Clive Arthur <clive@nowaytoday.co.uk> wrote in
news:tp0uv4$23lge$1@dont-email.me:

> On 03/01/2023 02:30, Brian Gregory wrote:
>> On 02/01/2023 13:48, Rink wrote:
>>> If you change the wires of one speaker you get a 180 degree
>>> difference on all audio frequencies.
>>> If from two speakers at the some radio (left and right), one is
>>> wrong connected, you can hear that at exactly the middle between
>>> the speakers where there is a fase-out for all audio
>>> frequencies.
>>
>> I've never heard an audible null between out of phase speakers.
>> It usually just ruins the stereo effect, makes it sound almost
>> like two separate lots of music (or whatever).
>>
> With a mono source (ie both channels the same phase) you lose a
> lot of bass in the centre. Block one ear and the effect is
> clearer and at higher frequencies too.
>
> A long time ago, when stereo records were often mixed with the
> vocals in the middle and other instruments to the sides, inverting
> one channel and adding it to the other channel (ie subtracting it)
> could do a not-too-bad job of removing the vocals for karaoke.
>

And then, in 1971 IIRC, came "quadrophonic"

Heathkit had an amp and there were quadrophonic album pressings.