Binaural

3.3.2.1

Binaural: Fundamentals

Binaural: “having or relating to two ears”. Binaural recording is a form of stereo recording that takes into account a head-related transfer function thus allowing the listener to localise sound sources. Recordings can be made using a dummy head with two external microphones, internal microphones at the centre of the model outer ears, or at the inner end of a model ear canal. Furthermore, b-format recordings may be decoded to binaural [89]. In short, this complex process involves decoding the b-format signals to virtual loudspeakers whose output is modified by a head-related transfer function (HRTF) relating to the relative postion of the virtual loudspeaker and virtual listener.

Playback is normally through in-ear or head phones. Closed headphones will offer the best insulation. It is possible to use two loudspeakers with crosstalk cancellation but the channel separation cannot reach 100% and unwanted colouration occurs [90]. Ultradirection loudspeakers are theoretically valid for binaural playback but, again, further development is required.

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Headtracking can be used with headphone listening. The movement of the listener’s head is monitiored and the signal outputs are modified accordingly such that the soundfield appears static relative to the physical listening environment. This is easiest performed with a simulated soundfield of virtual sound sources (within a programming environment such as MaxMSP or Visual Studio). The location of the virtual sources relative to the virtual listener determines the HRTF by which the signal is modified. Moving the relative postion of the virtual sources and virtual listener – cued by the headtracker – utilises the relavant HRTF. B-format recordings can be decoded to binaural in a similar fashion to simulated soundfields (whereby the virtual loudspeakers are the virtual sources).

3.3.2.2

Choosing Binaural

The use of binaural technology in this research stemmed from the need for Phase Three’s real-time in situ reproduction. Thus isolating the effect of the ‘electroacoustic ear’ enables a more detailed understanding of how soundscape perception in situ and reproduction soundscape perception differ. An ambisonic configuration would have several drawbacks for an application such as Phase Three. The captured soundfield and the real-time reproduced soundfield would need to be far apart to avoid feedback. Also there would always be the background noise – something that earphones are able to reduce. Most significantly, the use of binaural provides an interesting comparison to ambisonics.

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3.3.2.3 Physical Setup

The Phase Two reproduction was of a two minute clip on continual repeat. The recording was made using a Brüel & Kjær dummy head, Brüel & Kjær pre-amplifier, and Zoom 2- channel digital recorder. The dummy head was positioned on a concrete bench, with foam under its metal supports in an effort towards insulating from any ground vibration. The location of recording was only several meters from where the ambisonic recordings were previously made and thus of a similar soundscape.

The clips were played from a laptop through an M-Audio 2-channel soundcard and Sennheiser CX300ii in-ear headphones. The participants also wore a pair of 3M ear defenders. The same reproduction equipment was used for Phase Two and Three – the only difference being that in Phase Three the binaural capture was relayed live. Acoustically insulating the binaural reproduction for the participants was crucial for Phase Three and whilst the listening room binaural test of Phase Two did not require such insulation, it was important to retain continuity. (The ear defenders affect the frequency response as discussed in section 3.3.1.4.) During Phase Three, a 98Hz high-pass filter was implemented to avoid wind noise.

The combination of in-ear headphones and ear defenders was selected as the best means of blocking out the non-electroacoustic soundscape and hence isolating the binaural reproduction. Whilst open headphones tend to offer better sound quality, closed

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headphones offer better isolation. Basic testing with the closed headphones available to the researcher concluded that none had sufficient enough isolation such that the non- electroacoustic soundscape would be inaudible. Furthermore, the soundscape is too varied for active noise cancelling to be a viable option.

3.3.2.4

Calibration

The earphones/ear defenders combination was calibrated at a reference of 1kHz. A calibration signal of 1kHz at 80dB was played directly into the left microphone of the Brüel & Kjær dummy head, with the outer ear removed and the correct adapter to give the calibrator a flush fit. Using the same equipment and setting as per the field recording, the calibration signal was recorded (r1). The same calibration signal of 1kHz at 80dB was played directly into the left ear of the Kevlar dummy head and, again, recorded (r2). This signal, r2, was fed back to the laptop through the M-audio soundcard and analysed with the software WigWare. A note was made of the sound level identified at 1kHz, Lr2 (which was greater

than 80dB). The r1 signal was then played back from a laptop through the M-Audio soundcard and the Sennheiser CX300ii left earphone. The earphone was placed in the intricately modelled ear canal of a Kevlar dummy head, with the 3M ear defenders over the top. The output signal, r3, was sent back to the laptop where the sound level was compared to that of the input signal, r1. For Phase Two, the binaural soundfield recordings were adjusted accordingly, with the software Audacity, by reducing the level by LN dB, where LN

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Both the Brüel & Kjær and the Kevlar dummy heads offer flat frequency responses (variation < 1dB). The earphones were expected to peak at 5kHz and dip towards the upper and lower ends of the audible frequency spectrum. However, the effect of the ear defenders was unknown and therefore investigated. White noise at 90dB was sent to the earphone fitted in the left ear of the Kevlar dummy head, returning a signal that was analysed using WigWare’s fast Fourier transform analyser, ‘FFT Analyzer’. Screenshots showing the frequency response of the earphones in the Kevlar dummy head without ear defender can be found in section 7.1.1.1 of the appendices and with ear defenders in 7.1.1.2. The effect of the ear defenders was to increase the overall levels as well as raise the level of the upper and lower ends of the audible frequency spectrum.