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Do all wideband drivers have rising response?

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Posted on February 15, 2015 at 20:38:44
PaulF70
Audiophile

Posts: 1425
Location: Midwest
Joined: June 30, 2006
 The ones I have measured (Fostex, Lowther) do, except for one the 8" Supravox field coil. That was pretty flat from 50-8K. Though that was a long time ago.

A rep for SEAS once told me that it is a matter of physics that any truly wideband driver will have rising response (and I'm not talking about peaks, but a rise in mean level from, say, 100 Hz to 2 Khz+). He said this after they had just released their own highly-regarded 8" widebander.

Is it true that the flux density required for a high-eff, wideband cone will always produce a rising response, to some degree at least? Why?

(Note: I'm aware that in-room response of many a widebander-based speaker can be surprisingly flat, due to various factors - I'm just taking about the drivers.)

(Note 2: This type of rising response can't really be corrected by rear-horn-loading, which only affects a couple octaves. Now, if you front-load it too, like a Beauhorn...)

 

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RE: Do all wideband drivers have rising response?, posted on February 16, 2015 at 10:15:43
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 You can't really get anything useful from this without allowing for directivity and consequently some kind of room-acoustics assumption.

The effect you describe is a consequence of using cones of real-world stiffness - it is a cone resonance phenomenon.

 

RE: Do all wideband drivers have rising response?, posted on February 17, 2015 at 19:13:53
PaulF70
Audiophile

Posts: 1425
Location: Midwest
Joined: June 30, 2006
 Paul, is this a correct translation of your statements?

1) We should really consider only in-room FR/power response - on-axis response is irrelevant.

2) Yes, all wideband high-eff cone drivers do exhibit a rising frequency response.

Thanks.

 

RE: Do all wideband drivers have rising response?, posted on February 17, 2015 at 19:40:52
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 I would not go that far - speakers and how they react in a room seem to have many variations. There is a good body of psychoacoustic research about the effects of direct sound, early sound, late and/or reverberant sound, etc. Both first-arrival (on-axis) and late arrival (reverberant) spectra are important in their own way. I'm just saying that these issues are especially important with a large-diameter "tweeter"!

Most, but not all, wideband paper cone drivers, and most high efficiency midranges for that matter, do seem to have that rising response, especially if they are high efficiency. Phillips produced some excellent work IIRC in the seventies on how cone resonances behave with frequency, and that work supports the notion that the rising response is usually a characteristic. A few drivers look better than most, so not all, but most.

 

RE: Do all wideband drivers have rising response?, posted on February 25, 2015 at 14:36:49
Retsel
Audiophile

Posts: 1239
Joined: April 17, 2000
 No. I have Lowther DX4s and on open baffle, yes there is a rising response wich required a crossover to correct.

However, if you mount the DX4s in a back-loaded horns (I have Hedlund Horns), the backloaded horns can be configured (based on the size of the compression chamber and the use of felt in the compression chamber and in the backhorn) to balance out the sound to something that resembles a flat frequency response. I say resembles a flat response because I never measured the response, but the high frequencies were clearly not overstated relative to the rest of the frequencies.

One issue is the infamous Lowther "shout." This most likely is a spike in output around 2k hz that exists for the 8 ohm drivers. The 15 ohm Lowthers do not have such a response, so the genre of Lowther drivers does make a difference.

Retsel

 

RE: Do all wideband drivers have rising response?, posted on February 25, 2015 at 15:36:15
PaulF70
Audiophile

Posts: 1425
Location: Midwest
Joined: June 30, 2006
 Back-horn-loading doesn't take care of the general rising response issue with widebanders, or Lowthers in particular. (That's why Beauhorn front-loaded the driver as well. They claimed that's how Lowthers were designed to be used, and I thought they had about the best implementation.)

I think the shout is caused by both the rising response (we're talking about a general, continuous rise from 100-200 Hz all the way up to 4K or higher) as well as peakiness caused by cone-breakup and/or whizzer interactions. In any case most seem to point the finger at the whizzer regarding peakiness.

 

RE: Do all wideband drivers have rising response?, posted on February 27, 2015 at 09:20:31
Retsel
Audiophile

Posts: 1239
Joined: April 17, 2000
 "Back-horn-loading doesn't take care of the general rising response issue with widebanders, or Lowthers in particular. (That's why Beauhorn front-loaded the driver as well. They claimed that's how Lowthers were designed to be used, and I thought they had about the best implementation.)"

It does, but you have to know how to adjust the speakers to do it. The Hedlund Horns were designed for the Lowther DX2 and the frequency response was quite flat for that speaker. But when I purchased the DX4s, I needed to add a couple small bags of sand to the compression chamber to reduce its size and "tip down" the frequency response.

All speakers work the same way. The size of the box that a speaker is mounted in determines the system Q, which determines the frequency response. A speaker mounted in an open baffle does not change its Q, thus, crossovers must be used to adjust its frequency response, but the compression chamber does.

I suspect that Beauhorns were designed with larger compression chambers and relied on the front horn to balance the frequency response. Beauhorns very well may be the best implementation of the backhorn speakers, although I never understood how it could work because the front horn was too small to raise the lowest frequencies. I think that the lack of folds in the Hedlund Horn put it in the category of better backhorn designs for Lowthers.

However, Lowthers mounted on open baffles clearly outperforms all backloaced designs, but eliminates the possibility of using the Lowther as a full range, do it alone, speaker.

Retsel

 

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