Acoustic Treatment vs Soundproofing: Differences & Which is Best

Acoustic treatment and soundproofing have different metrics to measure their effectiveness or desired outcome.

Acoustic treatment uses the following metrics to quantify and assess the sound quality of a given space:

• Reverberation Time (RT60): the most important parameter in acoustics and is used both in acoustic treatment and soundproofing methods. It measures how long (in seconds) it takes for sound to decay to 60 decibels in a given space. Different rooms have different ideal RT60 values depending on the purpose of the room.

Ideal RT60 values are:

• Home Theater: 0.3 – 0.6 seconds
• Recording Studio: 0.3 – 0.6 seconds
• Classroom: 0.3 – 0.6 seconds
• Office: 0.5 – 1.0 seconds
• Conference Room: 0.5 – 0.7 seconds
• Concert Hall: 1 – 2 seconds
• Church or Worship Space: 1.5 – 2.5 seconds
• Restaurant: 0.8 – 1.2 seconds
• Hospital Wards: 0.3 – 0.7 seconds
• Gymnasium: 0.4 – 0.6 seconds
• Auditorium: 1.5 – 2.5 seconds

Early Decay Time (EDT): measures how long (in seconds) it takes for sound to decay the first 10 decibels in a given space.

Clarity Index (C50 or C80): helps assess how clear or distinct speech or music is in a given space. Technically it’s the ratio of energy between early reflections (occurring within 50ms or 80ms) and later arriving reverberant sound. It measures the clearness and definition of the sound source.

Sound Pressure Level (SPL): quantifies the amplitude of sound waves in decibels (dB) to determine if a room has spots of excessive loudness or dead spots where sound waves cancel each other.

Frequency Response: measures how a room treats different frequencies. Ideally an acoustically treated room will have a balanced frequency response that doesn’t overemphasise or suppress any particular range of frequencies.

Speech Transmission Index (STI) or Speech Intelligibility Index (SII): STI or SSI aims to quantify how well a listener would understand spoken words in a given space. It considered the clarity and quality of the speech itself as well as background noise and reverberation that might affect it from being understood. It ranges from 0 to 1 with higher values indicating more understandable speech.

Subjective assessment: acoustic engineers usually use subjective evaluations to give final recommendations.

 Soundproofing methods use the following metrics to quantify and assess the sound quality of a given space:

• Sound transmission class (STC): quantifies the effectiveness of a material or construction element in reducing airborne sound transmission. STC is the most important metric for evaluating the soundproofing capabilities of walls, ceilings, floors, doors and windows.

Higher STC rating indicates better soundproofing performance. For example, a wall rated at STC 30 will allow most sentences to be understood easily from one side to the other, while a wall with an STC rating of 60 should block loud conversational or ambient noise.

Common STC values are:

• Bedrooms: STC 45-50 – minimise conversation and TV noise from adjacent spaces.
• Bathrooms: STC 40-45 – Sufficient to muffle most household noises.
• Open Plan Offices: STC 35-40 – Adequate for a general buzz but not for speech privacy.
• Closed Offices: STC 45-50 – Designed for confidentiality and minimal disturbance.
• Conference Rooms: STC 50-55 – Higher STC values are necessary for confidential conversations.
• Classrooms: STC 45-50 – Necessary for good speech intelligibility and concentration.
• Lecture Halls: STC 50-55 – To ensure audibility even in the back rows and to minimise disturbance from adjacent spaces.
• Healthcare Patient Rooms: STC 40-45 – To provide a quiet environment for rest.
• Consultation Rooms: STC 45-50 – For patient confidentiality.
• Theatres: STC 60-65 – To completely isolate from external noise and ensure a pure auditory experience.
• Recording Studios: STC 55-65 – These values help to prevent sound leakage, maintaining recording integrity.
• Factories: STC 40-45 – To reduce machinery noise spreading to office spaces or the exterior.
• Airports, Train Stations: STC 40-50 – To minimise engine noise, announcements, and people.

Noise Criterion (NC) or Noise Rating (NR): a measure of the noise level within a space. Lower numbers are quieter.

Impact Insulation Class (IIC): evaluates the sound insulation of impact noises such as footsteps or dropped objects. Higher IIC numbers indicate better impact sound insulation.

Field Sound Transmission Class (FSTC): measures sound reduction on site and is a way of evaluating the effectiveness of installed soundproofing installations. Higher numbers mean better soundproofing

Sound Reduction Index (SRI): similar to STC but mostly used in Europe. Measures the reduction of sound passing through a material at various frequencies in decibels (dB).