THE APPLICATION OF ILLUSIONS AND PSYCHOACOUSTICS TO SMALL LOUDSPEAKER CONFIGURATIONS RONALD M. AARTS Philips Research Europe, HTC 36 (WO 02) Eindhoven, The Netherlands An overview of some auditory illusions is given, two of which will be considered in more detail for the application of small loudspeaker configurations. The requirements for a good sound reproduction system generally conflict with those of consumer products regarding both size and price. A possible solution lies in enhancing listener perception and reproduction of sound by exploiting a combination of psychoacoustics, loudspeaker configurations and digital signal processing. The first example is based on the missing fundamental concept, the second on the combination of frequency mapping and a special driver. INTRODUCTION applications of even smaller size this lower limit can A brief overview of some auditory illusions is given easily be as high as several hundred hertz. The bass which serves merely as a ‘catalogue’, rather than a portion of an audio signal contributes significantly to lengthy discussion. A related topic to auditory illusions the sound ‘impact’, and depending on the bass quality, is the interaction between different sensory modalities, the overall sound quality will shift up or down. e.g. sound and vision, a famous example is the Therefore a good low-frequency reproduction is McGurk effect (‘Hearing lips and seeing voices’) [1]. essential. An auditory-visual overview is given in [2], a more general multisensory product perception in [3], and on ILLUSIONS spatial orientation in [4]. The influence of video quality An illusion is a distortion of a sensory perception, on perceived audio quality is discussed in [5]. revealing how the brain normally organizes and Two illusion based applications will be considered in interprets sensory stimulation. While illusions distort more detail, in particular for the application of small reality, they are generally shared by most people [7]. loudspeaker configurations. Illusions can occur with each of the human senses, but visual illusions are the most well known and In many sound reproduction applications it is not understood. However, we will focus on auditory possible to use large loudspeakers, due to size and/or illusions. First some overview is given on auditory cost constraints. Typical applications are portable illusions and then we will discuss how to utilize them. audio, multimedia, TV and public address systems, to Two examples will be given; both have to do with bass name just a few, see [6]. Hence the devices are often reproduction. The first exploits the virtual pitch small in size, and therefore the transducers are phenomenon the second the inaccuracy of human pitch inherently small as well. Needless to say, the detection at low frequencies. competitive market also dictates these products achieve the highest possible audio quality. The most well- A special issue of the J. Audio Eng. Soc. on auditory known characteristic of small loudspeakers is a poor illusions is in [8]. Another overview or specific low-frequency (bass) response. In practice this means examples are given in [9,10]. Many auditory that a significant portion of the audio signal may not be demonstrations including some illusions are on a CD reproduced (sufficiently) by the loudspeaker. For [11] and a special CD on musical illusions and loudspeakers used in such applications reproduction paradoxes is [12]. On the Internet one can find the below 100 Hz is usually negligible, whereas in some most well-known demos [wl1-wl6]. ILLUSIONS IN SOUND – AES 22ND UK CONFERENCE 2007 7-1 AUTHOR Headphones vs. loudspeakers Shepard's of pitch circularity It might seem trivial, but an important difference One of the most widely used auditory illusions is between headphones and loudspeaker listening is that Shepard’s [25,26] demonstration of pitch circularity, the left channel sound radiated by a single left which has come to be known as the ‘Shepard Scale’ loudspeaker enters both ears, while with headphones demonstration. The demonstration uses a cyclic set of listening, this sound reaches the left ear only. This has complex tones, each composed of 10 partials separated many consequences for sound localization; this is well by octave intervals. The frequencies of the partials are treated in [13]. Another important difference is that shifted upward in steps corresponding to a musical with loudspeaker listening, room acoustics plays an semitone (≈ 6 %). This creates the auditory illusion of important role, including the change of phase relations a tone that continually ascends or descends in pitch, yet between the left and right signals or the phase between which ultimately seems to get no higher or lower. components within one signal. Jean-Claude Risset subsequently created a version of Spatial illusions the scale where the steps between each tone are Ordinary stereo reproduction is an example of a spatial continuous, and it is appropriately