1. Disputes about TV occur everyday in many homes [1], where young listeners require
a lower volume than that needed by older listeners to improve speech intelligibility
[2]. Whilst the vast majority of TVs produce an omnidirectional radiation pattern,
this could be improved by boosting the sound received by the hearing impaired
listener using personal sound [3]. This personal sound is created by a directional
loudspeaker array that reinforces the sound produced by the TV in a certain
direction. An example of this can be observed in Fig. 1, where the sound radiated by
the TV is shown in blue, and the sound radiated by the directional source is shown in
red, and is focused towards the hearing impaired listener.
The performance of the line array has been measured in a low reverberation
environment, representative of a carpeted living room. The set up of the
measurements performed can be observed in Fig. 3., together with the measurement
geometry used to test the array. This measurement geometry is formed by 8
microphone positions, representing the ears of 4 viewers. As performance metric,
the acoustic contrast between a hearing impaired and 3 normal hearing viewers is
used.
At the moment the subjective performance of the array is being evaluated using a
hearing loss simulator, emulating the hearing capabilities of older listeners. The aim
of the experiments is to show that the speech recognition of a “hearing impaired”
listener placed on the beam of the array, is similar to that of a young listener outside
of the beam of the array. The conditions of the experiment are shown in Fig 6.
1. V. Yule, “Golden Oldies”, New Scientist, p 32. March 2011, Opinion letter.
2. Coren, S., MOST COMFORTABLE LISTENING LEVEL AS A FUNCTION OF AGE. Ergonomics, 1994. 37(7): p. 1269-1274.
3. Druyvesteyn, W.F. and J. Garas, Personal sound. Journal of the Audio Engineering Society, 1997. 45(9): p. 685-701.
4. L. E. Kinsler, A. R. Frey, A. B. Coppens and J. V. Sanders, Fundamentals of Acoustics, L.E. John Wiley & Sons, 2000.
5. Simón Gálvez, M.F. and S.J. Elliott, The design of a superdirective array in a room, in AES 52nd conference in sound
field control, 2013: Guildford, UK.
6. Simón Gálvez, M.F., S.J. Elliott, and J. Cheer, A superdirective array of phase shift sources. The Journal of the
Acoustical Society of America, 2012. 132(2): p. 746-56.
7. International Standards Organization, ISO 7029-Statistical distribution of hearing thresholds as a function of age, in
Acoustics, 2000.
8. Sandlin, R.E., Hearing Aid Amplification, 2000, Singular, Thomson Learning: San Diego. p. 70-73.
The aim is to achieve a beam of sound in the required direction without too much
excitation of the reverberant field. To this end:
• Individual elements are constructed as phase shift devices, with hypercardioid
directivity.
• Columns of elements are driven in phase to give greater vertical directivity.
• Each of the elements across the array is driven by a superdirective beamforming
algorithm.
Figure 1. Directional radiator (red) boosting the sound produced by the TV (blue) which is uniform all
around the space.
The beaming of the array’s energy towards the bright zone (the zone where the
hearing impaired person is placed) is very important, considering that when the array
is introduced inside a normal, reverberant environment, reflections arise from all of
the room surfaces, which reduces the original directivity of the array[5].
The array presented here uses 32 sources. The sources are set in vertical groups of 4,
so that each group of 4 vertical sources is driven by an independent digital filter. This
allows the creation of a steerable beam in the horizontal plane, whilst a fixed beam
is obtained in the vertical plane, which minimizes reflections from the floor and
ceiling. The radiation from the back of the array is minimized by using phase-shift
sources, a special kind of radiator that produces a hypercardioid directivity [6].
To further improve the operation of the array, digital signal processing is applied to
the input of each group of 4 vertical sources. A Least Squares optimization that
maximizes the acoustic contrast between the hearing impaired and normal hearing
listeners is used to create the 8 filters of the array. Using 8 independent channels
instead of 32 lowers the cost of the array, and avoids poor robustness.
Figure 3. Superdirective array with 32 sources, arranged in groups of 4 vertical sources.
A highly directional loudspeaker array has been designed and built using a reduced
number of independent sources. The array has been shown to give a high acoustic
performance in a low reverberant environment. Whilst the subjective performance is
currently being evaluated, the measured acoustic results show a performance that
should be enough to perform the required boost in speech intelligibility.
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Figure 6. Conditions to be used on the subjective tests.
Figure 4. Set-up of the measurements in the reverberant environment (left), and
measurement geometry used to estimate the array’s performance (right). The blue ears
represent young listeners, and the red ears represent the bright zone where the hearing
impaired old listener is placed.
The directivities that the array obtains in the reverberant environment of Fig. 4 are
shown in Fig. 5, together with the acoustic contrast between the young and old
listeners shown in the right hand side plot of Fig. 4. The resulting acoustic contrast is
similar to the amplification prescription required by a 70 years old listener [7,8].
Marcos Felipe Simón Gálvez1 and Stephen J. Elliott1
1 Signal Processing and Control Group, Institute of Sound and Vibration Research, University of Southampton, Southampton, Hampshire,
SO17 1BJ, United Kingdom.
Figure 5. Directivities of the line array at different frequencies measured in the reverberant
environment (left) , and the acoustic performance (right).