Dubious about SPL used by objective testers

[hardisj]

I think the sound from the speakers above the (mono) bass frequencies are probably closer to uncorrelated than correlated.

That seems reasonable.

 

[MrPeabody]

This is the method ANSI/CTA-2034-A uses in its in-room SPL estimation. The standard uses a listening distance of 4 m (13 ft), assume 2 channels of uncorrelated signal (+3 dB) and room gain of +6 dB (3 dB higher than the calculator Erin linked to):

View attachment 100671

CTA-2034A also provide adjustment guidance. Note that the adjustments are based on the listening level category. For example, if the speakers are rated for listening level "Loud" using the formula above, but the room is large (≥ 600 ft²) with normal furnishing, they will be derated 1 step to "Moderate".

View attachment 100672

As implied here, the oft-quoted +6 dB rule for two speakers applies only for wavelengths long enough such that the two wavefronts are coherent (in phase).

The rule that says that SPL drops by 6 dB with every doubling of distance is applicable only when there are no boundaries that will upset the perfectly spherical expansion of the wavefront. This rule is of course derived from the fact that the surface area of a sphere increases as the square of the radius. (If you double the radius, the area quadruples, which means that the density of any quantity distributed uniformly on the surface will decrease by a factor of 1/4.) This rule assumes anechoic conditions and does not apply otherwise. When conditions are not anechoic, -3 dB per doubling of distance may be more useful, however it is going to depend on the number of boundaries, and on the distances relative to the wavelength. It will not be the same both for wavelengths that are shorter that the mean distance from the speaker to the walls and for wavelengths longer than this mean distance.

 

[NTK]

... When conditions are not anechoic, -3 dB per doubling of distance may be more useful, however it is going to depend on the number of boundaries, and on the distances relative to the wavelength. It will not be the same both for wavelengths that are shorter that the mean distance from the speaker to the walls and for wavelengths longer than this mean distance.

The beauty of following standard is: Don't argue with me, go argue with the standards committee

 

[hardisj]

So, we all agree. We are to test at 80dB @ 1m. Right?

 

[Senior NEET Engineer]

But you've already gained the +3dB once you put it in to a room. So when you move twice the distance away from the speaker, you still lose 6dB. You don't get to keep adding 3dB because the room has already been factored in at the beginning.

Example: Start with 96dB @ 1m anechoic. In a room near a wall it's closer to 99dB. Now move 2 meters away and the SPL is roughly 6dB down or 93dB in-room. At 4 meters it's 87dB.

So, like I said, if you take the 96dB anechoic then that's closer to 84-87dB at typical listening distance (anechoic). Add a few dB for room gain, sure. But the difference between 1 meter and typical listening distance of 3-4 meters is -9 to -12dB. That's why a 96dB @ 1m output measurement isn't over-reaching in loudness levels; it's right in the sweet spot where I find most listen once you factor in how far their MLP is from the speakers.

Now, if we want to add the additional speakers and all that... sure... that is correct. I still contend 96dB is a good number to test a single loudspeaker with assuming it can handle that. In one of my recent drive unit tests I maxed out at 95dB because the sensitivity of the DUT was ~84dB on average and I limit my IMD testing to about +10dB (iirc).

At this point, you and I may be "arguing" the same thing. I just want to be clear for others who may read that this is how the math works.

Though, I still do my testing at 7 different levels ranging from the standard 2.83v/1m to -6dB up to +14dB/1m to try to make sure I cover a wide range of output levels.


Edit: I use this site as a sanity check for in-room SPL levels at varying distances. Love this site:
https://myhometheater.homestead.com/splcalculator.html

No we are not arguing the same thing. I just want to make it clear to others that you are mis applying free field acoustics to small rooms.

 

[hardisj]

Noted. Thank you for the information.

 

[Deleted member 14468]

These are the words of NRC: very loud and considered far beyond normal listening levels

I don't know how these statements get misconstrued, but that's the text from one of our sites, so it's not NRC writing it.

Typical listening levels are in the 70 to 80dB range, with two speakers playing in a real room. When we perform a 90dB THD test, that's a single speaker playing in an anechoic chamber. That's extremely loud. Two speakers increase output by 3dB, so that's already 93dB if we had two speakers playing. The room would add at least 2 more dB, so say 95dB. The measuring distance is 2 meters, and if we cut it to 1 meter, that would be 6 more dB. You can see how these things add up -- a single speaker measured at 90dB from a 2-meter distance IS very loud and far beyond "normal" listening levels. Sure, so movie soundtracks play in excess of 100dB in some rooms people have, but that's a group of at least 5 speakers, again in a real room.

Doug Schneider
SoundStage!

 

[Deleted member 14468]

The reason they don't measure all speakers at the quieter level may be time/money related. I'm guessing they have to pay for time spent using NRC's anechoic chamber.

While I do think quieter distortion measurements would be useful data for all speakers, they're certainly more useful for speakers who can't really handle the louder test.

Hi,

1) Yes, we do pay for the time.

2) We could measure at lower levels--and we have for some speakers severely restricted in terms of their output--but our 90dB measurement (2-meter distance) is a good place to start. If the speaker isn't distorting too badly, we push it to 95dB, but that's typically the limit.

Doug Schneider
SoundStage!

 

[Deleted member 14468]

I haven't looked into the details. You could ask @Doug Schneider.I'm sure there is some back-and-forth, but the editors has the final say about what's published and what's most meaningful. One of the most difficult things in publishing is satisfying multiple kinds of audiences simultaneously. You will have people writing you and demanding more graphs and others who ask to put that content to the side, sometimes very emotionally. It's not a scientific publication after all, but it's good that they take the time to make those measurements, despite whatever reservations a part of their readership or they themselves might have about their efficacity, and have also prepared lists online of the tests. This is better than the Stereophile approach, which does not keep a separate listing or database, and bury the measurements in reviews.

Hi,

You got our process right in many ways. Our goal isn't to try to ascertain the entire measured performance of a loudspeaker under test. Instead, it's to take a good number of measurements that begin to give a lesson in terms of what a speaker will sound like in a room, as well as to see if it's performing to spec and if any gross design problems are showing up.

Doug Schneider
SoundStage!

 

[Deleted member 14468]

I'm glad they show distortion at those levels, and I wish they would actually show distortion levels quite a bit louder. The loudest peaks I hear will be in the 105dB range at a listening distance of almost 4m, so their test is far too quiet for me, personally.However, I do agree with you that they should show the quieter distortion measurements for all speakers, and not just the smaller ones.

Insofar as going louder goes, the way our tests run -- swept sine wave that's fairly long in duration -- we'd blow up most speakers we test if we pushed them to, say, 100dB. I know this from experience (i.e., we have). The 95dB test is very difficult for most speakers that come in. Not all, but most.

Doug Schneider
SoundStage!