Hearing sound behind a loudspeaker is a phenomenon known as backwave radiation or rear-wave radiation. It occurs due to the design and construction of the loudspeaker enclosure and the interaction between the front and back waves produced by the speaker. Here’s a detailed explanation of how we can hear sound behind a loudspeaker:
- Loudspeaker Design:
- Loudspeakers are designed to convert electrical signals into audible sound waves. They consist of a diaphragm (or cone), a voice coil attached to the diaphragm, a magnet, and a rigid enclosure. When an electrical signal passes through the voice coil, it interacts with the magnetic field produced by the magnet, causing the diaphragm to vibrate.
- Front Wave:
- The vibrations of the diaphragm create compressions and rarefactions in the air, generating sound waves known as the front wave. The front wave travels away from the loudspeaker and is the intended sound that listeners are meant to hear.
- Back Wave:
- Simultaneously, as the diaphragm vibrates, it also pushes against the air inside the loudspeaker enclosure. This action generates a second set of sound waves known as the back wave. The back wave travels into the enclosure and, in some cases, can be directed toward the rear of the speaker.
- Enclosure Design:
- The enclosure plays a crucial role in determining the direction and behavior of the back wave. Different types of enclosures, such as sealed enclosures, ported enclosures, or open-baffle designs, can affect the interaction between the front and back waves.
- Ported Enclosures:
- In ported enclosures, a specific opening or port is designed to allow the back wave to escape. This can result in the perception of sound behind the loudspeaker, especially if the listener is positioned close to the rear of the speaker.
- Reflections and Diffractions:
- The surfaces and materials around the loudspeaker can influence the behavior of the back wave. Reflections and diffractions of the back wave off nearby surfaces can contribute to the perception of sound behind the loudspeaker.
- Interference Patterns:
- Interference between the front and back waves can create complex patterns of reinforcement and cancellation. This interference can affect the sound field around the loudspeaker, leading to variations in perceived sound depending on the listener’s position.
- Distance and Positioning:
- The distance between the listener and the loudspeaker, as well as the listener’s position relative to the loudspeaker, will influence the perception of the back wave. The effect may be more noticeable in close proximity to the rear of the speaker.
- Room Acoustics:
- The acoustic characteristics of the room, including its size, shape, and the presence of reflective surfaces, also contribute to the perception of sound behind the loudspeaker. Room acoustics can enhance or diminish the effects of backwave radiation.
In summary, hearing sound behind a loudspeaker is a result of the interaction between the front and back waves produced by the speaker. The design of the loudspeaker enclosure, the presence of ports, and the surrounding acoustical environment all contribute to the complex patterns of sound radiation that listeners may perceive behind the speaker.