Measuring the sound radiation of speaker cabinet walls



Last updated: Nov. 25, 2017

Measuring sound radiation of speaker cabinet walls is a real challenge. The conventional frequency response measurement methods fail, so we have to look for other ways. Unfortunately, most of these methods require professional tools, and the majority of DIY loudspeaker builders are discouraged from measuring sound transmission loss. Around 2013 I came up with a new idea how to measure sound insulation of speaker cabinets with a very simple method, using a closed back speaker only.

Several attempts and methods are known for measuring the sound radiation of speaker cabinets. Some of them are based on vibration measurement data, while others use a microphone for the measurement. Note that the panel vibration doesn't represent the acoustic output directly so additional calculations have to be performed.

Here is a brief description of the best-known methods:

I came up with a very simple method how to measure speaker cabinet sound insulation. I choose a tweeter with very low resonance frequency, and during the sound insulation measurements I've turned it inwards the cabinet. Theoretically any kind of loudspeaker with a low resonance frequency and with closed chamber suits, if it is possible to achieve 90dB/m sound pressure from 100Hz upwards without distortion. Another criteria is that the cabinet should not alter the response of the speaker, more closely the T/S parameters, when the speaker faces inwards. Fortunately, this criteria is true if we use closed back tweeters or midranges.


cabinet sound insulation, panel vibration, transmission loss measurement - Vifa DX25TG05-04

The test cabinet and the Vifa DX25TG05-04

For the tests I used a Vifa DX25TG05-04 tweeter (Fs: 650Hz, Qts: 0,67). The test signal was a sine sweep with 2V RMS. I made the recordings by ARTA software.

When I measured the reference signal the tweeter was turned outwards (as a normal speaker in a normal cabinet). I made the reference signal measurement from a distance of 5cm and 20cm from the tweeter's faceplate and on axis. When I measured the side walls I turned the tweeter inwards the cabinet. I covered the tweeter with a flowerpot which was filled with sound absorbing material. The sound radiation measurements of the cabinet walls made from a distance of 5cm and 20cm from the center of the panels.

Finally I corrected the response between 200Hz and 1000Hz because the tweeter response shows a gradual decrease below 1000Hz (see lower figure). The results are appreciable above 300Hz - it's enough because the side panel resonances are higher in frequency.

cabinet sound insulation, panel vibration, transmission loss measurement - Vifa DX25 frequency correction

Frequency correction between 200Hz and 1000Hz

The external dimensions of the test cabinet: 27cm * 27cm * 39cm. The larger sides of the cabinet are made of MDF, particle board, birch plywood and OSB sheets with thickness of 18mm (I measured these only). The net volume of the cabinet is 19,3 litres. Unfortunately, before the measurements I added side bracing to the particle board, so I do not report measurement results for this.

The air closed into the enclosure shows standing wave resonances at 485Hz, 970Hz, 1455Hz in the longitudinal direction, and at 1470Hz in lateral direction. These resonances (especially the one at 1470Hz) are very strong due to the bad geometry, in a well-designed cabinet they would be smaller by 6-10dB.


cabinet sound insulation, panel vibration, transmission loss measurement - test cabinet for sound insulation measurements

The test cabinet

The real advantage of this method, that we can measure the reference signal and the sound transmitted through the panels with the same tools. What we have to do only is to flip the speaker and that's all. We don't need computer simulation software and acceleration sensor or an extra cabinet, only a tweeter or a closed back midrange with low fs. The drawback of this method is that the mechanical vibrations generated by the woofer are missing from the measurements. (Side note: I think that the resonance at 200 Hz comes from the magnet vibrations of the Vifa DX)

In a few words on the graphs. The light green is the reference, the blue and the black are the sound radiation measurements of the panels. The blue is measured with empty cabinet, the black is measured with cabinet filled 60% with sound absorbing material. The difference between the reference and the sound radiation is the sound transmission loss (sound insulation) in decibels.


OSB measured from 5cm - speaker cabinet sound insulation, panel vibration, transmission loss measurements

OSB measured from 5cm


OSB measured from 20cm - loudspeaker cabinet sound insulation, panel vibration, transmission loss measurements

OSB measured from 20cm


MDF measured from 5cm - loudspeaker cabinet sound insulation, transmission loss measurements

MDF measured from 5cm


MDF measured from 20cm - speaker enclosure sound insulation, transmission loss measurements

MDF measured from 20cm


plywood measured from 5cm - loudspeaker enclosure sound radiation measurements

Plywood measured from 5cm


plywood measured from 20cm - loudspeaker cabinet sound radiation measurements

Plywood measured from 20cm


And finally, two graphs for comparing MDF with plywood. The MDF has a stronger panel resonance at 640 Hz.


comparison of MDF and plywood 5cm, empty cabinet

Plywood vs. MDF from 5cm, empty cabinet


comparison of MDF and plywood 5cm, cabinet filled 60%

Plywood vs. MDF from 5cm, cabinet stuffed with polyester fill in ~60%


It is visible from the measurements that the sound absorbing material plays a very important role in reducing the sound pressure level inside the enclosure and hence decreasing sound radiation from the cabinet. Due to the foam the intensity of the sound transmitted through the walls decreased by 10dB on average. The sound insulation for the plywood with filling is between 30-40dB, which is not so bad.

There's only one question left: What is the best wood for speaker enclosures? The difference is not so great between wood types as commonly believed. Plywood has better sound transmission loss than MDF in the resonance region, but it's hard to recognize the difference in subjective tests.

Csaba Horvath

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