Propagation Delay

The speed of sound in air depends primarily on temperature: c = 331.3 + 0.606 × T (m/s), where T is the temperature in Celsius. At 20°C, c = 343.5 m/s. At 30°C (common in outdoor venues), c = 349.5 m/s — about 2% faster. Humidity has a smaller effect (about 0.3% increase at 80% relative humidity) and is usually ignored. For system alignment, the practical rule is approximately 1 ms per foot (or 3 ms per meter). A speaker 10 meters from the listener has a propagation delay of about 29 ms. A delay tower 50 meters from the main PA system needs approximately 146 ms of electronic delay to synchronize with the main speakers. The Haas effect (precedence effect) states that the first-arriving sound determines the perceived direction of a source. When a delayed speaker is within 25-35 ms of the main speaker, the listener perceives the sound as coming from the first-arriving source (the mains) even if the delayed speaker is closer and louder. Delay towers exploit this by adding electronic delay equal to the propagation delay from the mains, plus a few milliseconds of offset, so the mains always arrive first. Temperature changes during outdoor events shift propagation delay. A 10°C temperature change alters the speed of sound by about 1.8%, which at 50 meters translates to approximately 2.6 ms — enough to misalign subwoofers at 100 Hz. This is why outdoor systems require re-alignment as conditions change. In measurement, propagation delay introduces a linear phase component that must be removed ("delay compensation") before analyzing the intrinsic phase response of the device under test. SonaVyx detects the propagation delay from the impulse response peak and offers automatic compensation.