The 7th topic
Struggle with ultrasonic sound --- underwater

Almost no radio wave can be reached underwater. So Logosease uses ultrasonic sound of around 32 KHz for communication.
Audibility zone for humans are up to around 20KHz. We can not hear the higher frequencies than that.
However, if ultrasonic sound of around 32KHz exists underwater, Logosease will collect the sound and release from a speaker as noise.
One good example is sound of fish sonar. The ultrasonic sound of "tong tong tong ,,," released in 1 sec. interval is quite large volume. This is what we introduced before in the column in the Logosease web site. (Please refer to the 5th column).
We can say that Logosease is a detector of ultrasonic sound.

To our surprise, ultrasonic sound are filled around divers much more than we had imagined. The ultrasonic sound are captured by Logosease and turned into noise to our ears. Let us introduce you how we have struggled with ultrasonic sound. Most ultrasonic is released from regulators. Various sounds are released from the passage of breathing air. Since its design is for the air to enter from a tank and to be released from mouthpiece, air flow makes sounds as instrument of bugles or whistle. Sounds varies depending on manufacturers, models, aged degradation level of regulators.

Our first struggle with ultrasonic sound was breathing sound.
Right after starting sales of Logosease, we received complains from customers that the noise of "zah---" was annoying .We asked them model names of regulators they used, and we purchased them by ourselves, and conducted test with hydrophone to check what kind of noise was released actually. We found ultrasonic sound was released around 30KHz to 50KHz in breathing as in the Table 1 It is called white noise.
Japanese regulators are smaller and almost no ultrasonic sound is released in breathing. On the other hand, many foreign-made regulators are bigger and some of them release large ultrasonic sounds of "shu---, ha---" in breathing. If breathing hard, the ultrasonic sound is released stronger. When a diver got tired after diving and he/she would be out of breath. Then the he/she breathes hard. If breathing hard, ultrasonic sound gets louder. We can not enjoy quiet diving unless we delete the noise of ultrasonic sound in breathing. We made digital noise canceller with the two purposes of below.
1. To enjoy quiet diving, ultrasonic sound generated in breathing will not be released from speakers.
2. When breathing sound and received voice sound are lapped over, the only breathing sound will be deleted so that receiving voice sound is easier to hear.
After testing and evaluating several regulators of world top 3 makers, we released the software on June 7, 2013.

Our next tackle was to deal with noise as whistle "pi,,,,,".
We purchased the regulator our customer told and collected the noise with hydrophone. The big noise as musical instrument was released.
As seen in the Table 2, extremely strong sound was released at around 35KHz and 70KHz. This regulator's special design at the passage for the air to go through releases sound like musical instrument. This special design is the main character of this regulator but at the same time, it becomes the source of large ultrasonic sound . The sound pattern is not fixed but changes depending on the breathing conditions as stronger/weaker and longer/shorter, just like a musical instrument.
They are completely different pattern from voice. We released software with a countermeasure by recognizing frequency change of non-voice and deleting it.

We also found several sources releasing ultrasonic sounds as air released from BC and dry suits, and alarm released from dive computers.
There might be other regulators releasing strong ultrasonic sound. If you hear something dissonant, please inform us.

Human can not hear ultrasonic sound. No-one including designers and customers feel anything wrong even ultrasonic sound is released.
Because we can not hear the noise so it is no problem. Only ultrasonic sound detecting devices as Logosease can get it.
But dolphins or marine mammals must hear it clearly. I wonder how they feel. They may be so interested to hear like " I hear pi--- pi--- every time they breath. what's that?" or maybe they are annoyed with the noise, or don't mind at all. We wish we could ask them.