It is very difficult to measure how much unwanted sound is radiated by a cabinet in use because it is swamped by the loud wanted sound from the drivers. How this is done in industry is by measuring the displacement/velocity/acceleration at a large number of points over the whole of the cabinet surface, solving a boundary integral equation to obtain the pressure at those points and then summing the sound radiated by every point on the surface to points of interest outside the cabinet like the listening position in front of the speaker. It provides the complete sound field around the speaker radiated by the cabinet which is what one really wants to know.
If you have a lot of time then you can do this by mapping a grid over the whole of your speaker (the more points the higher the frequency resolved), taking a measurement at each point using, say, an accelerometer (mind numbingly tedious but doable), solving for the pressure (free software available to do this) and summing to get the sound field outside the cabinet (free software available to do this). Here is an example of a student doing it with a laser vibrometer but there others if you search.
If you have a lot of time then you can do this by mapping a grid over the whole of your speaker (the more points the higher the frequency resolved), taking a measurement at each point using, say, an accelerometer (mind numbingly tedious but doable), solving for the pressure (free software available to do this) and summing to get the sound field outside the cabinet (free software available to do this). Here is an example of a student doing it with a laser vibrometer but there others if you search.
https://www.canadiantire.ca/en/pdp/...VrRitBh3uZAbLEAQYAiABEgLJuPD_BwE&gclsrc=aw.ds
If you get an accelerometer you will also need to build a pre-amp for it.
dave
If you get an accelerometer you will also need to build a pre-amp for it.
dave
Better just off the midpoint.
http://p10hifi.net/tlinespeakers/forum/panel-resonance.gif
And the subpanels the brace creates should have a higher aspect ratio that the panel being braced, and the 2 subpanels should not be equal.
dave
I have not had much luck, not near as quick as the sthescope but will give you some numbers.
dave
One of the reasons the stethoscope is so good, it isolates the sound of the box from the sound of the room/speaker.
Cheap, effective, it just won’t give you quantitiative results.
dave
yeah I was planning to put one brace just below the first woofer and the second brace in the middle of the second woofer.
I just tested two phone apps. They work well, and phyphox from Aachen Uni is versatile, it can do Fourier transform! But you must use sine wave generator (REW) and just watch the display of the app. As well setting the phone on various positions on a speaker's wall is tricky.
The problem is how to connect data to sine sweep signal that excites wibes.
Yesterday in another thread I recommended a contact mic, but obviously a USB version isn't available. Music instrument shops would be happy to sell also a sound card with usb-out...
The problem is how to connect data to sine sweep signal that excites wibes.
Yesterday in another thread I recommended a contact mic, but obviously a USB version isn't available. Music instrument shops would be happy to sell also a sound card with usb-out...
For that box not the right orientation, you ar ebreaking the rule.
But given you are going to be working thru the woofer hole your options are limited pretty much to “sticks”. which are the leats effective measure/
If i was doing this box from scratch there would be a vertical brace from top-to-bottom, just off centre, and bracing the woofers to the back of the box. And some braces side-to-side but not right behind the woofers.
dave
Cop hat off: I have tried to make a sort of crude DIY laser vibrometer. Basically it was an LED pointer that bounced off a mirror attached to a speaker cone or box. The laser movement was read either by a photocell or simple with a video camera. The results were not very useful. 🙁 The idea has promise, I think. Might just need a better implementation than I was able to give it. There are some very $$$$ laser vibrometers that I would use if I won the lottery. Not in the amateur budget.
Using piezo contact mics works well. As long as you have a high impedance preamp, they are failry wide band. Absolute frequency response isn't too important, as you will likely just be comparing two different panels with the same contact mic and the same signal. It's the difference that counts.
Using piezo contact mics works well. As long as you have a high impedance preamp, they are failry wide band. Absolute frequency response isn't too important, as you will likely just be comparing two different panels with the same contact mic and the same signal. It's the difference that counts.
Hmmmm
I do have a focusrite recording interface that supplies phantom power
maybe I can use that with a small accelerometer?
I do have a focusrite recording interface that supplies phantom power
maybe I can use that with a small accelerometer?
I don't see why. Just use a contact mic. The Focusrite usually have an "Instrument" input with an impedance of 1.5M That'll do nicely for piezo contact mics.
https://www.amazon.com/Agile-Shop-Contact-Microphone-Pickup-Guitar/dp/B07HVFTGTH/
https://www.amazon.com/Agile-Shop-Contact-Microphone-Pickup-Guitar/dp/B07HVFTGTH/