Summer is here (at least for the next few days). So what could be better than driving your convertible with the roof down at last? So good to feel the wind in your hair, not so good if you want to listen to your music. What used to sound great with the roof up is now hollow sounding with a weak bass response and turning the bass up just makes everything muddy sounding. This phenomenon also occurs with the windows wound down on a conventional car to a slightly lesser extent. So what’s going on and how can we fix it?
As we know sound waves are just compression ripples in the air, in a car these waves are generated by the speakers and when the air is still and contained in a small cabin we do not need much energy to get a decent sound level. However, if we create large holes in the structure by opening the windows or taking the roof off we have opened the speakers up to the outside world. Suddenly your small speakers are trying to fill the world with sound; on top of that, they have to compete with more road and wind noise as well as all the air turbulence.
It would be OK if all frequencies were affected evenly and all that was needed was more power but unfortunately, this is not the case. Without getting into the technical reasons what we find is that the higher frequencies tend to be less affected but the mid-range clarity is lost and the bass is severely reduced. Try it for yourself, listen to a piece of music you know well while driving and then open the windows. It is obvious from the pictures above that far less sound is going to reach your ears.
What we need is to be able to adjust the sound to compensate for the changes brought about by opening the car. Certainly we need more power by adding an amplifier but more importantly, we need to be able to adjust the sound for the different configurations of the car so that it always sounds the same.
To achieve this we have a range of digital signal processors at Car and Home Stereo that we can configure and program to solve the problem. The two leading units are the JL Audio TwK88 and the Audison Bit family of processor/amplifiers. These powerful processors can be programmed to allow precise tuning of the vehicle acoustics and most importantly have multiple pre-set capabilities which allow the user to switch between different tunings for different conditions.
So how do we do it?
The process requires taking acoustic measurements with the vehicle roof up and down. The same can be done for a non-convertible car with windows closed and open. We use a combination of spectrum analysers and FFT (Fast Fourier Transform) measurements to map the acoustic signature of the vehicle interior. By playing a test signal (usually pink noise) we can see what the placement of the speakers and vehicle interior is doing to the sound and of course, we can do the same with the car open.
Once we have the measurements we can start to tune the system. We start the process by adjusting time delays and phase controls to see if there are any cancellations being brought about by speaker positioning, we will also adjust speaker crossover points for the same reason. We do this first because no amount of EQ tuning would compensate for a phase issue and in fact, would probably make the sound worse. It also means less work for the processor which will ultimately give me more control. The goal is a clean, flat response to the test signal which we can use as a starting point for the real world tuning for the customer.
Once we have the desired response for the car in the closed condition we look at the measurements for the converted configuration. The difference in the original measurements can now be compensated for by re-tuning the system and saving to a second pre-set. Once again we start with phase adjustments as with the top off there will be far less reflected sound and the speaker phase conditions will be different.
The result is the same audio performance for both configurations of the vehicle and at the touch of a button, you can switch between the two presets meaning great sound regardless of hood position.
We mounted the Audison controller for this Mercedes in the ash tray making it quick and easy to change presets but invisible when the cover was closed.
This tuning process is not simple and many different factors have to be considered as the adjustments are made. It also requires accurate and quite sophisticated measurement tools and at Car and Home Stereo we have invested in these to ensure we can achieve the best possible performance for our customers. We can also use these tools to identify acoustic problems in audio systems and we are more than happy to run a diagnostic on your car to help you choose the right way forward to better sound.
If you would like to take advantage of our test equipment or wish to discuss any aspect of tuning please feel free to call me on 01625 432707 or even better call into our shop on Sunderland Street in Macclesfield for a chat and a listen to our demonstrator.