Audio Basics for Cinema Sound Engineers By Barry Ferrell Director of Product Strategy QSC Audio Products, Inc. Vice-President International Theatre Equipment Association ITEA Technical Seminar QSC Audio Products, Inc. 1
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. ITEA Technical Seminar QSC Audio Products, Inc. 2
Key Concepts and Terms The Decibel Amplifier power in d. BW Power Compression Inverse Square Law Half-Space Quarter-Space Mutual Coupling ITEA Technical Seminar QSC Audio Products, Inc. 3
The Decibel The decibel is a ratio between two values, NOT a unit of measure. Sound pressure level is expressed in decibel form, as d. B-SPL. The ratio of a given volume level to a reference volume level. The decibel may refer to electrical power and voltage ratios. ITEA Technical Seminar QSC Audio Products, Inc. 4
The Ratio of Power Levels Given by the following formula: ratio in d. B = 10 Log (W/Wr) W equals the measured power in watts Wr equals the reference power in watts ITEA Technical Seminar QSC Audio Products, Inc. 5
The Relationship Between Levels in d. B is not Linear For example: Assume a theatre has a background noise level of 30 d. B-SPL and a maximum level of 120 d. B-SPL can be achieved. At first glance, an increase from 30 to 120 d. B-SPL may look like only a four to one ratio. ITEA Technical Seminar QSC Audio Products, Inc. 6
What is the Actual Power Increase? This equation shows the relationship of changes in d. B to power. (W/Wr) = 10[(d. B-d. Br)/10] ITEA Technical Seminar where W equals the measured power and Wr equals the reference power and where d. B equals the measured SPL and d. Br equals the reference SPL QSC Audio Products, Inc. 7
![For Our Example, We have: (W/Wr) = 10[(120 -30)/10] = 10(90/10) = 109 =](https://present5.com/presentation/2bd285471f66463c70dc572f1860d688/image-8.jpg "For Our Example, We have: (W/Wr) = 10[(120 -30)/10] = 10(90/10) = 109 =")
For Our Example, We have: (W/Wr) = 10[(120 -30)/10] = 10(90/10) = 109 = 1, 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. ITEA Technical Seminar QSC Audio Products, Inc. 8
Common d. B Power Ratios A 3 d. B increase represents twice the power A 6 d. B increase represents 4 times more power A 10 d. B increase represents 10 times more power A 20 d. B increase represents 100 times more power ITEA Technical Seminar QSC Audio Products, Inc. 9
How Digital Soundtracks Increase Power Requirements Dolby A-type soundtracks can reproduce peaks of 91 d. B-SPL per channel. Relatively small amplifiers and speakers are capable of reproducing this level. ITEA Technical Seminar QSC Audio Products, Inc. 10
Digital Soundtrack Requirements Peak levels of 103 d. B-SPL, per channel, at all frequencies. A 12 d. B increase over the levels of a Dolby A-type soundtrack requiring 16 times the peak power. ITEA Technical Seminar QSC Audio Products, Inc. 11
Methods to Increase System Power Output Add More Amplifier Power Bi and tri-amplification Horns and Multiple Speaker Arrays Speaker Mounting and Location ITEA Technical Seminar QSC Audio Products, Inc. 12
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. ITEA Technical Seminar QSC Audio Products, Inc. 13
A Better Way to Look at Amplifier Power Convert it to d. BW, referred to a power level of 1 watt. We can see how power increases will affect SPL in a theatre. Power ratings in d. BW may be directly added to speaker sensitivity to get a theoretical peak output level. ITEA Technical Seminar QSC Audio Products, Inc. 14
For Example, What is the Output in d. BW of a 200 Watt Amplifier? Use the following equation. d. BW = 10 Log (W) where W equals the rated amplifier power in watts therefore, 10 Log (200) = 10 x 2. 3 = 23 d. BW ITEA Technical Seminar QSC Audio Products, Inc. 15
Power Output, in d. BW, for a Variety of Amplifier Sizes Note that doubling the power results in an increase in d. BW of only 3 d. B, a very moderate increase in SPL. ITEA Technical Seminar QSC Audio Products, Inc. 16
Speaker Limitations Power Compression Speakers 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 ITEA Technical Seminar QSC Audio Products, Inc. 17
Effect of Port Design ITEA Technical Seminar The Ideal response is linear. A 10 d. B increase in power produces a 10 d. B increase in SPL. Real World designs fall short of the ideal by varying degrees. Bigger ports are generally better. QSC Audio Products, Inc. 18
Power Compression Power compression of seven 15” woofers. Voice coil diameter and cooling techniques are key factors. ITEA Technical Seminar QSC Audio Products, Inc. 19
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 ITEA Technical Seminar QSC Audio Products, Inc. 20
Wavelength at a few Frequencies ITEA Technical Seminar QSC Audio Products, Inc. 21
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. ITEA Technical Seminar QSC Audio Products, Inc. 22
Horn Limitations Horns 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! ITEA Technical Seminar QSC Audio Products, Inc. 23
Beamwidth of LF Horn Note the loss of pattern Control below 200 HZ ITEA Technical Seminar QSC Audio Products, Inc. 24
Multiple Speaker Arrays Multiple direct radiator boxes may be combined to provide excellent realworld efficiency at low frequencies. Typical cinema sound systems are perfect examples of this approach to sound system design. ITEA Technical Seminar QSC Audio Products, Inc. 25
