dB calculations

[Ivan Beaver]

Re: dB calculations

One example would be when you want your subs to be more "in time" with your mains across a larger portion of the venue.


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That is another good reason my "go to " design is a MONO center cluster (with the subs) with exploded fills as needed.

But it all depends on "what you are after".

 

[Ivan Beaver]

Re: dB calculations

. I don't like the, "let's fix it at 2.83Vs".

Personally I like to publish the Impedance vs. Frequency plot, choose a actual impedance mid-band for the speaker (not on a resonance if present, come on people!!!), calculate the 1W drive level, and then produce the Sensitivity vs Frequency plot at 1m. For subs I like to do it ground plane, so the customer can actually verify the specs.

Personally I do like the 2.83V-and that is what we do.

The idea of 1watt- simply is not true-since the impedance varies quite a bit over the response of the loudspeaker.

Even in your example-you apply a SINGLE VOLTAGE and you are not varying it with freq to maintain the "1 watt" specified.

And it can be a bit more confusing if the impedance happens to be 6 ohms or so.

I prefer to show the impedance vs freq and the magnitude vs freq response.

With a constant 2.83V to all products, it gives (in my opinion) a better idea of real sensitivity.

With that the educated user can determine what they feel the specs should be, and not rely on the "simple numbers" that may or may not give a real indication as to the real performance of the loudspeaker in question.

HOWEVER, either way is MUCH better than just a "simple number" and no idea where it came from, which is my BIG gripe.

In MY opinion-those that provide just a "simple number" are trying to hide something.

 

[Mark DeArman]

Re: dB calculations

Personally I do like the 2.83V-and that is what we do.
The idea of 1watt- simply is not true-since the impedance varies quite a bit over the response of the loudspeaker.

I get the reasons. It's just for the type of work that I do, the comparisons work much better using a 1W fixed point. This is the number which is called out on the spec sheet as "the" sensitivity. The plot vs. frequency is relative to that one point, for the fixed voltage which is listed on the spec sheet. From a customer perspective, for the most part it leads to some numbers which are lower than other people get with 2.83Vrms as the stimulus. (I guess the marketing guys could complain, but since I write the spec sheets personally now... I got tired of them making things up. I look at some of the shit they made 10 years ago, and I can't believe anyone okay'd it.... uhg.)

Basing them all on voltage does have the advantage, for the small end-user, that voltage clipping will most likely limit their output. So he can choose a box and amp combination easier. But most people probably don't think about it that way.

 

[Declan Slater]

Re: dB calculations

Can you give an example of situation where L-R is better than center cluster? I love my job but if there's one thing I hate, its L-R subs.

Well, the "power alley" effect of two seperated subwoofers, or groups of subwoofers, can work out pretty nice sometimes in longer rooms with poor acoustics. Another effect at work here is the polar pattern or (somewhat) directionality of different subwoofer cabinet designs. I often use a pair of single 15" subs together on stage left, and another pair stage right. Indoors, the combined pattern gives a nice power alley that works well with large and deep floor areas. If outdoors, this effect leaves far too little low end in areas not between the mains. If I turn the subs out at around 45 degrees away from centerline, the "power alley" effect is often widened dramatically, and matched up much better with the polar pattern of the tops.

 

[Ivan Beaver]

Re: dB calculations

I get the reasons. It's just for the type of work that I do, the comparisons work much better using a 1W fixed point. This is the number which is called out on the spec sheet as "the" sensitivity. The plot vs. frequency is relative to that one point, for the fixed voltage which is listed on the spec sheet. From a customer perspective, for the most part it leads to some numbers which are lower than other people get with 2.83Vrms as the stimulus. (I guess the marketing guys could complain, but since I write the spec sheets personally now... I got tired of them making things up. I look at some of the shit they made 10 years ago, and I can't believe anyone okay'd it.... uhg.)

Basing them all on voltage does have the advantage, for the small end-user, that voltage clipping will most likely limit their output. So he can choose a box and amp combination easier. But most people probably don't think about it that way.

I will agree that either approach will work-AS LONG AS the reader knows how the test was being done.

I could see a "creative engineer or marketing guy" using the 1 watt number-but on a speaker that has a "higher than normal" impedance for the rating, and figuring out the average impedance across the freq band-then using the voltage to get 1 watt and it being higher than say 2.8V. So the user "thinks/assumes" that it is at 2.83V for the 8 ohm rated box, but the drive voltage for the graph may actually be higher-giving the impression it is louder than it really is.

