Atkinson and Phillips

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I am a member of the board of the Audio Society of Minnesota, and our guests at Sunday's meeting were John Atkinson (Stereophile editor) and Wes Philips (now with www.onhifi.com). The meeting was essentially an open discussion of two channel audio -vs- multi-channel audio. The good news is that John is holding on strong to two channel. John and Wes asked us questions as much as we asked them. After the formal gathering a got the chance chat with Wes, but I was running to late for dinner to talk to John. So the subject that I wanted to personally talk to him about, I submitted as a letter to the Editor, and is copied below.

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Editor:
Speaker efficiency is not the same as speaker sensitivity. Years ago, all speakers were specified as having an efficiency rating, now speakers seem to be specified with a sensitivity rating. I think that the speaker industry needs to get back to rating a speaker's efficiency, or at the vary least, we as Audiophiles need to understand the difference.
Efficiency is the true measure for speakers. It is like miles/gallon to the automobile. But speaker sensitivity is like saying how many miles an automobile can go on a tank of gas. If we don't know how many gallons of gas are in the tank, it is meaningless in terms of efficiency. This is the speaker industry's way of making 4 ohm speakers with 2 ohm impedance dips sound as if they are efficient. Sure a car may sound great when the sales person says that you can drive 500 miles between fills, but then when you find out that it takes 40 gallons of gas to refill the tank after you bought the car, you may feel like you didn't get the correct information. You will have wished that the sales person would have told you the true efficiency rating in miles per gallon (12.5 mpg in this case).
Speaker efficiency is specified as sound pressure per watt (like miles per gallon of gas). In other words, how much acoustical power is put into the air, for how much electrical power is put into the speaker. The rating is typically given as dB per 1 watt measured at 1 meter distance.
Speaker sensitivity is specified as sound pressure per volts (like miles per tank of gas). In other words, how much acoustical power is put into the air, for how much voltage is put into the speaker, regardless of its impedance. The rating is typically given as dB per 2.83 volts measured at 1 meter distance.
It is true that 2.83 volts RMS into 8 ohms is equal to 1 watt of power. But now the "sensitivity" game is played and a speaker is said to put out 90dB at 2.83 volts. The truth is that the 4 ohm speaker at its 2 ohm impedance dip, is requiring 4 watts of power, not 1 watt like they would like you to believe. Can you say 84dB efficiency?......I can!
It is true that speaker designers with only Solid State amps in mind, tend to fall into the "sensitivity" trap, if they need a little more output in the bass, "just parallel another woffer". The other reason the industry has fallen into low impedance speakers is to make them play louder at the same volume setting (with a Solid State amp). It is a way to get more power out of the SS amp at the same volume setting. Efficiency is poor, but its sensitivity made the sale.
To convert a 4 ohm speaker's sensitivity rating to an efficiency rating subtract 3 dB. For example 90dB sensitivity - 3dB = 87dB efficiency. To convert a 2 ohm speaker's sensitivity rating to an efficiency rating subtract 6 dB. For example 90dB sensitivity - 6dB = 84dB efficiency.
I suppose a sensitivity rating is OK to use if you have the type of SS amp that doubles its output power every time the load impedance is halved (100w @8 ohms, 200w @4 ohms, 400w @2 ohms, 800w @1 ohm).
A typical tube amp has multiply impedance taps that vary the output voltage (and current) to accommodate the load for consistent power into the different loads. At the same volume setting, a tube amp will put 2.83 volts out of the 8 ohm tap for 1 watt into 8 ohms, and will put 2.00 volts out of the 4 ohm tap for 1 watt into 4 ohms. A typical tube amp is "true" to output power and a speaker's true measure of efficiency should be used.

Stereophile measures a speaker's "voltage sensitivity" with 2.83 Vrms forced into it. Since voltage sensitivity is not equal to true efficiency, Stereophile also need to tell their readers what the actual efficiency rating is when the impedance is taken into account. This is so important, especially for a magazine that reviews many tube amps.

Paul Speltz
mcken001@tc.umn.edu

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Topic - Atkinson and Phillips - Paul Speltz 10:18:55 03/21/01 (2)

Re: Atkinson and Phillips - Ralph 13:51:09 03/21/01 (1)

Re: Atkinson and Phillips - Paul Speltz 14:28:22 03/21/01 (0)

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