Decibel question

Discussion in 'Speakers' started by Troy Swanson, May 16, 2005.

1. Troy Swanson Extra

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I know decibels are a measure of a speakers effciency.

Higher the number, less power required to achieve a desired SPL.

When and how much makes a difference.

I have a pair of Wharfedale diamond 8.1 bookshelves rated at 86 dB at 1 meter.

I am looking at replacing them with a pair of Klipsch B-2s rated at 92 dB at 1 meter.

How significant is 6 decibels from 86 to 92?

2. Wayne A. Pflughaupt Producer

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It’s roughly the equivalent of tripling your amplifier power.

Regards,
Wayne A. Pflughaupt

3. FeisalK Screenwriter

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Wayne, wouldn't it be like quadrupling?

4. Chris Quinn Screenwriter

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Yes, it would take quadrupling the power. Every 3dB increase requires twice the watts.

5. Mark Seaton Supporting Actor

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Mark Seaton

While many mislabel sensitivity as efficiency. Efficiency is a clearly defined concept in most any science, commonly noted the percentage of input power which is converted to work. Efficiency should be listed as a percentage. Specifications for raw drivers sometimes list no(%) which is in fact the efficiency of the driver as calculated from its Thiel-Small parameters.

Sensitivity is a measure of output as a result of some input, or the input required to produce some output. Loudspeakers are commonly measured for senstivity as it relates to SPL produced at 1m from a 1W of input power. 1W into an 8 Ohm resistive load requires 2.83V across the driver. Real loudspeakers are not resistive loads. All loudspeakers are of course not 8 Ohms either. Many companies will list sensitivity without a referenced drive voltage for a 4 Ohm loudspeaker. This gives the 4 Ohm speaker an apparent 3dB gain in sensitivity.

Our amplifiers work by providing and maintaining a drive voltage and whatever current the loudspeaker demands until either the voltage or current limits of the amplifier are reached. Using the "nominal" 2.83V drive voltage gives us a reference for what we will hear when we sit two speakers side by side, and connect one, then the other; without changing the drive level (volume setting). In reality, the 4 Ohm nominal loudspeaker will be making the amplifier work harder with the same volume setting. In real use we have to look at how our amplifier performs at lower impedances to see how much of the improved sensitivity we can put to use. Some amplifiers will almost double their power into 4 Ohms relative to 8 Ohms. Some will be almost the same.

If you want to consider maximum calculated clean capabilties from a loudspeaker and amplifier based on its rated power into the nominal load of your loudspeaker, you need a more appropriate 1W sensitivity for your loudspeaker. 2.0V into a 4 Ohm load allows you to directly calculate from the 4 Ohm power rating of your amplifier. If you are given a 2.8V sensitivity for a 4 Ohm loudspeaker, you need to subtract 3dB to put this in proper perspective with the available power into 4 Ohms from your amplifier, or calculate for 1/2 the 4 Ohm power rating. Of course calculating this maximum output ignores whether or not your speaker will go up in smoke or be heavily distorting at this level. Those are separate, but equally important matters.

For those curious where the rule-of-thumb 3dB per doubling comes from, here's the equation for relative power increase:
relative change in dB = 10 * log(P/P-i)

Where P is the new power or rated power, and P-i is the initial power or can be 1W if that is your reference.

When looking at Voltage The equation changes to:
relative change in dB = 20 * log(V/Vi)

Hope that helps,

6. Troy Swanson Extra

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Mark,

I am still confused a little.

her is my example

Speaker set 1: 86 dB@ 2.83v/1 meter 6 ohms

Speaker set 2: 92 dB@ 2.83v/1 meter 8 ohms

Which would be more efficient, or do you need my Receiver specs to complete the
story?

7. Jeremiah Dant Agent

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the one at 92db@ 1 watt would be more efficient. the speaker at 86 db@ 1 watt would require 4 times times the amount of power to reach the same volume as the 92db@1 watt. every 3 db requires double the power. here is another way to look at it.

speaker 1
86 db = 1watt
89 db =2 watts
92 db = 4 watts

speaker 2
92db= 1 watt

8. Greg Bright Second Unit

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Troy,
What this means, all (and I mean all) other things being equal, is that the Klipsch speakers will play louder and cleaner than the Wharfdales using the same amount of amplifier power. And that's what we all want. Right? Obvious answer: not necessarily.

9. Troy Swanson Extra

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Thank you for the replies,

I figured this was the case, it seems as though Klipsch has very high dB ratings, typically 4-6 dB higher.

I have been comparing Monitor Audio, Boston acoustics, Polk, etc. and they all have a lower dB rating than Klipsch.

I wanted to make sure before I purchase another pair of speakers.

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