Re: OT: Microphone experts mainling-list
Re: OT: Microphone experts mainling-list
- Subject: Re: OT: Microphone experts mainling-list
- From: Brian Willoughby <email@hidden>
- Date: Tue, 30 Jan 2018 18:50:41 -0800
I would like to clarify a few details on low frequency localization.
Low frequencies are not difficult to localize with regard to natural sounds.
The brain is perfectly capable of detecting phase differences for sounds
originating in a real, three-dimensional space, and arriving at each ear at
different times. It’s actually the middle frequencies where the human hearing
system has difficulty. For the average size head, frequencies above 800 Hz
start to overlap and errors occur in the phase difference calculations. From
there to about 1600 Hz, localization is difficult. So, we’re directionally deaf
for about an octave in the middle, although the psychoacoustics are quite
complex.
Very low frequencies are difficult to localize, but we’re talking about
frequencies below 80 Hz - the lowest octave of the human hearing range.
Non-natural sounds can be different, especially for music mixed with a
standard, amplitude-only panning system. Some people refer to this as
multi-mono recording, even though it’s typically called stereo. This is by far
the most common way to mix music, and the low frequencies have no inter-channel
delay at all, only amplitude changes which are useless. So, in that sense, low
frequencies in stereo music sources can be impossible to locate because the
necessary localization information is simply not present in the signal.
Finally, to bring this back to the topic of CoreAudio, Apple provides a 3D
Mixer (deprecated) and Spacial Mixer AudioUnits which do allow for
inter-channel delays. In addition, the output system of CoreAudio will map
spacial sound source onto the speakers you have according to their physical
placement (as set by the user in Audio MIDI Setup). This works for stereo,
binaural outputs as well as quadrophonic, 5.1 surround, or higher. Thus, for
audio from video games or music produced and mixed with CoreAudio, it would
indeed be possible to localize low frequency sounds. Of course, frequencies
below 80 Hz will be difficult to localize, but most sound sources have
relatively less content that low, except for electronic and experimental music.
Thus, it is more accurate to say that we only need one subwoofer in a surround
system because music has been produced for decades with no inter-channel delay.
But things are changing. Depending upon the frequency range of your subwoofer
(100 Hz?) and the slope of the crossover filters, there could be a reasonable
amount of directional information in surround content, depending upon the
manner in which the audio was produced. Of course, this assumes that your main
speakers do not extend below 80 Hz well enough to cover the directional
information in that range.
Brian Willoughby
Sound Consulting
On Jan 25, 2018, at 12:47 AM, Richard Dobson <email@hidden> wrote:
> That's a very cool idea ... but also very difficult. The overall topic is
> generally referred to as "localisation". You are describing "dummy head
> recording". Spectrum analysis will be an important tool, though in some cases
> inspecting the waveform may show how phase differences between the ears
> contribute to localisation. This "inter-aural difference" is an important
> element. It is known, for example, that localisation depends not only on
> distance, but also on pitch - low frequencies are virtually impossible to
> localise as the phase is not sufficiently different between the ears. Which
> is why we may need 5 speakers, in just the right positions, to hear music in
> "surround", but just one sub-woofer, which can be placed just about anywhere.
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Coreaudio-api mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden