Procuring an omnidirectional source: why check directivity by frequency band?

When procuring equipment for reverberation-time or building-acoustics measurements, a dodecahedral enclosure can make “omnidirectional” seem like an obvious box to tick. Its shape describes an arrangement, however, rather than proving that the source meets the selected method in the target bands. The procurement decision should be based on verifiable directivity, output capability and system configuration.

Define the task before the frequency range

ISO 3382-1 covers reverberation time and other room-acoustic parameters in performance spaces; ISO 3382-2 covers reverberation time in ordinary rooms; ISO 16283-1 addresses field measurements of airborne sound insulation between two rooms in a building.[1–3] A general “for building acoustics” description does not establish suitability for all these tasks.

The specification should identify the method and edition, lowest and highest analysis bands, and whether the analysis uses octave or one-third-octave bands. Check the stated frequency-response range, verified directivity range and usable output range separately. Frequencies at which the device produces sound are not necessarily bands in which its measurement suitability has been demonstrated.

Why a polyhedral source can still be directional

A real source has physical dimensions and multiple radiating elements. Research shows that a dodecahedron is an approximation to an omnidirectional source, and that interference in the array's sound field can cause departures from omnidirectionality as frequency changes.[4] A low-frequency polar plot therefore cannot substantiate the entire target range, and overall frequency-response equalization does not demonstrate uniform radiation in every direction.

For example, HBK's original source documentation presents band sound power, directivity evaluations and horizontal-plane polar plots at different frequencies separately.[5] These are manufacturer product data, not the standard itself. When reviewing a report, also check the measurement plane, distance, bandwidth and angular-averaging method. A horizontal-plane plot alone is not a complete qualification assessment.

High overall output can leave weak bands

As an engineering practice, examine the sound-power spectrum in the target bands, then establish the margin of the useful signal above background noise in the intended space. Reverberation measurements need a usable decay segment; sound-insulation measurements also depend on the signal in the receiving room. A broadband overall level does not guarantee sufficient margin in every band.[5]

Distinguish source sound-power level from sound-pressure level at a particular position, checking reference quantities and conditions when comparing data. Record the amplifier, excitation signal, equalization preset and sustained operating state as well. NTi's original manual distinguishes equalized and non-equalized signals and describes clipping, overheating and protective output reduction.[6] The loudspeaker model alone is insufficient to reproduce the complete system's performance.

Carry the limits into acceptance

The handover package should include directivity evidence for the target bands, output data for the corresponding configuration, and band-specific background-noise and trial-measurement records from a representative site. Trial measurements establish task feasibility; they do not replace source qualification under the specified conditions.

Check the full applicable standard for the project's tolerances and measurement arrangements; this article introduces no universal thresholds. If the existing configuration covers the task, an upgrade to maximum power is not automatically necessary. If a gap remains, identify the affected bands and conditions before deciding on additional verification or a configuration change.

Sources

[1] ISO 3382-1:2009, official scope. https://www.iso.org/standard/40979.html

[2] ISO 3382-2:2008, official scope. https://www.iso.org/standard/36201.html

[3] ISO 16283-1:2014, official scope. https://www.iso.org/standard/55997.html

[4] Quested et al., An analytical model for a dodecahedron loudspeaker applied to the design of omni-directional loudspeaker arrays, Applied Acoustics 85 (2014), public abstract. https://www.sciencedirect.com/science/article/pii/S0003682X14000875

[5] HBK, OmniPower Sound Source Type 4292-L, Product Data BP 2667–12, pages 2–4. https://www.bksv.com/media/doc/bp2667.pdf

[6] NTi Audio, Dodecahedron Speaker Set User Guide, pages 1–3. https://www.nti-audio.com/wp-content/uploads/download-manager-files/NTi-Audio-Dodecahedron-Speaker-Set-User-Guide.pdf

Sources checked: 25 October 2026.