Active Noise Cancellation
Active noise cancellation : another unlikely cure for tinnitus. After reading this don’t loss hope as a Sarasota Chiropractor offers hope with a treatment called NeuroCranial Restructuring. Make sure you watch the video attached with a testimonial on NCR and tinnitus.
The BTA has received an increasing number of enquiries about claims for a new method for alleviating tinnitus sensations. This method is known as active noise cancellation or active noise control (ANC for short). ANC is the name for a general technique which can be used to attenuate offending background sounds in the external environment. Many readers of Quiet may already have heard about this technique since headphone systems incorporating ANC technology have been widely advertised in the popular press and are available from a number of commercial outlets. These systems have proved to be highly successful in reducing unwanted ambient sounds that are typically generated in noisy enclosed spaces, such as airplanes and trains. For those people who suffer from tinnitus, the tinnitus-related noise can be described as yet another example of an aggravating sound, just like airplane noise. If so, then how successfully might ANC serve to reduce the level of the tinnitus noise? The aim of this article is to explain in simple terms some of the scientific bases behind the ANC technology and to explain why ANC has limited use as a treatment for tinnitus despite being successful for other forms of noise reduction. Yet again, the claims for a new and exciting breakthrough in tinnitus therapy are leading many vulnerable patients down another blind alley.
ANC is a far more complex process than simply attenuating a noise by wearing earmuffs or earplugs. It involves some serious science. Essentially, ANC works by capturing an ongoing sound pattern in the external environment and by simultaneously playing back a precise mirror-image of that same sound ― the “anti-noise” signal. If the counterbalancing sound pattern has the same amplitude but the opposite phase (or polarity), then it will exactly cancel the original signal with the net result of no sound at all. These are the basic theoretical concepts of ANC. The idea is much more simple to explain than the technique is to put into practice! Nevertheless, this has been achieved. First, tiny microphones, one on each earcup, detect ambient noise before it reaches the listener’s ears. Then, the noise-cancellation circuitry, usually housed in an external device containing a special type of computer, inverts the captured signal, turning the sound wave upside down. The noise-cancellation system must match the timing of the two signals precisely so that when the opposite of the external noise is played out by a speaker device in the earcup, they destructively interfere with one another. Physical limitations make absolutely perfect noise cancellation impossible. For example, if the original and the inverted signals are even slightly mismatched this will cause the combined noise level to increase rather than decrease. Often ANC devices include a microphone inside each earcup so that the “error” part of the signal that has not been cancelled by the speakers can be monitored and the “anti-noise” signal can be subsequently modified to improve the amount of cancellation that is achieved. Despite these challenges, many commercial ANC systems have been proved to reliably reduce the loudness of low-frequency background noise.
If the technical challenges of implementing an effective ANC device have been overcome, then why are there limitations in applying ANC as a modern therapy for tinnitus? There are two reasons.
The first explanation for the limited therapeutic benefit of ANC for tinnitus is that ANC technology requires a signal that can be detected by an electrical system. For example, a microphone turns sound waves into a varying electrical voltage that can be processed by an electric device. There is, as yet, no known way of accurately monitoring the tinnitus signal reaching the brain. Simple methods of sound matching (where the person is free to shift the frequency of an external sound in order to identify a frequency that most closely matches the pitch of their own tinnitus) are insufficient. What many of the therapeutic claims for ANC fail to address is that the form of the energy commonly underlying tinnitus sensations is neither air-conducted nor body-conducted and it is therefore not possible to obtain an electrical representation of it. The most common forms of tinnitus have no measurable acoustic representation and are widely believed to arise from internal states in the brain, such as abnormal neural firing patterns within the central auditory system. Consequently, there is absolutely no tinnitus-related air-borne signal for the ear microphone to detect, and hence for the device to cancel. Very occasionally, a rare case of tinnitus is diagnosed where the tinnitus noise is also audible to other people. In these cases of objective tinnitus, the noise can be linked to a clear physical cause such as a circulatory problem (like an aneurysm or tumour in a blood vessel), repeated spasms of the muscles in the middle ear or inner ear structural defects. The cause of these physical vibrations is generally external to the auditory system. Could ANC be used to treat such cases of objective tinnitus? Well, this question leads us to the second point about ANC.
In principle, it should be possible to use an ANC system to treat a case of objective tinnitus because ANC can be used to cancel any sort of vibration pattern. However ANC systems need to obtain a representation of the vibration energy travelling to the person’s ear BEFORE it has reached their ear. This is done using a microphone near the source of the noise. The computer in the ANC system can then calculate the required “anti-noise” from this sound. By the time the noise sound has travelled from the system’s microphone to the person’s eardrum, the computer in the ANC system has calculated and produced the “anti-noise” signal. In the case of objective tinnitus, we can see an obvious problem. There is no straightforward way to detect the vibration pattern and to calculate the “anti-noise” sound before the tinnitus sound has reached the ear and been heard. Thus standard noise control algorithms cannot be used for tinnitus noise control. There are other predictive noise control algorithms that could work in this scenario, but the problems caused by having the noise source and the place where you want to create quiet so close together would produce considerable difficulties. We have seen a website describing ANC therapy for tinnitus which claims that a therapeutic counterbalancing signal can be personally tailored to an individual, first by using psychophysical sound matching and second by making a recording of this sound onto a CD. Simply playing back the CD over headphones bypasses the essential active part of ANC and so we doubt that the CD would do more than simply mask the tinnitus sound by providing another sound distraction.
Although we would all like to see an effective and reliable cure for tinnitus, ANC is unlikely to provide a solution to the tinnitus problem. There are no published scientific studies to support any of the bold claims that you might read in newspapers, magazines and on the internet.
John Lieurance, D.C.
941 330 8553