DISCLAIMER: The following is for general informational purposes only. This site does not endorse any particular therapy. Any use of sound therapy should first be discussed with a medical professional to address proper protocol and associated risks. Negative effects from sound therapy can be delayed and, as a result, sound therapy should never be done aggressively and should not be done without guidance from a medical professional.
This section will largely focus on the details of pink noise as it is one of the more popular forms of sound therapy. There have not been studies proving that pink noise is the optimal form of broadband noise for hyperacusis therapy. However, for reasons that will be explained below, its frequency characteristics make it a reasonable choice.
White noise is commonly used for tinnitus masking and masking of unwanted sounds to aid with sleep. It is the simplest noise to generate as it simply a sequence of random numbers. However there are several reasons why pink noise is commonly chosen over white noise for hyperacusis therapy.
- Pink noise may be less irritating or painful because it has lower energy at high frequencies.
- Pink noise is a closer fit to sound experienced naturally.
- Pink noise contains the same amount of acoustic energy per octave. The cochlea is similarly distributed as it contains roughly the same number of hair cells per octave.
Below is a comparison of the frequency content of these two broadband noise types:
From the graph above you can see that the high frequency energy of pink noise is lower than that of white noise. Pink noise contains the same total amount of energy within each octave. Thus the total energy between 1 kHz and 2 kHz is the same as the energy between 2 kHz and 4 kHz. With white noise, the energy between 1 kHz and 2 kHz is equivalent to just half of the energy between 2 kHz and 4 kHz.
Softened Pink Noise and Brown Noise
For those who have severe hyperacusis, even pink noise can be irritating at very low volumes. Low pass filtering can be employed in early stages to reduce high frequency content further. One popular pink noise treatment uses low pass filtering of pink noise starting at roughly 5 kHz which yields a softened pink noise perhaps more similar to analog generated pink noise. Further reduction of high frequency components can be found by using brown noise. With brown noise, each octave contains half of the energy as the octave below it. Brown noise is far easier to tolerate and resembles the deep roar of a waterfall. Brown noise, however, does not stimulate the higher frequency regions of the auditory system to the same degree.
Broadband Noise Samples
Below are samples of the broadband noise described in the previous sections. Pink noise is most commonly recommended for hyperacusis treatment. White noise is simpler and contains more high frequency energy but those with hyperacusis can find it to be unpleasant. Brown noise is more pleasant but is not generally used for hyperacusis, perhaps because it does not contain as much high frequency energy as pink and white noise.
WARNING: As with any audio file, set to minimum volume, hit play, and gradually increase volume to an audible level. Please contact your doctor first if you have any question about your capacity to listen to unknown audio files.
|Download (10s Track)
|Download (1hr Track)
|Softened Pink Noise
|Download (1hr Track)
|Download (10s Track)
Some suggest the need for caution over using “low quality” pink noise. “Low quality” sources include low quality MP3s, using youtube for a pink noise source, or using an equalizer. The following sections analyze these three non-ideal sources. There have been no studies that investigate whether such “low quality” sources actually have a negative impact compared to “high quality” sources. Given that headphones or noise generators will have their own non-idealities, there is no such thing as delivering pure pink noise to the auditory system.
Broadband Noise and MP3 Compression
MP3 compression of an hour long pink noise file is a helpful way to minimize disk space but it must be done carefully. The concern with MP3 compression on pink noise (and brown noise) is that the compression algorithm will filter high frequency energy and will also cause changes in content across all frequencies. The following will show that MP3 compression of pink noise can be OK if done carefully.
Below shows a comparison of pure pink noise (wav) with 2 MP3s created using two common bitrate settings:
As hearing for most rolls off significantly by 16kHz, cutoff frequencies of 19kHz and above are reasonable. A quick inspection of the graph above also shows that frequency content throughout the rest of the spectrum is not significantly enhanced or reduced before the cutoff. As a result, the 256 kb/s stereo (128 kb/s mono) MP3 can be considered high quality pink noise and is equally as effective as the original file. Using MP3 compression, the file size of an hour of pink noise is more manageable:
It goes a bit against intuition that MP3s do not degrade pink noise quality. This is because MP3s are normally used to compress music where slight imperfections in voices and instruments are noticeable. With pink noise, it doesn’t matter if the compressed noise is different from the original as long as the frequency spectrum is still “pink”. This is not to say that all MP3s of pink noise are OK. It is best to check the cutoff frequency for yourself using a free audio analyzer such as Audacity or to download from a source that explicitly shows the cutoff frequency. If there is any doubt, 256 kb/s MP3s are almost always going to be OK and the lossless wav or CD format is a sure bet.
Pink Noise on Youtube
Youtube is one of the top results for pink noise in a google search leading some to use this pink noise for therapy. An analysis of the first 10 results for pink noise on youtube found the following
- 4/10 of “pink noise” on youtube was actually brown noise (much less high frequency content)
- 2/10 of “pink noise” on youtube was neither pink nor brown noise (see below).
- 4/10 of “pink noise” on youtube was actually pink noise
- All but one video had frequencies above 15.5kHz removed due to youtube’s standard audio compression.
WARNING: THUMBNAIL LINKS TO YOUTUBE PAGES CAN AUTOPLAY. ENSURE VOLUME IS LOW BEFORE CLICKING. Please contact your doctor first if you have any question about your capacity to listen to unknown video files.
The spectrum of the best (#10), worst (#6), and weird (#2) are shown in the plot below,
Clearly youtube generally does not provide ideal pink noise. Only video #10 is close to being a high quality source. Whether or not these differences result in a difference in treatment effectiveness is unknown.
Pink Noise through iTunes Equalizer
Equalizers are a common way to knock down high frequencies that may cause discomfort. Looking at pink noise that has gone through an equalizer is a good way to see exactly what common multi-band equalizers do to the frequency content of an audio file. Below is the equalizer settings set in iTunes in order to reduce the high frequency content of ideal pink noise:
The spectrum of the pink noise at the output of the equalizer is shown below:
The frequency content in the filtered region is not ideal. Keep in mind that when pink noise goes through headphones or speakers, there will be additional fluctuations in the frequency response. Pink noise will never be ideal.