When do you need more amplifier?

[PENG]

On SoundStageNetwork, Dynaudio Special Forty speakers come in with sensitivity at 83.6 dB averaged from 300 to 3,000 Hz. Their impedance low is roughly 5 or 6 ohms, similar to what Dynaudio claims. These Dynaudios are indeed power-hungry speakers.
View attachment 30299

Frequency response curve at 0°, 15 and 30° off-axis isn't bad. Unless that business between 500 and 800 Hz is audible noise.
View attachment 30300

That's consistent with Stereophile's, except the impedance dip to 4.9 ohm in JA's measurements, but the difference is still negligible, within the margin of error.

 

[Swerd]

Noise, from what?

I would guess that the stuff at ~550-750 is due to the placement in the room unless they were actually tested in a quasi-anechioc space.

The testing room used in the SoundStageNetwork measurements (shown in post #35) is the Canadian NRC's fully anechoic space.

The Stereophile reviewer of these speakers also had this to say:

"… I listened to the enclosure's panels with a stethoscope. While the top panel was quiet, the sidewalls were afflicted with several resonant modes that affected the midrange. The amplitudes of these spurious vibrations were sufficiently high that I could hear them with the stethoscope placed on the center pillar of the Blu-Tack–coupled stand. The audibility of this behavior will depend on the size of the area of the cabinet panel affected, the relative phase of the radiation, and how well that area couples to the air. However, the touch of midrange congestion was something I'd first noticed from my listening chair."​

The Stereophile review's measurements also confirmed this in figures 3 and 4.

"However [in figure 3], the top panel had a low-level but sharply defined mode at 730Hz. More significant, I found a very strong mode at 512Hz on the sidewalls, with more, lower-level modes just below 300Hz and between 600 and 750Hz (fig.3). This behavior correlates with the problem I heard in my auditioning."​

Fig.3 Dynaudio Special Forty, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

In Figure 4, another possible source of this noise was identified as the port tube and/or its opening.

"The port's output (red trace) peaks in textbook fashion between 30 and 100Hz, but its upper-frequency rolloff has a severe resonance spike at 700Hz. Fortunately, the port faces to the speaker's rear, which should reduce the audibility of this mode."​

Fig.4 Dynaudio Special Forty, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of: woofer (blue) and port (red),respectively plotted below 350Hz and 1kHz, and complex sum of nearfield responses plotted below 300Hz (black).

 

[TankTop5]

Someone near me just listed this for $50. I think it would do the job.


https://www.parasound.com/vintage/hca1000a.php

 

[highfigh]

The testing room used in the SoundStageNetwork measurements (shown in post #35) is the Canadian NRC's fully anechoic space.

The Stereophile reviewer of these speakers also had this to say:

"… I listened to the enclosure's panels with a stethoscope. While the top panel was quiet, the sidewalls were afflicted with several resonant modes that affected the midrange. The amplitudes of these spurious vibrations were sufficiently high that I could hear them with the stethoscope placed on the center pillar of the Blu-Tack–coupled stand. The audibility of this behavior will depend on the size of the area of the cabinet panel affected, the relative phase of the radiation, and how well that area couples to the air. However, the touch of midrange congestion was something I'd first noticed from my listening chair."​

The Stereophile review's measurements also confirmed this in figures 3 and 4.

"However [in figure 3], the top panel had a low-level but sharply defined mode at 730Hz. More significant, I found a very strong mode at 512Hz on the sidewalls, with more, lower-level modes just below 300Hz and between 600 and 750Hz (fig.3). This behavior correlates with the problem I heard in my auditioning."​

Fig.3 Dynaudio Special Forty, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

In Figure 4, another possible source of this noise was identified as the port tube and/or its opening.

"The port's output (red trace) peaks in textbook fashion between 30 and 100Hz, but its upper-frequency rolloff has a severe resonance spike at 700Hz. Fortunately, the port faces to the speaker's rear, which should reduce the audibility of this mode."​

Fig.4 Dynaudio Special Forty, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of: woofer (blue) and port (red),respectively plotted below 350Hz and 1kHz, and complex sum of nearfield responses plotted below 300Hz (black).

Thanks for the additional info.