Mutual Coupling Allows 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. ITEA Technical Seminar QSC Audio Products, Inc. 26
Mutual Coupling continued For every doubling of the number of speakers, the maximum possible output increases by 6 d. B-SPL. Half of the increase is due to doubling the power capacity of the system and the rest of the increase (another 3 d. B) is the result of mutual coupling. ITEA Technical Seminar QSC Audio Products, Inc. 27
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”. ITEA Technical Seminar QSC Audio Products, Inc. 28
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 d. B decrease in power needed for a given SPL compared to half-space mounting. A real world decrease of 3 to 4 d. B is not uncommon. ITEA Technical Seminar QSC Audio Products, Inc. 29
Speaker Sensitivity Expressed 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. ITEA Technical Seminar QSC Audio Products, Inc. 30
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. ITEA Technical Seminar QSC Audio Products, Inc. 31
Subwoofer Requirements Design for 113 d. B-SPL in the middle of the room. Subwoofers get an additional 3 d. B 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. ITEA Technical Seminar QSC Audio Products, Inc. 32
Typical Subwoofer Sensitivity Ratings ITEA Technical Seminar QSC Audio Products, Inc. 33
The Effect of Distance on SPL Outdoors, the sound level will drop 6 d. B 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 d. B cushion with pink noise. ITEA Technical Seminar QSC Audio Products, Inc. 34
Inverse Square Law ITEA Technical Seminar QSC Audio Products, Inc. 35
Inverse Square Law ITEA Technical Seminar QSC Audio Products, Inc. 36
Indoor SPL vs. Distance Worst case scenario given by free field calculation ITEA Technical Seminar QSC Audio Products, Inc. 37
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 d. B for every doubling of the distance, starting with one meter. Since 1 x 2 x 2 = 16, we need to double the distance four times. ITEA Technical Seminar QSC Audio Products, Inc. 38
Example continued Four times 6 d. B gives a loss of 24 d. B. Check this with 20 Log(D), where D is in meters. The amplifiers are 100 watts per channel or 20 d. BW. A small, single-woofer speaker with a passive crossover and a sensitivity of 97 d. B is used. ITEA Technical Seminar QSC Audio Products, Inc. 39
Projected Maximum SPL Add the amplifier power, in d. BW, to the speaker sensitivity and subtract the loss due to distance. In this case, 97 d. B + 20 d. B - 24 d. B = 93 d. B -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 d. B higher for short peaks. ITEA Technical Seminar QSC Audio Products, Inc. 40
Digital Requirements A 93 d. B-SPL capability is adequate for Dolby A-type soundtracks, which require only 91 d. B-SPL. However, it falls 10 d. B short of the needs of a digital soundtrack. Ten times the amplifier power, 2000 watts, would theoretically yield the desired 103 d. B -SPL. This doesn't work in the real world as the speaker power rating would be exceeded. ITEA Technical Seminar QSC Audio Products, Inc. 41
A Better Solution Double the woofer section and use biamplification to power the woofer and horn section separately. Mutual coupling increases the woofer sensitivity to 100 d. B. The power increase is shared by two woofers and the system needs only 7 d. B more power. ITEA Technical Seminar QSC Audio Products, Inc. 42
Better Solution continued Our initial amp power was 20 d. BW, adding 7 d. B gives us 27 d. BW. From the preceding chart, a power rating of 500 watts is equivalent to 27 d. BW. Check with W=10 d. BW/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. ITEA Technical Seminar QSC Audio Products, Inc. 43
The Real World Bi-amplification, room reflections and amplifier dynamic headroom will usually produce levels at least 3 to 6 d. B higher than this calculation. Real-world systems could probably use 350 watts per channel with little chance of power amplifier clipping or distortion. ITEA Technical Seminar QSC Audio Products, Inc. 44
Typical Subwoofer Recommendations Assumes 3 to 6 d. B increase in sensitivity from quarter space mounting Mutual coupling begins to fall off for 4 to 8 subwoofers due to the large array size 300 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 300 w per woofer may experience mechanical failure, even if thermal power rating is not exceeded. SPL is calculated at 113 d. B at ½ the auditorium depth. Amplifier headroom and room gain make these estimates fairly conservative. ITEA Technical Seminar QSC Audio Products, Inc. 45
Conclusions A 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. ITEA Technical Seminar QSC Audio Products, Inc. 46
A Few Guidelines Use 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. ITEA Technical Seminar QSC Audio Products, Inc. 47
Guidelines continued Take advantage of the benefits of bi and triamplification 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. ITEA Technical Seminar QSC Audio Products, Inc. 48
Any Questions? Contact QSC Audio at (714) 754 -6175 Ask For Sam Hynds Francois Godfrey Or Barry Ferrell ITEA Technical Seminar QSC Audio Products, Inc. 49
Recommended Reading: The Yamaha Sound Reinforcement Handbook by Gary Davis ITEA Technical Seminar QSC Audio Products, Inc. 50
QSC Audio The Power Behind the Pictures ITEA Technical Seminar QSC Audio Products, Inc. 51
ITEA Technical Seminar QSC Audio Products, Inc. 52
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