Re: Phasing of the speakers
- From: Patrick Turner <info@xxxxxxxxxxxxxxxxxx>
- Date: Sat, 25 Apr 2009 15:26:47 GMT
Alex wrote:
A thread about phase alignment reminded me of another question:
Does speaker connection polarity (+/-180 degrees of phase) make any
difference?
I remember in my young years we experimented and sort of came to a
conclusion that the correct phase is when the cones of the speakers suck in
with the first beat of a big drum. The sound was "deeper" this way. However,
the difference was marginal and barely perceplible. Personally I did not
hear any difference.
What is the modern take on that? Do manufacturers of the top-end CD players,
amplifiers and speakers specify their gear as "inverting" or non-inverting"?
(I believe they do not) Do the recording studios observe "polarity" from the
microphone to the digital values actually recorded on CDs? (Probably not)
Regards,
Alex
When a drum is beaten, the skin is forced down creating a suction wave.
Meanwhile a compression wave is created out the bottom of the drum.
The two waves sum together and the mic records the pressure changes then
the signal is processed in the studio gear and recorded onto CD maybe
and sent to the record cutting head amp to make an LP.
Its anyone's guess which way the initial drum bang is phased. In
amplifiers at home there may be several stages of amplification and
maybe the signal gets inverted or maybe not.
I don't know what importance is given to phase when recordings are made.
The initial bang of a drum consists of a large number of sine wave
harmonics which start together but with all rapidly decay in amplitiude.
Most people would not notice if all or some of these wave forms were
phased oppositely to others; we would merely groove along with the
obvious bang of a drum and all the more so if there is a suitable rythym
of drumming.
If we recorded an explosion then you'd expect to be hit by a blast of
blowing air rather than a sudden suction towards the source of the
explosion sound. A hand grenade let off in a living room will make a
hell of a bang which may rupture both your ear drums. It'd be impossible
to recreate the sound with a normal hi-fi sound system. Both sucks or
blows of serious magnitude would be perceived as awful noise. Music is
carefully arranged noise which is meant to have use muse, and be
delighted, and distracted from the world or work and troubles. I have
two 12" bass reflex drivers in reflex speakers capable of going down to
20Hz, and the Fb is about 30Hz, and just what phase reversals occur is
something I have either forgotten or never measured. At below about 40Hz
the sound comes from the port of the box. But In perceive the bass to be
quite excellent and very much like bass heard from bass instruments or
large drums. I just spent another pleasant evening listening to music at
a friend's place where he has a sub-woofer and bass speakers also
capable of going down to 25Hz. I built all the amps and speakers and the
sub and when I set up the sub position I wanted to get maximum flatness
of response at the listening chair and so we tried the sub in many
positions and with reversal of speaker leads at the sub amp to get the
best flat response with minimal peaking and troughing due to phase
adding and cancelations. The sub amp filter is has only 50 degrees of
phase shift at the -3dB point but then has a much increasing rate of
attenuation. Some filters might have a huge phase "turnover" at the -3dB
point, making the integration of sub and existing bass speakers very
hard to achive without unwanted peaks and troughs in the response.
I like bass I get with my two bass units which cover 20Hz to 250Hz with
perhaps a slight broad lump in the low bass response. Perhaps the drum
sound I get is better than what my friend has, but its hard to say
because our rooms are different. I've been to some loud rock music gigs
wearing ear plugs because ot the SPL and found my chest heaving with low
bass and the boom chucka boom. I never bothered to ask myself is my
chest being compressed at each drum beat or stretched; you just feel
pummelled by the sound anyway.
The transition reponse between bass and midrange and how the filter
crossovers are set up will affect the perceived transient sound quality.
Sometimes one has to reverse phase connect mids relative to bass, then
treble relative to mids, depending on the response. Its a very tricky
business to set up speaker drivers for minimal phase change and yet
achieve a smooth response through the crossover regions without a big
peaks each side of the crossover F or two or a big trough where each
driver tries to cancel the sound at the trough.
I think getting the smooth response with rapid cut offs at least
12dB/octave away from the Xover F is the most important thing to get
right.
This means a slightly over damped second order filter. Big old woofers
meant to be cut off at sat 250Hz may need much more steep filtering
beyound 500Hz lest the transients muddy the sound meant to be handled by
the smaller midrange driver.
Patrick Turner.
.
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