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The Price of 3 DB

A 3 DB LEVEL INCREASE is a modest one. Subjectively, one might describe it as being a ?tweak? to the loudness of a system. Most infrared remote controls for home entertainment systems are about 1.5 dB per step, and most people who go to the trouble to pick one up and point it will change the level by at least two steps. Quit reading and go try it now....

A 3 DB LEVEL INCREASE is a modest one. Subjectively, one might describe it as being a “tweak” to the loudness of a system. Most infrared remote controls for home entertainment systems are about 1.5 dB per step, and most people who go to the trouble to pick one up and point it will change the level by at least two steps. Quit reading and go try it now....

At low levels

At low playback levels, a 3 dB change can be made without regard for its effect on the audio system's components. The loudspeaker is likely operating well below its thermal limits, so any increase in heat is insignificant. Also, the amplifier is likely operating far below its clipping point, so the loudness can be increased without undue concern regarding distortion.

The logical way to increase the level of a sound system is to start low and slowly advance the level control until the desired level is attained. Most experienced sound people do this by instinct, and they know when to stop.

Because level increases come so easily initially, it's easy to forget that a 3 dB level increase exacts an increasingly greater toll in terms of system headroom and loudspeaker heat dissipation.

Efficiency and heat

Loudspeakers are notoriously inefficient. More than 95 percent of the applied power is wasted as heat, which increases the operating temperature of the loudspeaker, rather than converted into sound. If you turn it up 3 dB, it will get a little louder and run hotter.

The loudness increase is logarithmic — twice the applied power always produces the same 3 dB loudness increase. The temperature increase is linear — each additional watt has the same heating effect. So, if you double the power from 1 W to 2 W, and then from 2 W to 4 W, the loudness will change by 3 dB. But, there will be a larger temperature increase from the second power change, even though it was proportionally the same as the first one. As level is increased in equal loudness increments, the temperature increments get progressively larger with each increase.

The price per watt

Let's say that you're amplifier shopping, and paying one dollar per watt for amplifier power. Loudspeaker sensitivity ratings are usually referenced to 1 watt (actually 2.83 Vrms, but that's s a subject for another column). Let's start there and go up. The power must be doubled to 2 watts for a 3 dB level increase. Four watts gives another 3 dB, and 8 watts yet another. The level increases are relatively inexpensive, and it appears that the sky is the limit regarding the potential loudness of the system. But if you look at the trend, you'll see that you'll soon run out of money. Every 3 dB increase is twice the cost of the previous. Eventually you won't be able to afford another 3 dB!

If 3 dB is just noticeable, 6 dB can be considered the smallest “loudness upgrade” worth pursuing. A 6 dB increase represents twice the voltage across the loudspeaker, which quadruples the power that must be dissipated as heat.

Here are some real-world examples of the expected cost of a 6 dB upgrade for different types of sound systems. In each case, I'm assuming that only the amplifiers are being upgraded, and that the existing loudspeaker system will be used.

  • Paging system in a small office: 30 W amplifier to 120 W amplifier — about $100
  • Medium-sized portable PA system: 100 W amplifier to 400 W amplifier — about $300
  • Typical playback system for weekend rock band: 1,000 W to 4 kW — about $3,000
  • Large house of worship system: 20 kW to 80 kW — about $60,000
  • Touring system: 50 kW to 200 kW — about $150,000 (and three more trucks)

    Note that in each example the same loudness increase (6 dB) was achieved, but at a vastly different cost. Now consider the owner of the medium-sized portable PA system that runs sound at the large house of worship. The relatively low-cost upgrade to the smaller system translates into a major investment in the large one. It's all about “proportional change.” What you need depends on what you start with. This is why experienced audio people use the decibel system to rate and select audio components — the logarithmic relationships are built-in.



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