Another reason for using the same drive level (2.83V) is that is exactly the test that people do when they simply move a cable from one box to another to see which on has the higher sensitivity.

As with other things-there is a whole list of "games" that can be played to "suck people in", but in the end-it comes down to what does it sound like when turned up, and how loud can it get before "crapping out sonically".

But I have not seen any spec sheets on that. I have thought about some ways to do it, but different people have different standards of "crapping out" and there could be all sorts of ways to get around it.

But I would be open for that sort of test-AS LONG AS everybody used the same testing procedure.

 

[Mark DeArman]

Re: dB calculations

I could see a "creative engineer or marketing guy" using the 1 watt number-but on a speaker that has a "higher than normal" impedance for the rating, and figuring out the average impedance across the freq band-then using the voltage to get 1 watt and it being higher than say 2.8V. So the user "thinks/assumes" that it is at 2.83V for the 8 ohm rated box, but the drive voltage for the graph may actually be higher-giving the impression it is louder than it really is.

Well now, this is how everyone cheats on the power numbers too ;-) Like we've been saying, without a fixed impedance, it's hard to have a standard test for sensitivity and power.

Although, I have developed a power measuring system based on RMS averages and instantaneous quantities, like they use in the power industry, which is highly repeatable, and give results which are consistent with models of the box, but it gives a number a couple hundred watts below the AES2 spec for an average box. So we just use it for development.

 

[Scott Bolt]

Re: dB calculations

But the problem with center subs is that that very often you end up with a very different levels throughout the room.

It is much louder close to the subs than at seats further away.

Sometimes split subs (even with the interference) provides an overall more even response.

It really depends on the situation and what is expected as to what approach is best-for THAT situation. What is right for one is wrong for another.

I have found that using L/R subs and intentionally creating the power alley can be useful in some instances. Creating a chest thumping, shirt moving sub impact directly in the center of the dance floor can be very cool while leaving the sound levels off to the sides more muted for speaking works as well.

Of course, when real rooms are used that have real walls and real ceilings (vs the free speaker simulation programs), and people are on the dance floor absorbing energy, lots of this gets much less easy to predict.

 

[Ivan Beaver]

Re: dB calculations

Although, I have developed a power measuring system based on RMS averages and instantaneous quantities, like they use in the power industry, which is highly repeatable, and give results which are consistent with models of the box, but it gives a number a couple hundred watts below the AES2 spec for an average box. So we just use it for development.

I have some "old school" Simpson watt meters.

They go inline and being analog, they "average" the wattage, which takes into account the voltage and the impedance vs freq with the current being pulled.

It can be interesting to watch-especially as the freq changes and the impedance varies. A sine wave sweep is also very interesting.

I will try to get a photo tomorrow. I have 4 of different wattages so that different bands can be monitored at the same time.

I am sure there are other more sophisticated methods, but these are interesting to watch.

During the "crossover design" phase-I place them after the crossover and before the individual bands.

Tacking some more "old school" HP 400 volt meters across the outputs makes it even more interesting and educational.

 

[Mark DeArman]

Re: dB calculations

I have some "old school" Simpson watt meters.

Hah cool, I used to have an ole' Simpson 260 when I was a kid. I loved that bakelite construction and smell of old circuit boards it had. Unfortunately the switch broke in it, and it wasn't worth replacing.

Yes, the neat thing about measuring RMS average and instantaneous power simultaneously is you can measure the real and imag power components.

I'm in the process ( I might have mentioned this a couple times over the past year... I just keep putting it off, because it requires some mechanical design,) of making an even more advanced power/impedance meter right now. The key component, made by some start-up company in the valley, I'm told can measure the current phase with 0.1% linearity over the full audio band. I'm going to package this up with a 500W class-D amp with has an attenuated output coherent with the driven channel, a six channel ADC, some calibration hardware, and a high stability signal generator. Should be able to make very precise network measurements, and then use the sixth channel to correlate with either a laser or SPL input. It's kind of a fun project, it has just taken me some time to select the right analog components to do the voltage and current measurements with.

 

[Ivan Beaver]

Re: dB calculations

Hah cool, I used to have an ole' Simpson 260 when I was a kid. I loved that bakelite construction and smell of old circuit boards it had. Unfortunately the switch broke in it, and it wasn't worth replacing.

.

Yeah the old 260 was THE STANDARD in VOMs 'back in the day".

Not to good for tube circuits however (due to the lower impedance loading of the voltmeter), but they were like a "timex watch", not fancy-but they kept on "ticking".