ITEA Technical SeminarQSC Audio Products,Inc.1 Audio Basics for Cinema Sound Engineers By Barry Ferrell Director of Product Strategy QSC Audio Products,

ITEA Technical Seminar

QSC Audio Products,Inc. 1

Audio Basics for Cinema Sound Engineers

By

Barry FerrellDirector of Product Strategy

QSC Audio Products, Inc.

Vice-President

International Theatre Equipment Association



Introduction

Digital film soundtracks have greatly increased the demands on cinema power amplifiers and speakers.

To better understand the magnitude of this increase and how to deal with it, it is necessary to understand some basic audio terms and concepts.



Key Concepts and Terms

The Decibel

Amplifier power in dBW

Power Compression

Inverse Square LawHalf-Space

Quarter-Space

Mutual Coupling



The DecibelThe decibel is a ratio between two values, NOT a unit of measure.

Sound pressure level is expressed in decibel form, as dB-SPL.

The ratio of a given volume level to a reference volume level.

The decibel may refer to electrical power and voltage ratios.



The Ratio of Power Levels

Given by the following formula:

ratio in dB = 10 Log (W/Wr)

W equals the measured power in watts

Wr equals the reference power in watts



The Relationship Between Levels in dB is not Linear

For example: Assume a theatre has a background noise level of 30 dB-SPL and a maximum level of 120 dB-SPL can be achieved.

At first glance, an increase from 30 to 120 dB-SPL may look like only a four to one ratio.



What is the Actual Power Increase?

This equation shows the relationship of changes in dB to power.

(W/Wr) = 10[(dB-dBr)/10]

where W equals the measured power and Wr equals the reference power and where dB equals the measured SPL and dBr equals the reference SPL



For Our Example, We have:

(W/Wr) = 10[(120-30)/10] = 10(90/10) = 109 = 1,000,000,000

The power ratio is an astounding 1 billion to one, not the 4 to 1 ratio that may have been assumed at first glance.

The loudest level is 1 billion times more powerful than the quietest level.



Common dB Power Ratios

A 3 dB increase represents twice the power

A 6 dB increase represents 4 times more power





How Digital Soundtracks Increase Power Requirements

Dolby A-type soundtracks can reproduce peaks of 91 dB-SPL per channel.

Relatively small amplifiers and speakers are capable of reproducing this level.



Digital Soundtrack Requirements

Peak levels of 103 dB-SPL, per channel, at all frequencies.

A 12 dB increase over the levels of a Dolby A-type soundtrack requiring 16 times the peak power.



Methods to Increase System Power Output

Add More Amplifier Power

Bi and tri-amplification

Horns and Multiple Speaker Arrays

Speaker Mounting and Location



More Amplifier Power

An obvious way to increase the SPL capability.

However, the magnitude of the increase and the practical power capacity of loudspeakers indicates that a 16 times increase in amplifier power is not a viable choice.



A Better Way to Look at Amplifier Power

Convert it to dBW, referred to a power level of 1 watt.

We can see how power increases will affect SPL in a theatre.

Power ratings in dBW may be directly added to speaker sensitivity to get a theoretical peak output level.



For Example, What is the Output in dBW of a 200

Watt Amplifier? Use the following equation.

dBW = 10 Log (W) where W equals the rated amplifier power in watts

therefore,

10 Log (200) = 10 x 2.3 = 23 dBW



Power Output, in dBW, for a Variety of Amplifier Sizes

Note that doubling the power results in an increase in dBW of only 3 dB, a very moderate increase in SPL.

100 W = 20.0 dBW 350 W = 25.4 dBW 800 W = 29.0 dBW150 W = 21.8 dBW 400 W = 26.0 dBW 1,000 W = 30.0 dBW200 W = 23.0 dBW 450 W = 26.5 dBW 1,200 W = 30.8 dBW250 W = 24.0 dBW 500 W = 27.0 dBW 1,500 W = 31.8 dBW300 W = 24.8 dBW 600 W = 27.8 dBW 2,000 W = 33.0 dBW



Speaker Limitations

Power CompressionSpeakers are not linear devices

More power input may not result in proportionally more SPL output

Voice coil heating, cone excursion and box vent size and shape can limit maximum total output



Effect of Port DesignThe Ideal response is linear. A 10 dB increase in power produces a 10 dB increase in SPL.

