Confused about speaker "sensitivity" when using multiple speakers

[J M Fahey]

Speakers *are* measured in a "standard baffle", not alone hanging from a hook.
Here's specs for 8" to 15" speakers.
Not the best baffle in the world, not the worst either.

[Leo_Gnardo]

Speakers *are* measured in a "standard baffle", not alone hanging from a hook.

Now that's more like it! Thanks Juan for clarifying this baffling question.

[R.G.]

Just to add one more confusing dimension -
Modern speakers are designed assuming they're driven by a source with a much lower impedance than the speaker itself. That is, the amp is a decent stand in for a voltage source. Given the lack of standards for speakers and measuring sensitivity, some are quoted at single frequencies, some with white or pink noise as an output, and some nominal 4 or 2 ohm speakers quoted at a specified rms drive voltage, not a drive power.

Specifying with an input voltage started as a way to make the 2 and 3.2-ohm car speakers look better on low-voltage amplifiers that were actually delivering more power to the speakers on the limited 12V power from a car battery. However, specifying a speaker at a voltage is, perversely, probably more sensible for speakers designed for voltage drive.

Finally, combining speakers gets back to this voltage drive thing. Thomas Organ played this for all it's worth in their Vox amps. All of the bigger amps used exactly the same power supplies (+/-31V) and changed output power by changing the speaker impedance connected to it. That voltage supply is enough to get about 30W in a nominally 8 ohm speaker. The 30W variants used two 16's in parallel. The 60W variants used two 8s in parallel. The 120W amps used four 8's in parallel. With a given voltage across it, a speaker puts out a notional power. Parallel connection sucks more power out of the amp if the amp can supply the current. Series connection cuts the voltage per speaker and lowers the voltage per speaker nominally by half, and cuts the power delivered to each speaker.

[printer2]

No free lunch, costs you more in money and in weight. But the +3dB gain in SPL for doubling speaker area is real. It has been a long time (maybe 30 years) but I recall reading it is something along the line of a horn enclosure better coupling the high pressure of a driver to the air which is a low pressure medium. This is where memory fails me, think it is the high pressure is converted to higher velocity, might not be the right word. But if you think of the horn as a transformer converting the energy from a low impedance device (the driver) to a high impedance fluid (the air).

The size of the horn mouth is important, too small and you do not get an efficient transfer of energy. The same thing goes with a loudspeaker cone. The more diaphragm area the more efficient the transfer of energy. Do you get increased 3 dB gains until infinity? No, as you get closer to the wavelength size the gains get smaller. I think part of the gain is also partly due to the directivity of the sound wave as said, less gets wasted to the sides and back. But at relatively long wavelengths where the distance between two drivers in a normal sized cabinet is much shorter than the wavelength we still get our 'free' 3 dB. So it can not be all attributed to less sound getting wasted around the sides and back.

All the above is vague recollections and theories, how about some real world testing? We had some discussions about running extra speakers together and what would the final spl result. While I was somewhat confident the above is how things worked most of what I knew was the result of reading and I had no proof it was true. So I went out and set up a pair of speakers and did my own testing. I used two 15" Altec speakers in identical cabinets and used a PA amp with stepped level controls, sine wave generator, a SPL meter, two DMM's and started testing.

I measured each speaker separately to confirm they had identical spl numbers at 1W. Of course seeing is always more believable so I took pictures of my testing.

Two Amp SPL Test 00 2,83V 1W


Two Amp SPL Test Setup 01


Two Amp SPL Test 02 Left spl 1W


Two Amp SPL Test 03 Right spl 1W


Two Amp SPL Test 04 Both spl 1W Each


Two Amp SPL Test 05 Both spl 2W Left Skr


Two Amp SPL Test 06 spl 1W Left Skr

[printer2]

Only seven pictures per post, here are the rest.

Two Amp SPL Test 07 spl 1W Both Skr one Channel


Of course this convinced many people and since the topic always pops up couple times a year I always direct the discussion to the thread with my testing. Last year there was one person that attributed the 3 dB gain I saw to improper testing and that I was just seeing a node where the waves reenforced each other and that a corresponding trough would produce a -3 dB reading. The poster said that single frequency testing is worthless and that a broadband frequency signal is the only proper method of testing.

Well being that I sweeped the appropriate band of my signal generator and I spent a few years in a test lab I could not let it go with doubt hanging over my test. I set up a FM receiver and fed the amp inter-station hiss as a broad spectrum signal.

SPL Two Speaker Test With FM Receiver White Noise


SPL 1W FM hiss two channels to 15 inch Altecs


1W FM hiss one channel to 15 inch Altec


SPL 1W FM hiss one channel to 15 inch Altec



Sorry for the long post.