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Basic Audio Calibration


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#1 MarkTecher

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Posted 02 March 2006 - 04:44 PM

In 2000, I worte a short course on surround sound and delivered it to students at the Logan TAFE.

In 2004, I presented a modified version to CEDIA.

I am posting a slightly revised version here for those that need a quick how to on audio calibration. It is not meant to be the complete be all to end all technical paper, but rather a quick summation of how I recommend to calibrate your HT system using just a SPL meter. This is based on almost 16 years of HT experience hopefully will hepl others achieve a better listening experience...

Excerpt from "UNDERSTANDING SURROUND SOUND - 5.1 And Beyond..." writen by Mark Techer, July 2000
SYSTEM CALIBRATION.

AUDIO ADJUSTMENTS.

A well designed controller will have a few standard items. Apart from different surround sound decoding algorithms, your audio processor should have level trims for all channels including the FRONT LEFT and FRONT RIGHT.

For those systems that do not, you will need to find the level where the Left channel that reads +75dB and remember or mark the master volume at that point. That number will your 00dB level, even though it may not be close to 00dB.

Also, not all volumes display in relative (negative dB figures), some display in “absolute” (number that increase with the loudness). If your system is such, then choose a “round” number and then adjust each channel trim to read +75dB on the meter.

The worst case scenario will be a unit that does not offer either trims on the front left and right channels or a master volume displayed in absolute values…

In this digital age, usually both the trims and the master volume control will be digital and the trims should be in half dB adjustments. Full dB adjustments are too course and may not allow total precision.

All well designed decoders will also have an internal noise generator. The tone emitted from the noise generator will be equivalent to an audio tone recorded at –30dBFS for film sound (18BIT) which is the same as 105dB – 30dB or +75dB. Using a Sound Pressure Level meter and these tones, you will be able to precisely adjust the acoustic output level of your system to equal that of a cinema or dubbing stage.

The end result will be that you will hear the sound track they way it was heard in the studio that created it both from channel to channel sound as well as absolute volume level.

The SPL meter should have an analog display. The use of the analog model is preferred as reading can be under 1dB and depending on the type of level trim on your system, totally accurate.

The radio shack SPL meter is the same type used by the film industry. Independent tests revealed that this meter was the most accurate model from a group of meters tested including more expensive models.

The new meter from JAYCAR is more expensive, but it has a calibrate feature. Whilst this meter is digital, it does display in 1/10th of a dB.

USING THE METER.

The meter has a rotating “range” dial and two switches. The range dial provides the SPL range you would like to measure in decibels starting form 60dB to 120dB. There is also a battery check “BATT” position just next to the “OFF” position.

The two switches are labeled “WEIGHTING” and “RESPONSE”. Set the “WEIGHTING” switch to the “C” position and the “RESPONSE” switch to the “SLOW” position.

Set the dial to the 70dB range. This will now allow you to read SPL from 60dB to 76dB.

EDIT: Position the meter in the prime listening position.

Hold the meter at arms length from the body. The meter should be at seated ear height, pointed up and slightly forward.

DO NOT POINT THE METER AT ANY ONE SPEAKER.

I use a tripod to hold the meter and move well away so that reflections from my body are not read by the meter and give false readings.

Activate the test tone generator on your surround sound processor. Well designed processors will default to a 0dB reference level position regardless of the actual volume position and start at the LEFT front channel.

Observe the level reading. It should be reading at +75dB/C/SLOW. If the audio level is too high, the meter’s needle will “peg” and you will have to reduce the trim level for that channel to read the desired +75dB/C/SLOW. If the audio level is too low, adjust the trim to read the correct level.

PEGGING OF THE NEEDLE IS NOT ADVISED AND MAY ACTUALLY DAMAGE THE METER.

Once you are satisfied with the level, proceed to the next channel, working in turn around the room.

SUBWOOFER AND LFE LEVEL.

While the standard level for all main channels (including the Surrounds) is +75dB/C/SLOW, setting the level for both the subwoofer and the LFE channel is a little more complex.

