ALCons in Sound: What It Is, How to Calculate It, and Why It’s Key for Speech Intelligibility
Speech intelligibility is a critical parameter in the design of professional sound systems, auditoriums, conference rooms, and public spaces. To measure how much information a listener loses when hearing a spoken message, the %ALCons (Articulation Loss of Consonants) index is used, indicating the percentage of consonant articulation lost.
A low %ALCons means the audience clearly understands every word. Conversely, a high value indicates significant comprehension loss, which can negatively impact both user experience and safety in environments where public announcements are critical.
What is %ALCons?
%ALCons is a percentage value that measures the loss of consonant intelligibility in an acoustic environment.
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0% indicates perfect intelligibility.
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Increasing values reflect progressive loss of understanding.
Consonants, although acoustically less powerful than vowels, are essential for distinguishing words. If they are lost, entire sentences can become incomprehensible, even if vowels are heard clearly.
Factors Affecting %ALCons
Several acoustic and technical elements influence %ALCons in sound:
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Reverberation Time (RT60): A room with excessive reverberation masks consonants.
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Source-to-listener distance: The farther the listener, the more reverberant sound dominates over direct sound.
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Room geometry and volume: Large, reflective spaces increase intelligibility loss.
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Sound source directivity (Q): Speakers with good directivity improve the ratio of direct sound.
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Signal-to-noise ratio (SNR): Background noise can obscure consonants, reducing message clarity.
Formulas for Calculating %ALCons
There are different methods to calculate %ALCons, the most well-known being the Peutz model, which relates intelligibility loss to:
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Reverberation time
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Listener distance
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Room volume
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Sound source directivity
Other algorithms also incorporate signal-to-noise ratio and the balance between direct and reverberant sound fields.
%ALCons Ranges and Interpretation
| %ALCons | Intelligibility Level | Practical Application |
|---|---|---|
| 0–3% | Excellent | Recording studios, sound control |
| 3–7% | Good | Conferences, classrooms, acoustically treated theaters |
| 7–15% | Acceptable | Multipurpose spaces, standard PA systems |
| 15–33% | Poor | Difficult comprehension, not recommended in emergencies |
| >33% | Unintelligible | Communication practically impossible |
For most professional sound applications, it is recommended to keep %ALCons below 10–15%. In critical environments, such as emergency evacuations or teaching, the target should be ≤ 5%.
Practical Example of %ALCons Calculation
In a conference room of 1800 m³, with:
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Reverberation time: 1.2 s
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Listener distance: 8 m
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Signal-to-noise ratio: +25 dB
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Speakers with moderate directivity (Q=2)
The estimated %ALCons is between 8–12%. This implies acceptable intelligibility, although it could be improved with acoustic treatment (reducing reverberation to 0.7 s) or by bringing sound sources closer to the audience.
How to Reduce %ALCons and Improve Intelligibility
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Acoustic treatment: Absorptive panels and diffusers to reduce reverberation.
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Directional speakers: Greater control over sound dispersion.
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Coverage design: Place additional reinforcement points closer to the audience.
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Background noise reduction: Minimize noisy ventilation or machinery.
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Mix optimization: Emphasize key frequencies (2–4 kHz).
Conclusion
%ALCons in sound is a key indicator for evaluating speech intelligibility in any space. Understanding and controlling it is fundamental for professional acoustic design, sound system installation, and public safety.
Maintaining low %ALCons values ensures every word reaches the listener clearly, improving communication experience and audio system effectiveness.
By actively monitoring and optimizing %ALCons, sound engineers and venue designers can ensure that every spoken word is delivered with maximum clarity. This not only enhances audience engagement and comprehension but also strengthens safety measures in public spaces. Implementing proper acoustic strategies and high-quality sound systems can significantly reduce articulation loss, creating an optimal listening experience for all attendees.