid on a crushed stone bed at a speed of 40 km/h is 62 dB (A)[2], and at a speed of 60 km/h – 69 dB (A). Noise levels during acceleration of the train increase to 72 dB (A), and during braking – to 74 dB (A).

The noise level of a subway train moving in a tunnel at a speed of 60 km/h is 90–100 dB (A). Poor track quality increases the noise level by 10 dB (A), and doubling the speed – by another 10 dB (A).

The noise at stations when the train accelerates and brakes (with a normal ceiling without sound-absorbing lining) reaches 80 dB (A), and with such lining — 74 dB (A);

The noise levels inside the latest models of subway cars when moving in a tunnel at a speed of 60 km/h do not exceed 75 dB (A), while in cars of earlier designs they reached 95 dB (A).

When the speed of travel is doubled, the noise level inside the car increases by 6-8 dB (A). Doubling the axle load of the car increases the noise level in the car by 3 dB (A).

Tram.

The external noise of a tram when moving on crushed stone tracks at a speed of 40 km/h at a distance of 7.5 m from the rails and a height of 1.25 m above the rails is 81 dB (A), and at a speed of 60 km/h — 86 dB (A).

If asphalt pavement is used between the rails, then the noise levels of the tram at a speed of 40 km/h are 87 dB (A), and at a speed of 60 km/h — 91 dB (A).

When a tram moves along a concrete bridge, noise levels increase by an average of 4 dB (A).

A car as a source of noise

Rolling noise.

Rolling noise is created when the tire comes into contact with the road surface, which is never perfectly smooth, and also when air is forced out of the tread grooves on the tire surface.

The sound power of rolling noise is approximately proportional to the third power of the driving speed, and the noise level increases by 9 dB for every doubling of the driving speed.

In a passenger car moving steadily in direct gear on a smooth road with summer tires, rolling noise overshadows other noise components at speeds above 50 km/h.

Winter and special tires are noisier than summer tires. The noise from winter tires includes a significant proportion of high-frequency components, which leads to an increase in the level on asphalt by 3 dB (A).

Smooth dry asphalt surfaces are the quietest.

Concrete and grooved asphalt surfaces are noisier by 3-5 dB (A), and paved ones by 8 dB (A).

Thin snow cover significantly reduces noise generation.

Wet surfaces are noisier [up to 10 dB (A)] than dry ones.

Maximum permissible levels of noise emission from vehicles.

In a number of countries, vehicle noise emission levels are limited by law. For example, German law sets maximum permissible levels of vehicle noise emission in dB (A) [4]:

These are the noise characteristics of some sound sources.

It should be emphasized that the list of sound sources can be continued as the list of investigative situations that require analysis of the sound environment of a crime event as a «set of sounds inherent in the place where the recording of the phonogram under investigation was made and characteristic of the time of its production…» [5] is expanded.

In any case, however, the data provided will be useful to experts in their daily work, since the selection of the materials provided was based on the real needs of investigative and forensic practice.

List of recommended literature

1.     Sharshunsky V.L. Diagnostics of the sound environment recorded on a magnetic signalogram //Expert practice. — M .: VNII MVD USSR, 1080. No. 16. — pp. 100-102.
2.     Yashchurinsky Yu.V. Forensic diagnostics of the sound environment Dis. Cand. of Law: 12.00.09. – Kyiv: 1990. – 327 p.
3. Gross H. Kriminalpsychologie. Leipzig, 1905
4. Handbook of Technical Acoustics edited by M. Heckl and H.A. Müller. – L., Sudostroenie, 1980.
5. Lozhkevich A.A., Snetkov V.A., Chivanov V.A., Sharshunsky V.L. Forensic Study of the Sound Environment Recorded on a Phonogram. – M.: VNII MVD USSR, 1981. – 48 p.