The Spontaneous Otoacoustic Emissions (SOAEs) are low-level narrow band signals recorded in the external meatus in the absence of any stimulus. SOAEs were primarily considered upon their discovery as cochlear pathology or a result of irregularities in the arrangement of OHC rows. It is hypothesized now that their generation mechanism is similar to that of TEOAEs; that is they are a reflection of the traveling wave energy at various points of the organ of Corti showing impedance perturbations. According to this hypothesis, these structural perturbations do not have any effect on the audiometric threshold.

     The prevalence of SOAEs is significantly lower than that of TEOAEs and DPOAEs. However, the use of more sensitive microphones over the past years has revealed a higher SOAE prevalence than previously was thought when they were first discovered. Currently, the use of better instrumentation shows that SOAEs are found in about 60 to 70% of young, normal hearing adults.

Main characteristics of SOAEs:

• When present, one ear can exhibit one or several peaks at different frequencies and of different amplitudes. The number of SOAEs varies from one ear to another and from one subject to another.
  
  
• SOAEs are very stable with time and do not fluctuate much in frequency, making them a good indicator of trauma that can occur in one ear, as with other types of OAEs. However, the amplitude of each SOAE can fluctuate by as much as 10 dB depending on the time of recording.
  
  
• When multiple SOAEs are present, the amplitudes of each of the peaks are different from one another. For example in newborns and infants, it is not rare to record SOAE amplitudes ranging from -20 dB SPL to 20 dB SPL in the same ear.
  
  
• SOAE amplitudes can be reduced and their frequencies shifted following acoustical stimulation of the medial olivocochlear system. It has been shown that the effect of a contralateral auditory stimulation on DPOAEs is stronger in the vicinity of SOAEs.
  
  
• Gender effect: The SOAEs demonstrate a gender-prevalence, in that they are significantly more prevalent in women than men and with higher numbers of SOAEs in females than in males when they are present.
  
  
• Asymmetry: There is a higher prevalence of SOAEs and a greater SOAE number in the right ear than in the left ear.
  
  
• Age effect: The prevalence of SOAEs has been shown to be at a higher level in infants, full-term and pre-term neonates than in adults. SOAEs can be recorded as early as 30 weeks of conceptional age in pre-term neonates. The highest incidence of SOAEs (85%) has been found in neonatal pre-term and full-term populations. Also, SOAE number is greater in infants than in adults.
        The majority of SOAE peaks are observed between 1 and 2.5 kHz in adults. In infants, SOAEs are mainly observed between 2 and 5 kHz. SOAE changes with time in newborns could possibly reflect on-going cochlear maturation. However, evolution of the physical characteristics of the outer ear is supposed to explain the main differences between infants and adults. The outer ear amplifies sounds more in younger subjects and for higher frequencies than in adults.
  
  
• Relation between SOAEs and auditory sensitivity: SOAE presence is suggested to be linked to higher auditory sensitivity. SOAEs cannot been observed in cases of hearing impairment of 25 dB or more in adults. Their existence is also an indicator of strong and robust TEOAEs. Moreover, SOAEs are more present in females than in males and in right ears than left ears in agreement with better hearing in females and in right ears. HOWEVER, their absence does not signify that the subject has a hearing loss and there is no known correlation between SOAE number and audiological assessment.
  
  

Test procedures:

• SOAE can be recorded using the same equipment as other types of OAEs. The same sensitive microphone is required.
  
  
• SOAEs can be recorded in the external auditory meatus in the absence of any stimulus.
  
  
• It is also possible to detect SOAEs by synchronizing them to low-level click stimulation. In this case, the recording window lasts longer than for TEOAE recordings (usually 80 ms instead of 20 ms) and SOAEs are observed in the 60-80 ms window while TEOAEs elicited by a click do not persist after 20 ms. This recording method is a default setting on the ILO system by Otodynamics. Several authors have pointed out that this technique allows recording of SOAEs AND Synchronized-SOAEs. Comparisons of both SOAE recording methods (without and with a stimulus) have shown that they both give quite similar results. One of the advantages of using a low-level stimulus is to ensure that a good fit is obtained in the outer ear canal by checking the stimulus waveform.
  
  
• SOAEs can be recorded at any time, even during sleep (which can be very useful in babies).
  
  
• Most OAE equipment allow SOAE recordings.
  
  
  

      Because SOAEs are not present in 100% of normal-hearing adults, their clinical use remains uncertain at the present time while their presence is a sign of a normal functioning cochlea, their absence does not signify hearing impairment.
      Research is still in progress to explain some of the relationships between the presence of SOAEs and higher auditory sensitivity. In any case, when present, SOAEs can be considered useful in monitoring and following any small changes in the mechanics of the cochlea.

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