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 Post subject: Just curious
PostPosted: March 29th, 2019, 4:59 pm 
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Joined: January 15th, 2015, 7:19 am
Posts: 867
Location: Baltimore MD
On this link Paul McGowen of PS Audio makes a statement that seems wrong. and just want to verify with the guys here in the know
about 3 mins in this video
[url]
https://www.psaudio.com/askpaul/how-we- ... rs-part-2/[/url]
Paul states that putting multiple tweeters in line will lower the low freq roll off

Just want to fact check


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 Post subject: Re: Just curious
PostPosted: March 29th, 2019, 5:09 pm 
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Joined: March 12th, 2013, 11:45 am
Posts: 47
Location: Glen Burnie, MD
Paul, Call Eric Alexander at Tekton Design. He uses tweeters in a vertical line (Pendragon) and in a circle array.


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 Post subject: Re: Just curious
PostPosted: March 29th, 2019, 5:57 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 266
Actually, it is quite simple. It is a power handling issue. The Fs of a tweeter may go down to a few hundred Hz, but they cannot move enough air or handle the power required to reproduce lower frequencies at a reasonable volume. By using an array of 8 tweeters, you have reduced the load on each tweeter, which means you can lower the crossover point (from 4KHz to 2KHz in the video) without overloading the tweeters. Now you also need to take into account the amplitude of typical music, which is higher at 2 kHz than 4 kHz, which is why you need eight (8) tweeters instead of just 1 tweeter. But you do not lower the frequency just by adding tweeters, though that might happen if using the same crossover network without adjusting component values for the change in impedance caused by either parallel or series connection of the drivers.

David


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 Post subject: Re: Just curious
PostPosted: March 29th, 2019, 7:39 pm 
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Joined: February 28th, 2013, 3:31 pm
Posts: 1125
David has hit the nail squarely on the head. SPL at any frequency is a volume velocity issue with the actual volume needed dependent on frequency and level. Remember that for every octave decrease in frequency if you are going to maintain the same SPL you need 4 times the volume displacement. So as David mentioned, if whatever driver you are using be it a tweeter, midrange, or woofer, if you halve the frequency (one octave) and stay with the same type of driver it would have to have four times the excursion to maintain the same SPL. If you are already excursion limited with the driver you can keep the same excursion and extend down an octave by using four of the same driver. However, this philosophy gets out of hand quickly because for example if you wanted to go down another octave you would have to have 16 drivers to achieve the same SPL.

A problem that arises with multiple drivers in a pseudo line array is that as the wavelength of frequency being reproduced starts to approach the distance between the driver center spacing, comb filtering will occur resulting in response irregularities along the array. A true line source such as the BG RD75 planar magnetic driver that has a continuous 75" long diaphragm does not have that comb filtering problem. Because it is continuous you can think of the diaphragm as essentially an infinite number of infinitely small drivers. Therefore for the wavelength of the reproduced frequency to have comb filtering problems the frequency would have to be infinitely high.


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 Post subject: Re: Just curious
PostPosted: March 29th, 2019, 9:32 pm 
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Joined: January 15th, 2015, 7:19 am
Posts: 867
Location: Baltimore MD
That makes sense. Thanks


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 Post subject: Re: Just curious
PostPosted: March 29th, 2019, 11:23 pm 
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Joined: July 8th, 2016, 4:34 pm
Posts: 276
Actually the problem of beaming and lobing occurs when the dimension of the array is comparable to the wavelength being radiated. If the array is flat (not curved properly or the individual drivers are not phased properly) and driven uniformly you will still get beaming and lobing.


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 Post subject: Re: Just curious
PostPosted: March 30th, 2019, 10:54 am 
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Joined: February 28th, 2013, 3:31 pm
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What I was talking about was comb filtering not particularly lobing. Lobing can be considered a subset of comb filtering as it is commonly referred to with two drivers at the crossover frequency where both are reproducing the same frequency. The MTM configuration with 18 dB/octave slopes was designed to reduce that effect. Lobing often has two or more lobes that have different amplitudes and radiate in different directions. I am including an image of that.

With multiple drivers all passing the same frequency you get a different result with multiple peaks and dips in power output all in the same direction and amplitude. The response looks like the teeth of a comb. There is a photo of that also included.

Both cause problems but they are different in results.


Attachments:
Two way speaker lobing.jpg
Two way speaker lobing.jpg [ 177.37 KiB | Viewed 731 times ]
Speaker comb filtering.jpg
Speaker comb filtering.jpg [ 70.72 KiB | Viewed 731 times ]
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 Post subject: Re: Just curious
PostPosted: March 30th, 2019, 12:51 pm 
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Joined: July 8th, 2016, 4:34 pm
Posts: 276
The following link may be of interest -

https://www.edn.com/Home/PrintView?cont ... Id=4210985


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 Post subject: Re: Just curious
PostPosted: March 30th, 2019, 5:03 pm 
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Joined: February 28th, 2013, 3:31 pm
Posts: 1125
Don's references are especially thorough and formed the basis for his CBT.


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 Post subject: Re: Just curious
PostPosted: March 30th, 2019, 8:33 pm 
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Joined: July 17th, 2016, 6:24 am
Posts: 600
Having multiple tweeters increases the SPL levels across the entire frequency range,not must the roll off range below Fs. The SPL difference between the rolloff range and designed operating range remains same irrespective of how many drivers are used.

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Shashi


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