Sound Cancelling System
There is no question that a small block ford running 4000 or 5000 RPMs a foot away from your ears is loud. Insulation has only a limited effect. We tried driving with our Bose sound canceling headphones and the affect was amazing. The roar of the engine was tamed dramatically, but you could still carry on a conversation with the passenger. So turning the entire passenger compartment into a sound cancelling chamber seemed like an interesting idea. Here is how to do it:
1. A speaker box needs to be fabricated. We made a box, round in shape, to mount under the front clip. It is completely sealed, except for the opening on the bottom. The sound radiates directly down into the front passenger compartment. (Remove the access panel).
2. A 12” sub woofer was mounted on the bottom of the box aimed downward. The round opening for the speaker is used for access to the amplifier. The sound comes out at the driver and passenger’s feet, but since this is a low frequency out-of-phase sound generator, placement of the speaker really does not make much difference.
3. A high power amplifier was mounted inside the speaker box. It is powered with a four gauge wire connected through a high power DPDT relay, per the wiring diagram. Most any amp will work. A Polk 250 Watt amp was obtained from Crutchfield (Polk Audio PA500.4). It has a good sub woofer circuit, which is the only circuit used in this application. It is approximately 10” x 18” fitting neatly on the top inside of the speaker box, off to one side, to clear the back of the woofer.
4. The Bose headphones need to be dismantled. The sound cancelling circuit has a connection for (1) power, (2) microphone and (3) headphone speaker. This tiny circuit board is also mounted in the woofer box adjacent to the power amp. We simply stuck it to the power amp with a piece of double sided tape. (One could use a less expensive sound cancelling headphone circuit, such as Sony, but I don’t know how effectively they work compared the Bose.)
5. The Bose headphone circuit is powered by a 1.5 volt battery. Since a lithium ion AA battery will likely last 40 hours or more, it seemed much easier to simply power it with a battery than to fabricate a circuit to reduce the power from 12 to 1.5 volts. This requires the use of a DPDT relay to power off both the battery and the amplifier, as per the wiring diagram. We located the battery under the dashboard in a battery holder purchased at Radio Shack in a location where it could be easily accessed.
6. A tiny external microphone, available at Radio Shack, should be mounted in the engine compartment. It needs to be physically separated from the passenger compartment (by the bulkhead in this application). We mounted ours on the nozzle for the fire suppression system because it was conveniently located just in front of the distributor. A length of shielded wire needs to be spliced since the wire supplied with the microphone is not long enough to reach the amplifier. (Twisting two strands of speaker wire may also work, but shielding reduces the risk of stray signals being picked up.) The wire was run under the carpet to the speaker box.
7. An unused dash switch was used to activate the system. The power source for the relay switch was tapped into the starter switch “on” terminal so the sound cancelling would only operate when the ignition was turned on. (See diagram). There is no adjustment: it is either on or off. It is amazing what happens when driving down the road and one flips that switch!
There may be some fine tuning one could do, like adding a potentiometer to vary the input from the microphone giving some control over the degree of sound cancelling. The power draw from that amplifier is significant and the load on the alternator likely costs a few horsepower, so one would likely want to flip the sound cancelling off on the track. Keep in mind that this system does not make the car seem silent. It only reduces steady low frequency sounds, like the exhaust note, wind noise, and engine drone. One can still carry on a conversation.
04/01/09