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Distortion Measurement of a Closed-Box Loudspeaker at Low Fequencies without an Anechoic Chamber

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Harmonic and intermodulation distortions of a loudspeaker are important factors in assessing the loudspeaker system. These distortions are commonly measured in an anechoic chamber to avoid reflections from the walls. In this paper, a method to measure the distortions of the loudspeaker at low frequencies without an anechoic chamber is proposed. This method utilizes the fact that the n-th harmonic distortion outside the enclosure is boosted by 40 log n dB compared to that inside the enclosure. By compensating for the effect of standing waves occurring inside the enclosure, it is possible to predict the distortions for wide frequency ranges below the first break-up frequency of the diaphragm. The causes of prediction error are also analyzed. The experiment confirms the validity of the proposed method. The method is applicable to the measurement of intermodulation distortion as well.

Zusammenfassung

Harmonische und Intermodulationsverzerrungen eines Lautsprechers sind wichtige Faktoren der Beurteilung eines Lautsprechersystems. Gewöhnlich mißt man diese Verzerrungen in einem reflexionsfreien Raum, um Wandreflexionen zu vermeiden. In dieser Arbeit wird eine Meßmethode für die Lautsprecherverzerrungen bei tiefen Frequenzen vorgeschlagen, die ohne reflexionsfreien Raum auskommt. Sie geht von der Tatsache aus, daB die n-te harmonische Verzerrung auBerhalb des Gehauses im 40 log n dB gegeniiber dem Inneren angehoben wird. Durch Kompensation der Wirkung stehender Wellen innerhalb des Gehäuses ist es moglich, die Verzerrungen in weiten Frequenzbereichen bis herauf zur Frequenz der ersten Membranunterteilung vorherzusagen. Die Ursachen fur Vorhersagefehler wurden ebenfalls untersucht. Das Experiment bestatigt die Gültigkeit der vorgeschlagenen Methode, die auch auf Intermodulationsverzerrungen angewandt werden kann.

Sommaire

Les distorsions harmoniques et d'intermodulation sont des facteurs importants d'appréciation des haut-parleurs. On les mesure generalement en salle anechoique pour eviter l'effet des reflexions. Nous proposons dans cet article une methode de mesure de distorsions a des frequences basses qui ne necessite pas l'usage d'une telle salle. Cette methode utilise le fait que la distorsion du n-ième harmonique à l'exterieur de l'enceinte est accentuee de 40 log n dB par rapport à l'interieur. En tenant compte de l'effet des ondes stationnaires qui s'etablissent dans l'enceinte, il est possible de prévoir les distorsions pour un large intervalle de fréquences, dans la limite de la premiere fréquence de résonance du diaphragme. On analyse également les causes d'erreurs de prévision. La méthode s'applique aussi à la distorsion d'intermodulation.
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Document Type: Research Article

Publication date: September 1, 1994

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  • Acta Acustica united with Acustica, published together with the European Acoustics Association (EAA), is an international, peer-reviewed journal on acoustics. It publishes original articles on all subjects in the field of acoustics, such as general linear acoustics, nonlinear acoustics, macrosonics, flow acoustics, atmospheric sound, underwater sound, ultrasonics, physical acoustics, structural acoustics, noise control, active control, environmental noise, building acoustics, room acoustics, acoustic materials, acoustic signal processing, computational and numerical acoustics, hearing, audiology and psychoacoustics, speech, musical acoustics, electroacoustics, auditory quality of systems. It reports on original scientific research in acoustics and on engineering applications. The journal considers scientific papers, technical and applied papers, book reviews, short communications, doctoral thesis abstracts, etc. In irregular intervals also special issues and review articles are published.
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