Real World designs fall short of the ideal by varying degrees.

Bigger ports are generally better.



Power CompressionPower compression of seven 15” woofers.

Voice coil diameter and cooling techniques are key factors.



Wave Length of Sound

At room temperature, the speed of sound, called “c” is 1130 ft/sec.

The wavelength of a sound wave is the velocity divided by the frequency.

Wavelength= c/f

for 80 hertz, the wavelength = 1130/80 = 14.1 ft



Wavelength at a few Frequencies

Frequency (Hz) Wavelength (ft)10,000 0.111,000 1.13100 11.380 14.150 22.520 56.410 113



Horns and Multiple Speaker Arrays

Used to increase the sensitivity of loudspeakers.

Horns increase on-axis sensitivity by directing the resulting sound over smaller, more well defined areas.

Concentrating energy on the audience has the added benefit of increasing the intelligibility of dialog.



Horn LimitationsHorns are typically employed for midrange and high frequencies.

The long wavelength of low frequency sound waves makes it impractical to build horns of sufficient length and mouth size to provide true horn loading and uniform pattern control at the lowest bass frequencies.

Ideally, horn mouth circumference should be 3 times the wavelength of the lowest frequency, 156 ft at 20 Hz!



Beamwidth of LF Horn

Note the loss of patternControl below 200 HZ



Multiple Speaker Arrays

Multiple direct radiator boxes may be combined to provide excellent real-world efficiency at low frequencies.

Typical cinema sound systems are perfect examples of this approach to sound system design.



Mutual CouplingAllows multiple, closely-space woofers to produce more sound per amplifier watt than single loudspeakers.

Works better at lower frequencies and when the placement of the speakers is as close together as possible.

Array size should be small, compared to the wavelength of the sound being reproduced.



Mutual Coupling continued

For every doubling of the number of speakers, the maximum possible output increases by 6 dB-SPL.

Half of the increase is due to doubling the power capacity of the system and the rest of the increase (another 3 dB) is the result of mutual coupling.



Acoustic “Space”An omni-directional point source radiates into “Whole” space.

A source on a large plane radiates into half of the total space, this is “half-space”.

A source on the junction of two right-angle planes radiates into “one- quarter space”.



Acoustic “Space” continued

Half-space is simulated by screen channels in a baffle wall. This is also the condition under which most speaker sensitivity specs are obtained.Quarter-space is simulated by subwoofers at the floor level in a baffle wall.

In theory, this results in a 6 dB decrease in power needed for a given SPL compared to half-space mounting.A real world decrease of 3 to 4 dB is not uncommon.



Speaker SensitivityExpressed as output SPL measured at one meter for an input of one watt.

Sensitivity has been used as an indication of speaker efficiency and maximum output capability.

However, a higher sensitivity may not mean a higher maximum output SPL.

Maximum power capacity and power compression also greatly affect maximum output levels.



Important Speaker Performance Issues

Low distortion

Uniform dispersion of all frequencies.

Smooth frequency response.

Linear input-to-output response over a wide power range.

Not all of these factors are compatible with high sensitivity.



Subwoofer Requirements Design for 113 dB-SPL in the middle of the room.

Subwoofers get an additional 3 dB boost (minimum) in sensitivity when they are placed on the floor at the base of the baffle wall behind the screen.

As many as 4 to 8 woofers exhibit good mutual coupling at the low frequencies of subwoofer operation when stacked in a tight array.



Typical Subwoofer Sensitivity Ratings

Number of Woofers

Sensitivity (1/2 space)

Sensitivity (1/4 space)

1 96 dB 99 dB2 99 dB 102 dB4 102 dB 105 dB8 105 dB 108 dB



The Effect of Distance on SPL

Outdoors, the sound level will drop 6 dB for every doubling of distance.

Using this rule, the resulting calculations will have a built-in reserve of capacity.

Guarantee headroom by calculating the power required for the woofer only, then use the same power for the HF driver of a bi-amp system.

Typically at least a 3 dB cushion with pink noise.



