- The sensitivity of a loudspeaker with 1 watt input measured at 1 meter is accurately specified and can be used without question in calculating power requirements to achieve the desired SPL at all frequencies.<\/li>
- The loudspeaker is a perfectly linear device. The corollary to this is that the speaker will still sound good at its full rated power.<\/li>
- That leads to the assumption that amplifier power in dBW can be added to the 1W\/1m sensitivity spec to determine the maximum SPL at 1 meter at all frequencies.<\/li>
- The inverse square law<\/a> can be used to determine the sound level at any given distance from a speaker. The assumption is that the sound level drops 6 dB for every doubling of distance from the source.<\/li><\/ul>\n\n\n\n
There are some major issues with these assumptions\u2026<\/strong><\/h4>\n\n\n\n
In reality, the sensitivity of a loudspeaker varies greatly with drive voltage, frequency, impedance and speaker mounting location. Sadly, many \u201cmarketing\u201d specifications greatly exaggerate the sensitivity compared to the real world performance. Physics dictates that similarly sized and priced transducers and cabinets rarely have dramatic differences in sensitivity, so \u201coptimistic\u201d specifications of some models should be viewed with suspicion.<\/p>\n\n\n\n
Maximum sound level calculations invariably assume that loudspeaker output is linear over the full power and frequency range, i.e., adding 20 dB more power yields a 20 dB increase in level. Unfortunately, this is certainly not true at all frequencies, and factors like voice coil heating and mechanical displacement limits affect the level and quality at high power. It is important to remember that speaker power ratings are tested to the point of destruction. A speaker on the ragged edge of destruction is not going to sound good. However, low frequency woofers used in cinema subwoofers and screen channel LF systems are usually limited by mechanical displacement long before the thermal power ratings are reached.<\/p>\n\n\n\n
It\u2019s important to recognize that the inverse square law is based on a theoretical point source in free field conditions (anechoic), such as outdoors. But a large screen channel speaker does not behave as a point source in the nearfield, and sound level does not continue to fall off in real rooms due to reverberation. QSC recommends using a \u201closs due to distance (D)\u201d formula based on 15*Log(D) instead of 20*Log(D), which is much more accurate for large rooms such as cinemas. Fortunately, this more realistic approach works in our favor in the real world, since less power is needed than the theoretical Inverse Square Law suggests.<\/p>\n\n\n\n
Inflationary Trends<\/strong><\/h4>\n\n\n\n
It\u2019s very dangerous to attempt to compare the published specifications provided by different manufacturers. It\u2019s also very dangerous to use those specifications to design cinema sound systems and then make assumptions about the performance of those systems. One problem is that not all manufacturers cite their specification in the same manner, with the same measurement parameters. There are many pitfalls to avoid, but here are three of the most common ones.<\/p>\n\n\n\n
1. Publishing numbers with an unspecified frequency range<\/strong><\/h5>\n\n\n\n
Sensitivity, especially with subwoofers and LF enclosures, falls off at lower frequencies. A sensitivity rating at 200 to 500 Hz for a subwoofer that is used below 100 Hz will give a specification that could be at least 3 to 6 dB greater than reality at those frequencies.<\/p>\n\n\n\n
This simulation of the frequency response of a low cost 15 inch woofer shows that 40 Hz is down almost 8 dB from the level we see between 100 to 200 Hz.<\/p>\n\n\n\n
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![\"\"](\"https:\/\/blogs.qsc.com\/app\/uploads\/sites\/5\/2020\/06\/Frequency-Response.png\")
<\/figure><\/div>\n\n\n\n![\"\"](\"https:\/\/blogs.qsc.com\/app\/uploads\/sites\/5\/2020\/06\/SPL.png\")
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