5th Congress of Alps-Adria Acoustics Association NOISE-INDUCED HEARING LOSS

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1 5th Congress of Alps-Adria Acoustics Association September 2012, Petrčane, Croatia NOISE-INDUCED HEARING LOSS Davor Šušković, mag. ing. el. techn. inf. Abstract: One of the most common types of hearing loss is noise-induced hearing loss. Such hearing loss is recognized by the characteristic audiogram on which is clearly visible a larger loss of hearing in the area around the frequency of 4 khz. Noise-induced hearing loss is the result of constant exposure to noise, of working in noisy environment, of frequent listening to loud music, etc. Since hearing loss is permanent, hearing can be obtained only with appropriate hearing aids. Today's digital technology enables sound processing strategies that can fulfil the requirements of this hearing loss. Recommendations are set for this hearing aids; to use open fitting, work in at least 6 channels, have advanced algorithms for noise supression and focusing on speech, and to use hearing aids on both ears. With FM technology hearing can be provided in the most difficult hearing situations. Key words: hearing, noise-induced hearing loss, hearing aids, open fitting, sound processing, channels, fm technology 1. NOISE Nowadays, noise is one of the main culprits for the majority of hearing losses. Exposure to noise during the day permanently damages our hearing. Noise generated by traffic, noise that we are exposed at work, listening to loud music and most importantly, the neglect of hearing protection are the main causes of noise-induced hearing loss (NIHL). In the context of the medical literature, the noise can be defined as extremely intense sound capable of producing damage to the inner ear. High levels of sound pressure cause damage to the cilia on the basilar membrane, which are responsible for the transmission of impulses to the auditory nerve. As the resonant frequency of the external ear canal is about 3200 Hz, the sounds of these frequencies come to the eardrum increased up to 20 db. With the greater sensitivity of cilia to the higher frequency sounds, it is expected that cilia responsible for the adoption of sound frequencies around 4 khz are the most vulnerable. In acoustic trauma, which is caused by short exposure to intense sound, hearing loss similar to the one caused by noise may occur. Level of the sound pressure capable to produce this kind of damage is around db and varies between individuals. The outer ear examination often shows no damage, except in rare cases of the damage of the eardrum. However, audiogram sometimes shows a curve characteristic for the noise-induced hearing loss, or more often down-sloping or flat curve. 2. NOISE-INDUCED HEARING LOSS Chronic noise-induced hearing loss, in contrast to acoustic trauma, is a disorder that worsens over time due to exposure to noise. The amount of sound capable of creating damage may be shown by the so-called Damage Risk Criteria. It has been shown that short exposure to loud noise has the same effect as longer exposure to noise that is somewhat quieter. Since the increase of the sound intensity of 3 db corresponds to a twofold increase in the sound pressure level, it has been concluded that the exposure to a the intensity of sound higher by every 3 db will result in a twofold decrease in the exposure time allowed. However, as the noise level varies over time, there is a prescription that leads to an increase of 5 db in half the time. For example, OSHA (Occupational Safety & Health Administration) prescribes the permissible noise exposure to 90 dba for 8 hours, or 115 dba for 15 minutes. HMA-03 Page 1 of 5

