Running CBM-170s as acoustic suspension?
Hi guys,
I'm the proud new owner of a pair of CBM-170s that I'm running full range as a "stereo" for TV, movies, and music.
Overall I'm very pleased with them. I have noticed that some bass lines sound a little smeared though. One example is the opening theme music to House of Cards. (You can find and listen to this on YouTube easily enough.)
I have a pretty significant amount of distance between the speakers and the wall behind them, about 18 inches. I suspect the wall is pretty reflective though, which I assume isn't great.
I was wondering if it would be reasonable to plug the ports?
I read somewhere that the drivers are different for ported vs. non-ported speakers. I wonder if bunging the ports would cause the speaker to distort, or worse, cause some kind of damage to the drivers? Or maybe put them at risk of overheating?
I don't know much about speakers any would appreciate any feedback about this idea. Thanks in advance!
Re: Running CBM-170s as acoustic suspension?
I should also add that the impacts from drums in the lower frequency range sound like "puffs" instead of distinct drum impacts. I think the problems are related.
Re: Running CBM-170s as acoustic suspension?
Do you have any type of room correction software running. It could help if there are problems with the rooms acoustics. It could also need tweeked if dynamic volume and dynamic eq are turned on.
Re: Running CBM-170s as acoustic suspension?
Quote:
Originally Posted by
natetg57
Do you have any type of room correction software running. It could help if there are problems with the rooms acoustics. It could also need tweeked if dynamic volume and dynamic eq are turned on.
No, I'm running my receiver in bypass mode. I want to hear the speakers in all their unprocessed glory. :)
I don't think equalization would help anything. Everything sounds like it's at the right levels. The bass just doesn't sound as tight as it could be. If I had to guess, I would say the issue is around 70Hz.
If I set the speakers to "small" on my receiver and set the cutoff at 80Hz, everything tightens up, but of course it's kind of a bummer without a subwoofer.
Re: Running CBM-170s as acoustic suspension?
Quote:
Originally Posted by
tomker
Hi guys,
I'm the proud new owner of a pair of CBM-170s that I'm running full range as a "stereo" for TV, movies, and music.
Overall I'm very pleased with them. I have noticed that some bass lines sound a little smeared though. One example is the opening theme music to House of Cards. (You can find and listen to this on YouTube easily enough.)
I have a pretty significant amount of distance between the speakers and the wall behind them, about 18 inches. I suspect the wall is pretty reflective though, which I assume isn't great.
I was wondering if it would be reasonable to plug the ports?
I read somewhere that the drivers are different for ported vs. non-ported speakers. I wonder if bunging the ports would cause the speaker to distort, or worse, cause some kind of damage to the drivers? Or maybe put them at risk of overheating?
I don't know much about speakers any would appreciate any feedback about this idea. Thanks in advance!
Hi Tomker,
What you are experiencing is simply the bass extension limitations of the 170's. You are asking the speaker to reproduce bass notes that it is not designed to accurately reproduce. With a ported speaker, (any ported speaker), if you send the speaker a frequency below the port tune frequency, the woofer will become unloaded from the cabinet (the internal volume of the cabinet no longer acts like a spring to assist in controlling the woofer cone movement). The woofer then moves back and forth creating a lot of air movement but it is not accurately reproducing that frequency. This is precisely what you are hearing...
Keep in mind these are not full range speakers. If you want accurate reproduction of deep bass notes, you will need to add a subwoofer or go with something like our Sierra-1, which has a much lower port tune.
Sealing the ports will make the situation worse, not better.. The woofers in the 170's are not designed for a sealed cabinet. Best option is to do exactly what you did, limit the deep bass going to the speaker by setting a high pass filter.
Re: Running CBM-170s as acoustic suspension?
Quote:
Originally Posted by
davef
Hi Tomker,
What you are experiencing is simply the bass extension limitations of the 170's. You are asking the speaker to reproduce bass notes that it is not designed to accurately reproduce. With a ported speaker, (any ported speaker), if you send the speaker a frequency below the port tune frequency, the woofer will become unloaded from the cabinet (the internal volume of the cabinet no longer acts like a spring to assist in controlling the woofer cone movement). The woofer then moves back and forth creating a lot of air movement but it is not accurately reproducing that frequency. This is precisely what you are hearing...
Keep in mind these are not full range speakers. If you want accurate reproduction of deep bass notes, you will need to add a subwoofer or go with something like our Sierra-1, which has a much lower port tune.
Sealing the ports will make the situation worse, not better.. The woofers in the 170's are not designed for a sealed cabinet. Best option is to do exactly what you did, limit the deep bass going to the speaker by setting a high pass filter.
Hi Dave, thank you very much for the informative reply.
Could you please give me (us) some guidance for receiver settings to get accurate LF output from the 170s?
By default, my receiver crossover is set to 80Hz. I don't know what the rolloff is but there is still audible output at 60Hz and below. Is this to be expected or should I investigate a software filter for my HTPC to get a steeper rolloff?
Am I correct in thinking that the undamped woofer movement you describe would result in irregularities in the frequency response graph? I see that the graph you have on the product page is very smooth. Are those measurements taken using a LF filter?
Thanks!
Re: Running CBM-170s as acoustic suspension?
Quote:
Originally Posted by
tomker
Hi Dave, thank you very much for the informative reply.
Could you please give me (us) some guidance for receiver settings to get accurate LF output from the 170s?
You can not improve the LF output of the speaker below the port tune frequency. If you are hitting the speaker with deep bass notes at a volume level that is significant enough so that you are hearing the increased air-flow due to the unloaded woofer, than setting the high pass filter in your receiver to 80Hz is recommended. You can also try setting it lower or higher, there is no "correct" setting -- whatever sounds best to you.
