1. How Speakers Turn Electricity into Sound and Microphones Do the Reverse
In a dynamic speaker, an electric current flows through a part called a voice coil. This coil moves back and forth because of how it interacts with a permanent magnet. Since the coil is attached to a cone, the cone moves too, shaking the air to create sound [1].
Inside a dynamic microphone, a small coil is attached to a diaphragm. When sound waves move the diaphragm, the coil moves through a magnetic field. This movement creates a changing voltage through electromagnetic induction [2].
When you look at them side by side, you see that both devices share three main parts: a moving diaphragm (or cone), a coil, and a magnet.
2. Can a Speaker Be Used as a Microphone Because Their Structures Are Similar?
Because the structures are similar, directing your voice at a speaker should move the coil and create electricity. In fact, standard speakers have a structure similar to dynamic microphones, so they are described as working as microphones when used in reverse [2].
Explanations of speakers state that the most common dynamic type has the same basic structure as a dynamic microphone. The microphone is described as working in reverse, like a generator [1]. A Japanese musical instrument shop also explains that dynamic microphones are made of coils and magnets and work exactly opposite to speakers [3].
Separate English and Japanese sources both explain that speakers and dynamic microphones have similar structures.
3. How Bell's Telephone Used Similar Devices to Send and Receive Voices
The Smithsonian’s 1876 telephone is made of an iron vibrating diaphragm, an electromagnet, and a permanent magnet. It is said to be one of two units used in a demonstration between Boston and Salem on November 26, 1876 [4].
When used for sending, sound at the mouth moves the diaphragm, creating a fluctuating current in the electromagnet. When used for receiving, the current moves the diaphragm, which shakes the air [4]. This means the same type of device was used for both the sending and receiving sides.
However, this telephone did not use a moving coil like a dynamic microphone. It used iron pieces and an electromagnet. It is not the same shape as a dynamic microphone, but the two are related in principle, even though their structures differ.
4. How Microphones Became Separate from the Original Telephone Parts
Bell’s telephone was used in commercial service in 1877, but the transmitter was later replaced by a carbon type. Meanwhile, the receiver kept the magnetic type [4].
In 1916, Wente of Western Electric invented the condenser microphone. It is described as the first true high-quality microphone that could capture sound naturally while having flatter characteristics than the previous carbon type [5].
5. The Strengths and Weaknesses of Using a Speaker as a Microphone
Using a speaker as a microphone does not always mean high quality. While cheap speakers have been used as microphones, one forum comment says they may not suit uses that need very accurate sound (high fidelity). This point was raised in a forum discussion, so it should be taken as one example.
Speakers are designed to move air to make sound, while microphones are designed to pick up small sounds. Even with the same parts, different design goals lead to different strengths and weaknesses. On the other hand, dynamic microphones have advantages like a simple structure, durability, and not needing power, which are listed in the Japanese instrument shop explanation [3].
An example of using this strength is the Subkick. For recording drums, there is an old technique of placing a speaker, such as the woofer from a Yamaha NS-10M, in front of a bass drum. The speaker cable is connected through a DI box to a microphone preamp to pick up low frequencies [6].
Yamaha made this into a product called the SKRM-100 Subkick. It holds a 6.5-inch speaker/microphone in a 10-inch shell. In measurements by a reviewer, there is a peak around 63Hz in the low frequencies. The reviewer stated that mixing it with a normal bass drum microphone helped support the low end [6].
6. How to Check for a Magnet in a Small Speaker Yourself
Check whether a small speaker has a magnet inside. If you have an unused small speaker, bring a paper clip close to the back. If the clip sticks, there may be a magnet inside. Do not push hard or take it apart. Keep magnets away from smartphones and cards.
If you want to read more, photos of Bell’s telephone are on the Smithsonian National Museum of American History collection page (in English). The story about drum recording is in a Tape Op review article (in English). In Japanese, the dynamic microphone explanation from the instrument shop is easy to read.