I have seen a few questions on the need to delay the front of house speakers in a setup of recent times on some of the Facebook forums I'm on so I thought I would put together a discussion about when and when not to use delay in front of house.

We will assume at this stage we are talking about an indoor setup.

To start with lets establish the speed sound travels.

The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium.

In dry air at 0 °C (32 °F), the speed of sound is 331.2 metres per second (1,087 ft/s; 1,192 km/h; 741 mph; 644 kn).

At 20 °C (68 °F), the speed of sound is 343 metres per second (1,125 ft/s; 1,235 km/h; 767 mph; 667 kn), or a kilometre in 2.91 s or a mile in 4.69 s.

What atmospheric conditions affect the speed of sound.

The speed of sound in an ideal gas (air in our case) depends only on its temperature and composition. The speed has a weak dependence on frequency and pressure in ordinary air, deviating slightly from ideal behavior.

In common everyday speech, speed of sound refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance: sound travels most slowly in gases; it travels faster in liquids; and faster still in solids. For example, (as noted above), sound travels at 343 m/s in air; it travels at 1,484 m/s in water (4.3 times as fast as in air); and at 5,120 m/s in iron. In an exceptionally stiff material such as diamond, sound travels at 12,000 m/s; which is around the maximum speed that sound will travel under normal conditions.

Seen as we are talking about in indoor venues we will focus on air as that is hopefully the substance in the building.

For a given ideal gas the molecular composition is fixed, and thus the speed of sound depends only on its temperature. At a constant temperature, the gas pressure has no effect on the speed of sound, since the density will increase, and since pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly. In a similar way, compression waves in solids depend both on compressibility and density—just as in liquids—but in gases the density contributes to the compressibility in such a way that some part of each attribute factors out, leaving only a dependence on temperature, molecular weight, and heat capacity ratio which can be independently derived from temperature and molecular composition. Thus, for a single given gas (assuming the molecular weight does not change) and over a small temperature range (for which the heat capacity is relatively constant), the speed of sound becomes dependent on only the temperature of the gas.

So to summarise temperature is the only thing that can change the speed of sound. Given today that most buildings are air conditioned to day at 18-24°C we will assume that our sound is going to move at 343m/Sec for simplicity sake.

Human ear recognition.

Lets establish the distance a human can detect uncontiously the difference of a sound from 2 sound sources. Ie the source of the original sound and the speaker reproducing the sound.

A remarkable fact is that the pinna, the cartilage-filled structure surrounding the ear canal (commonly simply called the "ear"), is a vital part of direction sensing. Test subjects can be trained to locate sound using only one ear. But when the ridges of the pinna are gradually filled in, the ability is lost, in proportion to the filled in area. Apparently the brain uses reflections from the ridges of the pinna to determine direction. The head and pinna have a major effect on the sound that arrives at the ear.

The significance of the pinna reflection experiments for a sound system designer is that time delays on the order of 0.1 millisecond can effect sound imaging. Time delays between the left and right ear are on the order of 0.5 milliseconds, and are quite important. On the other hand, researchers have found that echoes in the range of 1 to 50 milliseconds are lumped together by the brain with the direct sound, so they are not actually heard as distinct echoes. Delays greater than 50 milliseconds are heard as echoes.

Echoes in the range of 25 to 100 milliseconds give a "cavernous" quality to the sound. What is commonly called an "echo," a distinct repetition of the original sound, only occurs for echoes of 400 milliseconds or longer. 

Distances for an effect the brain can detect

So if the brain can detect sounds differences in 25mS range then the difference in distance from the speaker to the listener in relation to the original sound source to the listener needs to be in the order of 8.5m to get a "cavernous" effect. A distinct reputation of sound is in the order of 136m.

To calculate a difference in a room is very difficult as there is 3 demential planes to calculate the distances on, and the width is of multiple differences that dependent on where the person is sitting.

Therefore if we simplify it to a 2 demential calculation the first arguable distance a person could detect a difference is if the speakers were 8.5m in front of the mic source. 

Other Considerations.

Now the source volume can be drowned out by the speaker volume dependent on the sound level of the source. i.e. source db < amplified db.

With this in mind the brain is less likely to detect a difference between the source and the speaker due to volume differences.

With this in mind I believe there is no need to delay the FOH (first row of speakers) in a system.

How ever is you have a staggered speaker system e.g. a speaker stack at row 1 of seating from your source then a boost set of speakers at row 20 and so on these speakers need to be delayed in relation to the distance in line of site from the first stack of speakers form the source.

Research for this post has been taken from

http://www.silcom.com/~aludwig/contents.htm

https://en.wikipedia.org/wiki/Speed_of_sound

About the author

Daniel Rogers

Owner and director of Intelligence By Design New Zealand and Australia. Daniel is currently living in Chinchilla with his wife and son where Intelligence By Design is based. Daniel has a New Zealand Certificate in Engineering focusing on Micro Processor systems. 14 years experience for TASC Systems LTD designing, servicing and installing control systems for fruit handling equipment both in New Zealand and around the world. Contracted to assist in the ISO-9001 and CE accreditation of a medical cleaning device for 12 months. Has also developed Microsoft Windows applications. He has 20+ years experience in sound engineering for Churches in both countries. He is currently doing web design and sound engineering in Chinchilla Australia.