2 Even though the noise-induced hearing loss varies from individual to individual, American College of Medicine Occupational Noise and Hearing Conservation Committee specified the main characteristics: Hearing loss is always perceptive (sensorineural) Almost always symmetrical and binaural Rarely will cause deafness Will not deteriorate after exposure to noise The degree of hearing loss is reduced as the threshold increases Frequency of 4 khz is most affected, and also the higher frequencies (3-6 khz) are more vulnerable than lower (500 Hz - 2 khz) The maximum hearing loss occurs after years of exposure to noise Continuous noise is more dangerous than the one that is intermittent with silence Emergence of noise-induced hearing loss can be divided into two phases. The first phase is characterized by the Temporary Threshold Shift (TTS). Slight loss of hearing occurs after exposure to noise which is completely withdrawn after a period of rest. It is recognized by light distinctive "ringing in the ears" and occurs after, for example, listening to loud music. Recovery time can take up to 48 hours, depending on the duration of exposure to noise. This damage can be considered as ear fatigue and many studies show that there is no damage to the sensory cells. After frequent exposure to noise capable of producing TTS, the Permanent Threshold Shift (PTS) will occur. This is the second phase of the emergence of chronic noise-induced hearing loss that cannot be repaired. At this point, there is permanent damage to sensory cells - cilia. Fig. 1. Audiogram of the person with NIHL Comparing the position of the phonemes with the audiogram, it can be noticed that a patient with NIHL will have problems with hearing of occlusives, fricatives and affricates characterized by noise at higher frequencies. Also, due to the higher frequencies of basic formants of female speakers, there will be more problems with listening to female vocies than male ones. Such hearing loss is causing problems in understanding speech and creates difficulties in social contact. It is often the cause of separation from society because of the inability of full participation in the discussions. Understanding speech is even more harder if more people talk at the same time or if the conversation takes place in noisy environments (Fig. 2. and Fig. 3.) Characterstics of the Noise-Induced Hearing Loss NIHL is determined by the specific audiogram. The curve shows normal hearing at low frequency sounds and a decrease after 1 khz (about 20 db per octave) to a frequency of 4 khz, followed by growth towards higher frequencies (Fig. 1.). Fig. 2. Position of phonemes in relation to the NIHL audiogram HMA-03 Page 2 of 5

3 Fig. 3. Position of some sounds in relation to the NIHL audiogram 3. RECOMMENDED HEARING AIDS FOR PERSONS WITH NIHL With regard to the specific audiogram and requirements that the hearing aid must meet, from a wide range of the hearing aids offered by the industry nowadays, there are some that stand out because of their mode of operation and characteristics to meet the needs of these patients. Hearing aids can be roughly divided into two groups: ones that are worn behind the ear (BTE) and the ones that are wholly in the ear (ITE). BTE hearing aids conducts sound to the ear by the tube and the earmold which is placed at the beginning of the ear canal. ITE hearing aids can be more or less visible in the ear canal, depending on the construction of the housing. People with normal hearing of lower frequency sounds often have problems with a sense of blockage or occlusion when wearing hearing aids. It is the effect that occurs due to three sound sources brought to the eardrum: the amplified sound from the hearing aid, the direct sound through the vent of hearing aids and the sound that occurs due to bone conduction (Fig. 4.). The difference between the amplified and two natural sounds can lead to somewhat strange sound, and in these situations patients complain that their own voice sounds strange, too deep, boomy or hollow, with the frequent observation that sound as if speaking from the barrel to themselves. This problem is a result of the inability to conduct sounds of lower frequencies than can be heard, which are only listened by the bone conductivity. The solution to this problem is to expand the vent, but today the technology can overcome this problem with open fitting. Fig. 4. Three sounds that come to the eardrum On ITE hearing aids, on which insist patients who care about discretion, larger vents are done, often of different shapes and sizes in order to obtain sufficient acoustic mass which would anull the effect of occlusion. On the other hand, vent size is limited by the size of hearing aids as well as by hearing aid amplification due to the greater possibility of whistling. Calculation of optimum vent size and shape is done by a software based on algorithms that correspond to the theoretical setting, and on the results of various studies (Fig. 5.). Fig. 5. Various vents D shape and Shark bite HMA-03 Page 3 of 5

