Principles of electroacoustics

Sound and electroacoustics

Sound is a mechanical oscillation that propagates through solid bodies, water or gas (air). Technically speaking, these oscillations are periodic pressure variations caused by a sound source (e.g. a loudspeaker). We use the term acoustics for sound waves in the frequency range audible to the human ear. If the sound is transformed into electrical signals or vice versa, or if it is amplified, stored or transmitted, we use the term electroacoustics.

Sound pressure level

The magnitude of the pressure variations is referred to as sound pressure. The unit of pressure used to measure this magnitude is the pascal. In practice, however, it is more advantageous to deal with sound levels using decibels, which is why we use the sound pressure level, which has an auditory threshold of 20 μPa (micropascals) as its reference point. As a result, decibel levels (dB SPL) are unambiguous and can be compared to each other.


The frequency is the number of oscillations per second, and determines the pitch. It is measured in hertz [Hz]. 1000 Hz is also referred to as 1 kHz (kilohertz). Electroacoustics deals with the audible frequency range of 20 Hz to 20 kHz.

Room acoustics

Room acoustics relate to the effects of a room’s structural conditions on the acoustics. Room acoustics have a major impact on how the visitor perceives a room, and therefore also play an essential role in electroacoustic systems.

Sound propagation

Sound spreads out radially from a sound source. Some of the sound waves reach the listener directly, while some are reflected by the walls, ceiling and floor, and still others are absorbed, depending on how the room is appointed.

Direct and indirect sound, ambient noise

The human ear can locate sound sources through direct sound, as this is the sound that reaches the ear first (law of the first wavefront). Thanks to indirect sound, which is also referred to as reverberation, the human ear can perceive the size of a room and its characteristics. This is also referred to as diffuse sound, as it is generally distributed evenly throughout the room, statistically speaking. Ambient noise, in turn, refers to all sound events that have an interference effect on the ability to clearly hear the sound.

Reverberation time and speech intelligibility

What is characteristic of direct sound is that it dies down abruptly when the sound source is switched off, whereas indirect sound remains in the room for a short time as reverberation. The reverberation time is defined as the time that elapses until the sound pressure level has dropped by 60 dB.

Reverberation time is closely linked to speech intelligibility. When reflected sound dominates direct sound and dies down quickly enough, this can be perceived as something enjoyable during musical performances. But in a voice transmission, reverberation causes a drop in speech intelligibility. Therefore the basic rule is that speech intelligibility deteriorates as reverberation time increases. For this reason, with a long reverberation time, it is critical to convey as much direct sound from the loudspeakers to the listeners as possible and to avoid stimulating reverberation as much as possible.

User standards prescribe that an announcement made over a voice alarm system must be at least 10 dB above the ambient noise level. If the ambient noise level is 70 dB, the loudspeaker must produce at least 80 dB in the zone in which you want to make the public announcement.

The most common way of expressing speech intelligibility is the ‘Speech Transmission Index’ (STI), which uses a scale from 0 to 1. Under the European standards applicable in many countries, EN 60849 (to be replaced by EN 50849) or TS 54-32, the minimum value prescribed for electroacoustic emergency notifi cation systems is an STI of 0.5.

STI value
0.00 – 0.30 poor
0.30 – 0.45 weak
0.45 – 0.60 fair
0.60 – 0.75 good
0.75 – 1.00 excellent