Managing Audio Delay

January 24, 2018 by Ben Stowe, CTS

Sound travels fairly slow, with an approximate speed of 1128 feet per second (this speed changes with temperature and humidity, but not frequency). This can impact how we deploy our systems.

One of the ways it impacts DJs the most is during events where additional speakers are needed for coverage. For example, if we deploy speakers into a fairly long room and have one set of speakers near the head table of a wedding and another set 100’ back in the room to provide adequate coverage for people back there to hear we have a serious delay problem. The sound from the speakers at the head table will arrive approximately 90 milliseconds after the sound from the speakers placed 100’ back. This will create a substantial echo, and will greatly decrease the intelligibility of speech.

Speech clarity isn’t the only area of sound impacted by delay though, even short distances can contribute to delay problems because sound travels in waves. When waves of equal amplitude and frequency arrive at different times we have a reaction between those waves. This can cause the signal to increase, decrease or shift phase. The result to the listener is un-even sound levels, for example one part of the room is very boomy and another has no bass at all, or it just sounds weird to use a professional term.

This isn’t limited to speakers either. We experience this all the time with officiants and others who speak using lav microphones while holding a book. We hear the sound from their voice to the mic, and then reflected off the book and into the mic, and at slightly different times. This causes them to sound like they are using a (undesired) vocal effect because that audio is out of phase.

The solution with speakers is easy. We simply delay the further back speakers so that they “wait” for the sound to arrive from the ones at the head table to arrive. We can approximate this at .89 mS per foot, although most modern signal processors (including some built into speakers) will allow you to simply enter a distance in feet or meters. Noting that the speed of sound changes with temperature and humidity this may be slightly off, but should fall well within the imperceptible range at that point.

To learn more about how subwoofer placement can impact the coverage in a room, read my article on that here https://www.mobilebeat.com/subwoofer-placement/. A future article will detail time aligning subwoofers and high-frequency boxes to get tight, punchy, loud bass.

Ben Stowe, CTS Ben Stowe, CTS (25 Posts)

Ben’s love of electronics and technology led to years of schooling in Electricity, Electronics, Robotics and Lasers. Ben supported himself through school by building and selling strobe lights and other electronic devices. He built his first DJ show largely from scratch and scrap, often repairing broken items others had thrown away because he could not afford to buy new equipment. He holds a Minnesota electrical license, and his AV installs have been featured in almost every major industry trade magazine. His relentless passion for education has led to a number of other certifications and accreditations, including the most widely recognized one in the AV industry, the InfoComm CTS. His love for education inspired him to begin the ProAcademy educational sessions, focused on increasing understanding of AV technologies within the industry. Ben has been involved in a number of technical writings, lectures, presentations, as well as research and development assistance with a number of manufacturers for products, industry wide. He is also a regular contributing author to industry magazines in the United States and Europe. Ben’s presentations have been featured across the world both as a part of industry leading trade shows, and as a presenter for various groups and functions. Some of these events include BPM in the United Kingdom, Mobile Beat, the ADJA National Convention, Wedding MBA, and a national tour as a headlining presenter for an industry magazine. The United States Armed Forces branches have also called upon Ben to provide engineering and training assistance. His highly informational, slightly nerdy and always funny presentation style have made him a favorite at events, while his sincere desire to help people with their application of technology have made him a favorite with them after the event. Ben serves the industry as the President of NLFX Professional, an industry leading supplier of sound, lighting and video systems, a role he has maintained since founding the company in 1993.


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