Question 1

What is C50 and how does it relate to speech clarity?

Accepted Answer

C50 (Clarity, 50ms) is the ratio of early energy (first 50 ms after direct sound) to late energy (after 50 ms), expressed in dB: C50 = 10 × log10(E_early / E_late). Higher C50 means more energy arrives early, which improves speech clarity. A C50 above 2 dB is generally considered adequate for speech. Values above 5 dB indicate excellent clarity. Negative C50 means more energy arrives late (excessive reverberation), making speech muddy. C50 is measured from the impulse response and is related to RT60 but provides additional information about the balance between early and late energy. A room can have acceptable RT60 but poor C50 if early reflections are weak or misdirected. SonaVyx calculates C50 automatically from each impulse response measurement.

Question 2

What is C80 and why is it used for music?

Accepted Answer

C80 (Clarity, 80ms) uses the same formula as C50 but with an 80ms boundary between early and late energy. The longer window accounts for the fact that music tolerates more reverberation than speech before it becomes unclear. C80 above 0 dB is acceptable for symphonic music. C80 of 1-5 dB is preferred for most genres. Above 5 dB can sound dry for classical music but may be appropriate for amplified music. C80 below -2 dB indicates excessive late energy, making musical passages blurred. C80 and RT60 are inversely related (shorter RT60 means higher C80), but the relationship is not linear because C80 also depends on the room geometry, which determines early reflection patterns. SonaVyx reports both C50 (speech) and C80 (music) from each impulse response.

Question 3

What is D50 (Definition) and how does it differ from C50?

Accepted Answer

D50 (Definition or Deutlichkeit) is the ratio of early energy to total energy, expressed as a percentage: D50 = E_early(0-50ms) / E_total × 100%. While C50 is the log ratio of early to late, D50 is the linear ratio of early to total. D50 above 50% (equivalent to C50 above 0 dB) is considered the threshold for adequate speech intelligibility. In practice, D50 and C50 carry the same information presented differently. D50 is used more in European standards and older literature, while C50 is preferred in ISO 3382. The advantage of D50 as a percentage is its intuitive interpretation: D50 of 70% means 70% of the sound energy arrives within the first 50 ms. SonaVyx calculates both and displays them in the impulse response analysis panel.

Question 4

What is Ts (Centre Time) and what does it indicate?

Accepted Answer

Ts (Centre Time) is the time at which the cumulative energy of the impulse response reaches 50% of the total energy. It represents the temporal center of gravity of the room response. Lower Ts indicates that energy arrives earlier on average, correlating with better clarity and intelligibility. Typical values: below 60 ms is excellent for speech, 60-100 ms is acceptable, above 100 ms indicates a reverberant space where speech will be difficult. Ts correlates well with both C50 and STI and has the advantage of being a single number in milliseconds that is intuitive to understand. Unlike C50 and C80 which use a fixed time boundary, Ts adapts to the actual energy distribution. SonaVyx displays Ts alongside other ISO 3382 parameters in the impulse response results panel.

Question 5

How do I identify early reflections in an impulse response?

Accepted Answer

Early reflections appear as discrete peaks in the impulse response within the first 50-80 ms after the direct sound. On the IR waveform plot, the direct sound is the first and usually largest peak. Subsequent peaks within 50 ms are early reflections from the nearest surfaces: floor, ceiling, side walls, and any nearby objects. The time delay of each reflection corresponds to the extra path length: a reflection arriving 3 ms after the direct sound traveled an extra 1.03 m. Early reflections are beneficial for speech intelligibility (they reinforce the direct sound) but can cause coloration if a single strong reflection arrives within 1-5 ms (creating comb filtering). SonaVyx IR display marks the direct sound arrival and highlights the early reflection window (0-50 ms) versus the late reverberation.

Question 6

What is Sound Strength G and why is it important?

Accepted Answer

Sound Strength G (ISO 3382-1) is the ratio of the sound energy at the measurement point to the energy that the same source would produce at 10 m in a free field, expressed in dB. It quantifies how much the room amplifies the source. G typically ranges from 0 dB (very absorptive or large room) to 10+ dB (small reverberant room). Higher G means the room provides more acoustic gain, which is desirable in unamplified performance spaces (concert halls, theaters) where the audience needs to hear the performer without amplification. G depends on both room volume and RT60: smaller, more reverberant rooms have higher G. ISO 3382 specifies G as a required parameter for performance space evaluation. SonaVyx calculates G when a reference impulse response is provided.

Question 7

How do I detect flutter echo from an impulse response?

Accepted Answer

Flutter echo is a rapid repetitive reflection between two parallel surfaces. In the impulse response, it appears as a series of evenly-spaced decaying pulses. The time spacing equals twice the distance between the parallel surfaces divided by the speed of sound. For example, walls 5 m apart produce flutter pulses every 29 ms (10 m round trip / 343 m/s). Flutter echo decays more slowly than the diffuse reverberation because the energy is trapped between two surfaces rather than being distributed throughout the room. On the IR waveform, you can visually identify the regular repetitive pattern. Frequency-domain analysis shows comb filtering with dips spaced at 1/delay. Treatment: add absorption or diffusion to one of the parallel surfaces. SonaVyx problem detector identifies flutter echo patterns automatically.

Question 8

Can I derive the frequency response from an impulse response?

Accepted Answer

Yes. The frequency response is the Fourier transform (FFT) of the impulse response. This is mathematically exact: taking the FFT of the IR gives you the complex transfer function H(f), from which you can extract the magnitude response (gain at each frequency) and phase response (delay at each frequency). This is why a single IR measurement contains all the information about the system. You can also window the IR before taking the FFT to analyze specific time segments: a short window around the direct sound gives the anechoic speaker response, while a longer window includes room reflections. This technique is used in loudspeaker measurement to separate the speaker response from the room response. SonaVyx IR mode shows the derived frequency response as a secondary plot below the IR waveform.

Impulse Response Analysis: C50, C80, D50, Ts, and ISO 3382 Parameters

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