A Little Thing On Phase

For now 2 video’s and some data….More to follow

Spiking the 1ms 2ms 3ms and 4ms mic Positions

I configured a Transfer Function engine between the Mic onax (measurement signal) and the Amp output channel driving the loudspeaker via a POL DI to see the time between the source (a Fulcrum Acoustic RX699) and the Mic Onax

Start The first “mic @ 1ms”= 19.9cm to 20.5cm doesn’t match the +/-34cm you would expect.

(Speed of Sound/1000ms (=1sec) > 340mt/sec / 1000 = 34cm (For the purists @ 14.1º Celcius))

This mainly has to do with the distance measurement device I use (a tailors measure ribbon (?)) being attached to the grill of the loudspeaker in the center where the little horn is visible. The high driver within the coax is further to the back so it sort of makes sense.

Also there’s a 0.0208ms time thing going on which has to do with a 48K Sample Rate. This translates to a 0.7cm difference in distance before Smaart goes to the next sample

(T 1 sample @48kHz SR is 1000ms/48000(SR) = 0.0208ms per sample x 340mt/s (speed of sound) 0.7cm)

A : 1ms +/- 19.9cm to 20.5cm

B : 2ms +/- 54.1cm to 54.6cm

C : 3ms +/- 88.3cm to 88.9cm

D : 4ms +/- 122.7cm to 123.3cm (a bit more but welcome to the real world of loudspeakers and measurements in a room 😉 ).

And the screen capture on what this means when looking at the transfer function display

Moving the mic from the 1ms to the 4ms spiked positions in the 1st screen capture

So before the screenshots a couple of formula’s:

T=1/F (or F=1/T) which you use to calculate the Time period of a frequency of intrest

WaveLength = Speed of sound/Frequency Calculates the wavelength of a frequency of interest.

And the definition of phase:

Phase=Time or Distance vs Frequency expressed in degrees.

What should be clear after the screen captures:

If phase moves down under a angle the measurement signal is late compared to the reference signal which means delay (or the mic has been moved away from the speaker after synchronising in case of the 2nd screen capture’s start (green trace)).

If phase moves up under a angle the reference signal is late compared to the measurement signal which means negative delay (or the mic has been moved towards the speaker after synchronising in case of the 2nd screen capture’s (half way to the end bleu-ish trace)).

Now if I synchronise Smaart at the 1ms/19.9cm mark and move the mic to the 2ms 3ms and 4ms marks the phase will go down under a angle indicating the measurement signal is late compared to the reference signal and the phase shift per step is each time an extra 360º degrees

Smaart will not (always) display the calculated frequency exactly because MTW is configured in the 2nd screen capture. If you do not know what MTW means contact Rational Acoustics or a Local Distributor/Authorized Reseller and attend a Smaart Training 😉

So at 2ms (coming from the 1ms mark ∆T=1ms) there wil be a 360º phase shift up to 1kHz. Using the T=1000ms (1sec)/F or F=1000ms (1sec)/T you get the Frequency that has a Time period of 1ms being 1kHz (1000ms/1000Hz=1). 1ms@1kHz is 360º

At 3ms (coming from the 1ms mark ∆T=2ms) there wil be a 720º phase shift up to 1kHz and 360º up to 500Hz. Using the T=1000ms (1sec)/F or F=1000ms (1sec)/T you get the Frequency that has a Time period of 2ms being 500Hz (1000ms/500Hz=1). 2ms@500Hz is 360º

At 4ms (coming from the 1ms mark ∆T=3ms) there wil be a 1080º phase shift up to 1kHz and 360º up to 333.33Hz. Using the T=1000ms (1sec)/F or F=1000ms (1sec)/T you get the Frequency that has a Time period of 3ms being 333.33Hz (1000ms/333.33Hz=3). 3ms@333.33Hz is 360º

Regarding the 34cm per 1ms: You need to use the formula Wavelength = Speed of sound/Frequency: 340 meters per sec (14.1ºC) / Frequency. Per step of 1ms +/-34cm means Frequency = Speed of sound/Wavelength > 340/0.34=1000Hz

All of the above can be calculated with negative values as soon as you get to the 2nd part where I synchronised Smaart at the 4ms mark on both Transfer Engines where green uses tracking delay and blue doesn’t so when moving the mic back towards the driver the phase trace of the blue Transfer Engine shows a upward angle.

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for blogDownload transfer data 😉

More to follow