Acoustic principles

1. Perception of noise by the human ear

"In today's hectic and stressful age, with the pressure to perform and the double burden of career and household, people are becoming increasingly sensitive to noise. Moreover, the effects of loud noise cause hardness of hearing over time. For this reason, it is becoming increasingly imperative in private, leisure and industrial buildings to find solutions for both sound insulation and room acoustics that provide people with a safe and liveable environment."

Acoustics is the gauge of sound

Physical acoustics examines the vibrations and sound waves lying in the range of audibility (16 to 20,000 Hertz), their propagation, composition, reception and so forth, e.g. frequency, acoustic pressure, propagation speed, insulation and damping.

The first question is - What is noise?

Noise is a subjective term for sound which is perceived as disturbing. Thus noise is not a measurable, defined quantity, but the sounds perceived as noise.

Fig. 7.3.1 Examples of silence and noise

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2. Acoustic field, threshold of pain

Effect of noise

The ear sensory organ does not possess any natural protection mechanisms that prevent noise from affecting the ear. Whereas, for example, the pupil of the eye contracts in strong light, the ear is constantly "switched to receive". Noise therefore refers to noises (sound) which disturb, bother, damage health and which can increase the risk of accidents.

These observations necessarily lead to the question: what are the limits of hearing, when is the threshold of pain reached?

The hearing capacity of the human ear is most easily shown in the acoustic field diagram in Fig. 7.3.2.

The right-hand ordinate gives the sound intensity (l) in watt/m2.

The numeric range of the threshold of audibility 0 up to the threshold of feeling or pain is shown by the logarithmic scale on the left-hand ordinate in dB.

The light blue area represents the entire acoustic field of a healthy human ear. Only a fraction of this is covered by the field of speech (dark blue area).

In the bottom section, the acoustic field (see black boundary) is delimited by the threshold of audibility. The lateral delimitations show that the human ear can perceive sound between the frequencies of 16 - 16,000 Hz. Below 16 Hz we speak of infrasound, and above 16,000 Hz we speak of ultrasound (see Fig. 7.3.5 on page 7.3.4).

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In the top section, the acoustic field passes into the threshold of feeling or the threshold of pain. If this is exceeded (see black boundary in top section), hearing may be endangered and even damaged.

Fig. 7.3.2 Perception of noise by the human ear (hearing areas, thresholds, frequency ranges)

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3. Principles of the physical quantities

Sound results from vibrations from a sound source (e.g. vocal chords, tuning fork, loudspeaker membrane, machine, vehicle).

For its propagation, it requires a medium such as air or another matter.

Thus sound can also propagate in liquids such as water, or in solid bodies, such as walls, ceilings or pipes.

In the case of solid media, we speak of structure-borne noise. The vibrations from the sound source (emission) are transferred to the medium and cause this to vibrate as well.

The propagation of sound waves in liquid or gaseous substances occurs in a similar manner, with the matter being alternately compressed and decompressed.

The distance between two wave crests or wave troughs, or put another way, the period during which this wave occurs, is characterised by the wave length l.

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Fig. 7.3.3 Sound always results from vibrations, i.e. through regular to and fro movements of flexible matter.

Fig. 7.3.4 Sound waves, e.g. due to molecular movements of the air  top

4. Measurement of volume and sensitivity

Measurement of volume

Volume is measured in decibels (dB). It represents a subjective quantity, which does not always coincide with the physical quantity of the sound when measured by sound pressure level, for example.

This is because everyone has a different absolute threshold and pain threshold.

Many sound measurement procedures are therefore aimed at assessing sounds as they are perceived by the ear.

Sensitivity
The human ear is not equally sensitive for all frequencies.

The frequency is specified in Hertz = Hz, and defines the tone pitch. The maximum human auditory sensitivity is between 1,000 and 4,000 Hz.

The sensitivity decreases at lower and higher frequencies, i.e. frequencies below 1,000 Hz and above 4,000 Hz are perceived as being quieter.

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Fig. 7.3.5 Frequencies in the range of human hearing

5. Acoustic comparison measurements, speed of sound

Acoustic comparison measurements

In the case of on the spot sound pressure (sound level) measurements, the measurement results are adapted to the physiological perception of the human ear through frequency-dependent assessments.

In other words, the measured values for the sound pressure level are corrected in accordance with a defined curve.

This correction is made with the help of assessment curve A. In this curve, the auditory perception of the ear is defined in accordance with DIN 4109, e.g. 60 dB(A). If Index A is omitted, the value is absolute (see Fig. 7.3.6).
Special rules apply, on account of the logarithmic structure:

If one adds, for example, two sound levels of 50 dB, you do not get 100 dB, but 53 dB. An increase in the sound level by 6 dB corresponds to a doubling of the sound pressure. A decrease in the sound level by 6 dB corresponds to a halving of the sound pressure.

Speed of sound
The speed at which sound waves propagate is called speed of sound c. This is heavily dependent on the propagation medium:

c für Luft bei -20°C ca. 320 m/s
c für Luft bei 0° C ca. 332 m/s
c für Luft bei + 50°C ca. 362 m/s
c für Wasser bei +20°C ca. 1460 m/s
c für Stahl / Aluminium ca. 5100 m/s

Fig. 7.3.7 Table of sound speed c

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Fig. 7.3.6 Diagram of auditory comparison measurements

6. Reverberation time, sound absorption

Reverberation time

The reverberation time T is the most important measured variable for room acoustics. It is defined as the time within which the sound energy has dropped by 60 dB, i.e. to a millionth, in a room after the sound generation has been switched off.

Once again, the logarithmic calculation formula means that the respective decrease in the sound pressure level by 6 dB corresponds to a halving of the sound pressure.

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Sound absorption and absorption factor a s

The absorption factor (formerly sound absorption coefficient) specifies the ratio of the absorbed sound proportion to the incident sound energy.

a s = 0 = complete reverberation
a s = 1 = complete absorption

e.g.: a s = 0.8; i.e. 80% of the sound is absorbed, 20% is reflected.

Sound absorption is heavily dependent on the surface of the surrounding building structure. Smooth and hard surfaces reflect sound more strongly than soft surfaces, such as e.g. curtains and carpeted floors, which considerably reduce sound reverberation.

A diffuse sound field is formed, which impedes an increase in the volume through reverberation.

This process is called sound absorption. Part of the movement energy of the sound waves is converted into heat upon entry into matter, such as e.g. carpets, curtains etc.

In principle, energy is not lost, it can only be converted

Structural sound proofing measures can be implemented using suitable sandwich panels with an insulating core made of polyurethane and mineral fibre. Section 7.4 deals with the theme of "Sound proofing in lightweight constructions".

Autor
Christoph Köhler

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