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Let's talk Tweeters!

Another thread which talked about specific speaker brands was taken over, so I’d like to start a new one.

Mind you, I do not believe in a "best" type of tweeter, nor do I believe in a best brand of speaker, so lets keep that type of conversation out, and use this instead to focus on learning about choices speaker designers make and what that may mean to the end user.

There is no such thing as a speaker driver without trade offs. Some choices must be forsworn in exchange for another.

In the end, the materials used, magnet and motor structure, and crossover choices as well as the listening room come together to make a great speaker, of which there are many. In addition, we all listen for different things. Imaging, sweetness, warmth, detail, dance-ability and even efficiency so there is no single way to measure a driver and rate it against all others.

Also, please keep ads for your 4th dimensional sound or whatever off this thread. Thanks.
erik_squires
erik_squires OP
16,008 posts
 
Hi Duke,

I may be using the term incorrectly. Generally when I read about compression, it’s not thermal. This may be due to the fact that I don't read or do much with professional level gear, in which this is a major concern. Thermal compression can be seen with tone bursts, and you can see that after the first couple of cycles the output drops remarkably.

"Static" or common compression is measured by comparing the FR at 2 different input levels. Find your level for 70 dB at 1kHz. Measure the FR. Add 20 dB of input voltage. Measure FR. The output should be +20dB everywhere. Wherever the output is NOT +20 dB is evidence of compression.

Thermal compression is caused by heat, and therefore it’s effects are changing with previous input. What I’m calling "static" compression is related more to the limits of the driver’s linear travel.

Look at the "Deviation from linearity" charts. Here is a speaker with excellent "linearity" or lack of compression:

http://www.soundstagenetwork.com/measurements/speakers/paradigm_studio10_v5/

And here is one with relatively poor linearity:

http://www.soundstagenetwork.com/index.php?option=com_content&view=article&id=1564:nrc-measu...

Best,


E
audiokinesis
2,731 posts
 

Thank you for that information and for digging up those examples, Erik!

Interesting that the less expensive speaker beats the more expensive one not only in deviation from linearity, but also in radiation pattern smoothness - look at the 45-60-75 degree off-axis curves. I tip my virtual hat to Paradigm.

Considering how SoundStage makes their Deviation from Linearity measurement, I think it includes not only mechanical effects but thermal ones also, because at 90 dB the speaker is seeing 10 times as much excursion but 100 times as much wattage as at 70 dB. At any rate, given that peaks 20 dB above the average are quite common in recordings that aren’t overly compressed, the deviation from linearity going from 70 to 90 dB may just be the "tip of the iceberg" for real-world effects, if the speaker is driven to average levels higher than 70 dB/1 meter.

My impression is that excursion-related non-linearities decrease gradually with level until they reach a certain point and then they shoot up rapidly.

The same thing is more or less true for thermal effects: It’s not uncommon for a driver to exhibit less than 1 dB of thermal compression at 10% of its AES rated power, often rising to about 2 dB at 50% of its rated power and then maybe 3.5 dB at 100% of its rated power. In other words, in that last doubling of input power, we only get about half as much increase in SPL as we "should have" (1.5 dB instead of 3 dB), Now these ballpark figures come from eyeballing the spec sheets of those few prosound manufacturers who publish compression specs. These numbers are for long-term thermal compression rather than short-term thermal modulation (which has a very rapid onset and then a slower release, unless another peak comes along before the voice coil has had a chance to cool down). My assumption is that there is a correlation between the short-term thermal modulation behavior and the long-term thermal compression behavior.

(Note that the AES rated power is a fairly conservative yardstick; typically the "music program" power rating is double that, and then the "peak" power rating may be double the music program rating, and we probably don't know which of these the manufacturer is using... and real-world, the excursion-limited power handling may be significantly lower than any of these at low frequencies.)

Duke




erik_squires OP
16,008 posts
 
@audiokinesis

Like a lot of tests, there is no way to exclude all other dimensions or confounds.

You are absolutely right in that these tests cannot be completely devoid of thermal issues.

And yes, price is no guarantor. Wharferdale Diamond's for instance have outstanding tweeters in regards to linearity, and many have expressed surprise at how good they sound after inexpensive cap upgrades. :)

Best,


E
erik_squires OP
16,008 posts
 
And that expensive speaker, it's a real laboratory of measurements vs. experience. The off-axis sounds very good to me, but the overall tonal balance never did. I attributed the latter due to compression, but also really trying to stretch a small woofer beyond its capabilities. 

I won't go into it here, as it causes a lot of passion, but I encourage anyone who can to look at the specs and listen. It's interesting what does and does not correlate with experience.
audiokinesis
2,731 posts
 

Getting back to tweeters, I don't have a lot of experience with a lot of different current-generation domes and ribbon(-ish) drivers, but I've tried a fair number of different compression drivers and many different diaphragm types.  My preferences tend to be for polymer diaphragms (rather than metal diaphragms) up until you get to the price point where true beryllium diaphragms enter the game.  Since my design choices place a higher priority on the horn or waveguide than on the compression driver, and a higher priority on synergy than on excellence in isolation, matching up the exit angle of the compression driver with the entry angle at the throat of the horn or waveguide is a consideration, which does limit my compression driver choices somewhat.

Duke

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