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[Paul Dershem]

<SNIP> I think we will need something over spl 100dB. As you mention 120 would cause damage but it will save us from having to turn the amp right up and hopefully the sound level will be hovering around the 100 dB level.

How far away from the loudspeakers do you expect to measure 100dB SPL? 

100dB @ 1M becomes 94dB @ 2M, 88dB @ 4M, etc.

[Rob Spence]

One thing not mentioned so far is the effect by sensitivity. That is part of the "not all things equal". Many times, a speaker with lower impedance might also have a lower sensitivity so will take more power to make the same SPL.

[Ken McDonald]

Thank you everybody for your kind thoughts on this. I have obviously got a lot to think about.
In response to the question about the distance from the speaker for measuring the SPL. Well I'm not measuring it with anything it was the availbility of headroom. So if if the amp is set at 3/4 volume or something at least we be out of the way of output distortion....I think

Anyway once again many thanks indeed....

[Tim McCulloch]

Thank you everybody for your kind thoughts on this. I have obviously got a lot to think about.
In response to the question about the distance from the speaker for measuring the SPL. Well I'm not measuring it with anything it was the availbility of headroom. So if if the amp is set at 3/4 volume or something at least we be out of the way of output distortion....I think

Anyway once again many thanks indeed....

The amplifier input knob has no intrinsic affect on distortion as it only attenuates the input signal being sent to the amp.  Distorted signals can and do exist ahead of the amplifier, and they will be amplified along with the undistorted portions of the signal as well.

Speakers also introduce distortion of their own, and changes in the amp's input level control will not affect that, either.

[Ken McDonald]

The amplifier input knob has no intrinsic affect on distortion as it only attenuates the input signal being sent to the amp.  Distorted signals can and do exist ahead of the amplifier, and they will be amplified along with the undistorted portions of the signal as well.

Speakers also introduce distortion of their own, and changes in the amp's input level control will not affect that, either.

Thank you for your reply but my point really was that for whatever the reason, if you turn an amp up full it distorts. If you turn it up to 3/4 then it doesn't sound anywhere near as bad. So if I have a much more powerful set up then I wont have to turn it right up to maintain that level of output. .

Am I right with that thinking or have I got that wrong too?

Edit... Even with a TV set if I turn that up to full volume it distorts so it's not just professional stage gear I am talking about

[Brad Weber]

Another aspect to this is that speaker impedance varies with frequency, often quite significantly.  Unlike the nice resistors used to obtain the published amplifier ratings, speakers not only do not present simple resistance but rather complex impedance, but a nominal 4 Ohm speaker is also not 4 Ohms at all frequencies, instead it would typically be below 4 Ohms at some frequencies and typically many times that at other frequencies.

[Ivan Beaver]

Thank you for your reply but my point really was that for whatever the reason, if you turn an amp up full it distorts. If you turn it up to 3/4 then it doesn't sound anywhere near as bad. So if I have a much more powerful set up then I wont have to turn it right up to maintain that level of output. .

Am I right with that thinking or have I got that wrong too?

Edit... Even with a TV set if I turn that up to full volume it distorts so it's not just professional stage gear I am talking about

The distortion of an amp will be the same at 1/4 of the volume control or turned all the way up.

It is the INPUT signal that determines how much power an amp will put out.  NOT the level control.

In the case of your TV, you are simply sending to much level TO the amp.

It is the TOTAL of ALL of your "volume controls" that determine whether or not distrotion is present in the signal chain-input trim-channel level-tone controls-mix bss-master level on console-input processing-output processing level-eq adjustments in the processing and LASTLY (and the point that has the most minimal effect) is the level control on the amplifier.

  Screw up one of them, and you get distortion and every where on down the chain will "pass it on".

[John Roberts {JR}]

Thank you for your reply but my point really was that for whatever the reason, if you turn an amp up full it distorts. If you turn it up to 3/4 then it doesn't sound anywhere near as bad. So if I have a much more powerful set up then I wont have to turn it right up to maintain that level of output. .

Am I right with that thinking or have I got that wrong too?

Edit... Even with a TV set if I turn that up to full volume it distorts so it's not just professional stage gear I am talking about

No.. as others have tried to describe, the amplifier being driven into overload (distortion) is the product of it's level control, and the input signal level. Turned all the way up with no input signal will not distort.

Your Tv set or whatever, has it's gain structure established by design to overload where it does.

JR

[Ken McDonald]

OK thank you guys. I understand that now, many thanks.

[duane massey]

Back to your original question, why are there so many 8ohm speakers? From a (former) manufacturer's perspective, it's a cost-effective thing. If you market a single-woofer box and a dual-woofer box it would require 2 different woofers (4- and 8-ohm) to provide both as 4-ohm boxes.

[ProSoundWeb Community]