Project 184: The Inaudible Loudspeaker -
Unmasking audio sound information by applying monaural stereophonics

Datum bijeenkomst:
dinsdag 3 april 2007
aanvang 20:00 uur (19:30 uur koffie)

The Inaudible Loudspeaker

Stan Tempelaars, Leo de Klerk

RBS Studio
Aelbrechtskade 129
3023 JG Rotterdam
tel 010 4257477

Openbaar vervoer
RBS studio is te bereiken met tram 4 richting Marconiplein vanaf het centraal station in Rotterdam
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Martin Goosen, zie bestuur.

Bij voorkeur aanmelden via de website. Of stuur een mailtje of brief naar de secretaris (zie bestuur).

Stan Tempelaars zal het luidspreker octrooi inleiden van zijn vroegere leerling aan het Haags Conservatorium Leo de Klerk.
Stan Tempelaars, al weer enige jaren met pensioen, is nog altijd aktief in zijn vakgebied de psycho-akoestiek, en is met name geïnteresseerd in de invloed van de oorschelp op het gehoor. Tijdens het doornemen van het octrooi en het luisteren naar Leo de Klerk's luidsprekercontraptie, noemde hij het 'de luidspreker die je niet hoort'.

We kunnen ons hierover op de bijeenkomst, aan de hand van een prototype, zelf een oordeel vormen.

Leo de Kerk zal u vertellen welke problemen hij tegenkomt bij het gebruik van luidsprekers, en hoe hij met de hulp van theorieën van wetenschappers als van Békézy en Moore tot zijn ongewone maar wel originele toepassing is gekomen. De hierna volgende Engelse tekst licht dit toe ( tijdens deze bijeenkomst zal echter Nederlands gesproken worden):

A new application for co-axial loudspeakers

We can not recognise the size of the sound source in the reproduced sound
We are all familiar with Von Bekésy’s problem (1960): the ‘in the box sound’ effect seems to increase with the decrease of the loudspeaker’s dimensions. In an experimental research on the relation between acoustic power, spectral balance and perceived spatial dimensions and loudness, Von Bekésy’s test subjects were unable to correctly indicate the relative dimensional shape of a reproduced sound source as soon as the source’s dimensions exceeded the actual shape of the reproducing loudspeaker box. One may conclude that the loudspeaker’s spatio-spectral properties introduce a message-media conflict when transmitting sound information.

We listen to the properties of the loudspeaker
Why does the ear lock so easily to the loudspeaker characteristics? Based on the hypotheses of Brian Moore (2002) et al., we may conclude that this is because the ear, by nature, produces two dimensional nerve signals to the brain that reflect the three-dimensional wave interference due to direct interaction of both the ear’s and the physical sound source’s spatio-spectral properties. Therefore any three-dimensional spatio-spectral ‘message’ information embedded in the signal to be transmitted is masked by the physically present ‘media’ information related to the loudspeaker’s properties. One could say that in prior art solutions the input signal of the loudspeaker system merely functions as a carrier signal that modulates the loudspeaker’s characteristics.

A phantom sound source appears to be more realistic
We all experience a ‘phantom’ sound source reproduced by a stereo-system to appear more realistic than the same sound reproduced from one box. However, spatial sound reproduction by means of stereophonic ‘virtual sources’ is a partial solution because it fails for stereophonic information that does not meet the L=R requirement for perfect phantom imaging. In fact stereophonics is no more than improved mono with the listening area restricted to a hot spot.

A new approach
For a new approach of this problem we considered that one better could overrule the monaural spatio-spectral coding than trying to manipulate it (as in crosstalk suppression or Bose-like diffusion approaches), thus preventing inter-aural cross correlation which is the root mechanism for the listener’s ability to localize any non-virtual sound sources i.e. loudspeakers.

A phantom loudspeaker
The resulting patent application describes a sound system that produces coherent vertical phantom images that cannot monaurally nor binaurally be decomposed to their root sources i.e. the actual loudspeaker drivers. The loudspeaker becomes audibly non-localizable and visually transparent and any multi-channel stereophonic horizontal configuration of these virtual speakers is possible without adding masking ‘media’ properties. As a consequence the listening area is not anymore restricted to a hot spot. Loudness perception is dramatically improved because the huge ‘out of the box’ sound screen will now fit to even the largest sound source shapes.

A rotation symmetrical response
For the production of a coherent vertical phantom image that also must be able to interact coherently in the horizontal plane in order to produce stereophonic imaging, a driver structure with a rotation symmetrical off-axis response is a main requirement. Therefore our prototypes are constructed from modified co-axial drivers. The modification consists of a quite simple altered implementation of existing driver parts and does basically not need re-engineering of the chassis, however, the modification offers the advantage to implement further refining features (for instance mass-less Motional Feedback sensing) that were difficult to apply in the original design.

An innovative backward compatible sound system
Conclusively we may state that it is possible to create an innovative and non-conventional sound system that is fully backward compatible with existing audio reproduction formats. The system removes the localizability of any boxes from the equation regardless of the listener’s individual hearing properties, provides for a social listening area and improved loudness at equal acoustic power and encourages new approaches in design. The invention may serve a wide range of applications such as Home Theatre, Public Address, Music Theatre, Paging, Personal Monitoring and Auditory Display in general.