How to Successfully Calibrate Home Theater Audio Systems

(Part 1 of 2 – Control Audio Distortions)

 

 

BY PAUL NIES, 21st Century Technologies

 

"…a quality calibration job means more to good sound than any of the customer’s expensive components”

Gerry Lemay, President

Home Acoustic Alliance

 

After your customer has carefully auditioned and selected his choice of audio electronics, your opportunity for success as a home theater audio calibration professional (dare we say your duty?) is making sure that his home theater audio system provides an optimum listening experience. In previous articles we have covered specific calibration “How To’s”, such as calibrating speaker delay time, minimizing room modes, and reducing reflections.

 

Improve Home Theater Sound

/newsletter/May03/Delay1.html

Measure RT60 Reverberation Decay Time

/newsletter/Mar02/SP295RDT.html

Achieving a Flat Audio Response Quickly and Easily

/newsletter/Oct02/WhyMicMultiplexer.htm

In this article we’ll step back from the “nuts and bolts” and look at the total calibration process to see how to maximize your calibration success and customer satisfaction.

 

“... the room is the final audio component...any improvement is a tremendous asset to both loudspeaker manufacturers, and customers. Those people who know how to elicit good sound from loudspeakers in a room...are the ones who can truly serve their customers, by delivering something tangible: great sound.”

Excerpt from MAXIMIZING LOUDSPEAKER PERFORMANCE IN ROOMS, Floyd Toole, VP Acoustical Engineering Harman International Industries, Inc.

 

 

Let’s begin by stating two basic truths that are fundamental to a successful calibration

1)      An audio system is not just electronics

2)      The correct use of acoustic treatments and correct speaker placement must be based on sound measurements

 

An Audio System is not just electronics

A home theater audio system is more than just the amplifier, wires and speakers; an audio system includes everything inside the room, including the walls, furniture, carpet and pictures. Room characteristics are major factor of the sound quality in a home theater, contributing as much as 20-30 dB to the reproduced sound don’t forget this if you want to

 

successfully calibrate home theater sound. Room modes are the biggest symptom of room acoustics, but higher frequency reflections must also be properly dealt with. Optimizing these conditions often requires that you make adjustments to the room itself, such as moving furniture, moving speakers, or adding wall treatments (drapes, pictures or even acoustic panels).

 

Before starting the calibration, be sure your customer understands that optimizing his home theater sound may mean making some “physical” changes to the room and that any compromises mean less than idea sound. As a professional, your job is to recommend and implement the best acoustical and customer-friendly solution, which brings us to the second basic truth:

 

 

Correct use of acoustic treatments and exact speaker placement and spacing must be based on sound measurements

A big mistake that many audio calibration professionals make is relying too heavily on measurements and formulas to tune a room. Calculations are an excellent starting place, but every room is unique – the furniture is different sizes, the carpet has a different thickness, not to mention the placement of doors and windows. A successful calibration requires having the proper tools to analyze the sound within the room, understanding how to use them, and knowing what to do with the measurement results. At a minimum you need the following tools:

 

 

Tool or test

Use

 


The SP295 Audio Suite provides all of the necessary tools you need to successfully calibrate a home theater audio system.

Polarity tester

Insure proper wavefront  phase

 

SPL measurement, A, B, C weighted

Basic sound level measurements Calibrating speaker levels

 

RTA measurement, 1/12th octave resolution

Analyze frequency response

Identify and minimize room modes

Set crossover frequency

 

Spatial Averaged SPL & RTA measurements

(average of multiple locations)

Determine best overall performance

 

ETG graph

Calibrate processor time delay

Identify and analyze reflection problems

ITDG - quantify amount of acoustic treatment

 

Noise Criteria meter

NC checks

THX certification

 

Pink Noise generator

Reference for sound level measurements

 

Audio sweep generator

Identify resonant objects

 

Measurement worksheet

Track measurement scenarios to aid in identifying optimum sub placement and listening positions

 

 

 

 

 

Distortions that Ruin Home Theater Sound

 

When you calibrate a home theater audio system, you are removing (or at least minimizing) all of the distortions that keep the reproduced audio from matching what the director heard in the mixing studio. Since most movie mixing studios have identical room acoustics, you can calibrate home theaters to a known standard. To keep our discussion centered on the room, which you can calibrate, rather than on audio components that have their performance characteristics manufactured in, we’ll assume the audio system is build around good quality amplifiers, speakers, and cabling. Here is an overview of the distortions that inhabit an uncalibrated home theater system.

 

 

 

Spatial Distortions

 

A properly calibrated system has tightly localized soundstage imaging and centered dialog for all seating positions, and envelops the listeners with 3-D ambience and audio effects. Spatial distortions skew the soundstage image, making sounds originate closer or farther away, or from a different direction; ambient sounds become directional rather than subtle; and dialog fails to follow the on-screen actors as they move. Spatial distortion occurs when the reproduced sound from one (or more) loudspeakers reaches the listener at the wrong amplitude relative to the sound from the other speakers. There are three causes of spatial distortion.

 

 

 

Spatial Distortion Caused by Unequal Speaker Levels

A basic cause of spatial distortion is unequal sound level from all loudspeakers. The cure is simple – calibrate all speakers to reproduce the same sound level at the listening position. You’ll need an SPL meter for this, as explained in Calibration step #5. Ten Steps to Optimized Sound will be explained in part 2 of “How to Successfully Calibrate Home Theater Audio Systems” – see the next SENCORE News.

