Julat pendengaran

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For more detail on human hearing see Audiogram, kontor sama kuat dan kecacatan pendengaran.

Julat pendengaran biasanya menggambarkan julat frekuensi yang boleh didengari oleh haiwan atau manusia, sungguhpun ia juga boleh merujuk kepada tahap julat. Bagi manusia frekuensi julat pendengaran biasanya dikatakan antara 20 Hz (kitaran sesaat) sehingga 20 kHz (20,000 Hz), sungguhpun terdapat perbezaan besar dikalangan individual, terutamanya pada hujung frekuensi tinggi, di mana kemerosotan bersama usia dianggap normal. Kesensitifan juga berbeza dengan banyak frekuensi, sebagaimana ditunjukkan oleh kontor sama kuat, yang biasanya diukur bagi tujuan penyelidikan, atau penyiasatan terperinci. Penyiasatan biasa bagi kehilangan pendengaran biasanya membabitkan audiogram yang menunjukkan tahap ambang berbanding piwaian normal.

Menentukan ambang pendengaran[sunting | sunting sumber]

Audiogram pada manusia dihasilkan dengan menggunakan peralatan ujikaji yang dikenali sebagai meter audio, dan ini membenarkan frekuensi berbeza diberikan pada subjek, biasanya melalui fontelinga yang diselaras, pada tahap tertentu. Tahap ini bagaimanapun, tidak mutlak, tetapi disesuaikan melalui lengkung berat ("weighting curve") dengan frekuensi berkait dengan graf piwaian yang dikenali sebagai lengkung boleh dengar minima ("minimum audibility curve") yang bertujuan mewakili pendengaran 'biasa'. Ini bukanlah ambang terbaik bagi semua subjek, di bawah keadaan ujian ideal, yang diwakili oleh sekitar 0 Phon atau pendengaran ambang pada kontor sama kuat, tetapi merupakan piwaian dalam piwaian ANSI sehingga tahap lebih tinggi pada 1 kHz.[1] Terdapat beberapa takrifan bagi lengkung pendengaran minima, ditakrifkan secara berbeza pada piwaian antarabangsa berlainan, dan ia jauh berbeza, memberikan perbezaan pada audiogram menurut meter audio yang digunakan. Piwaian ASA-1951 sebagai contoh menggunakan tahap 16.5 dB SPL (Tahap Tekanan Bunyi) pada 1 kHz sementara piwaian ANSI-1969/ISO-1963 berikutnya menggunakan 6.5 dB SPL, dan ia biasanya membenarkan 10 dB poembetulan bagi orang lanjut usia.

Ambang pendengaran bagi manusia yang tidak mampu bekerjasama sepenuhnya dalam ujian audiometrik, dan mamalia lain boleh dilakukan dengan menggunakan ujian pendengaran tingkah laku atau ujian fisiologi.

Audiogram boleh didapati dengan menggunakan ujian pendengaran tingkah laku yang dipanggil Audiometri. Bagi manusia ujian membabitkan ton berlainan diberikan pada frekuensi tertentu (pitch) dan kekuatan. Apabila seseorang mendengar bunyi, mereka mengangkat tangan atau menekan butang agar penguji boleh menentukan bahawa mereka mendengarnya. Keamatan bunyi terrendah direkodkan.

Ujian berbeza bagi kanak-kanak; mereka bertindak balas pada bunyi dengan telengan kepada atau menggunakan alat mainan. Kanak-kanak belajar apa yang mereka boleh lakukan apabila mendengar bunyi, sebagai contoh mereka diajar agar apabila mereka mendengar bunyi, mereka meletakkan patung dalam bot. Teknik yang sama boleh digunakan apabila menguji sesetengah haiwan tetapi disebalik mainan makanan boleh digunakan sebagai hadiah kerana bertindak balas pada bunyi. Ujian fisiologi tidak memerlukan subjek bertindakbalas.[2] Sebagai contoh apabila melakukan potensi disebabkan pendengaran pangkal otak ("brainstem auditory evoked potentials") tindakbalas pesakit pangkal otak diukur apabila bunyi dimainkan pada telinga mereka.

Maklumat pada pendengaran mamalia berlainan didapati dengan ujian pendengaran tingkah laku.

Mamalia darat[sunting | sunting sumber]

Bahagian berikut menggambarkan julat frekuensi bagi pendengaran mamalia tertentu berbanding mamalia yang lain. Bunyi pic rendah adalah berfrekuensi rendah; bunyi pic tinggi adalah berfrekuensi tinggi.

Manusia[sunting | sunting sumber]

Audiogram menunjukkan perbezaan pendengaran yang kecil.