So, they could use an anechoic room to test the speakers, but didn't think of using a clamp to verify/minimize the effect of the side panel resonances. Not saying they should do R&D for anyone but it shows, to me, a lack of curiosity.

 

[Swerd]

So, they could use an anechoic room to test the speakers, but didn't think of using a clamp to verify/minimize the effect of the side panel resonances. Not saying they should do R&D for anyone but it shows, to me, a lack of curiosity.

The anechoic room measurements were done SoundStageNetwork – only measurements, no review. Stereophile reviewed and measured these speakers. I quoted some of the reviewer's comments, but not this part:

"I tried placing a cloth bag of lead shot atop each speaker, and while this did reduce the cabinet talk a little, it destroyed the Special Fortys' beautiful appearance. I then tried AudioQuest Sorbothane pads instead of Blu-Tack between speakers and stands. If anything, the enclosures' vibrational behavior worsened, then further worsened when I returned the bags of lead shot to the speakers' tops. I then replaced each Sorbothane pad with a trio of upturned Black Diamond cones under each speaker. In theory this should have made things even worse, as it allows the resonances to be fully developed. However, I felt that the midrange congestion was reduced in level, perhaps because, with the speaker/stand interface now providing no damping, the Q (Quality factor) of the resonances was increased, meaning that each resonance covered a narrower range of frequencies. With the upturned cones there was also less energy coupled into the stand. All I can suggest is that if you buy Dynaudio Special Fortys, you experiment with different interface materials between the speakers and your stands."​

 

[Swerd]

Someone near me just listed this for $50. I think it would do the job.
https://www.parasound.com/vintage/hca1000a.php

That should be just the ticket. A 250 W amp for $50 is pretty damn good, 20¢ per watt ! My cheapskate standards for a good price on a used amp is $1 per watt. Go get it, but make sure it works before you buy.

 

[highfigh]

The anechoic room measurements were done SoundStageNetwork – only measurements, no review. Stereophile reviewed and measured these speakers. I quoted some of the reviewer's comments, but not this part:

"I tried placing a cloth bag of lead shot atop each speaker, and while this did reduce the cabinet talk a little, it destroyed the Special Fortys' beautiful appearance. I then tried AudioQuest Sorbothane pads instead of Blu-Tack between speakers and stands. If anything, the enclosures' vibrational behavior worsened, then further worsened when I returned the bags of lead shot to the speakers' tops. I then replaced each Sorbothane pad with a trio of upturned Black Diamond cones under each speaker. In theory this should have made things even worse, as it allows the resonances to be fully developed. However, I felt that the midrange congestion was reduced in level, perhaps because, with the speaker/stand interface now providing no damping, the Q (Quality factor) of the resonances was increased, meaning that each resonance covered a narrower range of frequencies. With the upturned cones there was also less energy coupled into the stand. All I can suggest is that if you buy Dynaudio Special Fortys, you experiment with different interface materials between the speakers and your stands."​

It seems logical to me that if something resonates, DAMPEN THE PANEL THAT'S RESONATING! It's the reason bracing is used.

The sentence that begins with "However, I felt..." is the reason I don't read reviews. I don't care how the reviewer feels, I want to know what they think, know and measured. The energy coupled to the stand would have been mitigated by pressing on the sides.

 

[Verdinut]

It seems logical to me that if something resonates, DAMPEN THE PANEL THAT'S RESONATING! It's the reason bracing is used.

In the book "How to Build Speaker Enclosures" written by former Altec Lansing engineers, it says that one of the common fallacies about the design and construction of a speaker enclosure, is that the enclosure is some sort of a musical instrument -that it should "resonate" or have "tone". Various types of woods are thought to be more suitable because "they are used in musical instruments".

The first clear distinction that must be made with regard to speaker enclosures then, is that they are not musical instruments and that they should not generate musical tones. They should be more accurately defined as precision reproducers of musical tones. This means that ideally, they will add no tonal coloration of their own, but will remain passive, responsive only to the controlling input signal. The speaker and enclosure are as impersonal as a mirror and any sound emanating from them must be only those produced by the recording artist.

In some of their enclosure building booklets, Altec even admitted that some of their commercially sold enclosures did not have the ideal adequate bracing. I'm sure that they proceeded that way to cut on weight and shipping costs. But they also specified that, when you build your own enclosures, you can properly brace them so that they do not resonate.