 

 

 

 

 

 

Spatial Distortion Caused by Different Speaker Distances

The direct sound from each main speaker must reach the listener at the same time so their wavefronts will interact properly to create the desired sound image. Delay the sound from one speaker a few milliseconds and the resulting sound image becomes too loud or soft, or originates from a completely different location. Time delay calibration in surround sound processors compensates for different physical distances between the speakers and listener. To accurately calibrate time delay, you’ll want to set acoustic distance using an ETG measurement, rather than the physical distance, as explained in step #4 in the next SENCORE News.

 

 

 

 

 

 

Spatial Distortion Caused by Room Reflections

The sound in a home theater room is largely the result of sound waves reflecting off the walls, ceiling, floor, and other hard objects. The strength and timing of the reflections creates the room’s characteristic ambience. (Think of the sound in a large auditorium – long reflection/ reverberation time, vs. sound in a small room – short reflection/reverberation time). To prevent spatial distortion, the reverberation time and the Initial Time Delay Gap (time between the direct sound and the first strong reflection) of a home theater must closely match the mixing studio. Since home theater rooms are much smaller than auditoriums, adding wall treatments to achieve a 25-30 millisecond ITDG as explained in Calibration step #6, solves spatial distortion reflection problems.

 

 

 

 

 

 

Tonal Distortion

 

Tonal, or frequency response distortion, deals with the proper reproduction of audio tones that are close together in frequency. Ideally, the frequency response at all locations in a home theater room should be completely flat, but in reality, you’ll never obtain that ideal and you’ll need to strive for the best overall response instead. As you work towards a flat response, you will battle three factors.

 

 

 

 

 

 

Tonal Distortion Caused by Room Modes

The biggest contributor to tonal distortion in a home theater is room modes. Room modes are low frequency (<120 Hz) standing waves that are caused by the interaction of the room’s physical dimensions and the wavelength of the audio tone. Room modes cause areas of loud, boomy bass and other areas of weak or missing bass. You can not eliminate room modes, but you can minimize their acoustic effects at designated listening positions. To do this, you will need good resolution RTA, as explained in Calibration step #7. Ten Steps to Optimized Sound will be explained in part 2 of “How to Successfully Calibrate Home Theater Audio Systems” – see the next SENCORE News.

 

 

 

 

 

 

Tonal Distortion Caused by Room Reflections

A certain amount of reflected sounds are desirable to create the desired room ambience. However, strong amplitude reflections that arrive at the listener at the wrong time can add to or subtract from the direct sound and cause frequency suckouts or peaking, or “slap echoes” which sound like a pinging or whizzing at certain frequencies. To solve distortion caused by room reflections, add wall treatment to the reflection “mirror” points, as explained in Calibration step #6.

 

 

 

 

 

 

Tonal Distortion Caused by Speaker Response

The subwoofer needs to begin reproducing the audio frequencies that fall below the range of the main speakers. If the crossover frequency is set too low, there will be a hole in the low audio frequencies; set the crossover too high and both the subwoofer and main speakers reproduce some of the same frequencies, making them too loud. To properly set the crossover frequency, use an RTA, as explained in Calibration step #8.

 

 

 

 

 

 

Noise Distortion

 

 

 

Background Noise

Too much background noise causes listening fatigue, drowns out subtle ambient sounds, and distorts loud program content when the listener turns up the volume to hear the soft dialogue. Home theater rooms should have an NC rating of 25 or lower, while a THX certified theater should have an NC rating of 15 or lower as explained in Calibration step #1.

 

 

 

 

 

 

Resonant Objects

You’ve just completed a calibration job and the customer is watching his first movie - Apollo 13. Right in the middle of liftoff the left surround speaker that you mounted on the wall begins to vibrate. Will you be embarrassed when the customer gives you a call? You should be! To find resonant objects you’ll need to perform a low frequency sweep, as explained in Calibration step #9.

 

 

 

 

 

 

To properly calibrate a home theater audio system, you need to control the audio distortions that alter the reproduced sound. Successful calibration can be accomplished by following a logical 10 step approach, and using the proper measurement tools. The SP295 Audio measurement suite provides you with the tools that you need to successfully accomplish this task.

 

 

 


 

 


 

 

The MX299 Microphone Multiplexer couples several measurement microphones to the SP295’s input to allow fast measurements at distributed locations throughout the room.

 

/products/cp5000/MX299 Cut Sheet.pdf

 

The SP295 ‘SoundPro’ Audio Analyzer provides the acoustic measurements and tests that identify and resolve acoustic problems.

 

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The SP295 Report Software and Worksheets allow you to quickly analyze multiple room scenarios, and provide your customer with before and after documentation.

 

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For more information on how to successfully calibrate home theater sound systems, or to learn more about the SP295 Audio suite, call 1-800-SEN-CORE, or click this link: /forms/sp295pricing.asp.

 

 

 

 

 

Paul Nies has been associated with Sencore for over 23 years in various capacities, including Application Engineer, Technical Trainer, and Multimedia Producer. Paul conducts numerous seminars on home theater audio and video for Sencore, including the CEDIA regional classes. He also provides application engineering and training support for Sencore’s video and audio products.

 

 

 

 

 

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