Loop Gain—x2 It’s all about listening!


Last month’s piece about hearing aids and assistive listening devices seems to have struck a chord with many readers and, in an all-time first for this column, even the editor offered his opinion! There is no question that hearing loss is a growing and debilitating problem that can lead to the social exclusion of many people, not just the elderly, although it is perhaps more associated with that demographic. The need to understand speech is universal but for those who can hear, it is often difficult to imagine what those with a hearing loss or reduced hearing sensitivity have to endure. An evening out in a noisy restaurant or bar is about the closest many of us may get to this, where the background noise level can render conversation almost impossible…but now imagine this occurring all the time and as a one-way experience with everyone else around you still happily conversing.

However, merely installing an assistive listening system is only half the job. Although we have plenty of assistive listening systems over here in the UK (and elsewhere in Europe), too high a percentage of them either do not work or are not effective. For example, many stores display a hearing-loop-assistance symbol at their tills or customer-service counters. A quick visual scan, though, often reveals there to be no microphone in sight. And if you ask the staff whether the loop system is working, they all too often do not have a clue what you are talking about.

Figure 1. Frequency response of AFILS before and after correction.
Figure 1. Frequency response of AFILS before and after correction.

Education at all levels is therefore needed. Assistive listening systems, however, can be remarkably effective and have literally changed people’s lives. Although this often applies to fixed installations, such as a potential user’s local house of worship or theater/concert hall, here in the UK, all (and I mean all!) cinemas (OK, movie theaters to you) are fitted with deaf-aid loop systems and so enable this leisure activity to be enjoyed by a wider audience.

Apart from the social aspect to this, it also makes extremely good business sense, because now a wider audience can potentially be attracted to attend. To be effective and to provide their potential benefits, assistive listening systems must be installed and operated correctly. This means that installers and specifiers actually need to think through what it is these systems are intended to do: that is, to “collect and transmit intelligible speech” to a person with impaired hearing. This means that the sound pick-up must be impeccable. Yet, many times no thought whatsoever seems to go into the design or specification of such systems.

In many live-performance theaters, for example, the assistive listening system is merely fed from the back-of-house stage relay system, which is there primarily to let the back-of-house staff and actors/performers know roughly what is happening on stage, so the intelligibility doesn’t have to be particularly good; it is only the general sense of what is happening or what part of the performance has been reached that is required. Yet this often abysmal feed is provided for the hearing impaired! (It’s about the equivalent of providing the physically impaired with a ski lift for access in a mall, hotel or station, etc.).

So, slinging a microphone over the front of the balcony in a theater or concert hall is just not good enough, but this regularly happens. But why? In a word: ignorance or, more precisely, ignorance and lack of guidance. There are several standards (e.g., IEC 60118-4 or BS 7594) that relate to the performance of Audio Frequency Induction Loop Systems (AFILS) and, if complied with, there would be a reasonable chance of providing a useful and usable system.

Unlike the British Standard, the international standard does not ascribe a target value for the potential intelligibility of the loop system, yet this is fundamental to the use and functioning of the installation. There are many other essential parameters, though, such as the required field strength, the frequency response and acceptable background noise levels (and, hence, the effective signal-to-noise ratio) to be specified. Interestingly, there are no such equivalent standards or codes of practice for infrared or wireless ALS.

Clearly, some sort of code of practice or standard is required. Then, perhaps our illustrious editor would have had more positive experiences than he noted last month. However, it’s not just about the installation, but also the way in which an assistive listening system is operated. Many systems can effectively be set and left alone, but here is a recent experience I had in a West End theater at a very well known and popular musical show. Sadly, for my sins, whenever I go to the theater these days, I take an infrared and AFILS receiver with me so I can listen in at almost any venue.

For this theater and show, the system worked extremely well, so 10/10 for the technology, installation and sound quality…but no one had really thought about the users and their practical requirements. There I was, two minutes before curtains up, busily swapping the receivers to see if I could pick anything up. Not a glimmer (assuming that audio can shine, of course). Being a West End theatre, there was a very high probability that it would be an IR system, so I stuck with this. Then, as the band struck up, wow, there was the audio! But, had I actually needed to use the ALS for real, I would have been sitting there wondering if the receiver I had been given or the system were actually working.

Five minutes into a show is not the time to start getting up and trying to make one’s way out of the auditorium in order to find someone who might be able to get the system switched on, and then retracing one’s steps to see if it had been! No, what this otherwise excellent system needed was a confidence signal or acoustic pickup within the auditorium so a potential user could tell if it was switched on and working before the show started. I suggested this to the management and, after a while, they understood my point, though doubted how they could implement it. Five minutes later, they knew exactly what to do and seemed a little surprised at the technical ability of one of their customers!

Merely listening to ALS is not enough to tell if it is really acceptable and, if an AFILS, if it meets the standards. Some objective measurements are required. Figure 1, for example, shows the frequency responses of a loop system before and after it was corrected for the effects of structural metal loss. The initial response with the diminished high-frequency content sounded quieter but perfectly acceptable to most unimpaired listeners. However, as can be seen, it was well down from where it should have been (i.e., flat). A small adjustment was all that was required, but an objective measurement was needed to really show up the problem. It’s amazing what the application of science can do!

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