Bagi manusia, gelombang bunyi disalurkan ke dalam telinga melalui salur telinga luar dan mengenai selaput tympanik / gegendang telinga. Hasilnya mampatan dan rarefaksi gelombang menggerakkan selaput nipis ini, menyebabkan tulang telinga tengah ( osikles; maleus, incus dan stapes) bergerak. Bilangan getaran tahap bunyi (gelombang sonik) sesaat mewakili frekuensi. Frekuensi infrasonik (bawah tahap pendengaran), sonik (aural), dan ultrasonik (atas pendengaran) diukur dalam Hertz (Hz); satu Hertz adalah satu kitaran gelombang (atau satu tekanan gelombang dalam audionik) sesaat. Khususnya, manusia memiliki julat aural maksima yang bermula serendah 12 Hz di bawah keadaan makmal ideal,[3] to 20,000 Hz bagi kebanyakan kanak-kanak dan sesetengah dewasa, tetapi julat menyusut sepanjang hayat, biasanya bermula sekityar usia 8 dengan frekuensi tinggi jadi semakin menghilang. Gelombang bunyi tidak didengari boleh dikesan (dirasai) oleh manusia melalui getaran fizikal badan pada julat 4 hingga 16 Hz. Terdapat perbezaan pada kesensitifan pe3ndengaran antara jantina, dengan wanita biasanya memiliki kesensitifan pada frekuensi lebih tinggi berbanding lelaki.[4] Getaran pada jaringan osikular menggantikan cecair basilar dalam kokhlea, menyebabkan rerambut dalamnya, dipanggil Stereocilia, untuk bergetar. Selaput rerambut menyelitupi kokhlea dari pangkal ke puncak, dan bahagian terangsang dan kekuatan rangsangan memberi petunjuk pada asal bunyi. Maklumat yang dikumpul dari sel rerambut dihantar melalui saraf bunyi untuk diproses pada otak.

Effects of high frequency limit[sunting | sunting sumber]

So-called "Nelson" dog whistles exploit this phenomenon by producing sounds at frequencies higher than those audible to humans but well within the range of a dog's hearing.

When compressing a digital signal, an acoustic engineer can safely assume that any frequency beyond approximately 20 kHz will not have any effect on the perceived sound of the finished product, and thus use a low pass filter to cut everything outside this range. The sound can then be sampled at the standard CD sample rate of 44.1 kHz (or 48 kHz in DAT), set somewhat higher than the calculated Nyquist-Shannon rate of 40 kHz to allow for the cut-off slope of a reasonable low pass filter.

When additional compression of sound is required, higher frequencies are usually cut off first, because regular adults' hearing in those areas is often even less than 20 kHz. This is due to loss of hearing in the high-frequency range, due to either hearing damage (e.g. from listening to loud music) or aging. For instance, the commonly used MP3 coding often cuts sounds above 18 kHz, or when compressing as high as 128 kbit/s, at 16 kHz.[5]

Dogs[sunting | sunting sumber]

The hearing ability of a dog is dependent on its breed and age. However, the range of hearing is approximately 40 Hz to 60,000 Hz,[6] which is much greater than that of humans. As with humans, some dog breeds become more deaf with age,[7] such as the German Shepherd and Miniature Poodle. When dogs hear a sound, they will move their ears towards it in order to maximise reception. In order to achieve this, the ears of a dog are controlled by at least 18 muscles. This allows the ears to tilt and rotate. Ear shape also allows for the sound to be more accurately heard. Many breeds often have upright and curved ears, which direct and amplify the sounds. As dogs hear much higher frequency sounds than humans,[7] they have a different acoustic perception of the world. Sounds that seem loud to humans often emit high frequency tones that can scare away dogs. Ultrasonic signals are used in training whistles, as a dog will respond much better to such levels. In the wild, dogs use their hearing capabilities to hunt and locate food. Domestic breeds are often used as guard dogs due to their increased hearing ability.[6]

Bats[sunting | sunting sumber]

Bats require very sensitive hearing to compensate for their lack of visual stimuli, particularly in a hunting situation, and for navigation. Their hearing range is between 20 Hz and 150,000 Hz. They locate their prey by means of echolocation. A bat will produce a very loud, short sound and assess the echo when it bounces back. The type of insect and how big it is can be determined by the quality of the echo and time it takes for the echo to rebound; there are two types; constant frequency (CF), and frequency modulated (FM) calls that descend in pitch.[8] Each type reveals different information for the bat; CF is used to detect an object, and FM is used to provide information regarding the nature of the object and its distance. The pulses of sound produced by the bat last only a few thousandths of a second; silences between the calls give time to listen for the information coming back in the form of an echo. There is also evidence to suggest that bats use the change in pitch of sound produced (the Doppler effect) to assess their flight speed in relation to objects around them.[9] The information regarding size, shape and texture is built up to form a picture of their surroundings and the location of their prey. Using these factors a bat can successfully track change in movements and therefore hunt down their prey.

Mice[sunting | sunting sumber]

Mice have large ears in comparison to their bodies. Mice hear higher frequencies than humans; their frequency range is 1 kHz to 70 kHz. They do not hear the lower frequencies that humans can; they communicate using high frequency noises some of which are inaudible by humans. The distress call of a young mouse can be produced at 40 kHz. The mice use their ability to produce and hear sounds out of our and other predators' frequency ranges to their advantage. They can alert other mice of danger without also alerting the predator to their presence. The squeaks that we can hear a mouse make are lower in frequency and are used by the mouse to make longer distance calls, as the low frequency sound can travel further than the high frequency sounds.[10]

Marine mammals[sunting | sunting sumber]

Marine mammals are mammals that inhabit the oceans, bays, and some rivers. As aquatic environments have very different physical properties than land environments, there are differences in how marine mammals hear compared to land mammals. The differences in auditory systems have led to extensive research on aquatic mammals, specifically on various kinds of dolphins.

