Film-Tech Forum ARCHIVE: Checking room EQs with P35-MF?

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Posted by John Hawkinson (Member # 1135) on 05-09-2003, 05:49 PM:

Is it legitimate to check room equalization by running P35-MF (SMPTE multifrequency test film) and measuring the results on an RTA? Is there some reason that is not a good idea?

It seems a reasonable way to try to compensate for some of the concerns with pink noise (J ^HFA, etc.).

Since P35-MF is designed for checking levels at the amplifier output, I assume that I should expect it to be flat all the way, rather than having a rolloff after 2k?

Thanks.

--jhawk

Posted by Mark Gulbrandsen (Member # 72) on 05-09-2003, 08:17 PM:

Since it is not random noise you will not be taking anything other than frequency responce of the B chain into context. You can certainly try it and only remember that your ears actually calculate the final eq.
Mark

Posted by John Hawkinson (Member # 1135) on 05-09-2003, 08:48 PM:

Like I said...as a "check." Not as something to actually do the EQ from.

--jhawk

Posted by Gordon McLeod (Member # 33) on 05-09-2003, 10:54 PM:

An RTA displays a flat line from a pink noise source and a riseing line from white noise
Simplistically Pink noise is equal energy per octave compares to white noise which is equal energy per frequency
A sweep tone such as the multi frequency test film would produce a rising level displayed

Posted by John Hawkinson (Member # 1135) on 05-10-2003, 01:59 AM:

P35-MF is not a "sweep tone." It's ~10 seconds of one tone, ~10 seconds of the next tone, etc., etc.

--jhawk

Posted by Gordon McLeod (Member # 33) on 05-10-2003, 08:09 AM:

Jhawk said "P35-MF is not a "sweep tone." It's ~10 seconds of one tone, ~10 seconds of the next tone, etc., etc"
same thing it is equal energy per tone.
It is for setting up eq of preamps only

Posted by Christopher Seo (Member # 7) on 05-10-2003, 12:59 PM:

John,

Where did you get P35-MF? It's listed as "out of stock indefinitely", along with about everything else, on SMPTE's website. How many tones, at what frequencies, does it have?

I would think that because of problems with standing waves it wouldn't be such a good idea. Plus the fact that levels could vary considerably even within a 1/3-octave band, such that a single tone in that band would not provide an accurate picture of the average level of that band.

Posted by John Hawkinson (Member # 1135) on 05-10-2003, 02:39 PM:

Chris,

P35-MF came from our storage cabinet, series 47000 dating from September 1978. It has 15 different tones. It starts with a 1k tone for reference, and then goes 40 Hz, 70 Hz, 100 Hz, 300 Hz, 500 Hz, 1 kHz, 2 kHz, 2.5 kHz, 3 kHz, 3.5 kHz, 4 kHz, 5 kHz, 6 kHz, 7 kHz, and 8 kHz. I'm not too sure on the 10 second bit, that's from memory, and might be off by ~2x.

--jhawk

Edit: threw in the date.

[ 05-10-2003, 08:38 PM: Message edited by: John Hawkinson ]

Posted by Ray Derrick (Member # 1001) on 05-11-2003, 05:40 AM:

Chis has hit the nail on the head.

Firstly, tones are not used for acoustic frequency response measurements (except in anechoic chambers) because single tones produce standing waves (which are peaks and nulls) throughout the room, particularly at low frequencies. These peaks and nulls occupy different physical positions for each different frequency. Try playing Dolby tone and move around the room. You will find the tone will be louder in some spots than others as you move from an area of a peak into an area of a null. In fact the tone may come and go by simply moving your head.

Secondly, an RTA is designed to read noise, not single frequency tones. Each band on an RTA has a response peak at the ISO frequency and the response falls rapidly off either side of that frequency. If you were desperate enough to try to equalise a room using tones then you should use a sound level meter to read the levels.

Gordy, the power for a sine wave at a given level is the same at any frequency. Sweep tones are used in anechoic chambers to measure the frequency response of microphones and speaker systems. Hence the response graph that comes with any studio microphone. (An anechoic chamber does not permit the generation of standing waves).

Posted by Gordon McLeod (Member # 33) on 05-11-2003, 09:12 PM:

To quote the audio cyclopedia I have
"when the output of a white noise generator is viewed on a oscilloscope or graph level recorder it displays a rising characteristic of 3db per octave Fig1-141a. To bring the response to an equal energy level (uniform output) a pink noise filter having an inverse frequencey characteristic is connected to the output of the signal generator"

Since energy is typically defined as the area under the waveform the smaller the timebase (ie higher frequency) the less energy is in the area under the waveform for the same peak amplitude
Part of the principle that a thyristor dimmer works on

Posted by Ray Derrick (Member # 1001) on 05-11-2003, 09:47 PM:

Gordy why are you quoting from the Audio Cyclopedia? I wasn't disputing what you had said about random noise.

What I am saying is that if you pump say a 100Hz sine wave of a given amplitude into a completely flat reproduce system, you will get exactly the same SPL reading as a 10kHz sine wave of the same amplitude. Why? Because the SPL is directly related to the RMS value of the sine wave, which is independent of frequency. Of course, if you take a single cycle of a sine wave then obviously the power at higher frequencies will be less than at lower frequencies, but SPL is averaged over a given time period, not a cycle at a time. Maybe I could have phrased my previous post in better terms.

Posted by Ray Derrick (Member # 1001) on 05-12-2003, 06:56 AM:

Just to further illustrate the independence of the power (or energy) of a sine wave from it's frequency, consider the following:

A 50Hz sine wave with peak amplitude of 100V has an RMS value of 70.7 volts (Vrms = Vpeak x 0.707). Now connect that voltage across a 100 ohm resistor and the resistor will dissipate 49.9849 watts of power (Power = Vrms squared / Resistance) . Now change the frequency to 20kHz. The sine wave still has an RMS value of 70.7 volts and the resistor still dissipates 49.9849 watts of power.

Gordy old mate, I rest my case. [Smile]

PS a thyristor dimmer works by taking a perfectly good sine wave and thoroughly mutilating it. Not quite the same principal. [Wink]