The MG hardtop was restored as described in the previous post: an interior layer of Dynamat and leather trim, an exterior fitted cloth, all seals replaced, a new tempered rear window, and replaced latches. With the top on the MGB, which has also undergone extensive sound treatment and interior upgrades, there is still a weak spot in the engine firewall area. While the hood is treated, the firewall is not 100% treated due to spacing issues and all the cutouts for the dash electronics. This is an area where some improvements could be made, but that's for a later task.
Without the top, on a six-lane interstate in San Diego, CA, you can measure 95dB at 65 mph—not ideal, but that is the extreme with no top. With the hardtop on, the dB level was lowered to 70dB, and let's remember that was with the overdrive engaged. These are impressive results given that a 3dB drop alone represents a halving of the sound intensity and is perceivable to the human ear. A drop from 95dB to 70dB is a 25dB reduction—wow!
Comparative Noise Levels
Referring back to the 1976 Road and Track Magazine reference, they did not mention if overdrive was engaged, if they were on a six-lane interstate, or if there were superchargers. They likely used stock mufflers and had a soft top, not a hard top, and no RPM was specified. Nevertheless, the measurements in my MG without overdrive engaged were as follows:
Speed | Road and Track dB | My MG dB |
Idle | 61dB | 56dB |
50 mph | 75dB | 72dB |
70 mph | 82dB | 76dB |
There is a significant improvement with the hardtop, but to be frank, I don’t see how the soft top stock MGB from 1976 could hit those dB numbers; they should be much higher unless they were coasting or lugging the engine.
The bottom line: you can significantly improve your audio experience with a treated hardtop paired with a treated car over the open-air roadster rolling down the road. The improvements are not just a drop in the ambient noise, which improves the audio contrast, but you will also experience benefits from “room treatment” via sound deadening and some reflection and diffusion from the hard surfaces and angles. All that should be expected and not a surprise. The real takeaway is how low you can drive the ambient dB in this environment.
For me, although I love great audio, the roadster is best with no top. I want to hear the engine, the wind, and the overall driving experience. I can always pick a quieter road and pump up the volume.
Enjoy the drive!
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