Inverse Square Law



Inverse Square Law



Indoor SPL vs. Distance

Worst case scenario

given by free field

calculation



A Theoretical Example A theatre is equipped for Dolby A-type soundtracks. The distance, in meters, to a point 2/3 of the way back from the screen is 50 ft or 16 meters.

The loss due to distance would be 6 dB for every doubling of the distance, starting with one meter. Since 1 x 2 x 2 x 2 x 2 = 16, we need to double the distance four times.



Example continuedFour times 6 dB gives a loss of 24 dB.

Check this with 20 Log(D), where D is in meters.

The amplifiers are 100 watts per channel or 20 dBW.

A small, single-woofer speaker with a passive crossover and a sensitivity of 97 dB is used.



Projected Maximum SPLAdd the amplifier power, in dBW, to the speaker sensitivity and subtract the loss due to distance.

In this case, 97 dB + 20 dB - 24 dB = 93 dB-SPL.

This represents the minimum SPL that one channel would be expected to reproduce at full output, a real system could be 2 to 3 dB higher for short peaks.



Digital Requirements

A 93 dB-SPL capability is adequate for Dolby A-type soundtracks, which require only 91 dB-SPL.

However, it falls 10 dB short of the needs of a digital soundtrack.

Ten times the amplifier power, 2000 watts, would theoretically yield the desired 103 dB-SPL.

This doesn't work in the real world as the speaker power rating would be exceeded.



A Better Solution Double the woofer section and use bi-amplification to power the woofer and horn section separately.

Mutual coupling increases the woofer sensitivity to 100 dB.

The power increase is shared by two woofers and the system needs only 7 dB more power.



Better Solution continuedOur initial amp power was 20 dBW, adding 7 dB gives us 27 dBW.

From the preceding chart, a power rating of 500 watts is equivalent to 27 dBW. Check with W=10dBW/10.

This is a practical amplifier size and the power is split between two woofers for a total of 250 W each, well within typical power ratings.



The Real WorldBi-amplification, room reflections and amplifier dynamic headroom will usually produce levels at least 3 to 6 dB higher than this calculation.

Real-world systems could probably use 350 watts per channel with little chance of power amplifier clipping or distortion.



Typical Subwoofer Recommendations

Assumes 3 to 6 dB increase in sensitivity from quarter space mountingMutual coupling begins to fall off for 4 to 8 subwoofers due to the large array size300 watts per woofer used as input power for all calculations due to mechanical excursion limits at low frequencies. Power ratings on specification sheets are typically the voice coil thermal limit and are usually much higher than the mechanical limit for typical cinema subwoofers. Systems designed to need more than 300w per woofer may experience mechanical failure, even if the thermal power rating is not exceeded.SPL is calculated at 113 dB at ½ the auditorium depth.Amplifier headroom and room gain make these estimates fairly conservative.

Quarter-SpaceSub Sensitivity (dB 1w, 1m) Recommended Maximum1x18 99-102 23 322x18 102-105 46 654x18 106 102 1028x18 108 135 180

Cinema Length (Ft)



ConclusionsA realistic understanding of the demands of digital soundtracks is necessary for the proper design of cinema sound systems.

All of the concepts discussed in this article can be applied to any actual theatre sound system.



A Few GuidelinesUse enough speakers to do the job correctly, most speakers don't get much louder after the first 300 watts per woofer.

Using more speakers has the benefit of spreading out the load and reducing the total power required due to mutual coupling, a real win-win scenario.



Guidelines continuedTake advantage of the benefits of bi and tri-amplification and buy the best speakers and amps you can afford.

Use Baffle walls and close-coupled arrays to increase speaker efficiency.

The investment is not likely to be obsolete, due to the longevity of the products and the high standards set by current digital film soundtracks.

Keep the audiences amazed and they will keep coming back for more.



Any Questions?

Contact QSC Audio at (714) 754-6175Ask For

Sam HyndsFrancois Godfrey

OrBarry Ferrell



Recommended Reading:

The Yamaha Sound Reinforcement Handbook by Gary

Davis



QSC Audio

The Power Behind the Pictures



Documents

ITEA Technical SeminarQSC Audio Products,Inc.1 Audio Basics for Cinema Sound Engineers By Barry Ferrell Director of Product Strategy QSC Audio Products,