Near-field sound measurement is unreliable when multiple source parts radiate simultaneously

Evidence

When sound radiates from a complex source like a loudspeaker what is measured and heard at different frequencies is different at short distances and long distances. Within what is called the near field the sound measured at any point is uncertain if multiple parts or portions of the source are simultaneously radiating at the same frequency. Acoustical interference between and among sounds from the radiating elements cause position-dependent irregularities, and both microphone angular orientation and distance become variables in what is measured. In the case of loudspeakers, we need directivity information, but would like it to be independent of microphone distance. Clearly the size of the “source” is a key variable, but in loudspeakers the source size varies with frequency. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1262

Categorization

• XREF: Interference between radiating elements causing position-dependent irregularities parallels the multi-speaker comb-filtering and interference physics covered elsewhere in this book.