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Old 30th March 2009, 03:35 PM   #1
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Default Decibel Watts VS Watts ?

Here`s a question for all amp wizards: just hypothetically, if we would have two otherwise identical loudspeakers, the only difference being the impedance rating (one 4 oHm, the other 8 oHm), driving them with an amp that is capable of 100 W into 8 oHms and 200 W into 4 oHms, which loudspeaker would achieve higher SPL with the same amp? Now, don`t forget that if we halve the impedance from 8 to 4 oHms, the thermal dissipation from the amp quadruples (and distortion level rises).
It is my suspicion that the only real significance to amplifier power is how many decibels of sound pressure it is capable of driving a speaker. Since speaker efficiencies are often specified as decibels with one watt of input, a more useful way of comparing amplifier output would be to reference their decibel output to one watt(dBW). This would give a much more accurate representation of amplifier output than the watt rating system. I fear that all this mighty watts talk serves only one purpose, the sales (and bragging) argument.
The way I understand things (I might be wrong) there is no real advantage in 4 oHms load compared to 8 oHms, although the electrical power doubles, the maximum available SPL is actually a little lower into 4 oHms, even if you use monster power amps.
For example there isn`t a single power amp from the Stereophile tests that would have higher dBW rating into lower impedances under the constant distortion requirement (1% THD+N) and dBW`s matter not watts, don`t they?
Check any of them here:
http://www.stereophile.com/solidpoweramps/

A "little" clarification would help

http://www.ringbell.co.uk/info/dbw.htm

http://www.giangrandi.ch/electronics...l/decibel.html
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Old 30th March 2009, 04:09 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Seeing dBW's is related to the voltage output of an amplifier an
amplifier will never have more dBW's into 4 ohms that it does 8.

4 ohm versus 8 ohm dBW's describes the voltage sag.

Regarding your speaker question all things being equal the 4 ohm
speaker will play 3dB louder. Amplifier dissapation is x 2 not x 4.

/sreten.

All things not being equal the 4ohm speaker could be the same level
as the 8ohm but have more bass, or the same in a smaller cabinet.
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Old 30th March 2009, 04:19 PM   #3
AndrewT is offline AndrewT  Scotland
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sorry but you've got it a bit mixed up.

There are two standard ways to specify the sensitivity/efficiency of a speaker.

1.) SPL @ dB/m/W (this is real watts, not the equivalent of 2.83V into 8r0)
2.) SPL @ dB/m/2.83V (some manufactures adopt dB/m/2V for 4ohm drivers but this is relatively rare).

Now look at your drive ability
100W (+20dBW) into 8r0 and 200W (+23dBW) into 4r0. Theoretically this is almost impossible but for this discussion I'll ignore that anomaly.

Select a speaker that has this spec.
90dB/W/m
Apply a maximum of 100W and the resulting SPL is 110dB/m less any compression effect.
But this is putting 100real watts into a speaker without knowing it's impedance.
Let's dissect the speaker rating.
It is exactly 8ohms across the whole frequency range and you supply it with 28.3Vac. It still has a maximum SPL of 110dB/m less the same compression effect.

If the speaker were really a 10ohm average across the whole frequency range and you apply 28.3Vac to it, you have available only 80W (+19dbW). The maximum SPL is 90+19=109dB less the same compression effect. The higher impedance has drawn less power from the amplifier and thus you end up with less SPL.

Let's select a 4ohm speaker
It's spec is 90dB/m/W.
Apply 100W and you get 110dB/m less the compression effect.
But to get this you only needed to use 20Vac to get that 110dB/m.
Increase the applied voltage to 28.3V (equivalent to 200W into 4r0) and the power delivered is +23dBW. the maximum SPL is 90+23=113dB/m less that compression effect.

The difficult bit in all of this is deciphering the manufacturers attempts to hide their real sensitivity if it suits their marketing.

A 7ohm speaker that is really 90dB/m/W looks better in the spec sheet if it's been quoted as an 8ohm speaker and they quote a voltage sensitivity of 90.58dB/m/2.83V. Their speaker appears to be ~0.6dB more efficient than another manufacturer who makes a real 8ohm speaker.
4ohm speakers gain 3dB using this same voltage sensitivity ruse. Don't be fooled. Ask what they mean by W and 2.83W if you suspect they are at it.
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Old 31st March 2009, 10:41 AM   #4
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by AndrewT

Now look at your drive ability
100W (+20dBW) into 8r0 and 200W (+23dBW) into 4r0. Theoretically
this is almost impossible but for this discussion I'll ignore that anomaly.

Hi,

100W into 8 ohms and 200W into 4 ohms are both +20dBW.

/sreten.
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Old 31st March 2009, 11:10 AM   #5
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Quote:
Originally posted by sreten


Hi,

100W into 8 ohms and 200W into 4 ohms are both +20dBW.

/sreten.
200W is 3dB (twice = 3dB) over 100W which is 20dB (100 = 2 zeros = 20dB) over 1W. Making a total of 23dBW. 53dBm if you like. The impedance is irrelevant. What planet are you on sreten?

w
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Old 31st March 2009, 02:58 PM   #6
infinia is offline infinia  United States
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Quote:
Originally posted by sreten


Hi,

100W into 8 ohms and 200W into 4 ohms are both +20dBW.

/sreten.

Simply wrong dBW is the ratio of power in watts to 1 Watt

10LOG (power in watts)/1watt
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Old 31st March 2009, 04:43 PM   #7
AndrewT is offline AndrewT  Scotland
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Sreten was referring to dBV and got it mixed up with dBW.
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Old 31st March 2009, 04:56 PM   #8
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100W into 8 ohms and 200W into 4 ohms are both +20dBV?

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Old 1st April 2009, 05:54 AM   #9
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Thank you guys for all your theories, but there seem to be no practical evidence that more SPL is possible into lower impedances, under equal loudspeaker conditions. It seems we only get more heat (dissipation) from amps

"Evolution 600 was a powerhouse, clipping with continuous drive at 610W into 8 ohms (27.85dBW), 1190W into 4 ohms (27.75dBW), and 2.2kW into 2 ohms (27.4dBW). I don't hold my AC wall voltage constant for these tests; at 125.8V with the amplifier quiescent, it had dropped to 121.8V with the amplifier clipping into 4 ohms, 119V with it clipping into 2 ohms. With its S/N ratio (input shorted, ref. 1W into 8 ohms) of 81.7dB (wideband, unweighted) improving to 92.3dB when A-weighted, the Evolution 600 offers a superbly wide dynamic range that well exceeds that of the 16-bit CD medium. "

http://stereophile.com/solidpoweramp...ll/index4.html
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Old 1st April 2009, 11:23 AM   #10
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by fivestring

Thank you guys for all your theories, but there seem to
be no practical evidence that more SPL is possible into
lower impedances, under equal loudspeaker conditions.
It seems we only get more heat (dissipation) from amps
Hi,

No. There is ample evidence a 4ohm speaker can exploit
the extra current and hence power from an amplifier.

If speaker efficiencies are the same it will be +3dB louder.

/sreten.
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