Ideally you would use a Real Time Spectrum Analyzer. The level of the Subwoofer can be set with the SPL meter by the following method. The level of the Subwoofer should also read +75dB on the meter, however the tone for the Subwoofer is a “Warble” and the level will very along with the frequency. It will be difficult to set the level at the 70dB range.

Turn the dial to the 80dB range. If your processor has a Sub test warble, adjust the trim to read an average of +79dB/C/SLOW with the lowest part reading at –5dB on the 80dB scale. The level difference will be around just 4dB and is equivalent to the average level of +10dB when using the RTA.

DO NOT SET THE SUBWOOFER TO READ 10 DECIBELS HIGHER THAN A MAIN CHANNEL WITH THE SPL METER.

The LFE channel is a fixed value in reference to the subwoofer level. That is if you increase the Subwoofer level, you will also increase the level of the LFE channel. Some decoders offer a separate trim for the LFE channel. It should be set to the same level as the Subwoofer. If the SUB trim is 0dB, then so is the LFE trim.

THE LFE TRIM WILL NOT GO ABOVE 0 REF DECIBELS. IF THE LEVEL IS NOT HIGH ENOUGH AT THE 0 REF DECIBEL LEVEL, INCREASE THE GAIN ON THE SUBWOOFER WHERE POSSIBLE.

PLAYBACK.

Once your system has been calibrated, you can enjoy a film. The 0dB reference level is the level you should watch films at. You may of course reduce this level if it is too loud.

DIALNORM may be seen on some format decoders. It is a part if the Dolby Digital coding and will advise you of the average level of dialogue in a program. If the DIALNORM reads a plus figure EG “DIALNORM +4”, you must reduce the level of the master volume by 4dB. When used correctly, you will find that the level of dialogue remains consistent across a wide range of program material. The peak levels however will vary occasionally resulting in some very high Sound Pressure Levels.

Enjoy your films and music…

Mark

Edited by MarkTecher, 02 March 2006 - 09:00 PM.


#2 Skid_MacMarx

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Posted 02 March 2006 - 04:51 PM

rhanks Mark

#3 MarkTecher

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Posted 02 March 2006 - 05:11 PM

rhanks Mark


Your welcome :blink:

HERE are two SPL meters. The one on the left is the Radio Shack (Tandy in Australia) meter and the one on the right is from Jaycar...

Mark

Edited by MarkTecher, 02 March 2006 - 05:12 PM.


#4 norpus

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Posted 02 March 2006 - 05:41 PM

Your welcome :blink:

HERE are two SPL meters. The one on the left is the Radio Shack (Tandy in Australia) meter and the one on the right is from Jaycar...

Mark

Hehe I have both these! (bought the digi b4 the RS -and I use the RS one most cos its got the VU meter style which is easier to judge the dynamics with) :P

#5 :)

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Posted 02 March 2006 - 05:46 PM

Hehe I have both the same ones! (bought the digi b4 the RS -and I use the RS one most cos its got the VU meter style which is easier to judge the dynamics with) :blink:


yep I've got the radio shack an absolute bargain from dse for the $30 or so it sells at.

Mark if you don't mind I'll put a link to this up in the HT FAQ sticky - still waiting on coral to unhitch some technical difficulties I'm having to do that but soon as I can I will. Should be most usefull to people not sure about the procedure.

#6 hornblower

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Posted 02 March 2006 - 05:57 PM

Set the dial to the 70dB range. This will now allow you to read SPL from 60dB to 76dB.

Hold the meter at arms length from the body. The meter should point up and slightly forward.

DO NOT POINT THE METER AT ANY ONE SPEAKER.

I use a tripod to hold the meter and move well away, so that reflections from my body are not read by the meter and give false readings.

Activate the test tone generator on your surround sound processor. Well designed processors will default to a 0dB reference level position regardless of the actual volume position and start at the LEFT front channel.



Enjoy your films and music…

Mark


Can you clarify the following:
How far should the meter be from the speaker or are you suggesting that the meter should be located at the listening position?

hornblower

#7 Wacko02

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Posted 02 March 2006 - 06:18 PM

yep I've got the radio shack an absolute bargain from dse for the $30 or so it sells at.

