Speech and intelligibility within hearing loop systems
For a hearing loop to be beneficial to the end user it is imperative that the quality of audio and speech intelligibility be maintained, there are many different influencing factors that directly affect the hearing loops audio intelligibility. In this article I hope to briefly cover the main factors that influence intelligibility and give a few tips on how you may improve it within your systems.
System selection that takes into account expected metal losses and area coverage issues
When you begin designing your hearing loop you need to consider the environment in which it will be incorporated into. For example, is it a large area? Does the building infrastructure use a lot of metal within its materials? Or will signal spill be an issue? These questions will help you to design a loop that is Standard Compliant within its environment. Different materials affect hearing loops in different ways, large amounts of metal for instance drastically reduce a perimeter loops capability to provide intelligible audio, a phased array system in this kind of scenario will overcome the resulting metal loss and create a consistent magnetic field. Choosing the wrong design and Loop Layout can result in an induction loop that fails to provide any value at all to the end user.
Input selection and separation from background noise, (garbage in garbage out principle) including microphone selection and line inputs.
It is essential to consider the different types of microphones that are available to provide audio input into the system and which are the most appropriate for the environment. You must, for example, consider the placement of the microphone. The aim of any microphone set-up is to isolate and capture only the desired sounds, to use a well know AV term – garbage in, garbage out.
We have written a whitepaper here to explain the various differences in microphones and their most appropriate use within various assistive listening environments.
Calculate loop impedance
Loop amplifiers have limits of current and voltage. Voltage limits are determined by the supply rails, if we exceed those limits the signal will clip. Some loop drivers can deliver high currents but have very limited voltage headroom which you will see from this calculation is counter intuitive to an induction loop system.
Voltage is related to wire length and wire size. If we were just dealing with DC current then this would be it. V=IR (Where V is Voltage, I is Current and R is Resistance). Because we are dealing with AC signals the calculation is V=IZ where Z is the impedance of the loop. Impedance is determined by inductance (L) of the conductor and frequency (F). Therefore as higher frequencies are required to be reproduced the overall impedance of the loop increases requiring higher voltage to reproduce these sounds V=IZ. Inductance is greatly affected by the number of turns in a loop and overall length of loop therefore as a loop becomes more complicated (such as a phased array systems) a higher voltage is required to drive these.
Some loop manufacturers use misleading figures to disguise the lack of voltage headroom, by using PK values and not true RMS values of voltage or worse Pk – Pk so as not to reveal the inadequacies of the equipment.
For example if an amplifier stated it could offer 47V peak to peak we know that this must be converted to a useful value to calculate its effectiveness at driving higher frequencies. First by dividing pk to pk by half to get the actual peak value 23.5V PK then multiplying by 0.775 which is the relation between peak and RMS with a full current sine wave the output is 18.21 VRMS. Now we shall apply this to the above equations and we see that driving the same 200m loop with 6 dB of loss we see the loop will begin to voltage limit driving a continuous sine wave of 2.15kHz and short term peaks at a frequency of 2.975 kHz
Amplifier selection and required characteristics for reproduction of human speech frequencies (speech banana) including voltage head room requirements and power characteristics
This graph is part of a hearing chart that can be found in just about any audiologist’s office. The vertical axis shows the hearing threshold in dB, and the horizontal axis show the frequency of sounds. The bands of hearing loss ranging from mild to profound are also displayed. The hearing loss definitions are set by the World Health Organisation.
The images representing various noises show what sounds may or may not be audible depending on the severity of the hearing impairment. It’s worth noting that regular exposure of over 15 minutes to anything over 90dB can lead to noise induced hearing loss. The pattern of human speech operates at different frequencies depending on the sound being made. For example the ‘mmm’ sound is quite low frequency, whilst the ‘th’ is relatively high in frequency. It is the higher frequencies that are most commonly lost in hearing impairment, which makes conversation difficult to understand. The frequency band which captures human speech is often referred to as the speech banana.
Make sure the receivers are adequate or hearing aid and positioning of telecoil in hearing aid
When an alternating electrical current is passed through a wire an electromagnetic field is generated. The lines of force, or magnetic vectors, in the electromagnetic field are directional; (fig1) portrays the current flowing towards the viewer, as denoted by the dot in the middle of the wire rather than a cross.
(Fig 1) An alternating current passing through a wire creating a magnetic field. Magnetic vector lines showing the direction of the magnetic field. The telecoil is 100% inductive as it is in line with the magnetic field.
When placed within the magnetic field a voltage is induced upon the Telecoil. As it is a coil it will be 100% inductive when the lines of force are moving through it and 0% inductive when they are travelling at 90 degrees (see Fig2).
(Fig 2) The telecoil is 0% inductive as it is perpendicular (90°) with the magnetic field.
Due to this principle, when a hearing aid user, wearing an aid with the telecoil mounted vertically, bends down or nods their head 90° to the loop wire they lose the signal generated by a standard perimeter audio frequency induction loop. To be at their most effective for the application of hearing loops telecoils should be mounted vertically within the hearing aid.
When the telecoil is at 45° to the magnetic field induction is significantly reduced.
Therefore when the Telecoil is oriented at 45° pickup of magnetic field is greatly reduced, and therefore the functionality of the hearing aid.
Modern hearing loop design can, to some extent, mitigate this effect to account for head movement and telecoil misalignment. However as a large number of hearing loop systems are still based on perimeter loop designs it is still advisable to mount telecoils in a vertical alignment within the hearing aid.
I hope that this article has helped to briefly describe some of the contributing factors to the audio intelligibility of a hearing loop and given some insight into how to develop systems that provide the greatest value to the end user.
by Jonathon Hoskins