Acoustic Quality Measurements

MEASUREMENT, ANALYSIS AND PREDICTION OF ROOM ACOUSTIC PERFORMANCE – ELECTROACOUSTIC MEASUREMENTS

Every enclosed space has certain acoustic parameters crucial to determine if it is suitable for a specific usage (e.g. music production, speech, etc.). In addition, these acoustic features frequently dictate the type and the specifications of a potential PA installation in this space.

In ALPHA Acoustiki, using state-of-the-art acoustic software and years of experience in the field, we can calculate and predict these acoustic characteristics of the room before it is even built.

 

omnidirectional sound source

Omnidirectional Sound Source (Dodecahedron)

However, using specialized equipment to perform on-site acoustic measurements, we can evaluate the room response in more detail. This is practically done by measuring the impulse response of the space at specific source/receiver positions using an omnidirectional sound source (dodecahedron speaker). Sine sweeps (or “Swept-sines) used as excitation signals provide a way to measure linear impulse response.

Using the impulse response of a room, we can generate and calculate various acoustic parameters such as:

  1. Reverberation Time; RT60, RT20
  2. Frequency Response
  3. Early Decay Time; EDT)
  4. Clarity; C50 C80
  5. Definition; D50 D80
  6. Sound Strength; G
  7. Sound Level; ??
  8. Percentage Articulation Loss of Consonants; %ALCons
  9. Speech Transmission Index & Rapid Speech Transmission Index; STI / RASTI

The sound perceived by a listener into an enclosed room can be divided into three fractions: direct sound, early reflections, and reverberation (delayed reflections).

Direct sound is waves that reach the listener’s ears directly from the source.

Early reflections from the side walls, the roof, etc., arrive afterwards, because they need to travel a greater distance. Early reflections are those that reach the listener up to 50 ms (100 ms for large rooms).

Finally, the delayed reflections, which are the sound energy that reaches the listener after the time limit of 50 ms (100 ms for large rooms), contribute to the phenomenon of reverberation.

waterfall-plot

Time / Frequency 3D graph representation (waterfall plot)

RASTI ALcons (%) intelligibility
0-0.3 >15 bad
0.3-0.45 15-12 fair
0.45-0.6 12-8 good
0.6-0.75 8-4 very good
0.75-1 <4 excellent

Objective and subjective room criteria

The acoustic parameters mentioned above are measured before the study and after the acoustic treatment. Depending on the type and the usage of a room (e.g. music, speech), each acoustic parameter has a different impact on the design or treatment.

In an acoustic treatment of a room designed for speech (e.g. lecture room), the key acoustic feature is speech intelligibility, i.e. the clarity of the individual vowels and consonants of the speaker in any position in the room. In contrast, a concert hall requires a different and often more complex approach.

During those electroacoustic measurements, we can identify various acoustic quality problems such as but not limited to; standing waves, flutter echo, comb filtering, room resonance, etc.

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