Sound Delay Explained
Delay of audio is one of the main design considerations for any audio application.
The concept is important in applications where multiple speakers are used through an audience area or venue. The concepts also applies to stage layout, microphone placement and loudspeaker design.This article will explain the concept and the main environments to which sound delay tactics apply.
What is sound delay?
Sound takes time to travel. This means that we hear things after they have happened. Sometimes we notice this and other times we do not. Sound delay is the time between when something happens and when we hear it.
A good example of this is lightning and thunder. We see lightning immediately but it takes longer for thunder to reach us. Another example is during very large concerts where the audience's response to music may have a delayed effect. For example everyone jumping on a beat can give a quick "wave" effect as people further down the arena hear the sound.
How fast is sound?
Sound travels at a speed of 340 metres per second (760 miles per hour). This is subject to small changes depending upon air density, temperature and humidity. The table below shows speed comparisons which helps us to understand relative speed:
- Fastest bird flight (Peregrine Falcon) - 68 Mph
- Boeing A380 cruising speed - 587 Mph
- Speed of sound - 760 Mph
- Land Speed record (Thrust SSC by Andy Green) - 763 Mph
- Concorde cruising speed 1354Mph
- F14 Fighter Jet max speed - 1544 Mph
- Speed of Light - 670,616,629 Mph
So while sound does travel quickly, the speed is one we can comprehend and visualise.
Why does this matter for live events?
Imagine yourself standing at one end of a football pitch. At the other end there is music being played from a speaker. On a quiet day you can hear the music and, although it sounds distant, you can hear what is playing quite clearly.
Imagine that another speaker is setup at the halfway line and playing the same music. What would the music sound like? You would hear the music twice, as one sound source will reach your ears followed quickly by the other.
The delay will be approximately 1/6 of a second. This will make the music much harder to hear (in the audio industry we call this "reduced intelligibility"). Now imagine two more speakers are added at different distances down the football pitch. Now you are hearing the music four times but at slightly different times (we call this "offset"). Consequently, the sound will become poor or "muddy". If this was the spoken word as opposed to music it would become hard to hear or to understand.
So sound delay can cause an issue and render a sound system almost useless.
Why Not just use one big speaker?
This is a very good question and there is merit in this practice. If we just had one very big speaker we might not need many smaller ones and all the issues that go with time delay are gone. The reason that this is not done is because of coverage.
Image this giant speaker sitting on the ground at one end of the football pitch. Now imagine how loud it would need to be to give the person at the end of the football pitch a good volume. If you were standing next to that speaker it would be extremely loud. Anyone living nearby would certainly not appreciate this super-loud speaker blaring away.
Instead we use distributed speakers. This means many speakers are placed throughout the audience area. As they are spaced out, the audience will be closer to the sound which means they do not need to be as loud therefore enabling the audience members to enjoy the sound at a comfortable volume.
So how are delays speakers used?
Return now to the football field example of four speakers.To counter the problem of lots of sound sources reaching you at different times we add a delay to each speaker. This means that the sound comes out of each speaker a little later than the first speaker. How much later depends upon how far it is from the first speaker. We call this process "delaying back".
The sound then travels down the pitch and, as the sound from the first speaker reaches the next speaker, then the second speaker makes the same sound and therefore adds this to the sound field.
The effect of this is that each speaker is almost indistinguishable from the other sound sources. The result is that the audience hears clear, accurate and precise audio.
A real world use of this method could be an indoor concert. We may be using a large main sound system, such as a Martin Audio Line Array. But it may also be necessary to have additional smaller loudspeakers further down the event space and under a balcony to provide even sound coverage. In such a case these smaller speakers would have time delay applied to align them with the main sound source
In speaker design
Audio delay is also a very important concept in loudspeaker design, although the delay effects are much more subtle than in multi-speaker arrangement.
If the components (such as woofers and tweeters) within a loudspeaker are not in line with each other (common in most speakers) the sound will arrive with the listener at different times. This means that high frequencies may be heard earlier/later than low frequencies. Although the time delay is minute, it is enough to "colour" the sound.
To counter this aberration, the individual components within a sound system may be "time aligned" to the component that is furthest from the listener. This way the sound at all frequencies is emanating at exactly the right time. This time alignment is done using sound delay equipment as part of the sound system's Signal Processing System.
Few people can distinguish a specific delay from one speaker component to the next but most people can tell that a correctly time aligned system sounds better than one that is not.
This practice is commonplace within professional audio but is rare in the semi-professional and hi-fi fields of audio. In our company we use time delays for all 2-way, 3-way and 4-way active systems.
How is time delaying done?
Every loudspeaker component we use is controlled by a Digital Signal processor (DSP). These devices have an array of control functionality of which one is time delay. We are able to time-align loudspeaker components individually, as well as entire sections of an audio system.
In cases where it is necessary to time-delay microphones (such as in orchestral sound reinforcement) delay equipment is inserted into the sound desk.
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