We hear a lot of sounds every day. Each type of sound has a different characteristic such as quality, loudness, pitch, wavelength, frequency, and velocity. Sound is a mechanical wave and it needs a medium to travel. Some of the major things that can happen to a sound wave while traveling through a medium are- Reflection, transmission, diffraction, and absorption. Sometimes, we hear the same sound repeatedly, at clear intervals. This is called an Echo and it happens when sound waves are reflected from a distant object until it reaches our ears. A reverberation takes place due to the reflection of sound waves by nearby objects, causing us to hear a prolonged sound wave even after the source has stopped making that sound.
The primary difference between an echo and a reverberation lies in the fact that our ears can differentiate the sounds coming from an echo as two separate events, whereas reverberation appears as one prolonged, continuous sound.
What is an echo?
Sound waves get reflected off an object to creating an echo
When we hear the same sound two or more times, after a short interval of at least 0.1 sec, it is called an echo. It is caused by sound waves bouncing off a distant object.
It is important to note that, the short interval should be of at least 0.1 sec, as below that, our brain does not consider the repeated sounds as separate events, but as one extended process.
This is because the impact of sound persists in our brain for 0.1 sec.
You may ask, if there is a minimum limit for time, there must also be a minimum distance for the reflecting object. The minimum distance for the reflecting object is 17.2 meters in order to hear an echo.
And we can also calculate this,
The speed of sound in air, at room temperature, is 343 m/s.
The minimum time interval between the original sound and echo heard is 0.1 s
If the distance between sound origin and the wall is "d", The total distance traveled by the sound wave is 2d, for both forward and return journeys.
We have to find ‘d’ to get the minimum distance
Taking the formula v = 2d/t , we have
343 m/s = 2d/0.1s
2d = 343 x 0.1
Therefore, d = 17.15, which is approximately 17.2 meters.
Are Echoes Desirable or Undesirable
Instances of echo can be easily found in nature, for example, when you are inside a cave or near a mountain, you can hear your claps or the sound of bird songs again and again. Here echo adds to the aesthetic value of nature.
Echoes are also desirable in sonars and some music productions.
But sometimes echoes can be undesirable. For instance, during telephonic calls, echoes can cause a lot of problems and lead to crosstalk.
Echoes are also undesirable in big halls and movie theatres because they interfere with the original sound and create confusion.
How to Prevent or Reduce Echoes
Echoes can be reflected by both hard and soft walls, but are more distinguishable when reflected by hard surfaces. This is because the sound waves are absorbed by soft surfaces. Thus, there is little sound energy that travels back to the listener.
Thus, it is recommended to use as many soft surfaces as possible to reduce echo inside rooms, big halls, or recording studios. Curtains, rugs, wall coverings, carpets all help in preventing unnecessary echoes. Furniture like sofas and bookshelves also help a lot in absorbing sounds. That is the reason why you can hear a lot of echoes in an empty flat. Many of you may have experienced it when your neighbors shifted to a different house.
Echo-proof studios are also fitted with foam-lined surfaces that can absorb sound. Sound-absorbing acoustic panels, made of foam and mineral wool, are used to line the walls of big halls, studios, and theatres to avoid echoes.
What is Reverberation?
Reverberations are short sound reflections that occur when the sound waves reflect from multiple hard surfaces inside an enclosed place. These sounds are reflected so quickly that they overlap each other and appear as though coming from a single source, and persisting beyond the normal time.
Reverberations may appear just like an echo, but in actuality, they are much shorter sound reflections compared to an echo. If the reflected sound reaches the listener in less than 0.1 sec of the original sound, we hear a reverberation.
Reverberation is generally heard in closed spaces such as gyms, small studios, and restaurants. It is also heard while we sing in the shower.
Although reverberations occur in open spaces and nature, they are usually not detected by the human ear.
Reverberations caused by sound waves reflecting off multiple surfaces
Reverberation does not happen endlessly. It eventually fades away and the time required for the sound to die down by 60 decibels, once the source of the original sound has stopped is called ‘Reverberation Time’.
Advantages and Disadvantages of Reverberation
Reverberations occur naturally when a person speaks or makes a sound in a hall or room. But high amount of reverberations is a disadvantage as they overlap the original sound. Thus, the listener cannot comprehend what the speaker is saying.
Generally, a limited amount of reverberation is desirable depending on the situation.
In music halls and orchestras, for example, reverberations add depth to the music and give it a soothing feel.
The precise amount of reverberation time also varies and is adjusted based on the needs.
Reverberations are also advantageous for many kinds of music sound mixers. Sound engineers often add these effects to songs, recordings, and musical instruments.
Thus, the right amount of reverberation adds volume and depth to the music or speech, but too much of it can cause the program to become garbled and incomprehensible.
How to Prevent or Reduce Reverberation
Though reverberations add a pleasant effect in live music or a recording, it is problematic in other instances, like podcasts. The solution is to use acoustical foam which can absorb the sound waves instead of reflecting them off the walls of a room.
Similarly, adding furniture, cushions, rugs, fabric panels and other sound-absorbing materials can help prevent reverberations inside a room.
In conference halls and dining rooms, cover the floors with wall-to-wall carpeting and the walls with fabric panels to avoid reverberations.
Difference Between Echo and Reverberation
Pictorial representation of difference between Echo and Reverberation
Echo:
It is caused when a sound is heard repeatedly at some interval, due to the reflection of sound waves by surfaces.
It is heard when the time interval between the source and the reflected wave reaching the ear is more than 0.1 sec.
The echo is clear and heard as two separate sounds.
The minimum distance of the reflecting object is 17.15 m
Reverberation:
It is caused by the reflection of sound waves multiple times to give a prolonged hearing effect of the original sound.
It is heard when the time interval is too small to distinguish.
Reverberation is comparatively less distinguishable.
There is no specified minimum distance.
Conclusion
To summarize what we have learned, reverberation is the persistence of sound after the source ceases to make a sound. It is caused by a large number of reflections and is heard as a single continuous sound.
Echoes are repeated sounds, heard distinctly after being reflected by a source at a considerable distance.
If the time interval between the two sounds is less than 0.1 sec, it is no longer considered as an echo, but a reverberation. This happens as the human brain cannot differentiate between the two events.