Mark if you don't mind I'll put a link to this up in the HT FAQ sticky - still waiting on coral to unhitch some technical difficulties I'm having to do that but soon as I can I will. Should be most usefull to people not sure about the procedure.

Can we just get this put up as a sticky?

And thanks very much Mark. It's really appreciated having this information/help available in this forum.

#8 Spearmint

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Posted 02 March 2006 - 06:32 PM

Great write up Mark,

It is well deserved of being a sticky.

Just one point that I feel needs to be clarified and I promise not to bother you with anything else in this thread.


PLAYBACK.

Once your system has been calibrated, you can enjoy a film. The 0dB reference level is the level you should watch films at. You may of course reduce this level if it is too loud.

Enjoy your films and music…

Mark


This is the “Reference Level” when using the test tones built into the receiver, it does not take into account variations in the source level or the software recording levels.

So don’t take the volume level as being gospel! Always use care.

#9 arushan

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Posted 02 March 2006 - 06:53 PM

great contribution mark, this stuff is gold for beginners, i think it should be posted on the sticky thingy on this forum..

thanks mate..

#10 skinXBWAU

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Posted 02 March 2006 - 06:56 PM

Cheers Mark. :blink:

You're a champ!

#11 MarkTecher

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Posted 02 March 2006 - 08:37 PM

yep I've got the radio shack an absolute bargain from dse for the $30 or so it sells at.

Mark if you don't mind I'll put a link to this up in the HT FAQ sticky - still waiting on coral to unhitch some technical difficulties I'm having to do that but soon as I can I will. Should be most usefull to people not sure about the procedure.

That would be great Al, thanks...

Mark

#12 JohnA

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Posted 02 March 2006 - 08:40 PM

great post Mark.
now i got to go and get me one of thos radio shack meters

looking at DSE they look a little different to the one you posted a pic of
http://www.dse.com.a...ducts/q1461.jpg

I gather its the same thing maybe a newer model

Edited by JohnA, 02 March 2006 - 08:41 PM.


#13 MarkTecher

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Posted 02 March 2006 - 08:40 PM

Can you clarify the following:
How far should the meter be from the speaker or are you suggesting that the meter should be located at the listening position?

hornblower

Yes the main listening positon is correct. I will change that if I have missed that point. When I wrote the course, it was intended to be delivered orally, so no doubt, if that point came up I would address it straight away...

Mark

#14 MarkTecher

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Posted 02 March 2006 - 09:04 PM

Great write up Mark,

It is well deserved of being a sticky.

Just one point that I feel needs to be clarified and I promise not to bother you with anything else in this thread.
This is the “Reference Level” when using the test tones built into the receiver, it does not take into account variations in the source level or the software recording levels.

So don’t take the volume level as being gospel! Always use care.


This is the level based on film sound, so yes other sources like a CD for example will vary. My theory on CD playback is no higher than -12dB, as that medium is just 16BIT (93dB) and film sound is based on 18BIT or 105dB.

Allot of DVDs are also encoded too high as well. THX transfers have been accused of not being loud enough. They are actually the correct level as most non THX transfered DVDs read +4dB, but opposite was the case with THE 6TH DAY, where the DN level was -6dB.

Other medium such as DTV has issues as well, as there are too many varibles. Whilst the Dolby Digital audio coding includes dialogue normalization in the meta data, not all format decoders display it, so again the variances between different program are unknown to most. Watching the display on my system for DTV, and the average program reads a DN level of +4, some at +2, and most adds at +9! No wonder adds seem louder then program. The truth here of course is that they are not louder, just the average level of dialogue in a commercial is pushed closer to the top end of the medium. This is done becuase an add on TV does not have loud sound effects or huge musical cresendos that require wide dynamic range, so sound engineers push the level closer to the top of the range. The result is that we percieve the level to be louder.

In todays digital world, I would like to think that TV stations know what the limits are and stay closer to a constant level. This seems to vary between channels as well. MPEG (16BIT) is of course another story...