4 Recommendations of experts for noise-induced hearing loss still refer to the "fit as open as possible" rule - that is, the use of open fitting technology in every possible case. Open fitting leaves enough space for the passage of audible sounds of lower frequencies in a natural way due to which the sound is more natural, it is easier to determine the direction of the arrival of the sound and there is no occlusion. Amplified sound from the BTE hearing aid is led by a thin, barely noticeable tube to the ear canal, and on the top of the tube there is a small dome that t serves as the protection against cerumen. A newer version of open fitting hearing aids separates receiver (speaker) of the hearing aid casing which reduces the dimensions of hearing aids. Even more importantly, gain losses are reduced by placing the receiver in the ear canal. This technology (CRT - Canal Receiver Technology, RITE - Receiver in the Ear or RIC - Reciever in the Canal) appears to be the best solution for people with NIHL (Fig. 6.). In addition to the required algorithms - options for noise suppression and focusing on the speech, another option provided by today's hearing aids is recommended. Algorithm that performs a nonlinear transformation of the higher frequency sounds to the lower frequency range could provide people with a higher degree of hearing loss with the ability to hear high frequency sounds (around 4 khz). Because of the relatively symmetrical hearing loss on both ears and in order to further increase the speech clarity, it is advisable to wear hearing aids on both ears. With binaural hearing aids fit today's technology allows communication between hearing aids and the use of the network of two pairs of microphones. In this case, the sound d from the front would be even more prominent over the background noise. Binaural wearing of hearing aids greatly helps in locating the direction of the arrival of sound and increases the user satisfaction FM technology Fig. 6. CRT hearing aid Phonak Audéo S SMART Today's technology and multi-channel digital audio processing, which is done by the hearing aids hardware, provides a variety of options which may greatly benefit patients with noise-induced induced hearing loss. Very specific audiogram curve that is broken in several spots and difficulty in understanding speech in difficult hearing situations create demands that today's hearing aids can fulfill. Firstly, consider the requirements for the number of channels, with reference to the number of points in which it is possible to set up gain of hearing aids. Although today's nonlinear fitting formulas describe very well different hearing losses, audiogram of NIHL makes the request of at least five points of gain tuning. Since the difference between the number of bands of signal processing (the number of channels), and the number of bands in which the gain is determined (the number of tuning points) is set only at the hearing aids with a smaller number of channels, a minimum of five to six bands in which gain could be determined is recommended. Because of the complex signal processing algorithms and applications for suppression of noise while focusing on the speech signals, at least six channels of signal processing are recommended, therefore six bands in which the noise is suppressed and speech sounds are focused on. Although advanced technology enables excellent speech understanding in difficult hearing situations, there are still situations where ambient noise makes it impossible to listen well. Distance from the speaker makes it difficult to listen with hearing aids and the technology that would solve this problem, or the problem to listen to speakers who are not facing the person with hearing loss, is necessary. Rooms in which there is an echo or reverberation, or even outdoor spaces provide a difficult task for the hearing aids. Technology of the transmission of the frequency modulated audio signal has been shown as an optimal solution in such a difficult hearing situations. A radio transmitter located near the speaker and a radio receiver as a part of the hearing aid allow transmission of the highquality speech signal directly to the hearing aid eliminating the problem of distance and ambient noise (Fig. 7.). Fig. 7. FM transmitter and FM receiver HMA-03 Page 4 of 5

5 FM technology proves to be particularly effective in solving problems with understanding speech that occurs in people with noise-induced hearing loss, but also in children during classroom instruction. Schools for children with special needs, as well as regular schools attended by children with hearing loss are increasingly equipped with FM systems that enable them to continue normal monitoring of school programme. REFERENCES [1] Baumann N. : Kinds of hearing losses; sses.htm; April [2] Cox R. : Hearing aid benefit in everyday environments, Ear Hear 12: ; [3] Gatehouse S, Noble W : The speech, spatial and qualities of hearing scale, Int J Audiol 43: 85-99; [4] Hogan C., Turner C. : High frequency audibility: benefits for hearing-impaired listeners; J Acoust Soc Am 104: ; [5] Rabinowitz P. : Noise-Induced Hearing Loss; [6] Somerville E.T. : Noise-induced hearing loss and industrial audiometry; pdf/jroyalcgprac pdf [7] Vonlanthen A., Horst A. : Hearing Instrument Technology, 3rd Ed.; Thomson Delmar Learning, [8] Walker Skinner M. : Speech intelligibility in noiseinduced hearing loss:effects of high-frequency compensation; Washington University School of Medicine; [9] Worker Health Protection Program : Noise Induced Hearing Loss; HMA-03 Page 5 of 5