called the illusion. Between and sometimes outside the continuous Risset scale or Shepard-Risset glissando loudspeakers, sources are audible, while there are no [wl6]. When done correctly, the tone appears to rise (or ‘real’ sources present, this is also covered in [13]. descend) continuously in pitch, yet return to its starting note. Risset has also created a similar effect with 3D sound, rhythm in which tempo seems to increase or decrease See ‘Spatial illusions’ above, and [14]. endlessly. (Stereo) Base widening The glissando illusion Many schemes exist to give a virtual widening to The glissando illusion was first reported and loudspeaker base, see [15]. demonstrated by Diana Deutsch [27]. The auditory illusion is created when a sound with a fixed pitch, Elevation such as a synthesized oboe tone, is played together The impression that the source is perceived to be with a sine wave gliding up and down in pitch, and located above the plane in which it is actually radiating they are both switched back and forth between stereo is called elevation, see [16]. loudspeakers. The effect is that the oboe is heard as switching between loudspeakers while the sine wave is Missing fundamental heard as joined together seamlessly, and as moving There is a vast amount of literature on this topic see around in space in accordance with its pitch motion. e.g. [17]. If there are higher harmonics but the Right-handers often hear the glissando as travelling fundamental frequency is missing, we will still from left to right as its pitch glides from low to high, perceive the pitch of that missing fundamental, which and then back from right to left as its pitch glides from in this case is also called virtual pitch. An application high to low. Lefthanders often obtain different is the low frequency percept by small loudspeakers, see illusions. below in the section ‘Virtual Pitch’ and in [18, 19]. Continuity Doppler illusion The illusory continuity of tones [28, 29] is the auditory The Doppler illusion occurs if the sound source is illusion caused when a tone is interrupted for a short moving with respect to the listener; see [20,21]. Strictly time (approximately 50 ms or less), during which a speaking this is not an illusion but a physical effect; narrow band of noise is played. Whether the tone is of nevertheless it is usually mentioned as an illusion. constant, rising or decreasing pitch, the ear perceives the tone as continuous if the 50 ms (or less) Octave illusion discontinuity is masked by noise. The octave illusion discovered by Diana Deutsch in 1973 [22, wl3] is an auditory illusion produced by McGurk effect simultaneously playing two sequences of two notes The McGurk effect [1] is a perceptual phenomenon that are spaced an octave apart, high to low, and low to which demonstrates an interaction between hearing and high, in separate stereo channels over headphones. vision in speech perception. It suggests that speech People who are right-handed tend to hear the higher perception is multimodal, that is, it involves pitch as being in their right ear while the results are information from more than one sensory modality. This mixed for left-handed people, see also [23, 24]. effect may be experienced when a video of one phoneme's production is dubbed with a sound- 7-2 ILLUSIONS IN SOUND – AES 22ND UK CONFERENCE 2007 PAPER TITLE recording of a different phoneme being spoken. Often, Sound-induced illusory flashing the perceived phoneme is a third, intermediate This is a sound induced visual illusion. The illusion is: phoneme. For example, a visual /ga/ combined with an when a single flash is accompanied with two beeps, the audible /ba/ is often heard as /da/. Further research has single flash is perceived as two flashes. The illusion is shown that it can exist throughout whole sentences. strongest when the flash is in the periphery but it also The effect is very robust; that is, knowledge about it works in the fovea. This illusion was discovered by seems to have little effect on one's perception of it. Ladan Shams, Yukiyasu Kamitani, and Shinsuke This is different from certain optical illusions, which Shimojo [wl1]. break down once one ‘sees through’ them. APPLICATIONS Precedence effect Virtual Pitch This effect states that it is the first acoustic information Pitch is a subjective, psychophysical quantity. that arrives at a listener that determines the location of According to the American Standards Association a sound [13]. pitch is ''that attribute of an auditory sensation in terms of which sounds may be ordered on a musical scale''. Clifton effect For a pure tone, where the fundamental frequency The effect, named after Rachael Clifton [30], corresponds to the frequency of the tone, the pitch is introduces a change to the classical precedence unambiguous and-if we neglect the influence of sound demonstration. Half way into the click train, the source level on pitch-one can identify pitch with the frequency and echo clicks are reversed.