Sound behaves in similar ways to water which is why it can be directed through tubes as was done in old ship's telephones. Between two and four speaking/listening stations can be installed and these are connected by underground pipes and a special sound distributor. It is possible to communicate over unusually long distances without the use of any electronic amplifiers. However, visual contact makes sense.
Recommended for
Barrier-free
Region availability
| Europe | Asia | North America | Africa | South America | Oceania |
|---|---|---|---|---|---|
Åland Islands | Afghanistan | Anguilla | Algeria | Argentina | American Samoa |
Albania | Armenia | Antigua and Barbuda | Angola | Bolivia | Australia |
Andorra | Azerbaijan | Aruba | Benin | Brazil | Cook Islands |
Austria | Bahrain | Barbados | Botswana | Chile | Federated States of Micronesia |
Belarus | Bangladesh | Belize | Burkina Faso | Colombia | Fiji |
Belgium | Bhutan | Bermuda | Burundi | Ecuador | French Polynesia |
Bosnia and Herzegovina | British Indian Ocean Territory | British Virgin Islands | Cameroon | Falkland Islands (Islas Malvinas) | Guam |
Bulgaria | Brunei | Canada | Cape Verde | French Guiana | Kiribati |
Croatia | Cambodia | Caribbean Netherlands | Central African Republic | Guyana | Marshall Islands |
Cyprus | Christmas Island | Cayman Islands | Chad | Paraguay | Nauru |
Czech Republic | Cocos (Keeling) Islands | Collectivity of Saint Martin | Comoros | Peru | New Caledonia |
Denmark | Georgia | Costa Rica | Côte d'Ivoire | Suriname | New Zealand |
Estonia | Hong Kong | Cuba | Democratic Republic of the Congo | Uruguay | Niue |
Faroe Islands | India | Curaçao | Djibouti | Venezuela | Norfolk Island |
Finland | Indonesia | Dominica | Egypt | Northern Mariana Islands | |
France | Iran | Dominican Republic | Equatorial Guinea | Palau | |
Germany | Iraq | El Salvador | Eritrea | Papua New Guinea | |
Gibraltar | Israel | Greenland | Eswatini | Pitcairn Islands | |
Greece | Japan | Grenada | Ethiopia | Samoa | |
Guernsey | Jordan | Guadeloupe | Gabon | Solomon Islands | |
Hungary | Kazakhstan | Guatemala | Ghana | Timor-Leste | |
Iceland | Kuwait | Haiti | Guinea | Tokelau | |
Ireland | Kyrgyzstan | Honduras | Guinea-Bissau | Tonga | |
Isle of Man | Laos | Jamaica | Kenya | Tuvalu | |
Italy | Lebanon | Martinique | Lesotho | United States Minor Outlying Islands | |
Jersey | Macau | Mexico | Liberia | Vanuatu | |
Latvia | Malaysia | Montserrat | Libya | Wallis and Futuna | |
Liechtenstein | Maldives | Nicaragua | Madagascar | ||
Lithuania | Mongolia | Panama | Malawi | ||
Luxembourg | Myanmar (Burma) | Puerto Rico | Mali | ||
Malta | Nepal | Saint Kitts and Nevis | Mauritania | ||
Moldova | North Korea | Saint Lucia | Mauritius | ||
Monaco | Oman | Saint Pierre and Miquelon | Mayotte | ||
Montenegro | Pakistan | Saint Vincent and the Grenadines | Morocco | ||
Netherlands | Palestine | Saint-Barthélemy | Mozambique | ||
North Macedonia | Philippines | Sint Maarten | Namibia | ||
Norway | Qatar | The Bahamas | Niger | ||
Poland | Saudi Arabia | Trinidad and Tobago | Nigeria | ||
Portugal | Singapore | Turks and Caicos Islands | Republic of the Congo | ||
Romania | South Korea | U.S. Virgin Islands | Reunion | ||
Russia | Sri Lanka | United States | Rwanda | ||
San Marino | Syria | Saint Helena | |||
Serbia | Taiwan | São Tomé and Príncipe | |||
Slovakia | Tajikistan | Senegal | |||
Slovenia | Thailand | Seychelles | |||
Spain | Türkiye | Sierra Leone | |||
Svalbard and Jan Mayen | Turkmenistan | Somalia | |||
Sweden | United Arab Emirates | South Africa | |||
Switzerland | Uzbekistan | South Sudan | |||
Ukraine | Vietnam | Sudan | |||
United Kingdom | Yemen | Tanzania | |||
Vatican City | The Gambia | ||||
Togo | |||||
Tunisia | |||||
Uganda | |||||
Western Sahara | |||||
Zambia | |||||
Zimbabwe |
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