Quote:
Originally Posted by
tomker
By default, my receiver crossover is set to 80Hz. I don't know what the rolloff is but there is still audible output at 60Hz and below. Is this to be expected or should I investigate a software filter for my HTPC to get a steeper rolloff?
This is correct, the high pass filter in your receiver typically sets a 12dB/octave slope. This is fine. What volume levels are you listening at? If you start to hear the woofer bottoming during deep bass notes, turn the volume down or raise the crossover frequency.
Quote:
Originally Posted by
tomker
Am I correct in thinking that the undamped woofer movement you describe would result in irregularities in the frequency response graph? I see that the graph you have on the product page is very smooth. Are those measurements taken using a LF filter?
No, measurements are always taken with a full range signal. When the woofer becomes unloaded (undamped is not the correct term and is something else entirely) it is reflected by a steep decrease in output. There would be no frequency response anomalies unless you were hitting the woofer with enough voltage such that the woofer was reaching its mechanical and/or thermal limits.
What you are hearing is not related to frequency response.
You can see the port tune frequency of the speaker by examining the impedance measurement.
hope this helps!
Re: Running CBM-170s as acoustic suspension?
Sorry for using the wrong terminology. I'm new to all of this and am running up against the limits of my knowledge.
To answer your question about listening volume, I have been listening with my receiver set to around -30 dB. The receiver is advertised to be 70 WPC with 2 channels driven, so I suspect -30 dB is equal to 0.07 watts.
Using an SPL meter app that I have for my iPhone, I measured the ambient sound pressure to be around 36 dB, and with music playing, the SPL was in the low to mid 60s (from my listening position about 2 meters away). I don't know about the accuracy of such an app though.
I'm afraid I don't know how to derive port tune frequency from an impedance graph. I see that there are two big spikes in the graph, around 25Hz and 83Hz. Does this mean the port tune frequency is... 83Hz? I have yet to find a good explanation of how bass reflex ports work online.
Re: Running CBM-170s as acoustic suspension?
Quote:
Originally Posted by
tomker
To answer your question about listening volume, I have been listening with my receiver set to around -30 dB. The receiver is advertised to be 70 WPC with 2 channels driven, so I suspect -30 dB is equal to 0.07 watts.
That is not quite how it works with regard to attempting to determine how many watts your receiver is sending to the speakers. But that is really not important as I do not suspect you are reaching the limits of your receiver.
Quote:
Originally Posted by
tomker
Using an SPL meter app that I have for my iPhone, I measured the ambient sound pressure to be around 36 dB, and with music playing, the SPL was in the low to mid 60s (from my listening position about 2 meters away). I don't know about the accuracy of such an app though.
This iPhone SPL apps are very inaccurate. Mid 60's for SPL is very low. Typical listening levels seem to average about 75dB. I asked you the original question just to be sure you are were not playing at very loud volume levels, which I don't believe you are -- so all is good :)
Quote:
Originally Posted by
tomker
I'm afraid I don't know how to derive port tune frequency from an impedance graph. I see that there are two big spikes in the graph, around 25Hz and 83Hz. Does this mean the port tune frequency is... 83Hz? I have yet to find a good explanation of how bass reflex ports work online.
The port tune frequency can be determined by the "saddle" between the two low frequency impedance peaks. In the case of the 170's, this is at approximately ~50Hz. With regard to understanding how a bass reflex speaker works, Try Googling "Helmholtz Resonator".
Hope this helps!
Re: Running CBM-170s as acoustic suspension?
Quote:
Originally Posted by
davef
...
The port tune frequency can be determined by the "saddle" between the two low frequency impedance peaks. In the case of the 170's, this is at approximately ~50Hz. With regard to understanding how a bass reflex speaker works, Try Googling "Helmholtz Resonator".
Hope this helps!
I see. This prompted me to download a tone generator. Regardless of my receiver settings, I'm getting minimal output from the speakers below 50Hz at my usual listening levels, so I don't know if those frequencies are causing any audible problems.
I did notice that the output around 60Hz seems MUCH louder than around 70Hz and above. Could this be something to do with output from the port, or a room mode, or both, or neither...?
Thanks again for your time.
Re: Running CBM-170s as acoustic suspension?
The speakers output is very linear. Any emphasis on a particular frequency will be caused by the room.
Re: Running CBM-170s as acoustic suspension?
Quote:
Originally Posted by
tomker
I see. This prompted me to download a tone generator. Regardless of my receiver settings, I'm getting minimal output from the speakers below 50Hz at my usual listening levels, so I don't know if those frequencies are causing any audible problems.
I did notice that the output around 60Hz seems MUCH louder than around 70Hz and above. Could this be something to do with output from the port, or a room mode, or both, or neither...?
Thanks again for your time.
This is due your room. The bass output of the speaker falls off linearly. I suggest you read about room modes. http://en.wikipedia.org/wiki/Room_modes
Re: Running CBM-170s as acoustic suspension?
Quote:
Originally Posted by
davef
Thank you. I just checked and the wavelength of 60Hz is 18 feet which is almost exactly the length of the room where I have the speakers. So that makes sense. I was wondering if the placement of the speakers relative to the rear wall was making a difference but I suppose that with wavelengths this long, it doesn't really matter. (I have them about 18" from the wall.) And from the Wikipedia article it sounds like any treatment I can put on that wall will be fairly pointless, again due to the wavelength. I suppose I will have to resign myself to the idea that my apartment will not sound like a concert hall. :) Thanks for the time and patience.