The auditory system of a land mammal typically works via the transfer of sound waves through the ear canals. Ear canals in the pinnipeds or seals, sea lions, and walruses, are similar to those of land mammals and may function the same way. In whales and dolphins, it is not entirely clear how sound is propagated to the ear, but some studies strongly suggest that sound is channeled to the ear by tissues in the area of the lower jaw. One group of whales, the Odontocetes or toothed whales, use the process of echolocation to determine the position of objects, such as prey. The toothed whales are also unusual in that the ears are separated from the skull and placed well apart, which assists them with localizing sounds, an important element for echolocation.

Dolphins

Studies[11] have found there to be two different types of cochlea in the dolphin population. Type I has been found in the Amazon River dolphin and harbour porpoises. These types of dolphin use extremely high frequency signals for echolocation. It has been found that the harbour porpoise emits sounds at two bands, one at 2 kHz and one above 110 kHz. The cochlea in these dolphins is specialised to accommodate extreme high frequency sounds and is extremely narrow at the base of the cochlea.

Type II cochlea are found primarily in offshore and open water species of whales, such as the bottlenose dolphin. The sounds produced by bottlenose dolphins are lower in frequency and range typically between 75 to 150,000 Hz. The higher frequencies in this range are also used for echolocation and the lower frequencies are commonly associated with social interaction as the signals travel much further distances.

Mamalia laut menggunakan vokalasi dalam pelbagai cara. Ikan lumba-lumba berhubung menggunakan klik dan siulan, dan paus menggunakan ngerangan frekuensi rendah atau isyarat denyutan. Setiap isyarat berbeza dari segi frekuensi dan isyarat berbeza digunakan bagi berhubung aspek berlainan. Pada ikan lumba-lumba, gemalokasi digunakan bagi mengesan dan mencirikan objek dan siulan digunakan bagi kawasan masyarakat dan peranti perhubungan.

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Nota[sunting | sunting sumber]

  1. Sataloff, Robert Thayer; Sataloff, Joseph (Feb 17, 1993). Hearing loss. Dekker. 
  2. Clinical Audiology (edisi ke-5th). Lippen-Cott Williams and Wilkins. 2002. 
  3. 20 Hz dianggap had frekuensi rendah normal bagi pendengaran manusia. Apabila gelombang sine tulin dihasilkan di bawah keadaan ideal dan pada bunyi tahap tinggi, pendengar manusia akan mampu mengenal pasti ton sebagai serendah 12 Hz. Olson, Harry F. (1967). Music, Physics and Engineering. Dover Publications. p. 249. ISBN 0-486-21769-8. 
  4. Gotfrit, M (1995). Range of human hearing. Zen Audio Project. 
  5. "Spectral Analysis". Diperoleh pada 2012-02-04. 
  6. 6.0 6.1 Elert, Glenn; Timothy Condon (2003). "Frequency Range of Dog Hearing". The Physics Factbook. Diperoleh pada 2008-10-22. 
  7. 7.0 7.1 Hungerford, Laura. "Dog Hearing". University of Nebraska. Diperoleh pada 2008-10-22. 
  8. Devorah A. N. Bennu (10 October 2001). "The Night is Alive With the Sound of Echoes". Diarkibkan daripada asal pada 2007-09-21. Diperoleh pada 2012-02-04. 
  9. Richardson, Phil. "The Secret Life of Bats". Diperoleh pada 2012-02-04. 
  10. Lawlor, Monika. "A Home For A Mouse". Society & Animals 8. Diperoleh pada 2012-02-04. 
  11. Ketten, D. R.; Wartzok, D. Thomas, J.; Kastelein, R., pengarang. "Three-Dimensional Reconstructions of the Dolphin Ear" (PDF). Sensory Abilities of Cetaceans: Field and Laboratory Evidence (Plenum Press) 196: 81–105. Diarkibkan daripada asal pada 2010-07-30. 

Rujukan[sunting | sunting sumber]

  • D'Ambrose, Chris (2003). "Frequency Range of Human Hearing". The Physics Factbook. Diperoleh pada 2007-02-28. 
  • Hoelzel, A Rus (2002) Marine mammal biology: an evolutionary approach, Oxford: Blackwell Science Ltd
  • Ketten, D.R. (2000) Cetacean Ears. In: Hearing by Whales and Dolphins. W. Au, R. Fay, and A. Popper (eds.), SHAR Series for Auditory Research, Springer-Verlag, pp. 43–108. http://www.whoi.edu/csi/research/publications.html
  • Richardson W J (1998) Marine mammals and noise London: Academic
  • Rubel, E. Popper, A. Fay, R (1998) Development of the Auditory System New York: Springer-Verlag inc.

Pautan luar[sunting | sunting sumber]