88 dB Ambient?????

[Flint]

These are established curves for making measurements accurately and consistently.

The A-Weighted curve is for perceived loudness - as in OSHA requirements - as it takes into account the sensitivity of how humans hear (based on the "equal loudness curves"). If you want to compare two environments and how they will affect people's hearing loss and/or stress levels, use an A-Weighted measurement of the sound.

The C-Weighted curve is intended for measurements of the acoustic energy similar to a perfectly flat response but with the lowest bass and highest treble filtered off to ensure consistency. If you have a flat measurement mic which can accurately pick up 10Hz as easily as 100Hz, then noise we cannot "hear" could confuse the measurement. For instance, if you live near a highway and attempt to make an SPL measurement with a flat, non-filtered system the high levels of acoustic energy below 20Hz could confuse the measurement. Perhaps the audible sound in the room is around 75dB SPL but the rumble from the highway below 20Hz which is inaudible to us until it reaches 90dB SPL could cause the meter to show 85dB SPL, which is not helpful.

So, the acoustics measurement world has adopted the A-Weighting curves for loudness and the C-Weighted curves for "flat" acoustic energy measurements. In both cases the meter is making a single SPL measurement for the entire spectrum within the curves' filters. If you are measuring with a frequency analyzer (like TrueRTA or similar), you don't need the A-Weighted filter or C-Weighted filter because each frequency range is measured separately and a high level at 10Hz won't impact the measurements above 20Hz.

 

[heeman]

So have these curves been developed based on meter/mic inaccuracy or what has been established for "flat" response for "C" weighting?

 

[Flint]

it has nothing to do with mic accuracy. The curves are developed standards for simple known common measurement situations. The assumption Nady is making is that their mic is accurate enough to meet the expectations of the industry their meter is designed for. It may not be perfect, but it is close enough that when it says 95dB SPL A-Weighted it will hold up in court at be accurate withing the common +/-10% range of deviation.

 

[heeman]

So this is the desired curved for the human ear to perceive a flat response?

Once we take our measurements we should adjust, per the curve, to the best of our ability to obtain this response.

One point with respect to frequencies <30Hz, there is very little material in music that is in this range, well at least the music that I listen to. I am sure there are other genres that do.

However, movies will have content at these frequencies and if they are boosted above the desired dB, we will probably not be negatively affected, because it could be explosions, etc. and there is no such thing as to loud of an explosion.........

Anyway, thanks Flint for all of your input on this.

This is a nice little SPL Meter for the $.

 

[Flint]

You are interpreting what I am trying to say incorrectly.

The A-Weighted curve is about how humans perceive environmental noise and how our hearing gets damaged a high SPLs. It is not about "flat" at all. It is used to enforce OSHA regulations and noise ordinance laws. It is not used for calibration of frequency balance at all, ever. It is simply a loudness or ear-damage measurement. The only time one would ever use the A-Weighted setting in a Home Theater is when adjusting the levels of the 5 or 7 surround speakers without any EQ adjustments. Simply getting all 5 or 7 speakers to the same general SPL with a calibration signal doesn't require a full bandwidth measurement. I guess you could also use the A-Weighted setting to measure how likely you are doing damage to your ears if you like to really crank up the volume.

The C-Weighted setting is for a broad range measurement - like adjusting a tweeter level to a midrange level to a bass level. It can also be used to measure relative SPLs, as when determining how loud your system might be able to play. You could measure the voltage feeding your speakers along with the resulting SPL in the room then increase the volume to get a higher voltage resulting in a measured higher SPL. Once the SPL stops increasing at the same ratio as the voltage, you know you have reached the acoustic limits of the system. Or, you can use it to calibrate the subwoofer assuming the LFE crossover is above about 60Hz. But, to get the best results a frequency analyzer is massively more effective. You could use the meter's C-Weighting to measure a series of frequencies between 60Hz and about 10kHz to manually plot a crude frequency response, but we don't know for sure if the meter is truly flat in the midrange. Some are, some are not.

A-Weighting is all about loudness and ear damage (and OSHA Regs / Noise Ordinances).

C-Weighting is about more hifi type stuff, but still not ideal for frequency response.

I hope that helps.

 

[Flint]

And both filters (C-Weighting and A-Weighting) have the deep bass filtered out to ensure no environmental noise affects the measurement of whatever sound source you are attempting to measure.

I have some ambient acoustic noise floor measurements made in my home with a frequency analyzer (TrueRTA). In my home the ambient noise below 30Hz is 74dB SPL RMS with peaks as high as 80dB SPL. That noise would completely throw off any measurement I attempt to make with a perfectly flat, unfiltered SPL meter if the source SPL is below 80dB. However, the C-Weighting will filter out that subsonic ambient noise to I can measure the actual sound from the source.

This is also why I always calibrate my system at 90dB SPL rather than 75dB, like many recommend.

 

[heeman]

So this is the desired curved for the human ear to perceive a flat response?

Once we take our measurements we should adjust, per the curve, to the best of our ability to obtain this response.

One point with respect to frequencies <30Hz, there is very little material in music that is in this range, well at least the music that I listen to. I am sure there are other genres that do.

However, movies will have content at these frequencies and if they are boosted above the desired dB, we will probably not be negatively affected, because it could be explosions, etc. and there is no such thing as to loud of an explosion.........

Anyway, thanks Flint for all of your input on this.

This is a nice little SPL Meter for the $.

The comment above was in reference to "C" weighting only.

I think that typed words may be getting in the way here.

They idea is to get equal Sound Pressure Level across the frequency band, then by adjusting the roll off at the upper and lower end for "C" weighted measurement. I understand that the SPL Meter readings are just a starting point and not the correct tool for proper calibration.

Last night I was experimenting with Test Tones as opposed to Pink Noise. It became obvious where the Peak and Nulls are with respect to my room and current speaker placement with respect to the sweet spot.

Interesting stuff, anyway, some day I may run True RTA or other software to see what my system/room is really doing.

Right now it is sounding the best that it ever has and I (we) are really enjoying it!