Mark


great post Mark.
now i got to go and get me one of thos radio shack meters

looking at DSE they look a little different to the one you posted a pic of
http://www.dse.com.a...ducts/q1461.jpg

I gather its the same thing maybe a newer model

I've seen a picture on an Americal thread (AVS?) and if it is the same electronis as the one I pictured, then I cann't see any problems with it...

Mark

#15 :)

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Posted 02 March 2006 - 09:14 PM

great post Mark.
now i got to go and get me one of thos radio shack meters

looking at DSE they look a little different to the one you posted a pic of
http://www.dse.com.a...ducts/q1461.jpg

I gather its the same thing maybe a newer model


it is the newer model johna. works just fine. I remember seing it at norpus' place. mine is hte older model as pictured newer one jsut a bit smaller, more modern looking and cheaper ! but no loss on the quality of it.

#16 JohnA

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Posted 03 March 2006 - 06:45 AM

it is the newer model johna. works just fine. I remember seing it at norpus' place. mine is hte older model as pictured newer one jsut a bit smaller, more modern looking and cheaper ! but no loss on the quality of it.



Thanks Al,
off i go to get one today then

#17 fRuItCaKe.

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Posted 03 March 2006 - 07:20 AM

I've got a digital one bought for work. I wonder if you need to apply the same dB correction factors when looking at low frequencies (ie sub calibrations) as you do when usung the DSE/Radio Shack meter??

I would assume yes, but would you use the same corrections (near enough) or would they need to be accurately determined??
Chriso

#18 norpus

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Posted 03 March 2006 - 09:03 AM

I've got a digital one bought for work. I wonder if you need to apply the same dB correction factors when looking at low frequencies (ie sub calibrations) as you do when usung the DSE/Radio Shack meter??

I would assume yes, but would you use the same corrections (near enough) or would they need to be accurately determined??
Chriso

I have found not (so much). The digi seems to register higher readings when doing low freq sub sweeps than the RS (which is known to be out in this range and has published factors as you know)
Whether either of my meters (even if within their manufactruibg spec of say +/- 1dB at best) are accurate I would doubt, given they have never been calibrated against a primary reference
I would use the digi in preference to the RS for bass for this reason- but would still not expect great accuracy. But it is all relative, so probably ok for this purpose

I like the peak hold feature on the digi - it will be useful to measure dynamic range when I get my IB going :blink:

#19 RodN

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Posted 03 March 2006 - 07:11 PM

I like the peak hold feature on the digi - it will be useful to measure dynamic range when I get my IB going :blink:


You won't need a meter for that mate, just watch everyone's cheeks flapping.

#20 Austen

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Posted 03 March 2006 - 10:48 PM

Great post thanks Mark.

When I brought my HT gear I managed to sneak a RS meter in the budget but I've never really had the chance to play with it, I basically used the Denons built-in press-go-and-it-does-everything-for-you button.

Next time I'm home I'll have a play with the meter now !!!

Thanks again,

Austen.

#21 sschen

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Posted 03 March 2006 - 11:06 PM

I have added this thread to my favourite. Very handy for quick reference. So thanks very much Mark.

I also have a couple of other links to HT calibration that others may find handy:

http://www.audioholi...r_placement.php

http://www.hometheat...p?postid=650553

#22 :)

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Posted 03 March 2006 - 11:10 PM

I have added this thread to my favourite. Very handy for quick reference. So thanks very much Mark.

I also have a couple of other links to HT calibration that others may find handy:

http://www.audioholi...r_placement.php

http://www.hometheat...p?postid=650553


thanks sschen again once sticky capabilities get rectified by coral, will update with these links in the HTFAQ sticky

#23 skinXBWAU

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Posted 06 March 2006 - 02:57 PM

Is this going to be made sticky or what?

#24 MarkTecher

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Posted 06 March 2006 - 05:00 PM

Is this going to be made sticky or what?


Hi SkinXBWAU, how did you go with calibrating your system?

Mark

#25 skinXBWAU

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Posted 06 March 2006 - 06:43 PM

I'm just about to upgrade so I'm holding off for a couple of weeks.